[SOUND] [MUSIC] A second location where volcanic activity happens is along rifts. Remember that rifts are places where continental crust is starting to break apart. It has not yet separated continents in two, so we have not yet formed a new ocean basin. But, because rifts stretch the lithosphere horizontally, they also result in thinning the lithosphere vertically. We can see that with a simple sketch. Let's imagine that we're pulling, I'll do it in 3D. Let's imagine that we start pulling a region of lithosphere apart. Okay we're stretching it that way. After some time, it'll be longer, I'm drawing this over-simplified. Not only does it get longer horizontally, but it also thins vertically. Now let's look at the consequence of this thinning to these things here below. Let's imagine that we have a block of asthenosphere outlined in white here. That's at fairly high pressure because it's deep down because remember, as you go deeper and deeper in the earth. You have the weight of all the overlying rock above you, so the farther down you go, the greater the pressure that's acting on you. Well, if we stretch that lithosphere and thin it, you can't create just an open space underneath. So, what happens is the asthenosphere has to flow upward to fill that space, now remember this flow happens very very slowly. And the order centimeters a year, so we're still dealing with solid rock. But, nevertheless, we can think of the asthenosphere as plastically flowing upwards to fill the open space created as the overlying lithosphere thins. Except, again, there's never any actual open space. The process all happens as a continuum. So as soon as the stretching takes place, the asthenosphere starts to flow. Now look what happens. We've taken rock that was once deep down, and have brought it up closer to the surface. It happens, even though very slowly, it happens fast enough so it can take place without the rock cooling very much. So we have decompression of a body of rock and as soon as that decompression starts, the rock starts to undergo partial melting. As soon as it starts to partially melt, it forms magma. And then, as we'll see, the magma rises goes up to the surface forming volcanoes. One place that this happens is in the East African Rift, where giant volcanoes like Mount Kilimanjaro have formed as a consequence of the stretching that's taking place in association with the rifting of Africa. The same thing happens beneath mid-ocean ridges. I'm going to draw a simple sketch of a mid-ocean ridge. Not to scale. Just to emphasize that beneath the axis of a mid-ocean ridge, I've shown a bunch of these normal faults that we talked about earlier. And this would be the crustal layer. And this would be the lithosphere mantle. Anyway, beneath the axis of the mid ocean ridge, the lithosphere's particularly thin. Asthenospehere is flowing up from below in order to fill that space. And again, it's happening as a continuum. There's not like an open space there, but as this phenomena of divergent plate motion takes place. So this plate's moving to the side, asthenosphere, Is moving up. Now once that asthenosphere goes from this depth to this depth, it's undergone decompression. It starts to partially melt, and so we start to form magma in this area, and eventually that magma starts to rise up. And spills out at the axis of the mid-ocean ridge. So, in fact, mid ocean ridges are places where a huge amount of melting and eruption of volcanic material is taking place. We're usually not as familiar with that or we don't see that everyday because it's submerged beneath two kilometers of water. Sea level's all the way up here. So we don't see this. But it's happening in spurts all the time. And all of the sea floor that we see on the planet today was formed by this type of activity over the past couple hundred million years. Because this lava is erupted at the floor of the sea, underneath water. It forms a very unusual structure called pillow lava. It erupts a little bit, forms a blob that freezes instantly because it is surrounded by cold water. Then that blob fills internally with more magma, until the pressure gets great enough and splits [NOISE], blob comes out and that freezes into a pillow like shape. Then it splits, another blob comes out, and so in effect you get these blob like, sometimes elongate blob like shapes, the combination or this pile of this stuff is called pillow lava. [MUSIC]
