At one time all of the continents were combined together to form a large supercontinent called Pangaea. Over time, Pangaea eventually separated to form two large landmasses called Gondwanaland and Laurasia. These landforms slowly drifted apart to form the continents that exist today. The theory of plate tectonics explains how our present-day continents arrived at their current location. It also explains the formation of mountains, volcanoes, and earthquakes.
The outermost layer of the Earth involved in plate tectonics is called the lithosphere. The name is derived from the Greek word “lithos” meaning rock. The lithosphere consists of two compositional layers, the crust and upper mantle. The crust and mantle are composed of the elements silicon and oxygen. The mantle is also made up of the elements iron and magnesium as well as peridotite.
The lithosphere is about 80 km deep and it consists of tectonic plates that fit together like a giant puzzle. There are at least eight major tectonic plates and several smaller ones that surround the Earth. These tectonic plates are slowly moving. This may be one reason why the lithosphere and asthenosphere are described as Earth’s mechanical layers. The asthenosphere is a hotter, softer layer of solid rocks underneath the lithosphere. The lower mantle is located in the asthenosphere. The asthenosphere deforms slowly under tremendous pressure and heat. The asthenosphere assists in the movement of the tectonic plates. The lithosphere floats above the asthenosphere.
The eight major tectonic plates are the North American plate, South American plate, Nazca plate, Pacific plate, Australian plate, Antarctic plate, African plate, and Eurasian plate. The boundaries among these plates interact to form geologic features on Earth such as mountains, volcanoes, and deep sea trenches in the oceans.
Evidence to support the Theory of Plate Tectonics
- Rock formation
In the early 1900s, a German scientist Alfred Wegener proposed his idea called continental drift to explain how the continents were once join together and drifted apart. Wegener was able to observe the same similarities in the layers of sedimentary rocks found in the Appalachian Mountains in the United States and in some similar mountains in Greenland and Europe. These layers of rocks were about 200 million years old. Therefore the same age and type of rocks were found in mountains that are separated by the Atlantic Ocean. These rocks may have originated at the same location when the continents were joined together.
The fossil remains of plants that live in warm, tropical climates were found in Antarctica. This also suggests that the continents were once joined together as a supercontinent.
The hypothesis of continental drift was not able to explain how the tectonic plates move above the asthenosphere. Many scientists believe there are convection currents within the mantle and they are the driving force for plate tectonics movement. Recall that the mantle is located within the Earth between the crust and core. The core is extremely hot and it supplies a lot of heat to the mantle. Radioactive decay from within the mantle and the core also helps produce this heat.
The heat from the core produces convection currents within the mantle. Convection is the transfer of heat energy from a warmer region to a cooler region. Convection currents are the ongoing movement of fluids caused by differences in density of fluids at different temperatures. The molten rocks and matter in the mantle moves in a circular pattern. As the matter closer to the core is heated up, it becomes less dense and rises toward the crust. The cooler matter near the crust is denser and will sink towards the center of the Earth. The rise and fall of molten material in the mantle is due to convection currents. This phenomenon is the mechanism that moves the tectonic plates a few centimeters per year. Plate tectonics is a theory that explains why and how the continents’ locations change over time. It is also the study of the formation of the Earth’s crust. Plate motion causes earthquakes, volcanoes, and mountain ranges.
Scientists believe that there are two other processes involved in plate movement. They are ridge push and slab pull. Magma from the asthenosphere rises to the surface of the ocean when plates pull away from one another at mid-ocean ridges. When the lithosphere moves away from the ocean ridge, the lithosphere cools and becomes denser. The lithosphere eventually becomes too dense and begins to sink into the asthenosphere. This downward force is called slab pull. Ridge push is the force that is exerted when the asthenosphere puts pressure on the plate . Ridge push is thought to have some effect on plate motion.