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left: https://upload.wikimedia.org/wikipedia/commons/3/31/ 06_Spiral_CFL_Bulb_2010-03-08_%28white_back%29.jpg right: https://en.wikipedia.org/wiki/ Compact_fluorescent_lamp#/ media/ File:Elektronstarterp.jpg Explain why nickel deposits are associated only with mafic magma and not with intermediate or felsic magma? What is the composition of the black smoke in a black smoker, and how does that relate to a volcanogenic massive sulphide deposit? How might an epigenetic gold deposit be related to a porphyry deposit? Oxidation and reduction processes are important to both banded iron formation deposits and unconformity- type uranium deposits. Explain the role in each case. A typical kimberlite in northern Canada may look something like the diagram shown below. In this case, the diameter at the surface is around 500 m, and the total depth is about 2,500 m. Bearing in mind that an open pit cannot typically be any deeper than it is wide, what mining method(s) might be most applicable to a deposit of this type? [SE] What mineral is typically responsible for acid rock drainage around mine sites, and why is this mineral so common in this setting? Explain why glaciofluvial gravel is more suitable than till as a source for aggregate. The raw material for making cement is lime (CaO), and this is typically produced by heating limestone (mostly CaCO3) to about 1,000°C. Why is this an environmental issue? Name some important industrial minerals that form in an evaporite setting. If organic matter accumulates at an average rate of 1 mm per year, and if 10 m of organic matter is

Describe the general makeup and ages of the provinces of Laurentia, Laurentia’s journey over the past 650 Ma, and the processes by which additional rocks were added on its eastern, northern, and western margins during the Phanerozoic to form the continent of North America Explain the timing and depositional environments of mid- and late-Proterozoic sedimentary rocks in western Canada Describe the depositional environments and types of sedimentary rock that accumulated on the western margin of North America and in the Western Canada Sedimentary Basin (WCSB) during the Paleozoic Summarize the extents, geological origins, and migration of the accreted terranes of British Columbia and Yukon Explain how terrane accretion on the west coast during the Mesozoic contributed to the formation of the Rocky Mountains and how that in turn provided the source material for a thick sequence of Mesozoic sedimentary rock in the WCSB Describe the origins of the Mesozoic intrusive igneous rocks of the Coast Range and other areas within British Columbia Describe the geological effects of the accretion of the Pacific Rim and Crescent Terranes, the nature of WCSB deposition in the early Cenozoic, the ongoing volcanism and earthquake activity in western Canada, and the general effects of the Pleistocene glaciation in western Canada

Figure 21.1 Crowsnest Mountain in the southern Alberta Rockies is made up of Paleozoic rocks that were uplifted by continental convergence during the Mesozoic, and then eroded by glaciation during the Cenozoic [SE] Western Canada has a fascinating geological history with rocks ranging in age from the Archean to the Holocene. Over that time, almost every conceivable geological process has taken place here, resulting in the formation of a wide array of rock types, and some of the most important fossil deposits in the world. The region is also endowed with a range of geological resources, spanning the periodic table from beryllium to uranium, and the geological processes have produced awe-inspiring scenery and world- class recreational opportunities. This chapter focuses on the important geological history and geological features of western Canada, but includes an overview of Canadian geology as a whole, starting with the development and journey of the ancient continent of Laurentia.

Laurentia, which makes up the core of North America, is the largest and arguably the oldest of Earth’s cratons (regions of stable ancient crust). Some of the rocks are over 4 billion years old, and Laurentia has been together in its present form for the last billion years. Over the past 650 million years, Laurentia has moved along a zigzag path from deep in the southern hemisphere to close to the North Pole (Figure 21.2). During that time, it collided several times with other continents and was temporarily part of two supercontinents (Pannotia and Pangea).

Bodies of rock tend to be eroded and recycled through the processes of plate tectonics, including uplift leading to erosion and burial leading to melting, and thus there are very few areas of truly ancient rocks on Earth. The oldest undisputed rocks are those of the Acasta Gneiss from north of Yellowknife, Northwest Territories, aged 4.03 Ga. But there are some rocks that could be even older within the Nuvvuagittuq greenstone belt on the east coast of Hudson Bay, in Quebec. These have been isotopically dated at 4.28 Ga, although the reliability of that date has been questioned. Based on other data, it is acknowledged that the Nuvvuagittuq rocks are at least as old as 3.75 Ga. The Acasta and Nuvvuagittuq rocks are situated within the Slave and Superior Cratons respectively, the oldest parts of Laurentia (Figure 21.3). Although these ancient cratons are not consistently that old, they are generally older than 3 Ga, as is part of the Wyoming Craton. The Hearne and Rae Cratons are older than 2 Ga, while most of the other parts of Laurentia are aged between 1 Ga and 2 Ga. The various provinces of Laurentia were assembled by plate-tectonic processes between 1 Ga and 3 Ga. The areas of Figure 21.3 that are left uncoloured — the Appalachian, Innuitian, and Cordilleran fold belts — are geological regions that have been added to North America since 500 Ma. These are at least partly made up of sedimentary rocks that were deposited along the coasts and then folded, faulted, and uplifted during continental collisions.

Figure 21.3 The main provinces of Laurentia. The pink areas are the oldest; light yellow are the youngest. All of the areas south and west of the dotted red line are now covered with younger rocks. The white areas represent rocks that were added to North America since 700 Ma. [SE] Laurentia was part of the supercontinent Rodinia during the period between 1,100 Ma and 700 Ma. As Rodinia started to break up after 700 Ma, sediments derived from the erosion of the interior of the continent began to accumulate along its coasts, initially along the west coast, then the east coast at around 600 Ma, and finally on the north coast by around 550 Ma. This process continued for several hundred million years. By around 450 Ma, large areas of the interior of Laurentia were depressed below sea level — probably because of the downward pull of an underlying subducting plate — and marine sediments were deposited over parts of Quebec, Ontario, Manitoba, Saskatchewan, Alberta, and the Northwest Territories during the Ordovician, Silurian, and Devonian Periods (450 Ma to 350 Ma). These sediments are coloured various shades of blue on the geological map of Canada (Figure 21.4).

At approximately 350 Ma, the part of Gondwana that is now Africa collided with the eastern coast of North America, thrusting volcanic islands and sedimentary layers far inland to become the Appalachian fold belt. The Appalachian Mountains would have rivalled the Himalayas in extent and height during the Devonian. At about the same time, a smaller continent, Pearya, collided with the north coast, creating the Innuitian fold belt. At around 200 Ma, small continents that now make up the interior of B.C. and part of Yukon collided with the west coast of North America, starting the process of thrusting the sedimentary rocks inland and upward to form the Rocky Mountains. The west-central part of North America subsided once again at around 150 Ma, due to an underlying subducting plate, and this led to the deposition of more marine rocks across Manitoba, Saskatchewan, and Alberta, and north into the Northwest Territories and Yukon (the green areas in Figure 21.3).

Laurentia extends as far west as eastern B.C. (Figure 21.3), but the ancient rocks of the craton are almost completely covered by younger rocks in B.C., Yukon, and all of Alberta except the far northeast corner. Laurentia is well represented in northern Saskatchewan and across large parts of Manitoba, the Northwest Territories, and Nunavut (Figure 21.5). Where they are exposed, the rocks of the Canadian Shield are highly varied lithologically, typically strongly metamorphosed due to their deep burial at some time in the past, and in some cases, quite different from what could be expected to occur on Earth today. Starting from the south, in eastern Manitoba and adjacent Ontario, we have the ancient rocks of the Superior Province. On the map the Superior Province, rocks are mostly pink, representing granitic and gneissic rocks, with strips and blotches of green, representing metamorphosed sea-floor basalt and sediments, also known as greenstone belts. These rocks are widely interpreted to have deep crustal origins, and include large areas of granulite facies metamorphic rock formed at high temperatures and moderate to high pressures (see Figure 7.19). Superior Province greenstone belts in Ontario and Quebec host some of the world’s largest volcanogenic massive sulphide deposits. As described in Chapter 20, the Superior Province in northern Manitoba is host to important nickel deposits at Thompson. These formed from mantle-derived mafic magma that interacted with sulphur-bearing crustal rocks, and within which heavy-metal sulphide minerals formed. The Trans-Hudson Orogen (THO), as its name implies, extends through Saskatchewan and Manitoba and over to the eastern side of Hudson Bay. It represents the continent-continent collision zone between the Superior Craton to the south and the Churchill Craton (including the Wyoming, Hearne, and Rae Cratons) to the north; thus it’s a remnant of the initial formation of Laurentia at around 1.9 Ga. At the time of the collision, the THO would have been a major mountain range, and the rocks that we see there now — which evolved deep beneath those mountains — are highly metamorphosed sedimentary and volcanic rocks intruded by large granitic bodies. The important volcanogenic massive sulphide deposits around Flin Flon are within the THO.

