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PLATE TECTONICS is the theory that explains continental drift. CONVECTION currents in Earth's mantle are the driving force behind plate movement, causing divergent, convergent, and transform boundaries where Earth's plates meet.
CONVERGENT BOUNDARIES occur where plates collide. If one plate is more dense than the other, it will "sink" underneath and often form VOLCANOES as it pushes magma toward the surface. This is called SUBDUCTION.
If the plates are of similar densities, the edges will "crumple" and form high mountain ranges such as the Himalayas. This is called UPLIFT.
DIVERGENT BOUNDARIES occur where two plates are separating, or moving apart. Rising magma breaks through the Earth's crust and forces the plates apart before cooling. This process repeats itself over and over and results in a RIFT VALLEY if the boundary occurs on a continent or a MID-OCEAN RIDGE if the boundary occurs under the ocean.
This phenomenon is known as seafloor spreading. Scientists have several examples of evidence in support of seafloor spreading. When they analyze the rocks found along these boundaries, they find that the rock nearer to the mid-ocean ridge is younger, while rock found further away from the mid-ocean ridge is older. Also, they can see patterns in the direction of magnetic fields shown in opposite sides of a mid-ocean ridge, caused by periodic shifts in the direction of Earth's magnetic field.
TRANSFORM BOUNDARIES occur where two plates move past one another. Often, parts of the plates get "stuck" on one another. When they release, massive amounts of energy are dispersed through the plates in an event known as an EARTHQUAKE.
As a result of plate movement, earthquakes and volcanoes often occur along plate boundaries as shown in the map below. The region of intense volcanic activity surrounding the Pacific Ocean is sometimes called the "ring of fire".
EARTH'S LAYERS
CRUST - outer layer
LITHOSPHERE - crust and upper-most solid mantle, forms Earth's plates
MANTLE - hot layer of putty-like material, convection currents push/pull the plates along the surface
OUTER CORE - liquid, mostly iron
INNER CORE - solid due to intense pressure, mostly iron
Evidence for Plate Tectonics