The earth's crust is not entirely solid but is formed of a patchwork of solid segments called plates. The plates are composed of stone and can be anywhere between four and 40 miles deep. The earth has nine large plates, mostly named for the continents that rest upon them. It has a number of smaller pieces as well. These pieces float upon the hotter, molten rock that lies beneath them. As they float, convection currents, or currents caused by the transfer of heat, cause these plates to meet in some places and separate in others.
The results of this movement can be observed on the earth's surface.
In some places, the plates are moving slowly away from each other. These places are called divergent boundaries. Oceans often form or expand where plates move away from each other. It is believed that Iceland, for example, will eventually break apart into two separate land masses. This is because Iceland rests partially on the North American Plate and partially on the Eurasian Plate. North America and the North American Plate are moving slowly westward, while Eurasia is moving eastward. As the land separates, it is expected that the Atlantic Ocean will rush in to fill the space.
Convergent boundaries are places where the plates are colliding. Usually, when two plates collide, one is forced beneath the other. The areas where this occurs are called subduction zones. Volcanoes and mountains are common where plates converge. Occasionally, as was the case when India struck Asia, neither plate is forced downward. Rather than moving downward, the crust buckles or moves sideways. The Himalayas, the earth's tallest mountain range, were created by such a collision between the Indo-Australian Plate and the Eurasian Plate.
Transform-fault boundaries are commonly referred to as faults. Faults exist where plates are not colliding or diverging but are sliding past each other in opposite directions. Most transform-fault boundaries are beneath oceans. California's San-Andreas fault is a rare exception. The North American Plate is moving in a roughly southeastern direction. The Pacific Plate is sliding past it, moving northwest. Shallow earthquakes are especially common around transform-fault boundaries. Los Angeles's famed earthquake activity is a direct result of the San-Andreas fault.