Africa, for example, will eventually split along the Great Rift Valley system. Rift valleys are sites where a continental landmass is ripping itself apart. The two continents are moving away from each other at the rate of about 2.5 centimeters (1 inch) per year.
The North American and Eurasian tectonic plates, for example, are separated by the Mid-Atlantic Ridge. As the seafloor grows wider, the continents on opposite sides of the ridge move away from each other. Seafloor spreading is most dynamic along giant underwater mountain ranges known as mid-ocean ridges. In the process of seafloor spreading, molten rock rises from within the Earth and adds new seafloor ( oceanic crust) to the edges of the old. Some of the most dynamic sites of tectonic activity are seafloor spreading zones and giant rift valleys. The plates are always moving and interacting in a process called plate tectonics. Today, we know that the continents rest on massive slabs of rock called tectonic plates. One of the elements lacking in the theory was the mechanism for how it works-why did the continents drift and what patterns did they follow? Wegener suggested that perhaps the rotation of the Earth caused the continents to shift towards and apart from each other. Scientists did not accept Wegener’s theory of continental drift. These include Pannotia, which formed about 600 million years ago, and Rodinia, which existed more than a billion years ago. Today, scientists think that several supercontinents like Pangaea have formed and broken up over the course of the Earth’s lifespan. These pieces slowly assumed their positions as the continent we recognize today. Over millions of years, Pangaea separated into pieces that moved away from one another. By about 200 million years ago, this supercontinent began breaking up.
Pangaea existed about 240 million years ago. Wegener discovered that the Appalachian Mountains of the eastern United States, for instance, were geologically related to the Caledonian Mountains of Scotland. South America and Africa were not the only continents with similar geology. The east coast of South America and the west coast of Africa seem to fit together like pieces of a jigsaw puzzle, and Wegener discovered their rock layers “fit” just as clearly. The presence of these fossils suggests Svalbard once had a tropical climate.įinally, Wegener studied the stratigraphy of different rocks and mountain ranges. These fossils were of tropical plants, which are adapted to a much warmer, more humid environment. These plants were not the hardy specimens adapted to survive in the Arctic climate. Wegener also studied plant fossils from the frigid Arctic archipelago of Svalbard, Norway. The presence of mesosaurus suggests a single habitat with many lakes and rivers. Mesosaurus, a freshwater reptile only one meter (3.3 feet) long, could not have swum the Atlantic Ocean. For example, fossils of the ancient reptile mesosaurus are only found in southern Africa and South America. Wegener, trained as an astronomer, used biology, botany, and geology describe Pangaea and continental drift. Wegener was convinced that all of Earth’s continents were once part of an enormous, single landmass called Pangaea. He called this movement continental drift. In the early 20th century, Wegener published a paper explaining his theory that the continental landmasses were “drifting” across the Earth, sometimes plowing through oceans and into each other. The theory of continental drift is most associated with the scientist Alfred Wegener. Today, the theory of continental drift has been replaced by the science of plate tectonics. Continental drift describes one of the earliest ways geologists thought continents moved over time.
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