Figure 21.5 Geological features of the Canadian Shield of western Canada. A.B.: Athabasca Basin, T.B.: Thelon Basin, and TMZ: Taltson Magmatic Zone [ By SE after: geoscan/servlet.starweb?path=geoscan/ fulle.web&search1=R=208175] The Churchill Craton is lithologically similar to the Superior Craton, although not generally as old. It includes two important sedimentary basins: the Athabasca Basin in Saskatchewan and the Thelon Basin in Nunavut, both filled with rocks aged around 1.7 Ga. These consist primarily of sandstones and minor mudstones that are only weakly metamorphosed and essentially undeformed (not folded) because they are situated within a stable craton and so have not been subjected to significant tectonic forces. The Athabasca Basin is economically important for its large and rich unconformity-type uranium deposits (see Chapter 20). At its western end, there is the remnant of a large extraterrestrial impact, the 40 km diameter Carswell Crater. When the meteor struck at this location, at around 115 Ma, the impact and subsequent rebound of the crust was enough to bring metamorphic rock up to surface from beneath about 2,000 m of Athabasca Group sandstone. There is no connection between the Carswell Crater and the much older (~1.2 Ga) uranium deposits. The Taltson Magmatic Zone (TMZ), which forms the boundary between the Churchill and Slave Cratons, consists primarily of granitic rock. One interpretation is that the TMZ formed along a convergent boundary, although this is not universally accepted. The Slave Craton is dominated by granitic rocks and metamorphosed clastic sedimentary rocks. On its western edge, there is a large area of very old gneissic rock that includes the Acasta Gneiss, dated at 4.03 Ga, which, for the time being at least, is the oldest rock in the world (Figure 21.6).

The Wopmay Orogen, interpreted as the site of another ancient continent-continent collision, lies to the west of the Slave Craton. Although mostly composed of felsic igneous rocks and gneisses, the Wopmay Orogen includes a body of mafic and ultramafic igneous rock called the Muskox Intrusion. Derived from a mantle plume and dated at about 1.1 Ga, the Muskox is comparable to a handful of other mafic and ultramafic intrusions around the world in that it has distinctive repetitive layering caused by settling of heavy metal-rich minerals within the low-viscosity magma. Muskox has high levels of nickel, copper, and chromium, and has the potential to have platinum and palladium like a similar body in South Africa. Ultramafic intrusions like Muskox do not take place on Earth today because the mantle is no longer hot enough. The oldest rocks in British Columbia are the strongly metamorphosed sedimentary, volcanic, and intrusive rocks of the Monashee Complex, situated to the west of the Columbia River near Revelstoke (Figure 21.7). Aged around 2 Ga, these may actually be part of Laurentia.

There are much more extensive Precambrian rocks within the Columbia and Rocky Mountains of southeastern B.C. and the southwestern corner of Alberta. The rocks of the Purcell Supergroup (a supergroup comprises more than one group) are present in the extreme southeastern corner of B.C. and adjacent Alberta, and extend well into the United States (as the Belt Supergroup). These are mostly unmetamorphosed clastic rocks deposited in rivers and lakes during the middle Proterozoic, at around 1,400 Ma, while Laurentia was still part of the supercontinent Columbia. When Columbia rifted apart, the division happened within the area of the Purcell/Belt rocks. Similar rocks of the same age are present in Tasmania and Siberia, and it is postulated that they were once part of the same depositional basin.

The Windermere Group rocks — also mostly clastic sedimentary — were deposited in the ocean along the western edge of Laurentia (Figure 21.7) in the late Proterozoic (around 700 Ma) after the breakup of Columbia. In fact, sedimentary rocks of this age extend all along the western side of the Rocky Mountains, well into Yukon. Deposition in this area was taking place during the late Proterozoic Snowball Earth glaciations, as can be seen in Windermere Group rocks of the Toby Formation from the area south of Cranbrook, B.C. (Figure 21.8). The Toby Formation is a fine-grained marine rock (mudstone) with numerous large angular clasts of limestone and quartz. The mud was deposited in the quiet water of a continental slope environment, and the large clasts were dropped from floating ice derived from glaciers on Laurentia. The Toby Formation is unique in this area; most of the rest of the late Proterozoic clastic sedimentary rocks in this region do not have glacial dropstones.

At the beginning of the Paleozoic (542 Ma), Laurentia was near the equator (Figure 21.2) and sedimentation was continuing on all of Laurentia’s marine margins, including the passive margin (not tectonically active) on what is now the west coast. The clastic sediments of the Windermere Group are succeeded by mostly limestone beds (represented by the blue areas in Figure 21.7) interbedded in some areas with mudstone and sandstone. The most famous Cambrian rocks in the Rockies are those around Field, B.C., within Yoho and Kootenay National Parks. The Burgess Shale of the Stephen Formation is considered by some to be the most important fossil bed in the world because of its spectacular preservation of detail in a wide array of organisms that are ancestors to many of today’s organisms and are not present in earlier rocks. The Walcott Quarry, on the pass between Mt. Field and Wapta Mountain has been known and studied for over 100 years (Figure 21.9). In 2012 a new Burgess Shale discovery was made at Marble Canyon, about 30 km to the southeast, by a team led by the Royal Ontario Museum (ROM). Fossils with similar levels of preservation are present, and several previously unknown organisms have been found. The ROM continues to work in the Marble Canyon area and some of their discoveries are described and illustrated on this website: . The Paleozoic strata of the Rockies also include Ordovician, Devonian, Carboniferous, and Permian sedimentary rocks. For example, Carboniferous limestone makes up most of the upper part of Crowsnest Mountain in southern Alberta (Figure 21.1).

While clastic and carbonate sediments were accumulating along the western edge of Laurentia, much of the interior of the continent was submerged under inland seas that were connected to ocean most of the time. This region is known as the Western Canada Sedimentary Basin (WCSB). The Paleozoic sediments that accumulated within this basin show up as the blue areas in Figures 21.4 and 21.5; however, their

575 Chapter 21 Geological History of Western Canada extent is much wider than that because Paleozoic sedimentary rocks also underly the Mesozoic rocks within most of the areas that are light green on those maps. By way of example, a schematic cross- section through the Paleozoic and Mesozoic rocks of southern Manitoba is given in Figure 21.10. The section extends from the Saskatchewan-Manitoba border on the left to just east of Winnipeg on the right, and shows the Paleozoic rocks overlain on the rocks of the Precambrian Superior Craton. Fifteen different Paleozoic formations, ranging in age from Ordovician to Carboniferous, are shown in Figure 21.10. Of these, 11 are dominated by carbonate rocks (limestone or dolomite) that very likely formed in an ocean-connected marine environment. The non-carbonate formations are the lowermost one (resting on Precambrian rocks), which is sandstone of marine origin; the Devonian Prairie Evaporite Formation (in red) — the same formation from which potash is mined in Saskatchewan; and the upper two Devonian formations (in yellow), which are shale. When the Prairie Evaporite formed, the basin was isolated from the open ocean, and the rate of evaporation was greater than the rate of input from precipitation and river inflow. During that time, probably at least several million years, there were numerous changes in sea level or land level that allowed additional ocean water — and therefore additional salt — into the basin. Figure 21.10 The Paleozoic sedimentary rocks of southern Manitoba along a section extending from the Saskatchewan border on the left to the Winnipeg area on the right. The section is 400 km wide and 1,800 m high, and the vertical exaggeration is about 100 times. The dip of the beds is also exaggerated by 100 times; their original and current attitudes are close to horizontal. [SE] There are Paleozoic rocks in the central and western parts of British Columbia and Yukon, but they formed far away and did not become part of North America until the Mesozoic. Subduction started along the western edge of Laurentia by the middle Paleozoic. That meant that oceanic crust was moving toward the continent, bringing small segments of exotic continental crust with it (Figure 21.11). These crustal blocks along western North America are called terranes, indicating that they are sections of the continent that have an exotic origin (Figure 21.12). Most of British Columbia is made up of terranes that include sedimentary rocks with fossils that imply an origin south of the equator, or volcanic rocks with magnetic orientations that indicate a southern-hemisphere origin.

Figure 21.11 The distribution of continents in the early Carboniferous, showing the terranes that later became attached to the west coast of North America. The light blue areas are continental shelves, the white is ice of the Karoo Glaciation, and the red line shows subduction of oceanic crust beneath Laurentia. Panthalassic is the name for the huge ocean that preceded the Pacific Ocean. [SE based on information from Christopher Scotese at

The Mesozoic extends over 187 million years from the beginning of the Triassic (252 Ma) to the end of the Cretaceous (65 Ma). It was a particularly important period for the geology of western Canada. During this time, several continental collisions occurred along the west coast, resulting in the formation of the Rocky Mountains and the accretion (addition) of much of the land mass of British Columbia, and continuing deposition within the WCSB.

Continued subduction along the western edge of North America carried a number of continental terranes toward the coast, with the first collisions taking place in the early part of the Triassic, as the Quesnel, Cache Creek, and Stikine Terranes combined to form the Intermontane Superterrane, so named because it forms the interior plateau of British Columbia, between the Rockies to the east and the Coast Range to the west (Figure 21.13). Figure 21.13 Model of the accretion of the Intermontane and Insular Superterranes to the west coast of North America during the Mesozoic. Subduction zones are the red-toothed lines. The dark-red triangles represent volcanoes. [SE] Approximately 100 million years later, another pair of terranes — Alexander and Wrangellia — collided to form most of Vancouver Island and Haida Gwaii, plus a significant part of Alaska. During the Cenozoic, additional terranes (the Outboard terranes) were added to the western edge of North America. An overview of the accreted terranes of B.C., Yukon, and Alaska is given in Figure 21.14.

