[MUSIC] The second type of plate boundary, so called conversion plate boundaries where two plates are moving together. Another term that geologists use for them, are use for these boundaries is subduction zone. Because, what happens is as I mention they just don’t push together like that. Rather one plate sinks and heads back into the mantel beneath the other plate. And that process is called subduction, now when subduction takes place, one plate shears against the other, as the plate bends and starts to subduct, it breaks. And that breaking, causes generation of some earthquakes, also as the subducting plate shears and slides against the base of the overriding plate, it also generates earthquakes. And as we'll see later, earthquakes can develop in the descending plate even deeper in the Earth, but the net result is that we get lots of seismic activity at a conversion plate boundary. And that shearing creates reverse faults, and those slip on those reverse faults generates the earthquakes. Of convergent plate boundaries, so we have this process called subduction that takes place on the convergent plate boundary, we'll see later that when the sinking slab or what geologist refer to as the downgoing slap reaches depth inside the mantle, it warms up sufficiently. That the process of melting is triggered, it's not actually that the plate melts. We'll see that what happens is that gasses come off of the plate, and they cause the overlying mantle to melt. But, are point is that melting often accompanies the process or always accompanies the process of subduction. And as a result, a chain of volcanoes forms above the downgoing slab, that chain forms on the edge of the overriding slab. We call that chain of volcanoes a volcanic arc, if an oceanic slab is going underneath another oceanic slab, then the volcanic arc that's formed is a chain of islands called an island arc. If the chain of volcanoes forms along the edge of a continent, then it's called a continental volcanic arc. So, let's take a little voyage, a little field trip to the west coast of South America and look at a map on Google Earth that shows South America. And what we see is the Pacific Ocean floor, it's moving to the east relative to the continent of South America, which is part of the South Atlantic Plate moving towards the west. The Pacific Ocean floor sinks underneath the continent, and that generates a chain of volcanoes along the Andes, it also creates a long linear, very deep portion of the ocean floor that's called a trench. In this particular case it's called the Peru-Chile trench, so where we have subduction taking place, it's marked topographically by a trench on the sea floor and geologically by a volcanic arch on the over-riding plate. And then of course, all the earthquakes that happen because two plates are shearing past one another. Here's an example of an island arc on the western side of the Pacific Ocean floor, where the Pacific Ocean floor is sliding back into the mantle or being subducted beneath the Philippine plate. It creates a very deep, trench called the Mariana trench and a chain of volcanoes called the Mariana island arc. The Mariana trench includes the deepest parts of the ocean floor, because there's no continent nearby to shed sediment and to fill it up, and also the ocean flow is very old and fairly low elevation to start with. So this is where the deepest part of the ocean floor is there are places in the Mariana trench where the ocean floor is about 11 kilometers deep, as opposed to most of the ocean where it's only about four to five kilometers deep. Now, we've already noticed that lots of earthquakes occur at convergent plate boundaries. All you need to do is look at a map of epicenters to see the seismic belts that are associated with convergent plate boundaries, such as in the Peru Chili trench and at the Mariana trench. If we now look instead at a cross-section showing the depth of earthquake foci, we see that there are some shallow earthquakes due to the faulting that happens where subducting plate bends, or where it sheers against the base of the overriding plate, but in addition there are some intermediate and deep earthquakes in the downgoing slab. They actually occur within the subducted plate down to great depth. Down as far as maybe 600 kilometers or so into the mantel, and it's now recognized that these earthquakes can be used. To track the location of the plate as it sinks back down into the mantle, the discovery of that was attributed to two seismologists Wadati and Benioff, and that's sometimes called the Wadati-Benioff zone, or the Benioff-Wadati zone depending on where you're from. In recent years, new seismological techniques have actually allowed geologists to image the position of the downgoing plate. Because the downgoing plate tends to be cooler earthquake waves travel through it more quickly than they do through the surrounding warmer as thinnest prior and thus this shows up as a seismic velocity anomaly. Basically, it means that the earthquake waves travel at a different rate than what they would travel at if the plate were not as cool as it is. Using these techniques, geologists are able not only to see the plate down to 600 kilometers, or so where there are also earthquakes, but in some cases, contract the plates even deeper into the mantel. In effect we can see that in the deeper part of the earth, in the lower mantle there is a grave yard of old plates that have accumulating over a long period of geologic time. [MUSIC]
