[SOUND] [MUSIC] So let's look at a cutaway of the planet. Geologists recognize, or seismologist recognize that the internal portion of the Earth has distinct layers. The outer layer is called the crust, that's what we live on. Beneath that, is the greatest volume of the planet, a layer called the mantle. And then in the center of the earth is a dense ball called the core. So again, the nature of the earth the internal structure of the earth is one in which the planet is somewhat like an onion. Now, modern studies have shown fairly accurately where the boundaries between these layers are. And I won't go into the details of how these are determined. But for our purposes, we can recognize that the outer layer, the crust, is really quite thin. Beneath oceans it's only on the order of about five to ten kilometers thick. On average about seven kilometers thick. That's really not very far at all. You can run that distance in under an hour easily. Beneath continents it can be thicker. It ranges, the thinnest is maybe on the order of 25 kilometers the thickest is maybe on the order of 70 kilometers. But most of continental area is the crust is on the order of 35, 38 kilometers thick. Again not very thick indeed. if you compare to the overall diameter of of the earth which is 6371 kilometers, there abouts. Beneath that is the mantle. Geologists divide the mantle into two layers, the outer mantle and the inner mantle. The combined thickness of these two is about 2885 kilometers. The boundary between the two lies at a depth of around 660. At a fairly discrete boundary at the base of the mantle, suddenly things change radically, and we enter another realm called the core. The core really differs from the rest of the earth in that the core is composed primarily of an alloy of metallic iron. Whereas the mantle and the crust are composed primarily of rocks. Now, I mentioned that the core can be separated into an inner core and outer core. The inner core is solid iron alloy and the outer core is liquid iron alloy. In fact it's the circulation of the iron alloy in the outer core that generates the magnetic field that we have on this planet. Now using techniques again that are a little bit beyond our scope here, seismologist have been able to come up with a model for how earthquake velocity changes or earthquake wave velocity changes as a function of depth as you go into the earth. And you can see that gradually, the velocity increases as you go from the crust into the upper mantle and down to the base of the mantle. And then there's a sudden decrease in velocity when you enter the iron core and then the velocity increases again as you go down towards the center of the earth. So it's these sudden jumps in the velocity of seismic waves, which seismologist refer to as seismic discontinuities that define precisely where the boundaries are between the different layers inside the earth. Now, so far, we've been talking about the layering of the earth from two angles. We've said that the boundaries are defined by sudden jumps in velocity, and we can also say that those sudden jumps in velocity represent changes in composition, fundamental changes in composition. So the crust has a different composition from the mantel, and the mantel has a different composition when compared to the core. Specifically, the crust, well beneath the oceans, it's composed primarily of a dark gray rock called basalt. Beneath continents, on average, it has the composition of a lighter colored rock called granite, and then in the mantle, it's composed of a very dense, very dark rock called peridotite. Basically, if we use those three rock names we're accounting for most of the volume of the rock on this planet. And then as i mentioned earlier, as soon as we cross the core mantle boundary and get into the core, we're in a realm where everything is composed of iron alloy. You can imagine a soft material that's like molasses molasses or honey, and if you push on it, it will flow. You can imagine in contrast, a rigid material, say like a block of wood or a block of wood or a block of rock at the surface of the earth. If you push on it, it's too hard to flow. It doesn't flow. It's too hard to flow. So we can also make distinctions between the nature of the interior of the earth by looking at the mechanical behavior. Sometimes we use the fancier term, rheological behavior. How a material responds to a push or pull, or in other words how a material responds to a stress. And we make the fundamental distinction between plastic materials, which are able to flow, even if they flow very, very slowly, and rigid materials which cannot flow. So if we take that distinction we find that we can divide the uppermost part of the earth into two different layers that behave fundamentally differently. The outer layer, is called the lithosphere. Lithos from the Latin for rock. The lithosphere includes all of the crust, but it also includes the uppermost part of the underlying mantle. That's because the uppermost part of the mantle is the cooler part, where the rock, even though it's a different composition and everything, the main point is that that in the upper most part of the mantle is cold enough. It's still pretty hot. It's actually really hot, so you'd burn yourself if you touched it, but it's cool enough so that it's not able to float easily. Beneath that, at a boundary that's effectively defined by a temperature. On the order of 1,280 degrees centigrade. At temperatures greater than that, the peritotite of the mantle, is able to flow. And, we mark that boundary, as the boundary between the lithosphere above, and another layer, called the asthenosphere, below. The asthenosphere, is the layer of the mantle, that is able to flow relatively easily. We'll be seeing that the lithosphere is what makes up the plates in plate tectonics. So, understanding the concept of lithosphere is very important. [MUSIC]
