The idea that the earth’s continents move has been around for centuries. As early as the sixteenth century, cartographers noticed the puzzle-like similarities between the coastlines of Africa and North America. The theory of continental drift, however, was not popularly accepted by the geologic community until the 1960’s. One reason geologists found the theory difficult to believe was that no one had yet proposed a suitable explanation for how the continents were moving.
When a German meteorologist, Alfred Wegener, first proposed his theory of continental drift, he was ridiculed. Popular belief in the early twentieth century was that the great landmasses of earth had always been in their present locations. There was no known earthly power strong enough to move continents. Wegener’s evidence that the continents once stood connected was intriguing, but geologists of the day resisted his ideas. Evidence such as similar plant and animal life or rock formations on shores separated by an ocean was dismissed. Regrettably, Wegener was known widely as a zealot, and was greatly disliked by the geologic community. His universal lack of respect made geologists less likely to consider his work legitimate. Although Wegener was largely ignored during his liftime, his idea that the continents move stood the test of time.
The modern theory of continental drift is known as the theory of plate tectonics. Plate tectonics is simply the idea that the earth’s crust consists of plates that float on top of the plastic mantle and move about, into and past each other. The earth’s crust is a very thin layer that is composed of two types of rock, continental and oceanic. Continental crust is less dense, less flexible and much thicker than oceanic crust. The continents consist of the landmass visible above the ocean, plus the continental shelf they rise above. The continental plates are analogous to an iceberg, with a smaller fraction of the plate visible above water, and the rest underwater. What could be so powerful that it could move such behemoths across the surface of the earth? The most accepted reason is simple: mantle convection.
The layer directly beneath the crust is the mantle. The mantle consists of denser rock that is hot enough to melt, but is still solid because it is under great pressure. The mantle is a solid, but often behaves as a fluid. There are convection currents within the mantle that carry warmer mantle toward the surface and pull cooler mantle downward. Temperature differences are not all that drive mantle convection. There are other processes at work as well. Radioactive degradations of materials within the mantle release energy as heat, which results in areas that are warmer and more fluid. Minerals that compose the mantle behave differently under high pressure, becoming suddenly more or less fluid. These conditions and others combine to create the convective currents within the mantle.
There are three types of plate boundaries where the plates interact with each other: divergent, convergent and transverse. As warmer mantle rises, the pressure exerted upon it decreases, allowing it to melt into magma. Underneath the oceans are large rifts, or cracks, within the oceanic crust. The magma oozes out of the rifts onto the ocean floor. As the magma cools, it becomes denser and heavier. Because oceanic crust is so thin and flexible, as the magma cools, it sinks. The sinking motion causes it to move away from the rift. This type of boundary is a divergent boundary because the sides are moving away from each other. Most divergent boundaries occur in the oceanic crust, but there is a large divergent boundary in Africa where two sections of crustal plate are being driven apart by rising mantle.
Sea floor spreading often pushes oceanic plates into continental plates. Since oceanic plates are denser than continental plates, the oceanic plate will be subducted, or overridden, by the continental plate. The subduction of oceanic crust is a source of volcanic activities, most notably on the western coasts of North and South America. Because oceanic crust is constantly subducted, there is none older than approximately 190 million years old.
In some cases the densities of the crusts are similar, so rather than one being subducted, they are both pushed upward to form mountains. Crustal boundaries where the two plates are being pushed together are called convergent, or collisional boundaries. The grand mountains called the Himalayas were formed by such a boundary. The United States has had several mountian-building events caused by island arc accretions. Island arc accretions occur when independent pieces of continental crust, or ‘islands’ encounter another continental mass at a collisional boundary. The Rocky Mountains of the American interior are an example.
The third type of plate boundary is the transverse boundary. Transverse boundaries are formed when two plates slide past one another rather than into or away from each other. Transverse boundaries are usually located underwater and are associated with high earthquake frequency. A well-known, land-bound transverse boundary is the San Andreas Fault in the United States, where the Pacific plate and the North American plates interact.
As with many advances in human understanding of the natural world, Wegner’s theory of continental drift has taken a long journey from ridiculed idea to accepted science. While the theory of plate tectonics is generally accepted as truth, the explanations of its processes are as yet unproven. Most geologists believe the continents drift because they are moved by convection currents in the mantle. There is still debate over whether or not this theory is true. Although the exact cause is still debatable, science has proved beyond doubt that the crust of the earth is constantly on the move.
1. Decker and Decker, Volcanoes. San Francisco, W.H. Freeman and Company, 1981.
2. Clayton, Keith, The Crust of the Earth: The Story of Geology. New York, The Natural History Press, 1967.