Iapetus Sea Educational Materials

IAPETUS SEA During the late Precambrian and early Cambrian, a proto-Atlantic Ocean called Iapetus opened between divided continents, forming the vast Cambrian inland sea. The puddle submerged much of the surrounding ancient North American continent called Laurentia and the ancient European continent called Baltica. The Sea of Iapetus is about the same size as the North Atlantic and occupied the same general location about 500 million years ago (Fig. 69). The continuous coastline stretching from Georgia to Newfoundland between about 570 and 480 million years ago suggests that this ancient coast overlooked a vast, deep ocean. Iapetus stretches for at least 1,000 miles from east to west and borders a much larger body of water to the south. It was dotted with volcanic islands and resembled the present-day Pacific Ocean between Southeast Asia and Australia. The shallow waters of this ancient marine coast from the Cambrian to the mid-Ordovician, about 460 million years ago, contained abundant invertebrates, including trilobites, which account for about 70 percent of all species. Eventually, trilobites faded, while mollusks and other invertebrates extended throughout the ocean. The closure of this ancient ocean basin in the Ordovician as Baltica approached Laurentia signifies the formation of Laurasia. Large-scale mountain building followed the closure of Iapetus when the continents flanking the sea collided, pushing mountains in northern Europe and North America, including those that evolved into the Appalachians. A spate of mountain buildings may have triggered an explosion in species diversity. The greatest increase was the Ordovician radiation of marine species about 450 million years ago. The foreland basin is filled with thick sediments eroded from nearby mountains. Erosion from the mountains may have pumped nutrients into the ocean, triggering an explosion of marine plankton, thereby increasing the food supply for higher creatures. Therefore, the number of genera of mollusks, brachiopods, and trilobites (Fig. 70) increases dramatically, since organisms with abundant food are more likely to thrive and diversify into different species. An island arc located between two colliding landmasses was picked up and plastered at the edge of the continent as the two continents collided. The oceanic crustal plates that carry the islands dive beneath Baltica in a process known as subduction. That subduction saw the islands collide with the continent and deposit previously submerged rocks onto Norway's present-day west coast. Slices of land called terranes located in western Europe migrated to Iapetus from ancient Africa. In a similar way, chunks of crust from Asia traveled across the ancient Pacific Ocean called Panthalassa, Greek for "universal sea," to form most of western North America. Most of the Alaskan panhandle, known as Alexander Terrane, began its existence as part of eastern Australia about 500 million years ago. About 375 million years ago, it broke away from Australia, crossed the protoPacific Ocean, stopped briefly on the coast of Peru, slid past California, and crashed into the upper North American continent about 100 million years ago. The entire state of Alaska is an agglomeration of terranes which are pieces of ancient oceanic crust. The Terranes are well exposed in the Brooks Range (Fig. 71), the major east-west-trend mountain belt that forms the spine of northern Alaska. Basaltic underwater mountains that add up to the margins of North America travel halfway across the ocean that precedes the Pacific. Terranes (Fig. 72) are error-delimited blocks. They range in size from small crustal fragments to subcontinents, with geologic histories very different from neighboring blocks and adjoining continental masses. (Note: Don't confuse the term terrane with the word terrain, which means landform.) Terranes are usually bounded by faults and differ from their geological environment. The boundary between two or more terranes is called the suture zone. The composition of terranes generally resembles oceanic islands or plateaus. Others consist of consolidated conglomerates of gravel, sand, and silt accumulated in ocean basins between colliding crustal fragments. The Terranes, which are 1 billion years old, were dated by analyzing fossils of trapped radiolarians (Figure 73), marine protozoa that lived in deep waters and were abundant from about 500 million to 160 million years ago. Different species also define specific areas of the ocean where terranes originate. Many terranes travel great distances before finally following the continental margins. Some North American terranes have western Pacific origins and evacuated thousands of miles to the east. From the Cambrian to the end of the Paleozoic, the western edge of North America ended near present-day Salt Lake City. Over the past 200 million years, North America has expanded more than 25 percent during the prime pulse of crustal growth. Much of western North America was assembled from an arc of oceanic islands and other crustal debris slid off the Pacific plate as the North American plate headed west after the breakup of the supercontinent Pangaea. Terranes exist in all shapes and sizes, ranging from small pieces to subcontinents like India, itself a single large terrane. Most terranes are elongated bodies that change shape when colliding with continents. Terranes created on oceanic plates retain their shape until they collide and accrete to the continents. They then undergo crustal movements that modify their overall dimensions. Clusters of terranes in China are being stretched and moved in an east-west direction due to India's continuous pressure exerting into south Asia as it increases the Himalayas. Granulite terraines are high-temperature metamorphic belts that form in deeper parts of continental fissures. They also consist of the roots of mountain belts created by the collision of continents, such as the Alps and the Himalayas. North of the Himalayas is a belt of ophiolites, which marks the boundary between the sewn continents. Terrane boundaries are generally characterized by ophiolite belts, which consist of marine sedimentary rocks, pillow basalts, layered embankment complexes, gabbro, and peridotite. Terranes also play a major role in the creation of mountain chains along convergent continental boundaries. For example, the Andes appear to have been raised by the accretion of oceanic plateaus along the margins of the South American continent.Along the mountains of western North America, terranes are elongated bodies as they slice through the crust by a network of northwest-trending faults. One of them is the San Andreas Fault in California, which has undergone a displacement of about 200 miles in the last 25 million years. About 500 million years ago, North America was the lost continent. South America, Africa, Australia, Antarctica, and India have converged into the supercontinent Gondwana. However, North America and some smaller continental fragments float freely on their own. At this time, North America was located several thousand miles off the west coast of South America, placing it on the western side of Gondwana.About 750 million years ago, North America was located at the core of an earlier supercontinent called Rodinia, when Australia and Antarctica bordered the west coast of the North American continent. North and South America apparently bordered each other at the beginning of the Ordovician (Figure 74), placing what would become present-day Washington, D.C.The limestone formations in Argentina contain different species of trilobites typical of North America but not of South America. Fossil evidence suggests that the two continents collided about 450 million years ago, creating ancestral Appalachian ranges along eastern North America and western South America long before the current Andes formed. Later, the continent split, transferring a piece of land containing trilobite fauna from North America to South America.