Figure 21.14 A generalized overview of the accreted terranes of B.C., Yukon, and Alaska. The Intermontane terranes are in green, the Insular terranes in purple, and the Outboard terranes in yellow. The Coast Plutonic Complex (CPC) formed in situ and is not a terrane. [SE after Yukon and BC Geological Surveys] During the Jurassic, the Intermontane Superterrane acted like a giant bulldozer, pushing, folding, and thrusting the existing Proterozoic and Paleozoic west coast sediments eastward and upward to form the Rocky Mountains (Figure 21.15). The same process continued into the Cretaceous as the Insular Superterrane collided with North America and pushed the Intermontane Superterrane farther east. Folding in Rocky Mountain rocks, like that shown in Figures 21.16 and 21.17, is one of the results of this process.

Figure 21.15 Cross-section of the accretion of the Intermontane Superterrane to the west coast of North America and the resulting compression, folding, and thrusting of North American sedimentary rocks. In the Late Cretaceous, it was the accretion of the Insular Superterrane pushing against the Intermontane Superterrane that did most of the work. [SE]

Thrusting is another important process in the formation of fold-belt mountains, as described in Chapter 12. During plate convergence, entire sheets of sedimentary rock are slowly pushed over top of other sheets, resulting in situations where older rocks lie on top of younger ones. One of the best known examples of this is at Mt. Yamnuska, near Exshaw, Alberta (Figure 21.18), where the older Cambrian rocks were pushed east by a total of 40 km, over top of younger rocks.

Figure 21.19 The Lewis Thrust at Crowsnest Mountain near Frank, Alberta. Carbonate rocks of Devonian and Carboniferous age have been pushed 80 km to the east and thrust over top of Cretaceous mudstone. [SE] Not only did the subduction of oceanic crust beneath North America during the Mesozoic deliver geologically exotic terranes to the western edge of the continent, it also resulted in massive amounts of volcanism along the boundary (Figure 21.13). The upper-crustal magma chambers that fed those now- eroded volcanoes slowly cooled into granitic and dioritic stocks, and those stocks gradually coalesced into batholiths that extend from the southwest corner of B.C. all the way into Yukon and Alaska (Figures 21.20 and 21.21). Most of the granitic rocks of this region fall into two main age ranges: many are middle-Jurassic to early Cretaceous in age (~ 170 Ma to 140 Ma), while others are late Cretaceous to Paleogene (~50 Ma to 90 Ma). Many of the older bodies intruded into the terranes they are on before they arrived on the North America coast. This applies to those on Vancouver Island and Haida Gwaii. Some of the older ones formed in situ when the subduction zone was farther east (see Figure 21.13). Most of the younger bodies formed in situ when the subduction zone was close to where it is now (west of Vancouver Island) or slightly to the east. Although these intrusive igneous rocks cooled at depth in the upper crust, they now form some of the highest peaks in Canada, many of them hundreds of metres higher than those of the Rocky Mountains (including Mt. Waddington, the highest peak entirely within British Columbia at 4,091 m). It is estimated that over the past 100 million years some of these igneous bodies have been uplifted in the order of 8,000 m. Much of that uplift is a result of the relative low density of the granitic rocks compared with the surrounding rocks.

The construction of the Rocky Mountains during the Jurassic and Cretaceous — and their ensuing erosion — created a significant new source of sediments for the WCSB. Based on the ages, distributions, and thicknesses of the sedimentary layers (Figure 21.23), it is evident that the greatest volumes of sediment were produced in the Upper Cretaceous (100 Ma to 65 Ma) and into the Paleocene (65 Ma to 55 Ma). The sediments accumulated in a basin that is thought to have been at least partly formed by the presence of a subducting slab of oceanic lithosphere underneath this part of North America. However, Physical Geology 584 the ongoing uplift of the Rockies through this time period also led to isostatic depression of the crust. The western edge of the basin, which has about 4,500 m of Mesozoic rock alone, is a foreland basin. (See Chapter 6 for more on the origins of basins.) Figure 21.22 The distribution of Mesozoic sedimentary rocks in the Western Canada Sedimentary Basin. J stands for Jurassic and Pc for Paleocene. The lines of the cross-sections of Figures 21.10 and 21.23 are shown. [SE after Alberta Geological Survey] From time to time during the Mesozoic, the WCSB was filled to varying degrees with marine water. The Jurassic rocks at the base of the sequence are marine in origin, although the Jurassic sequence in Manitoba also includes evaporite layers. Most of the Middle Cretaceous rocks across the basin are marine, but the majority of the Upper Cretaceous rocks are of terrestrial origin, deposited within the flood plains and deltas of rivers. Some of these terrestrial sediments include coal layers; as described in Chapter 20, there are significant coal deposits in central Alberta. The Paleozoic sediments within the WCSB were buried deeply beneath the Mesozoic sediments and were heated enough to form both oil and gas. There are large petroleum resources in reservoir rocks of various ages extending from northeastern B.C. to southwestern Manitoba.

585 Chapter 21 Geological History of Western Canada Several of the terrestrial Cretaceous formations in the WCSB are host to important dinosaur fossils. Some are within B.C. and Saskatchewan, but the most famous are in Alberta, including the Dinosaur Park Formation (Figure 6.1), the Scollard Formation, and the Horseshoe Canyon Formation (Figure 21.24). The Dinosaur Park Formation has one of the greatest concentrations of dinosaur fossils of any rock on Earth, with at least 50 genera of dinosaurs represented, ranging from tiny Hesperonychus to giant Albertosaurus. The Hilda Bone Bed, situated about 80 km to the east of Dinosaur Park, is estimated to have the remains of approximately 1500 ceratopsians, all of which are interpreted to have died in a flood related to a tropical storm. A few of the larger herbivorous dinosaurs found at Dinosaur Park are illustrated in Figure 21.25.

Figure 21.25 Depiction of some of the large herbivorous dinosaurs from the upper part of the Dinosaur Park Formation. Left to right: the ceratopsian Pentaceratops , the hadrosaur Lambeosaurus eating from a tall tree, the ceratopsian Styracosaurus, the ankylosuar Scolosaurus, the hadrosaur Prosaurolophus (in the distance), the ankylosaur Panoplosaurus, and a herd of Styracosaurs in the background. [by J.T. Csotonyi at https://upload.wikimedia.org/wikipedia/commons/e/e9/ Dinosaur_park_formation_fauna.png]

Several important depositional basins existed in British Columbia during the Mesozoic, including the large Jurassic-aged Bowser Basin north of Terrace, and the smaller late-Cretaceous Nanaimo Basin between Vancouver Island and the mainland. In both cases, the rocks are mostly clastic, with both terrestrial and marine deposition.

Two additional relatively small terranes collided with North America early in the Cenozoic. At around 55 Ma, metamorphosed sedimentary and volcanic rocks of the Pacific Rim Terrane were forced a few tens of kilometres underneath the west coast of Vancouver Island (Figure 21.26). These rocks are distributed along the west coast of the island and in the area around Victoria (Figure 21.27). At around 42 Ma, sea-floor pillow basalt and gabbro of the Crescent Terrane accreted to the southern margin of Vancouver Island and also to the adjacent part of Washington State. These terranes are shown as Outboard terranes in Figure 21.14. Figure 21.26 East-west cross-section showing the accretion of the Pacific Rim and Crescent Terranes beneath Vancouver Island, and the ongoing subduction of the Juan de Fuca Plate. The dashed lines are inactive faults. [SE after Geological Survey of Canada]

Physical Geology 588 to the North American mainland, resulting in the uplift of the sediments deposited within the Nanaimo Basin to form islands in the Strait of Georgia (Figure 21.28) and mountains on Vancouver Island.

Following these events, the subduction of the Juan de Fuca Plate, which is a remnant of the former, much larger, Farallon Plate, was re-established at its current location farther to the west of Vancouver Island. This subduction, and that of the North America Plate beneath Alaska, has produced recently active volcanoes in Alaska, and all along the west coast from north of Vancouver Island to northern California (Figure 21.29). In southwestern B.C., there are several dormant volcanoes of Pleistocene age (including Garibaldi and Meager) that trend along a line that also passes through Mt. Baker in Washington State. About 40 km to the east is a trend of slightly older igneous complexes (Pliocene to Oligocene). The displacement between these belts could be explained by a westward shift in the position of the subduction zone over that time period.

Figure 21.29 The current plate situation along the western edge of northern North America. Blue lines are divergent boundaries, red lines are transform boundaries, and black lines with teeth are subduction boundaries. The dark red triangles are volcanoes. [SE]

Physical Geology 590 Sedimentation in the WCSB continued into the Cenozoic (Figure 21.22) with deposition of the Paskapoo Formation adjacent to the Rockies in Alberta (Figure 21.31), the Ravenscrag Formation in the Cypress Hills of southern Alberta and Saskatchewan, and the Turtle Hills Formation in southern Manitoba. All of these strata were deposited in terrestrial fluvial and deltaic environments, and all of them include coal deposits. Numerous mammalian (and other) fossils have been found in these rocks in Alberta and Saskatchewan. The mammals include primitive ungulates (ancestors to the deer and their relatives), a type of pangolin, a colugo (a gliding mammal that was possibly a primate ancestor), and some true primates in the suborder Plesiadapiformes, which became extinct and are not ancestors to any modern primates.

