Earth Science: Origins and Geology – Plate Tectonics
Earthquakes in Afghanistan, volcanoes in Java, ice-capped mountains in South America, and mid-ocean ridges and trenches are all natural phenomena caused by shifting continents or forming oceans. They are the results of hidden geological forces within the Earth.
Earth in Motion
How did the Atlantic Ocean spread, reaching all the way to Africa before splitting about 30 million years ago? Today the model of plate tectonics explains the movement of the Earth’s crust.
Earth’s uppermost mantle and crust make up the approximately 62-mile (100-km)-thick lithosphere. Earth’s solid outer shell, the constitution of which varies between the oceanic and continental lithospheres, is a mosaic of plates—seven large and about ten smaller sections.
Beneath the plates lies the asthenosphere. Soft and partly molten, the asthenosphere provides a base, over which these plates slowly glide. The cause of this gliding movement lies deep within the interior of the Earth.
The force behind plate tectonics
Radioactive decay heats the lower layers of Earth’s mantle, causing them to rise to cooler areas. There the molten rock cools and sinks, only to rise again as part of a cycle called mantle convection. The molten mantle material, or magma, pushes up Earth’s crust, ultimately breaking it open and solidifying along the resulting lines of fracture. Intense volcanic activity can cause magma to spill out onto Earth’s surface, creating lava flows. Such active faults and volcanic flows have formed the Mid-Atlantic Ridge on the floor of the ocean.
The solidification of magma forms new ocean crust as old cracks break open again and again, each time increasing the amount of crust. In this way, two oceanic lithospheric plates have formed, each of which is gradually growing and drifting away from the other. In much the same way, when the crust within a continent breaks open, new oceans can eventually form. For example, in about ten million years the East African trench could divide Africa.
CONTINENTAL DRIFT
The Atlantic coasts of Africa and South America fit into each other like jigsaw pieces and are also home to many of the same sediments and rep tiles. Similarly, closely related plants made their homes in India, Australia, and Antarctica 200 million years ago.
These observations led German meteorologist and geoscientist Alfred Wegener to propose in 1911 that all the continents were once one big landmass that later split up. 50 years later his theory was confirmed by the model of plate tectonics.