Why can you fit South America and Africa together like a jigsaw puzzle?
If you have ever looked at a map of the Atlantic Ocean, you have probably noticed that the coastlines of Africa and South America seem to fit together like pieces of a jigsaw puzzle. The fit between the two coastlines is even better when the edges of the continental shelf are compared. For many years, scientists thought this was just a coincidence, because no one could think of a way that the continents could slide around.
In fact, 220 million years ago, all the continents were joined all the continents were joined together to form one super continent called Pangaea. There was also only one giant ocean, Panthalassa.
In time, Pangaea split into smaller land masses, and the continents began to drift towards their present positions. In the future, the movements of the tectonic plates will cause the continents to drift even more.
This will result in the Pacific Ocean shrinking, the Atlantic Ocean getting wider, and Africa splitting into two.
image source: wikimedia commons
Evidence for Continental Drift
Evidence that South America and Africa might once have been joined to each other came from the research of the German geographer, Alexander von Humboldt. Von Humboldt traveled throughout South America, Africa, and other parts of the world, collecting plant and animal specimens and studying geography and geology. He observed many similarities between South America and Africa in addition to the apparent fit of continental coastlines. For example, von Humboldt noticed that the mountain ranges near Buenos Aires, Argentina, match mountain ranges in South Africa.
Other mountain ranges in Brazil extend to near the seashore and stop. Similar mountain ranges begin at the corresponding seashore in Ghana in Africa. All of these mountain ranges appear to have the same age and to be formed of the same kinds of rock. The rock strata in these and other mountain ranges would match perfectly if the coastlines of the two continents were lined up. Von Humboldt also observed similar patterns among mountain ranges in Europe and North America.
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 Today, the theory of continental drift has been replaced by the science of plate tectonics.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 is most associated with the scientist Alfred Wegener. 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. He called this movement continental drift.
Wegener was convinced that all of Earth’s continents were once part of an enormous, single landmass called Pangaea.
Wegener, trained as an astronomer, used biology, botany, and geology 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.
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.
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.
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. (It doesn't.)
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. The North American and Eurasian tectonic plates, for example, are separated by the Mid-Atlantic Ridge. The two continents are moving away from each other at the rate of about 2.5 centimeters (1 inch) per year.
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.