Rocks younger than Paleocene (i.e., younger than 55 Ma) are relatively rare across the prairies, but there are widespread Eocene-aged volcanic and sedimentary rocks in central and southern B.C. The Kamloops Group includes the Tranquille Formation of lacustrine sediments (lake deposited), overlain by the Dewdrop Flats Formation of basaltic and andesitic volcanic flows and breccias. The Tranquille Formation includes the McAbee Beds and a number of other important sites with Eocene fossils (Figure 21.32).

The earliest Pleistocene glaciation in Canada started at about 2.64 Ma (late Pliocene) in the Klondike area of Yukon. This was part of the Cordilleran Ice Sheet. The Laurentide Ice Sheet started to form shortly afterward, and within 200,000 years had covered a large part of Canada and extended well into the United States. The Pleistocene glaciations had a major impact on the topography and geology of western Canada, creating extraordinary glacial erosion features in the mountainous regions of the west (Figure 21.33), and leaving enormous volumes of glacial sediment and glacial depositional features throughout the region (Figure 21.34).

Which ancient continent collided with North America to form the Innuitian fold belt, and when did that take place? Explain why the ancient sedimentary rocks of the Athabasca and Thelon Basins are generally unmetamorphosed and undeformed. Explain why ultramafic intrusions, like those of the Muskox Intrusion, are relatively common in Archean rocks, but rare in Phanerozoic rocks. Use the Internet to find out why Cambrian marine organisms are so well preserved in the rocks of the Burgess Shale of British Columbia. The Prairie Evaporite Formation overlies marine carbonate rocks of the Winnipegosis Formation and is overlain in turn by marine carbonate rocks of the Dawson Bay Formation. What type of changes might have led to the accumulation of evaporites during this period of marine deposition? What features of the Intermontane Superterrane have been used to indicate that these rocks formed south of the equator? What is the connection between terrane accretion on the west coast and the relatively rapid accumulation of sediments within the WCSB? Why is the WCSB considered to be a foreland basin during the Mesozoic? The four main terranes of the Intermontane Superterrane are Cache Creek, Quesnel, Stikine, and Yukon- Tanana. Referring to Figure 21.14, determine the order in which these terranes are likely to have reached North America. The presence of Nanaimo Group sedimentary rocks far inland and at relatively high elevations on Vancouver Island is attributed to the accretion of the Pacific Rim and Crescent Terranes. What is the likely connection?

The story of how Earth came to be is a fascinating contradiction. On the one hand, many, many things had to go just right for Earth to turn out the way it did and develop life. On the other hand, the formation of planets similar to Earth is an entirely predictable consequence of the laws of physics, and it seems to have happened more than once. We will start Earth’s story from the beginning — the very beginning — and learn why generations of stars had to be born and then die explosive deaths before Earth could exist. We will look at what it takes for a star to form, and for objects to form around it, as well as why the nature of those objects depends on how far away from the central star they form. Earth spent its early years growing up in a very rough neighbourhood, and we will discuss how Earth’s environment influenced its development, including how it got its moon from what was quite literally an Earth-shattering blow. This chapter will also discuss the hunt for Earth-like exoplanets (planets that exist outside of our solar system).

The story of how Earth came to be is a fascinating contradiction. On the one hand, many, many things had to go just right for Earth to turn out the way it did and develop life. On the other hand, the formation of planets similar to Earth is an entirely predictable consequence of the laws of physics, and it seems to have happened more than once. We will start Earth’s story from the beginning — the very beginning — and learn why generations of stars had to be born and then die explosive deaths before Earth could exist. We will look at what it takes for a star to form, and for objects to form around it, as well as why the nature of those objects depends on how far away from the central star they form. Earth spent its early years growing up in a very rough neighbourhood, and we will discuss how Earth’s environment influenced its development, including how it got its moon from what was quite literally an Earth-shattering blow. This chapter will also discuss the hunt for Earth-like exoplanets (planets that exist outside of our solar system).

According to the big bang theory, the universe blinked violently into existence 13.77 billion years ago (Figure 22.1). The big bang is often described as an explosion, but imagining it as an enormous fireball isn’t accurate. The big bang involved a sudden expansion of matter, energy, and space from a single point. The kind of Hollywood explosion that might come to mind involves expansion of matter and energy within space, but during the big bang, space itself was created. Figure 22.1 The big bang. The universe began 13.77 billion years ago with a sudden expansion of space, matter, and energy, and it continues to expand today. [KP, modified after NASA/ WMAP Science Team, At the start of the big bang, the universe was too hot and dense to be anything but a sizzle of particles smaller than atoms, but as it expanded, it also cooled. Eventually some of the particles collided and stuck together. Those collisions produced hydrogen and helium, the most common elements in the universe, along with a small amount of lithium. You may wonder how a universe can be created out of nothing, or how we can know that the big bang happened at all. Creating a universe out of nothing is mostly beyond the scope of this chapter, but there is a way to think about it. The particles that make up the universe have opposites that cancel each other out, similar to the way that we can add the numbers 1 and -1 to get zero (also known as “nothing”). As far as the math goes, having zero is exactly the same as having a 1 and a -1. It is also exactly the same as having a 2 and a -2, a 3 and a -3, two -1s and a 2, and so on. In other words, nothing is really the potential for something if you divide it into its opposite parts. As for how we can know that the big bang happened at all, there are very good reasons to accept that it is indeed how our universe came to be.

The notion of seeing the past is often used metaphorically when we talk about ancient events, but in this case it is meant literally. In our everyday experience, when we watch an event take place, we perceive that we are watching it as it unfolds in real time. In fact, this isn’t true. To see the event, light from that event must travel to our eyes. Light travels very rapidly, but it does not travel instantly. If we were watching a digital clock 1 m away from us change from 11:59 a.m. to 12:00 p.m., we would actually see it turn to 12:00 p.m. three billionths of a second after it happened. This isn’t enough of a delay to cause us to be late for an appointment, but the universe is a very big place, and the “digital clock” in question is often much, much farther away. In fact, the universe is so big that it is convenient to describe distances in terms of light years, or the distance light travels in one year. What this means is that light from distant objects takes so long to get to us that we see those objects as they were at some considerable time in the past. For example, the star Proxima Centauri is 4.24 light years from the sun. If you viewed Proxima Centauri from Earth on January 1, 2015, you would actually see it as it appeared in early October 2010. We now have tools that are powerful enough to look deep into space and see the arrival of light from early in the universe’s history. Astronomers can detect light from approximately 375,000 years after the big bang is thought to have occurred. Physicists tell us that if the big bang happened, then particles within the universe would still be very close together at this time. They would be so close that light wouldn’t be able to travel far without bumping into another particle and getting scattered in another direction. The effect would be to fill the sky with glowing fog, the “afterglow” from the formation of the universe (Figure 22.1). In fact, this is exactly what we see when we look at light from 375,000 years after the big bang. The fog is referred to as the cosmic microwave background (or CMB), and it has been carefully mapped throughout the sky (Figure 22.2). The map displays the cosmic microwave background as temperature variations, but these variations translate to differences in the density of matter in the early universe. The red patches are the highest density regions and the blue patches are the lowest density. Higher density regions represent the eventual beginnings of stars and planets. The map in Figure 22.2 has been likened to a baby picture of the universe. Figure 22.2 Cosmic microwave background (CMB) map of the sky, a baby picture of the universe. The CMB is light from 375,000 years after the big bang. The colours reveal variations in density. Red patches have the highest density and blue patches have the lowest density. Regions of higher density eventually formed the stars, planets, and other objects we see in space today. [NASA/ WMAP Science Team ]

The expansion that started with the big bang never stopped. It continues today, and we can see it happen by observing that large clusters of billions of stars, called galaxies, are moving away from us. (The exception is the Andromeda galaxy with which we are on a collision course.) The astronomer Edwin Hubble came to this conclusion when he observed that the light from other galaxies was red-shifted. The red shift is a consequence of the Doppler effect. This refers to how we see waves when the object that is creating the waves is moving toward us or away from us. Figure 22.3 Duckling illustrates the Doppler effect in water. The ripples made in the direction the duckling is moving (blue lines) are closer together than the ripples behind the duckling (red lines). [KP, photo by M. Harkin (CC BY 2.0)

