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The crust consists of giant plates in constant motion, floating in the magma of the upper mantle. A constructive plate margin will be an area where magma is forced through the crust due to pressure along oceanic ridges. New ocean floor is formed when the magma solidifies. There are also destructive plate margins, otherwise known as subduction zones. It is the opposite of a constructive plate margin. Ocean floor is destroyed (hence, "destructive"), when plates collide. The edge of one of the plates is then dragged down into the mantle. The last type of boundary is called the transform fault. Plates just slide past each other at such areas. Plate movements at such boundaries can lead to earthquakes. Back (What is an earthquake) | Next (During an earthquake) |