Spreading Centers
Earth's crust is constantly changing, and plate material is always being created and destroyed. Continents and oceans do not ride atop the plates without changing. As the plates move and change so do the continents and oceans.

Click on the icons above to see the location of Earth's major volcanoes and earthquakes, the boundaries of the tectonic plates, and the spreading centers. Notice that about half of the plate boundaries are also spreading centers. Also notice that the plate boundaries that are coming together, rather than spreading apart, are the location of most major volcanoes and earthquakes.

The seafloor mountains that circle Earth are called the Global Mid-ocean Ridge. Magma is pushing up from Earth's interior all along this ridge and creating new crust. The same forces that pull the plates apart also allow magma from  Earth's interior to come up along the ridges and create new crust. Areas where new crust is created and plates are forced apart are called spreading centers.
Although the movement of the plates away from spreading centers is very, very slow (about one inch per year), it has been happening for millions of years. Thus, very small changes have become very large over time.
When scientists first discovered that new crust was being created continuously, they wondered if Earth were getting larger. Using mathematical models of Earth and sophisticated measuring tools scientists confirmed that it was not.
Two hundred million years ago the Atlantic Ocean was just a small bay between the continents of Africa, Europe, and the Americas. Now it is a huge ocean. The spreading center that runs through the earth beneath the Atlantic Ocean has been adding about 15 miles of new crust every million years, with the result that the Atlantic Ocean is now more than 3000 miles wide.
If this much new land has been created and Earth is no larger now than it was 200 million years ago, where is all the extra going? The answer is that it is being recycled.


When Plates Collide
Where tectonic plates are not being pushed apart, they are either smashing together or slowly sliding past each other.
What happens when plates collide depends on the type of crust involved. If one plate is continental crust and the other is oceanic crust, the lighter continental crust will be pushed up and the oceanic crust will slowly be forced under it. If both plates are oceanic crust, one also will be forced under the other.
When one plate edge sinks beneath another plate edge, the crust material is forced back into Earth's interior where it is destroyed. (It becomes part of Earth's mantle.) One plate sinking beneath another is called subduction, and these areas are known as subduction zones.

Volcanoes like Alaska's Mt. Mageik (above) occur along subduction zones, areas where one tectonic plate is sinking beneath another one.

A deep trench in the seafloor forms where a plate is subducted. Volcanoes are created in a line that parallels the trench. If volcanoes are created in an oceanic plate, they can produce islands as they grow. If the volcanoes are in a continental plate, they can produce mountains.
Over half of all Earth's volcanoes occur in a wide arc that roughly outlines the Pacific Ocean. This is because the Pacific Plate is being subducted under the North American Plate on its east and north sides and under the Philippine and Australian Plates on its west side. This area has so many volcanoes that it is known as the Ring of Fire.
When both plates being pushed together contain continental crust (which is relatively light), the crust tends to be pushed upward or sideways. Upward motion can create large mountain ranges. Both upward and sideways plate movements can cause large earthquakes.

A Look Beneath the Surface...