Figure 22.4 Red shift in light from the supercluster BAS11 compared to the sun’s light. Black lines represent wavelengths absorbed by atoms (mostly H and He). For BAS11 the black lines are shifted toward the red end of the spectrum compared to the Sun. [KP, spectra by Harold Stokes (public domain) ] Before we get to the Doppler effect as it pertains to the red shift, let’s see how it works on something more tangible. The duckling swimming in Figure 22.3 is generating waves as it moves through the water. It is generating waves that move forward as well as back, but notice that the ripples ahead of the duckling are closer to each other than the ripples behind the duckling. The distance from one ripple to the next is called the wavelength. The wavelength is shorter in the direction that the duckling is moving, and longer as the duckling moves away. When waves are in air as sound waves rather than in water as ripples, the different wavelengths manifest as sounds with different pitches — the short wavelengths have a higher pitch, and the long wavelengths have a lower pitch. This is why the pitch of a car’s engine changes as the car races past you. For light waves, wavelength translates to colour (Figure 22.4). In the spectrum of light that we can see, shorter wavelengths are on the blue end of the spectrum, and longer wavelengths are on the red end of the spectrum. Does this mean that galaxies look red because they are moving away from us? No, but the colour we see is shifted toward the red end of the spectrum and longer wavelengths. Notice that the sun’s spectrum in the upper part of Figure 22.4 has some black lines in it. The black lines are there because some colours are missing in the light we get from the Sun. Different elements absorb light of specific wavelengths, and many of the black lines in Figure 22.4 represent colours that are absorbed by hydrogen and helium within the Sun. This means the black lines are like a bar code that can tell us what a star is made of. The lower spectrum in Figure 22.4 is the light coming from BAS11, an enormous cluster of approximately 10,000 galaxies located 1 billion light years away. The black lines represent the same elements as in the Sun’s spectrum, but they are shifted to the right toward the red end of the spectrum because BAS11 is moving away from us as the universe continues to expand. So to summarize, because almost all of the galaxies we can see have light that is red-shifted, it means they are

If we were to take an inventory of the elements that make up Earth, we would find that 95% of Earth’s mass comes from only four elements: oxygen, magnesium, silicon, and iron. Most of the remaining 5% comes from aluminum, calcium, nickel, hydrogen, and sulphur. We know that the big bang made hydrogen, helium, and lithium, but where did the rest of the elements come from? The answer is that the other elements were made by stars. Sometimes stars are said to “burn” their fuel, but burning is not at all what is going on within stars. The burning that happens when wood in a campfire is turned to ash and smoke is a chemical reaction — heat causes the atoms that were in the wood and in the surrounding atmosphere to exchange partners. Atoms group in different ways, but the atoms themselves do not change. What stars do is change the atoms. The heat and pressure within stars cause smaller atoms to smash together and fuse into new, larger atoms. For example, when hydrogen atoms smash together and fuse, helium is formed. Large amounts of energy are released when some atoms fuse and that energy is what causes stars to shine. It takes larger stars to make elements as heavy as iron and nickel. Our Sun is an average star; after it uses up its hydrogen fuel to make helium, and then some of that helium is fused to make small amounts of beryllium, carbon, nitrogen, oxygen, and fluorine, it will be at the end of its life. It will stop making atoms and will cool down and bloat until its middle reaches the orbit of Mars. In contrast, large stars end their lives in spectacular fashion, exploding as supernovae and casting off newly formed atoms —including the elements heavier than iron — into space. It took many generations of stars creating heavier elements and casting them into space before heavier elements were abundant enough to form planets like Earth. Until recently, astronomers have only been able to see stars that already contain heavier elements in small amounts, but not the first-generation stars that started out before any of the heavier elements were produced. That changed in June of 2015 when it was announced that a distant galaxy called CR7 had been found that contained stars made only of hydrogen and helium. The galaxy is so far away that it shows us a view of the universe from only 800 million years after the big bang.1

A solar system consists of a collection of objects orbiting one or more central stars. All solar systems start out the same way. They begin in a cloud of gas and dust called a nebula. Nebulae are some of the most beautiful objects that have been photographed in space, with vibrant colours from the gases and dust they contain, and brilliant twinkling from the many stars that have formed within them (Figure 22.5). The gas consists largely of hydrogen and helium, and the dust consists of tiny mineral grains, ice crystals, and organic particles. Figure 22.5 Photograph of a nebula. The Pillars of Creation within the Eagle Nebula viewed in visible light (left) and near infrared light (right). Near infrared light captures heat from stars and allows us to view stars that would otherwise be hidden by dust. This is why the picture on the right appears to have more stars than the picture on the left. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

A solar system begins to form when a small patch within a nebula (small by the standards of the universe, that is) begins to collapse upon itself. Exactly how this starts isn’t clear, although it might be triggered by the violent behaviour of nearby stars as they progress through their life cycles. Energy and matter released by these stars might compress the gas and dust in nearby neighbourhoods within the nebula. Once it is triggered, the collapse of gas and dust within that patch continues for two reasons. One of those reasons is that gravitational force pulls gas molecules and dust particles together. But early in the process, those particles are very small, so the gravitational force between them isn’t strong. So how do they come together? The answer is that dust first accumulates in loose clumps for the same reason dust 605 Chapter 22 The Origin of Earth and the Solar System bunnies form under your bed: static electricity. Given the role of dust bunnies in the early history of the solar system, one might speculate that an accumulation of dust bunnies poses a substantial risk to one’s home (Figure 22.6). In practice, however, this is rarely the case.

As the small patch within a nebula condenses, a star begins to form from material drawn into the centre of the patch, and the remaining dust and gas settle into a disk that rotates around the star. The disk is where planets eventually form, so it’s called a protoplanetary disk. In Figure 22.7 the image in the upper left shows an artist’s impression of a protoplanetary disk, and the image in the upper right shows an actual protoplanetary disk surrounding the star HL Tauri. Notice the dark rings in the protoplanetary disk. These are gaps where planets are beginning to form. The rings are there because incipient planets are beginning to collect the dust and gas in their orbits. There is an analogy for this in our own solar system, because the dark rings are akin to the gaps in the rings of Saturn (Figure 22.7, lower left), where moons can be found (Figure 22.7, lower right).

Figure 22.7 Protoplanetary disks and Saturn’s rings. Upper left: An artists impression of a protoplanetary disk containing gas and dust, surrounding a new star. [NASA/ JPL-Caltech, 1E5tFJR] Upper right: A photograph of the protoplanetary disk surrounding HL Tauri. The dark rings within the disk are thought to be gaps where newly forming planets are sweeping up dust and gas. [ALMA (ESO/NAOJ/NRAO) Lower left: A photograph of Saturn showing similar gaps within its rings. The bright spot at the bottom is an aurora, similar to the northern lights on Earth. [NASA, ESA, J. Clarke (Boston University), and Z. Levay (STScI) Lower right: a close-up view of a gap in Saturn’s rings showing a small moon as a white dot. [NASA/JPL/Space Science Institute,

In general, planets can be classified into three categories based on what they are made of (Figure 22.8). Terrestrial planets are those planets like Earth, Mercury, Venus, and Mars that have a core of metal surrounded by rock. Jovian planets (also called gas giants) are those planets like Jupiter and Saturn that consist predominantly of hydrogen and helium. Ice giants are planets such as Uranus and Neptune that consist largely of water ice, methane (CH4) ice, and ammonia (NH3) ice, and have rocky cores. Often, the ice giant planets Uranus and Neptune are grouped with Jupiter and Saturn as gas giants; however, Uranus and Neptune are very different from Jupiter and Saturn.

Figure 22.8 Three types of planets. Jovian (or gas giant) planets such as Jupiter consist mostly of hydrogen and helium. They are the largest of the three types. Ice giant planets such as Uranus are the next largest. They contain water, ammonia, and methane ice. Terrestrial planets such as Earth are the smallest, and they have metal cores covered by rocky mantles. [KP, after public domain images by FrancescoA, WolfmanSF and NASA These three types of planets are not mixed together randomly within our solar system. Instead they occur in a systematic way, with terrestrial planets closest to the sun, followed by the Jovian planets and then the ice giants (Figure 22.9). Smaller solar system objects follow this arrangement as well. The asteroid belt contains bodies of rock and metal. Bodies ranging from metres to hundreds of metres in diameter are classified as asteroids, and smaller bodies are referred to as meteoroids. In contrast, the Kuiper belt (Kuiper rhymes with piper), and the Oort cloud (Oort rhymes with sort), which are at the outer edge of the solar system, contain bodies composed of large amounts of ice in addition to rocky fragments and dust. (We will talk more about smaller solar system objects in a moment.)

