The land below us is always in motion. Plate tectonics studies these restless effects to give us a better understanding of the Earth and its past. New molten rocks are poured out in the form of magma from the mid-ocean ridges. The rock is recycled and re-entered back into the earth in deep ocean trenches through convection current. The convection current in the mantle drives plates around either against or away from each other. These collisions give rise to earthquakes, volcanoes, mountains, and continental drift. The crashing and spreading of the plates forms the landscape of the Earth as we see it today. The positions of the land masses today is a result of continental drift. During the Earth\'s existance, the magnetic fields have never been stable. Solidified magma containing magnetic imprints reveal periods of time when the Earth\'s magnetic fields have actually been reversed.
Approximately 4.55 billion years ago, the Earth was just a ball of molten material. Since then, parts of the Earth have cooled forming the solid crust-mantle. This process has been occurring for roughly about 3.8 billion years. The mantle is about 2900 km. thick, which lies above a layer of molten magma that still exists today. The immense heat from the magma (approximately 2700(C) causes convection in the mantle (Figure 1). Convection is caused by non-uniform temperature in a fluid and density differences. This continuous convection is the cause of plate movement. Each complete cycle, called a convection cell, drives the plate in the direction of the cell. How does a \'solid\' mantle move? The mantle may be solid but, as with most solids, it will deform if long term stress is applied; "...like Silly Putty which seeps into the rug when left unattended, mantle material flows when subjected to small long-term stresses."1
Presently, there are more than fourteen plates in the Earth\'s crust (Figure 22). Upwelling hot magma flows out from mid-ocean ridges and then cools down when exposed to the cooled environment outside; the layer of cooled magma forms the lithosphere. When magma flows out from the ridges, the crust is fractured and a new ocean floor is built spreading perpendicularly away from the ridge. Because of this constant upwelling, the ocean is relatively shallow in these areas. Sea floor spreading and continental drift are the products of this continual upwelling. The cooled magma will, in time, sink back down into the Earth in the deep ocean trenches. The mantle sinking down produces subduction zones or Benioff zones. The deepest part of the ocean resides in these areas. There are three types of boundaries where plates meet: divergent boundaries -- the upwelling of magma; convergent boundaries where the plates collide producing mountains, volcanoes, and earthquakes; and transform boundaries -- lateral movement. Transform plates are caused by fracture zones. When a rift opens from the upwelling of magma it causes a crack in the crust. As new magma rises to the surface, the crack increases caused by the pressure, resulting in a horizontal faulting. The fractured plate pieces travel in the same direction as the original plate was traveling -- away from the ocean ridge.
During the early 1900\'s, a theory of a \'super-continent\' was developed by Alfred Wegener. He was ridiculed for his ideas that continental drift produced the present positions of the continents from a single \'super-continent\' called Pangea. This theory is widely accepted today, however. There was abundant evidence for Wegener to believe in the existance of Pangea. The shape of the continents could be pieced together like a giant jigsaw puzzle suggesting that the continents were once \'glued\' together. The fossils found on the continents were not distinct to that particular land, but were also found in lands that were separated by thousands of kilometers of water. Fossils indicated that identical species existed in different continents. Geological structures also demonstrated that the continents were, in fact, one giant land mass; old mountain ranges from one continent matched with those from another (i.e., South America and Africa).
Ocean spreading has always been moving the continents towards or away from each other. About 200 million years ago during the Jurassic period, Pangea began to separate (Figure 33). Pangea\'s continental crust was subjected to many faults and rifts. Hot magma would flow out, splitting the land apart and creating a