Continental Crust Study Materials

CONTINENTAL CRUST At the beginning of the Proterozoic, more than 80 percent or more of today's continental crust already existed (Figure 41). Most of the material currently locked in sedimentary rocks is on or near the surface. Therefore, many sources of Archean rocks are available for erosion and redeposition. Sediments that come directly from primary sources are called graywackes, often described as dirty sandstone. They usually occur on folded sedimentary rocks such as those in the Alps and Alaska. Most Proterozoic graywackes consist of sandstones and siltstones derived from Archean greenstone. Another common rock type is fine-grained quartzite. It was a metamorphic rock derived from the erosion of coarse silica rocks such as granite and grained sandstone called arcose. Conglomerates, the consolidated gravel equivalent, also abound in the Proterozoic. Nearly 20,000 feet of Proterozoic sediment make up Utah's Uinta Range (Figure 42), and Montana's Proterozoic belt system contains sediments more than 11 miles thick. The Proterozoic is also known for its widespread terrestrial red layer, so named because sediment grains are cemented with red iron oxide. Their appearance about 1 billion years ago suggests that the atmosphere has substantial oxygen levels today. Weathering of primary rocks produces solutions of calcium carbonate, magnesium carbonate, calcium sulfate, and sodium chloride, which are then deposited into limestone, dolomite, gypsum, and halite. Higher concentrations of carbon dioxide in the Precambrian may explain the prevalence of dolomite over limestone. These minerals appear to be primarily chemical deposits and are not of biological origin. In the Mackenzie Mountains in northwest Canada, dolomite deposits range up to 6,500 feet. In the Alps, huge chunks of dolomite soar into the sky. Carbonate rocks such as limestone and limestone, produced mainly by organic processes involving the shells and skeletons of simple organisms, became more common in the late Proterozoic beginning about 700 million years ago. In contrast, they are relatively rare in the Archeans due to the rarity of chalk-secreting organisms. The Proterozoic continent consists of the Archean kraton. The North American continent gathered from seven cratons about 2 billion years ago, making it the oldest continent. Kraton welded to what is now central Canada and north-central United States. Meanwhile, continental collisions continued to add large areas of new crust to the growing proto-North American continent (Fig. 43). At Cape Smith in Hudson Bay is a 2-billion-year-old piece of oceanic crust that accretes inland, suggesting that continents collided and closed off ancient oceans. An arc of volcanic rock also weaves through central and eastern Canada into the Dakotas. A region between Canada's Great Bear Lake and the Beaufort Sea holds ancient mountainous roots formed by the collision of North America and an unknown landmass between 1.2 and 0.9 billion years ago. A possible continental collision with South America created the Grenville orogeny between 1.3 and 0.9 billion years ago, which formed a 3,000-milelong mountain belt in eastern North America from Texas to Labrador. Meanwhile, continental collisions continue to add large areas of new crust to the growing protoNorth American continent. Much of the continental crust underlying the United States from Arizona to the Great Lakes to Alabama was formed in one large wave of crustal generation unparalleled in North America since. The rapid buildup of new crust results from periods of tectonic activity that are more energetic than any subsequent time in Earth's history. The assembled North American continent is stable enough to withstand a billion years of stampede and rift. It continues to grow by adding pieces of continents and island arcs to its edges. The large mass of volcanic rock near the eastern margin of North America implies that the continent was the core of a larger supercontinent called Rodinia that formed during the late Proterozoic. The interior of this land is heating up and erupting with volcanism. As a result, the warm, weakened crust broke up into several large continents between 630 and 560 million years ago, although their shape differed from today's continents. The breakup of the supercontinent created thousands of miles of new continental margins, which probably played a major role in the explosion of new life at the beginning of the Phanerozoic. After covering life in the Proterozoic, the next chapter discusses invertebrate life forms and rocks from the Cambrian period.