Figure 22.9 Our solar system. Top: The solar system shown with distances to scale. Distances are in astronomical units (AU), where 1 AU is the average distance from Earth to the Sun. The edge of the Kuiper belt extends to 50 AU (7.5 billion km), but this distance is minuscule compared to the size of the solar system as a whole, which extends to the edge of the Oort cloud, thought to be 15 trillion km away. Bottom: Solar system with the Sun and planets to scale. The gas giants are the largest planets, followed by the ice giants, and then the terrestrial planets. Note that the planets in this diagram likely do not reflect the entire population of planets in our solar system because evidence suggests that large planets are present beyond the Kuiper belt. [KP, planet photographs courtesy of NASA via 1M89xPs, Milky Way photo ForestWanderer (CC BY SA 3.0) Part of the reason for this arrangement is the frost line (also referred to as the snow line). The frost line separated the inner part of the protoplanetary disk closer to the sun, where it was too hot to permit anything but silicate minerals and metal to crystalize, from the outer part of the disk farther from the Sun, where it was cool enough to allow ice to form. As a result, the objects that formed in the inner part of the protoplanetary disk consist largely of rock and metal, while the objects that formed in the outer part consist largely of gas and ice. The young sun blasted the solar system with raging solar winds (winds made up of energetic particles), which helped to drive lighter molecules toward the outer part of the protoplanetary disk. The objects in our solar system formed by accretion. Early in this process, particles collected in fluffy clumps because of static electricity. As the clumps grew larger, gravity became more important and collected clumps into solid masses, and solid masses into larger and larger bodies. If you were one of these bodies in the early solar system, and participating in the accretion game with the goal of becoming a planet, you would have to follow some key rules: Keep your velocity just right. If you move too fast and collide with another body, you both smash up and have to start again. If you move slowly enough, gravity will keep you from bouncing off each other and you can grow larger. Your distance from the Sun will determine how big you can get. If you are closer, there is less material for you to collect than if you are farther away. 609 Chapter 22 The Origin of Earth and the Solar System To begin with, you can only collect mineral and rock particles. You have to grow above a certain mass before your gravity is strong enough to hang onto gas molecules, because gas molecules are very light. As your mass increases, your gravity becomes stronger and you can grab material from farther away. The bigger you are, the faster you grow.

In the early stages of the game, the protoplanetary disk is turbulent, and you and other objects can get thrown into different orbits or at each other. This might be a good thing, or it might not, depending on how the rules above apply to you. If the game progresses to the point where there is no more material within your reach and you are not yet a planet, then it’s game over. If you slow down too much (e.g., from bumping into other objects), you could spiral into the Sun (game over). If another planet gets big enough, it can: Rip you apart and then swing the pieces around so fast that for the rest of the game you collide too hard with other pieces to grow any bigger (game over) Fling you out of the solar system (game over) Grab you for itself (game over) Trap you in an orbit around it, turning you into a moon (game over, and incredibly humiliating) The outcome of the game is evident in Figure 22.9. Today eight official winners are recognized, with Jupiter taking the grand prize, followed closely by Saturn. Both planets have trophy cases with more than 60 moons each, and each has a moon that is larger than Mercury. Prior to 2006, Pluto was also counted a winner, but in 2006 a controversial decision revoked Pluto’s planet status. The reason was a newly formalized definition of a planet, which stated that an object can only be considered a planet if it is massive enough to have swept its orbit clean of other bodies. Pluto is situated within the icy clutter of the Kuiper belt, so it does not fit this definition. Pluto’s supporters have argued that Pluto should have been grandfathered in, given that the definition came after Pluto was declared a planet, but to no avail. Pluto has not given up, and on July 13, 2015, it launched an emotional plea with the help of the NASA’s New Horizons probe. New Horizons sent back images of Pluto’s heart (Figure 22.10). On closer inspection, Pluto’s heart was discovered to be broken.

Figure 22.10 Photographs of Pluto. Left: The heart-shaped region called Tombaugh Regio is outlined. This region is named after Pluto’s discoverer Clyde Tombaugh [KP, NASA/APL/SwRI , 1MOuT3m]. Right: False-colour images show compositional variations in Tombaugh Regio. [KP, NASA/APL/SwRI ,

Proximity to the Sun explains why the terrestrial planets are so much smaller than the gas giant and ice giant planets. Mars is smaller than it should be, given the rule that distance from the Sun determines how much material a body can accumulate, and this can be explained by its proximity to Jupiter. Jupiter’s immense gravity interfered with Mars’ ability to accrete. Further evidence of Jupiter’s interference is the debris field that forms the asteroid belt. From time to time, Jupiter still flings objects from the asteroid belt out into other parts of the solar system, some of which have collided with Earth to catastrophic effect. The Kuiper belt is an icy version of the asteroid belt, consisting of fragments left over from the early solar system. The material in the Kuiper belt is scattered because of Neptune’s gravity. From time to time, Jupiter interferes here as well, flinging Kuiper belt objects toward the Sun and into orbit. As these objects approach the Sun, the Sun causes dust and gas to be blasted from their surface, forming tails. We know these objects as comets. Comets may also come from the Oort cloud where gravitational forces from outside of the solar system can hurl objects from the Oort cloud toward the Sun.

that result to a diagram for easy comparison. It is useful to approximate the structure of a terrestrial planet as having two parts: a metal core and a rocky mantle. If we know the density of the planet as a whole, and the densities of the materials making up the rocky mantle and the core, we can find out how much of the planet is core and how much is rocky. The density of the planet is the sum of the percent having the density of the core and the percent having the density of rock. This can be written as follows: planet density = % core/100 x core density + (1- %core/100) x rock density Rearranging the equation gives us: % core = (planet density – rock density)/ (core density- rock density) x 100 Step 1. Find the percent core for each of the terrestrial planets using the data in Tables 22.1 and 22.2. For our calculations the planet density will be the uncompressed density of the planet. Uncompressed density is the density after removing the effects of gravity squeezing the planet together. (Notice that the density we mentioned for Earth is 5.51 g/cm3, but Earth’s uncompressed density is only 4.05 g/cm3.) The first one is done for you.

Step 2. Once we have the percent of core, we can use it to find the volume of the core for each planet. The core volume is the percent of core times the volume of the planet. Use the planet volumes in Table 22.3 to calculate the core volume. Record your answers.

*Unsqueezed” values Step 3. We can get the radius of the core from its volume by using the formula for the volume of a sphere (volume = 4/3pr3, where r is the radius). This calculation is done for you in Table 22.4. From these values, express each radius as a percentage of the total radius. To do this, divide the core radius by the planet radius and multiply by 100. Using your results, fill in the diagrams at the bottom of Table 22.4 by drawing in the boundary between the core and mantle.

One of the terrestrial planets is thought to have been involved in collisions that resulted in the permanent loss of a substantial amount of its mantle. You might be able to guess which one it is from the uncompressed densities of the planets. It should also be clear from your diagrams. Which planet is it?

If you were to get into a time machine and visit Earth shortly after it formed (around 4.5 billion years ago), you would probably regret it. Large patches of Earth’s surface would still be molten, which would make landing your time machine very dangerous indeed. If you happened to have one of the newer time- machine models with hovering capabilities and heat shields, you would still face the inconvenience of having nothing to breathe but a tenuous wisp of hydrogen and helium gas, and depending on how much volcanic activity was going on, volcanic gases such as water vapour and carbon dioxide. Some ammonia and methane might be thrown in just to make it interesting, but there would be no oxygen. Assuming you had the foresight to purchase the artificial atmosphere upgrade for your time machine, it would all be for naught if you materialized just in time to see an asteroid, or worse yet another planet, bearing down on your position. The moral of the story is that early Earth was a nasty place, and a time machine purchase is not something to take lightly.

Chapter 9 explains that Earth’s heat comes from the decay of radioactive elements within Earth, as well as from processes associated with Earth’s formation. Let’s look more closely at how those formation processes heated up Earth. Heat came from the thermal energy already contained within the objects that accreted to form the Earth. Heat came from collisions. When objects hit Earth, some of the energy from their motion went into deforming Earth, and some of it was transformed into heat. Clap your hands vigorously to experience this on a much smaller (and safer!) scale. As Earth became larger, its gravitational force became stronger. This increased Earth’s ability to draw objects to it, but it also caused the material making Earth to be compressed, rather like Earth giving itself a giant gravitational hug. Compression causes materials to heat up. Heating had a very important consequence for Earth’s structure. As Earth grew, it collected a mixture of silicate mineral grains as well as iron and nickel. These materials were scattered throughout Earth. That changed when Earth began to heat up: it got so hot that both the silicate minerals and the metals melted. The metal melt was much denser than the silicate mineral melt, so the metal melt sank to Earth’s centre to become its core, and the silicate melt rose upward to become Earth’s crust and mantle. In other words, Earth unmixed itself. The separation of silicate minerals and metals into a rocky outer layer and a metallic core, respectively, is called differentiation. The movement of silicate and metal melts within Earth caused it to heat up even more.

Although Earth had swept up a substantial amount of the material in its orbit as it was accreting, unrest within the solar system caused by changes in the orbits of Saturn and Jupiter was still sending many large objects on cataclysmic collision courses with Earth. The energy from these collisions repeatedly melted and even vaporized minerals in the crust, and blasted gases out of Earth’s atmosphere. Very old scars from these collisions are still detectable, although we have to look carefully to see them. For example, the oldest impact site discovered is the 3 billion year old Maniitsoq “crater” in west Greenland, although there is no crater to see. What is visible are rocks that were 20 km to 25 km below Earth’s surface at the time of the impact, but which nevertheless display evidence of deformation that could only be produced by intense, sudden shock. The evidence of the very worst collision that Earth experienced is not subtle at all. In fact, you have probably looked directly at it hundreds of times already, perhaps without realizing what it is. That collision was with a planet named Theia, which was approximately the size of Mars (Figure 22.11). Not long after Earth formed, Theia struck Earth. When Theia slammed into Earth, Theia’s metal core merged with Earth’s core, and debris from the outer silicate layers was cast into space, forming a ring of rubble around Earth. The material within the ring coalesced into a new body in orbit around Earth, giving us our moon. Remarkably, the debris may have coalesced in 10 years or fewer! This scenario for the formation of the moon is called the giant impact hypothesis.

