Transcript of Earth Layers & Isostasy

[music] To understand the principles of Plate Tectonics,   the dynamic surface-altering process that is  the source of all the world’s volcanoes, major   earthquakes, and major mountains, we have to look  more closely at the layers of the Earth, first   discussed in the lecture on Earth Formation. Let’s review the basics. First, due to density   separation, the core, mantle, and crust – the  primary compositional layers – were formed.   Let’s look closer at the core. It’s made mostly of  iron and is separated into two physically distinct   layers. They are compositionally quite similar –  mostly made of iron. However, physically, we have   a small solid inner core at the very center of the  earth, where, though temperatures are higher than   anywhere else inside our planet, pressures are  so high that liquid iron can’t exist. Surrounding   the inner core is a much larger, liquid iron  outer core. Because pressures are less here,   iron can be stable as a liquid. For comparison,  the outer core is almost twice as thick as   the inner core. Combined, however, these two  layers represent about ½ the radius of Earth.   Now let’s go out to the crust, which is so  thin that it would be impossible to see it   in any whole-earth drawing shown to scale.  So in all cases, we have to exaggerate it.   The earth has a radius of about 6400 km.  The crust is at most 50 km thick at most   1/128th the radius of the earth. The rest of  the earth, most of it in fact, is mantle!   Let’s look more closely at the crust. It is  composed of two kinds – denser oceanic crust,   which is thin and mostly made of a rock  called basalt. Its density makes it sink low,   which is why it is the crust that underlies  the lowest basins on the planet – the oceans.   Continental crust is much thicker and less dense  than oceanic crust. It is made out of many kinds   of rocks, but its average composition is  similar to that of a rock found on the   continents – called granite. The buoyancy and  thickness of continental crust makes it float   high above the oceans, creating the continents. To understand how crust floats, we need to look   more closely at the top of the mantle and how it  interacts with the crust. Because of the water   content of the mantle and the temperatures and  pressures encountered in the zone between about   100 and 300 km depth, the region there, called the  Asthenosphere, behaves like a plastic solid – and   is capable of flow over long periods of time. [music] Thus it convects. Hotter, less dense material rises,   displacing colder denser material, which  sinks. The rest of the upper mantle plus all   of the crust are fused together. We call that  combination of crust + upper mantle that sits   above the asthenosphere, the lithosphere.  The lithosphere is broken up into pieces   that we call plates. Some of the pieces contain  continental crust, some oceanic crust – and some   contain both side by side. Pause now. [music] The plasticity of the underlying asthenosphere  allows the overlying lithosphere to sink into it,   much like icebergs or wood floating in the water.  The denser lithosphere portions that contain   oceanic crust will sink lower and be thinner. The  less-dense lithosphere that contain continental   crust will be much thicker and ride higher as  well as extend deeper (again, like an iceberg).   We call this process of lithosphere sinking  into asthenosphere, isostasy. How do we know?   Scientists take images of the earth (like x-rays),  by using seismic waves that travel through the   earth and reflect and refract off major boundaries  and return to the surface. The first major   boundary that is encountered during this process,  off which waves reflect, is called the Moho,   after a Yugoslavian scientist who first discovered  it, whose last name was Mohorovicic. (Moho is   a shorter, easier-to-remember label!). Since  the moho marks the boundary between the crust   and the mantle, what layer does it sit within?  The moho sits in the middle of the lithosphere,   because the lithosphere contains all of the crust  plus the upper-most mantle. Does the moho have   anything to do with the asthenosphere? No. And what do we notice when we study the Moho   across the planet? It appears very close to the  surface, at depths as low as 3-5 kilometers,   beneath the oceans. It appears as deep as  50 kilometers below the highest mountains.   As material is added to or removed from  the crust, it will adjust isostatically,   again, much like icebergs or ships in the  ocean. When cargo is added to a cargo ship,   what happens? It sinks lower into the water.  When the cargo is removed? The ship rises.   So what happens when the tops of mountains are  eroded? Material is removed, so the crust rises   upward. And what happens when the eroded sediment  is carried to the coast and dumped on the edges   of the continental shelf? The land there will  sink under the weight. So earth’s surface is   continually rising and sinking isostatically as  weight is removed or added by erosion, deposition,   volcanic eruptions, mountain building processes,  and other processes related to plate tectonics.   Pause now. [music] For more information and more detail,   continue on to the next video in the series. [music]