Continental Drift Theory
The evolution of the Earth surface has been a matter of curiosity and debate among scientists for centuries now. Continental drift describes one of the earliest ways geologists thought continents moved over time. Today, the theory of continental drift has been replaced by the science of plate tectonics.
The theory of continental drift was proposed by geophysicist and meteorologist Alfred Wegener in 1912. Wegener published a paper explaining his theory that the continental landmasses were “drifting” across the Earth, sometimes ploughing through oceans and into each other. He called this movement continental drift.
Pangaea: A Supercontinent
He proposed that Earth must have once been a single supercontinent before breaking up to form several different continents. This explained how similar rock formations and plant and animal fossils could exist on separated continents. Modern science recognizes this ancient supercontinent called Pangaea did exist before breaking up about 200 million years ago, as Wegener theorized.
As per Wegener, Pangaea existed about 240 million years ago. By about 200 million years ago, this supercontinent began breaking up. Over millions of years, Pangaea separated into pieces that moved away from one another. These pieces slowly assumed their positions as the continent we recognize today.
Today, scientists think that several supercontinents like Pangaea have formed and broken up over the course of the Earth’s lifespan. These include Pannotia, which formed about 600 million years ago, and Rodinia, which existed more than a billion years ago.
Evidence:
Wegener, trained as an astronomer, used biology, botany, and geology to describe Pangaea and continental drift. For example, fossils of the ancient reptile Mesosaurus are only found in southern Africa and South America. Mesosaurus, a freshwater reptile only one meter (3.3 feet) long, could not have swum the Atlantic Ocean. The presence of Mesosaurus suggests a single habitat with many lakes and rivers.
Wegener also studied plant fossils from the frigid Arctic archipelago of Svalbard, Norway. These plants were not the hardy specimens adapted to survive in the Arctic climate. These fossils were of tropical plants, which are adapted to a much warmer, more humid environment. The presence of these fossils suggests Svalbard once had a tropical climate. Some of the examples were Glossopteris, Lystrosaurus and Cynognathus.
Grooves and rock deposits left by ancient glaciers are found today on different continents very close to the equator. This would indicate that the glaciers either formed in the middle of the ocean and/or covered most of the Earth. Today glaciers only form on land and nearer the poles. Wegener thought that the glaciers were centered over the southern land mass close to the South Pole and the continents moved to their present positions later on.
Similarly, Coral reefs and coal-forming swamps are found in tropical and subtropical environments, but ancient coal seams and coral reefs are found in locations where it is much too cold today. Wegener suggested that these creatures were alive in warm climate zones and that the fossils and coal later had drifted to new locations on the continents.
Finally, Wegener studied the stratigraphy of different rocks and mountain ranges. 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. South America and Africa were not the only continents with similar geology. Wegener discovered that the Appalachian Mountains of the eastern United States, for instance, were geologically related to the Caledonian Mountains of Scotland.
Rejection:
Scientists did not accept Wegener’s theory of continental drift. 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 which was faulty. Today, we know that the continents rest on massive slabs of rock called tectonic plates. The plates are always moving and interacting in a process called plate tectonics.
The continents are still moving today. Some of the most dynamic sites of tectonic activity are seafloor spreading zones and giant rift valleys. 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. Seafloor spreading is most dynamic along giant underwater mountain ranges known as mid-ocean ridges. As the seafloor grows wider, the continents on opposite sides of the ridge move away from each other.
Rift valleys are sites where a continental landmass is ripping itself apart. Africa, for example, will eventually split along the Great Rift Valley system. What is now a single continent will emerge as two—one on the African plate and the other on the smaller Somali plate. The new Somali continent will be mostly oceanic, with the Horn of Africa and Madagascar its largest landmasses.
The processes of seafloor spreading, rift valley formation, and subduction (where heavier tectonic plates sink beneath lighter ones) were not well established until the 1960s. These processes were the main geologic forces behind what Wegener recognized as continental drift.
Continental drift was a revolutionary theory explaining that continents shift position on Earth's surface. Though scientifically unsound on various grounds, Wegener’s theory is a significant milestone in the study of tectonics, and it laid a strong foundation for future theories like seafloor spreading and plate tectonics. Wegener never lived to see his theory gain wider acceptance. He died in 1930 at the age of 50. Scientists confirmed some of Wegener's ideas decades later, which are now part of the widely accepted theory of plate tectonics.