Earth’s first experiment with having an atmosphere didn’t go well. It started out with a thin veil of hydrogen and helium gases that came with the material it accreted. However, hydrogen and helium are very light gases, and they bled off into space. Earth’s second experiment with having an atmosphere went much better. Volcanic eruptions built up the atmosphere by releasing gases. The most common volcanic gases are water vapour and carbon dioxide (CO2), but volcanoes release a wide variety of gases. Other important contributions include sulphur dioxide (SO2), carbon monoxide (CO), hydrogen sulphide (H2S), hydrogen gas, and methane (CH4). Meteorites and comets also brought substantial amounts of water and nitrogen to Earth. It is not clear what the exact composition of the atmosphere was after Earth’s second experiment, but carbon dioxide, water vapour, and nitrogen were likely the three most abundant components.

Figure 22.11 Artist’s impression of a collision between planets. A similar collision between Earth and the planet Theia might have given us our Moon. Fortunately for us, the collision that gave us the moon was a glancing blow rather than the direct hit shown here. Earth might not have survived a direct hit. [NASA/ JPL-Caltech, 1IkP069] One thing we can say for sure about Earth’s second experiment is that there was effectively no free oxygen (O2, the form of oxygen that we breathe) in the atmosphere. We know this in part because prior to 2 billion years ago, there were no sedimentary beds stained red from oxidized iron minerals. Iron minerals were present, but not in oxidized form. At that time, O2 was produced in the atmosphere when the Sun’s ultraviolet rays split water molecules apart; however, chemical reactions removed the oxygen as quickly as it was produced. It wasn’t until well into Earth’s third experiment — life — that the atmosphere began to become oxygenated. Photosynthetic organisms used the abundant CO2 in the atmosphere to manufacture their food, and released O2 as a by-product. At first all of the oxygen was consumed by chemical reactions, but eventually the organisms released so much O2 that it overwhelmed the chemical reactions and oxygen began to accumulate in the atmosphere, although present levels of 21% oxygen didn’t occur until about 350 Ma. Today the part of our atmosphere that isn’t oxygen consists largely of nitrogen (78%). The oxygen-rich atmosphere on our planet is life’s signature. If geologic process were the only processes controlling our atmosphere, it would consist mostly of carbon dioxide, like the atmosphere of Venus. It is an interesting notion (or a disconcerting one, depending on your point of view) that for the last 2 billion years the light reflected from our planet has been beaming a bar code out to the universe, similar to the ones in Figure 22.4, except ours says “oxygen.” For 2 billion years, our planet has been sending out a signal that could cause an observer from another world to say, “That’s odd… I wonder what’s going on over there.”

If by that you mean, are there other planets where we could walk out of a spaceship with no equipment other than a picnic basket, and enjoy a pleasant afternoon on a grassy slope near a stream, then that remains to be seen. On the other hand, if you are asking if other planets exist that are rocky worlds approximately Earth’s size, and orbiting within their star’s habitable zone (the zone in which liquid water, and potentially life, can exist), then many planet hunters are cautiously optimistic that we have found at least 12 such worlds so far. As of July 2015, NASA’s Kepler mission has detected a total of 4,696 possible exoplanets. The Kepler spacecraft has an instrument to measure the brightness of stars, and looks for tiny variations in brightness that could be caused by a planet passing between the star it orbits and the instrument observing the star. Potential candidates are then examined in more detail to see whether they are in fact planets or not. So far 1,030 of those candidates have been confirmed as planets.1 Of those, 12 satisfy the criteria of being one to two times the size of Earth, and orbiting their star within the habitable zone.2 The uncertainty about the 12 possible Earth-like worlds is related to their composition. We don’t yet know their composition; however, it is tempting to conclude that they are rocky because they are similar in size to Earth. Remember the rules of the accretion game: you can only begin to collect gas once you are a certain size, and how much matter you collect depends on how far away from the Sun you are. Given how large our gas giant and ice giant planets are compared to Earth, and how far away they are from the Sun, we would expect that a planet similar in size to Earth, and a similar distance from its star, should be rocky. It isn’t quite as simple as that, however. We are finding that the rules to the accretion game can result in planetary systems very different from our own, leading some people to wonder whether those planetary systems are strange, or ours is, and if ours is strange, how strange is it? Consider that in the Kepler mission’s observations thus far, it is very common to find planetary systems with planets larger than Earth orbiting closer to their star than Mercury does to the Sun. It is rare for planetary systems to have planets as large as Jupiter, and where large planets do exist, they are much closer to their star than Jupiter is to the Sun. To summarize, we need to be cautious about drawing conclusions from our own solar system, just in case we are basing those conclusions on something truly unusual. On the other hand, the seemingly unique features of our solar system would make planetary systems like ours difficult to spot. Small planets are harder to detect because they block less of a star’s light than larger planets. Larger planets farther from a star are difficult to spot because they don’t go past the star as frequently. For example, Jupiter goes around the Sun once every 12 years, which means that if someone

617 Chapter 22 The Origin of Earth and the Solar System were observing our solar system, they might have to watch for 12 years to see Jupiter go past the Sun once. For Saturn, they might have to watch for 30 years.

The operational definition of “other Earths,” which involves a terrestrial composition, a size constraint of one to two times that of Earth, and location within a star’s habitable zone, does not preclude worlds incapable of supporting life as we know it. By those criteria, Venus is an “other Earth,” albeit right on the edge of the habitable zone for our Sun. Venus is much too hot for us, with a constant surface temperature of 465°C (lead melts at 327°C). Its atmosphere is almost entirely carbon dioxide, and the atmospheric pressure at its surface is 92 times higher than on Earth. Any liquid water on its surface boiled off long ago. Yet the characteristics that make Venus a terrible picnic destination aren’t entirely things we could predict from its distance from the sun. They depend in part on the geochemical evolution of Venus, and at one time Venus might have been a lot more like a youthful Earth. These are the kinds of things we won’t know about until we can look carefully at the atmospheres and compositions of habitable-zone exoplanets.

Let’s try this out for the Earth-like exoplanet called Kepler-452b. The first thing we need to know is the size of the host star Kepler-452. We can get that information by comparing its surface temperature and brightness to that of the sun. Start by calculating the ratios of the sun’s temperature to the star’s temperature, and the star’s luminosity to the sun’s luminosity using the data in Table 22.5. Record your answers in the table. Then find the star’s radius using the following equation, and record your result:

Astronomers looking for some of the earliest stars in the universe were surprised to find a planetary system called HIP 11952, which existed 12.8 billion years ago. This was very early in the universe’s history, when stars still consisted largely of hydrogen and helium. Do you think there were terrestrial planets in this system? Why or why not?4. Summarize the trends in size and composition of objects in the solar system.5. What is the frost line, and what does it help to explain?5. This cartoon shows three of the same type of solar system object. One goes on an adventure and comes back the worse for wear. What are the objects, and where might they be located?

6. Why is Pluto not considered a planet?7. What is differentiation, and what must happen to a planet or asteroid for differentiation to occur?9. The exoplanet Kepler-452b is within the habitable zone of its star. In our solar system, planets a similar distance from the Sun are terrestrial planets. Why can we not say for certain that Kepler-452b’s distance from its star means it is a terrestrial planet? 8. Of the planetary systems discovered thus far, none are exactly like our solar system. Does this mean our solar system is unique in the universe?

aggregate (20) unconsolidated materials (typically sediments) that are used in the construction industry albedo (19) the reflectivity of a surface of a planet (expressed as the percentage of light that reflects) albite (2) sodium-rich plagioclase feldspar alpine glacier (16) a glacier formed in a mountainous region and confined to a valley (same as valley glacier)

anaerobic (18) processes that take place without oxygen andesite (3) a volcanic rock of intermediate composition anion (2) a negatively charged ion angular unconformity (8) a geological boundary at the base of a sedimentary layer where the sedimentary rock beneath has been tilted or folded and then eroded

atoll (18) a ring-shaped carbonate (or coral) reef or series of islands atomic mass (2) the total number of neutrons plus protons in an atom atomic number (2) the total number of protons in an atom attitude (12) the orientation of a sloping geological feature, such as a bedding plane or fracture

breakwater (17) a structure built offshore in order to deflect the energy of waves breccia (6) a sedimentary- or volcanic-rock texture characterized by angular clasts brunisol (5) a relatively immature forest soil, lacking in well-defined horizons

caliche (5) a white calcium-carbonate rich layer within soils in arid regions calving (16) the loss of ice from the front of a glacier by collapse into water Canadian Shield (21) the exposed part of the continent Laurentia carbonate (2) a mineral in which the anion is CO3-2 carbonate compensation depth (18) the depth in the ocean (typically around 4000 m) below which carbonate minerals are soluble

Cordilleran Ice Sheet (16) the continental glacier that covered part of western North America, including almost all of British Columbia, part of the Yukon, and part of northern Washington, during the Pleistocene glaciations

craton (21) a region of ancient (typically Precambrian) crystalline rock (equivalent to a shield) creep (15) the very slow (mm to cm per year) flow of unconsolidated material on a gentle slope crest (17) the highest point on a wave crevasse (16) an open fissure on the surface of a glacier

formation (6) a unit of sedimentary rock that is lithologically consistent and sufficiently thick and extensive to be shown on a geological map at the scale that is typically used in the area in question

