Volcanic Activity and Plate Motions
>> Hi, everyone. We're going to look in this animation at how the movements of the Earth's plates controls volcanic activity on the Earth. Let's look first at what's called continental rift volcanism. Now, in places where forces are causing the Earth's plates to stretch, especially in the continents, a couple of different things can happen. In this part of the animation, you can see there's what's called a rift valley where the crust is ripped apart and it sinks down to form a valley. That's kind of like the East African Rift Valley, or much of the Western United States in Nevada. Or you can simply get cracks in the crust that will allow magma from the mantle to rise up.
In either of these situations, what we call continental rifting, we get volcanoes to form because as the plates separate, it allows the mantle to rise upward. Really what happens is the separation of the plates depressurizes the mantle underneath. There's not as much weight on top. And that reduction in pressure allows the hot rock of the mantle to actually melt. Now, more important even than that is the kind of volcanic activity that happens as associated with subduction.
In this animation, we're going to subduct some ocean floor underneath the edge of a continent. And as that happens, as you know, you get an ocean trench. But at a certain depth, about 80 miles down in the Earth, the subduction of the plate triggers melting of the mantle. And that magma rises and a whole series of volcanoes begins to pop up along the edge of the continent parallel to the trench.
We call this a CONTINENTAL VOLCANIC ARC. The Andes Mountains are a classic example of such a system. Now, another thing we can do is have instead of ocean floor subducting underneath continent, we could have ocean floor subducting under other ocean floor. In this case, the animation is flipped around, but it doesn't really matter; ou can imagine either of these in a mirror image. It's the same process. We're going to subduct the ocean floor underneath this other ocean floor. We're going to generate magma. The magma's going to rise and a whole series of volcanic islands is going to pop up in the ocean parallel to the trench.
You see how it's very similar to the continental situation. The only difference is that the volcanoes are popping up out of the ocean parallel to the trench instead of on land parallel to the trench. An example of this would be the Aleutian Islands, say, or the Philippines, or the Marianas Islands in the Pacific. These are all what we call VOLCANIC ISLAND ARCS. Now, the other main source of volcanic activity on Earth are what we call HOT SPOTS. In a situation like this, there's magma being generated all along a long line. Hot spots are situations where magma forms in just one local area. And we think these hot spots form because of large blobs, or columns, of magma that rise from deep in the mantle. We call these, MANTLE PLUMES. And as tectonic plates move across mantle plumes, if mantle plumes stay in one place (and there's controversy about whether they do).
But let's assume the plume stays in one place. It'll make a volcano. The volcano will get carried away. Another volcano will form; it'll get carried away on the moving plate. And yet another volcano will form and it'll get carried away. And you get a chain of hot spot volcanic islands. This is how, for example, the Hawaiian Islands formed, all in a line, one after the other, as they moved across a hot spot. Let's back this up again and we'll show one volcano, then another, and then another forming over the hot spot. So those are the main reasons, and processes, by which the Earth makes volcanic activity.