Laurasia Study Notes

LAURASIA During the Silurian, all the northern continents collided to form Laurasia, which includes the interiors of North America, Greenland, and Northern Europe.The ancestors of the North American continent called Laurentia gathered from several colliding microcontinents starting about 1.8 billion years ago. Most continents evolved in a relatively short period of time, only 150 million years. Most of the continental crust underlying the United States from Arizona to the Great Lakes to Alabama was formed in one large wave of crustal generations that have no known similarities. This was probably the most energetic period of tectonic activity and crustal generation in Earth's history, when more than 80 percent of all continental masses were created. The best exposure of these Precambrian metamorphic rocks is the Vishnu Schist on the floor of the Grand Canyon (Figure 85). Laurentia is stable enough to withstand another billion years of jostling and rift. It continues to grow by plastering pieces of continental pieces and island arcs to the edges. The presence of large amounts of volcanic rock near the eastern edge of Laurentia implies the continent was once the core of a larger supercontinent. The central part of the supercontinent is far from the cooling effect of subduction plates, where the Earth's crust sinks into the mantle. As a result, the interior of the supercontinent warmed up and erupted with volcanism. After the rapid formation of continents, the interior of Laurentia experienced extensive frozen activity from 1.6 to 1.3 billion years ago. The broad belt of red granite and rhyolite, which is igneous rock formed by molten magma frozen underground as well as on the surface (Table 6), extends several thousand miles across the interior of the continent from southern California to Labrador. Laurentian granite and rhyolite are unique in their sheer volume, suggesting the continent is stretching and thinning almost to its breaking point. These rocks are currently exposed in Missouri, Oklahoma, and several other areas. However, they are buried under a mile thick sediment in the middle of the continent. In addition, large amounts of molten basalt poured from large tears in the crust that flowed from southeastern Nebraska to the Lake Superior region about 1.1 billion years ago. An arc of volcanic rock also weaves through central and eastern Canada into the Dakotas. The large outpouring of igneous rocks in the interior of this continent suggests that Laurentia was part of a supercontinent that formed about 1.6 billion years ago and broke apart about 1.3 billion years ago, coinciding with frozen activity. The supercontinent acts like an insulating blanket over the upper mantle, allowing heat to collect underneath. About 1.1 billion years ago, large amounts of molten basalt flowed from large tears in the Earth's crust that flowed from southeastern Nebraska to the Lake Superior region. About 700 million years ago, Laurentia collided with another large continent on its southern and eastern borders, creating a new supercontinent called Rodinia.A superocean located roughly at the site of the current Pacific Ocean surrounds the supercontinent. The collision pushed a 3,000-mile-long mountain belt in eastern North America called the Grenville orogeni (Fig. 86). Similar mountain belts also occupy parts of western Europe. About 670 million years ago, thick ice sheets scattered over most of the land during the largest glaciation period known to Earth. By this time, Rodinia might have passed through one of the poles and collected a thick sheet of ice. Rodinia split between about 630 and 560 million years ago, and its constituent continental blocks drifted away from each other. As continents disperse and subside, seawater floods the interior, creating large continental shelves on which diverse organisms evolve. The rapid evolution of species at this time is remarkable. Another remarkable episode of explosive evolution occurred when a supercontinent called Pangea broke apart about 400 million years later. The ancient North American continent welded together from crustal remains called cratons about 2 billion years ago. The continents of Africa and South America did not come together until about 700 million years ago. Over the past half billion years, about a dozen individual continental plates have come together to form Eurasia. It is the youngest and largest modern continent and is still held together by chunks of crust that arrived from the south, riding highly mobile tectonic plates. Rodinia split and separate continents opened a protoAtlantic Ocean called Iapetus. The rift process formed a vast inland sea, which submerged most of Laurentia about 540 million years ago, as evidenced by the presence of the Cambrian coast in places like Wisconsin. Iapetus had the same location and size as the North Atlantic and was dotted with volcanic islands, resembling the present-day southwestern Pacific Ocean. When continents reached their maximum spread about 480 million years ago, seafloor subduction beneath the North American plate began a period of volcanic activity and mountain building. From early Siluria to Devonian, about 420 million to 380 million years ago, Laurentia collided with Baltica, an ancient European landmass, and closed Iapetus.The collision united the two continents into the supercontinent Laurasia (Figure 87), named for the Canadian province of Laurentian and the Eurasian continent. This collision of the Paleozoic continent lifted enormous masses of rock into several mountain belts around the world. The seams joining the mainland are preserved as the eroded core of the ancient mountain range. When Laurentia united with Baltica, an island arc in the proto-Pacific Ocean called Panthalassa began to collide with the western margins of what is now North America.The collision caused Antler orogeny, which severely damaged rocks in the Great Basin region from the California-Nevada border to Idaho (Figure 88). It is named after Antler Peak near Battle Mountain, Nevada. It consists of widespread coarse clastic sediments and a prominent eastward thrust fault known as Roberts Mountain Thrust. Laurasia occupies the northern hemisphere, while its counterpart Gondwana inhabits the southern hemisphere. Most of Gondwana is in the south polar region from the Cambrian to the Silurian.The continent of Australia today sits on the equator on the northern edge of Gondwana. A huge body of water called the Tethys Sea, named after the mother of the ocean in Greek mythology, separates two supercontinents. Evidence of the existence of vast seas between the landmasses comes from a unique specimen of flora called Glossopteris (Figure 89) found in southern soils but absent in Laurasia. The continents were degraded by erosion. The shallow sea flooded the land, covering more than half of the land area today. Heavy sediments form deep depressions in the oceanic crust called geosyncline. Sedimentary rocks were then uplifted into the large mountain belt surrounding the Mediterranean Sea as Africa pounded Eurasia. Inland seas and wide continental margins along with a stable environment provide excellent growing conditions and allow marine life to develop and spread throughout the world. During the late Siluria, Gondwana wandered into the south polar region about 400 million years ago and acquired a thick layer of ice. The glacial center extends in all directions. The ice sheet covers most of east central South America, South Africa, India, Australia, and Antarctica (Figure 90). During the early part of glaciation, maximum glacial effects occurred in South America and South Africa. Later, the major glacial centers shifted to Australia and Antarctica, providing compelling evidence that the southern continents were wandering locked together at the South Pole. In Australia, Silurian period marine sediments were found interspersed with glacial deposits. Tillites, consisting of large rocks and glacially deposited clay, are separated by coal seams. This suggests that periods of glaciation were punctuated by warm interglacial spells as vast forests grew. The Karroo series in South Africa consists of a sequence of late Paleozoic lava flows, tillites, and coal seams, reaching a total thickness of 20,000 feet. Among the coal seams are fossilized leaves of the extinct fern Glossopteris. Since this plant is found only in the southern continent, it is one of the best proofs for the existence of Gondwana. After the earliest examination of land plants from the Silurian, the next chapter will focus on the marine life of the Devonian period.