Gaia hypothesis (19) the hypothesis advanced by James Lovelock that the organisms have affected the atmosphere and oceans such that conditions on Earth have been kept habitable, in spite of significantly changing energy received from the Sun

glacier (16) a long lasting (centuries or more) body of ice on land that moves under its own weight glaciofluvial (16) referring to sediments deposited from a stream that is derived from a glacier glaciolacustrine (16) referring to sediments deposited within a lake in a glacial environment glaciomarine (16) referring to sediments deposited within the ocean in a glacial environment glaucophane (7) a blue-coloured sodium-magnesium bearing amphibole mineral that forms during metamorphism at high pressures and relatively low pressures, typically within a subduction zone

index fossil (8) a fossil with a distinctive appearance and a wide geographic range but from a relatively restricted time range, thus making it useful for dating a correlating rocks from different regions (the most useful index fossils are from organisms that lived for less than a million years)

Intergovernmental Panel on Climate Change (19) (IPCC) an international body established in 1988 by the UN’s World Meteorological Organization and the UN Environment Program to prepare periodic reports on the status of global climate change and its mitigation

lava tube (4) a tube that forms as mafic lava flows along a channel and lava leveés build up on either side, eventually forming a roof (once a lava tube forms it insulates the flowing magma, allowing it to stay hot a liquid for longer and therefore flow much further)

mantle plume (3) a plume of hot rock (not magma) that rises through the mantle (either from the base or from part way up) and reaches the surface where it spreads out and also leads to hot-spot volcanism

metamorphism (3) the transformation of a parent rock into a new rock as a result of heat and pressure that leads to the formation of new minerals, or recrystallization of existing minerals, without melting

plate tectonics (1) the concept that the Earth’s crust and upper mantle (lithosphere) is divided into a number of plates that move independently on the surface and interact with each other at their boundaries

polar wandering path (10) a path of varying magnetic pole positions defined by paleomagnetic data (in fact it is now understood that the continents have wandered, not the poles, so a more appropriate terms is “apparent polar wandering path”)

positive feedback (19) a process that results in an increase in that process (in the context of climate change it is a process that enhances the change in climate, such as the reduced reflectivity of the Earth’s surface when ice melts)

radioactivity (9) the natural transformation of unstable isotopes into new elements radiolaria (18) microscopic (0.1 to 0.2 mm) marine protozoa that produce silica shells Rayleigh wave (11) a surface seismic wave, with vertical motion recharge (14) the transfer of surface water into the ground to become groundwater

rock basin lake (16) a lake situated in a rock basin carved at the upper end of an alpine glacier rock cycle (3) the series of processes through which rocks are transformed from one type to another rock fall (15) the near-vertical fall or bouncing of rock released from a steep slope rock slide (15) the translational motion of an essentially intact body of rock down a slope (rock slides are typically slow, because once they start to move fast the rock body becomes fragmented and then flows as a rock avalanche)

salatation (13) the bouncing of particles along a stream bottom or desert floor sand (6) a mineral or rock fragment ranging in size from 1/16th to 2 mm sandstone (1) a rock that is primarily comprised of sand-sized particles sandur (16) an extensive region of sand and gravel deposited by streams flowing out of a glacier (same as outwash plain)

sector collapse (4) the sudden collapse of a significant part of the flank of a volcano sedimentary rock (3) rock that has formed by the lithification of sediments sediments (3) unconsolidated particles of mineral or rock seismic (11) pertaining to earthquakes

shear force (15) the component of the gravitational force in the direction parallel to a slope shear strength (15) the strength of a body of rock or sediment that counteracts the shear force shear stress (12) the stress placed on a body of rock or sediment adjacent to a fault sheeted dykes (10) a series of near-vertical dykes formed in the vicinity of a spreading ridge when magma from depth flows into fractures formed by extensional forces

slaty cleavage (7) the tendency for slate or phyllite to split into sheets (note that this is the only situation in this textbook where the term “cleavage” is applied to a rock as opposed to a mineral)

streak (2) the mark left on a porcelain plate when a mineral sample is ground to a powder by being rubbed across the plate (typically considered to provide a more reliable depiction of the colour than the whole sample)

stress transfer (11) the change in the pattern of stress on a region of rock as a result of an earthquake (typically stress is reduced in the area of a rupture zone, but is increased elsewhere in the vicinity)

supergroup (21) a stratigraphically-continuous series of related groups superterrane (21) a number of terranes that are contiguous supraglacial (16) on the surface of a glacier surf zone (17) the near-shore zone where waves are breaking into surf

symmetrical (12) a fold in which the limbs are at the same angle to the hinge syncline (12) a downward fold where the beds are known not to be overturned synform (12) a downward fold where it is not known if the beds are overturned T

tuya (4) a flat-topped volcanic hill or mountain that formed when an eruption took place beneath a glacier and the melting led to the formation of a lake that then resulted in the wave-erosion of the top of the volcano

zone of accumulation (16) the part of a glacier, above the equilibrium line, where there is net gain of ice mass because not all of the snow that falls each winter is able to melt during the following summer

Steven Earle was born in the Okanagan Valley of British Columbia. He earned a BSc in geology from the University of British Columbia in 1975 and a PhD in geochemistry from Imperial College (London University) in 1982. He worked as a geologist and geochemist in the mineral exploration industry in western Canada from 1978 to 2000. For 20 years he developed and taught a wide range of earth science courses at Vancouver Island University. He currently designs and teaches distance courses for Thompson Rivers University (Open Learning), and also helps to grow food and drive the Community Bus on Gabriola Island. He maintains that the best way to see rocks is from a kayak.

The periodic table is a list of all of the elements arranged in groups according to their atomic configuration. In this table the elements are colour-coded according to their chemical and physical properties.

Geology involves integration of various different sciences (chemistry, physics, and biology for example), but also requires an understanding of the importance of billions of years of geological time. Paleontology is an important aspect of geology and requires an understanding of biology, including evolution, the physiology of animals and plants and ecological relationships. Geologists provide information to reduce the risk of harm from hazards such as earthquakes, volcanoes, and slope failures; they play a critical role in the discovery of important resources; they contribute to our understanding of life and its evolution through paleontological studies; and they play a leading role in the investigation of climate change, past and present and its implications. Halite is composed of sodium (Na) and chlorine (Cl) with the Na+ and Cl- ions alternating with one another in all three directions within a cubic structure. A mineral has a specific chemical composition and lattice structure. Rocks are made out of minerals, and most rocks contain several different types of minerals. The main component of Earth’s core is iron (Fe). Transfer of heat from the core to the mantle leads to heating of lower mantle rock. When heated, the rock expands and its density is reduced. Because the mantle is plastic, this lower-density material tends to rise toward the surface, and cooler denser mantle material moves in to take its place. Mantle convection creates the traction that can force plates to move around on the surface. Hot mantle rock moving toward the surface partially melts because the pressure is reduced. The magma produced moves upward into cracks in the crust and is extruded onto the sea floor. 215 – 65 = 150 Ma. Since the age of the Earth is 4570 Ma, this represents 150/4,570 = 0.033 or 3.3% of geological time. At 1 mm/y 30,000,000 mm would accumulate over that 30 million years. This is equivalent to 30,000 m or 30 km. Few sequences of sedimentary rock are even close to that thickness because most sediments accumulate at much lower rates, more like 0.1 mm/y.

Charges: proton: +1, neutron: 0, electron: -1, Masses: proton: 1, neutron: 1, electron: almost 0. The element’s atomic number will determine the extent to which its outer layers are populated with electrons. If the outer shell is not quite full, the atom may gain electrons to fill them and become an anion (negative charge). If the outer shell has only a few electrons, it may lose them and become a cation (positive charge). Cations and anions attract each other to form molecules with ionic bonding. Helium and neon (and the other noble gases) have complete outer shells and therefore no tendency to form ionic bonds. Electrons are transferred from one atom to another to form an ionic bond. Electrons are shared between atoms to form a covalent bond. An anion has a negative charge and a cation has a positive charge. Minerals are classified into groups based on their anion or anion group. Name the mineral group for the following minerals: An unbonded silica tetrahedron has one Si ion (+4 charge) and 4 oxygens (-2 charge each) so the overall charge is 4 – 8 = -4 for SiO -4 Magnesium can substitute freely for iron in olivine and several other minerals because they have similar charges (+2) and similar ionic radii. Pyroxene is made up of single chains of tetrahedra while amphibole is made up of double chains. Biotite includes iron and/or magnesium in its formula, while muscovite does not. The two end-members of the plagioclase series are Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8) In quartz each silica tetrahedron is bonded to four other tetrahedra, and since oxygens are shared at each bond the overall ratio is silicon (+4) to two oxygens (2 x -2 = -4), which is balanced. Some minerals have distinctive colours, but many have a wide range of colours due to differing impurities. Glass has a Mohs hardness of about 5.5 while porcelain is close to 6.5. The mineral is between these two, so it must be close to 6.