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2. Lecture Two - September 13.docx

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Jessica Hawthorn

THE COMPOSITION AND FORMATION OF EARTH Why do we care about how the Earth formed, and what does this have to do with dinosaurs (since this is fundamentally a course on dinosaurs)? Well, the Earth is integrated with the things living on it. The evolution of life is dependent of the evolution of the Earth itself...which have to do with its composition. The heavier elements are towards the center of the Earth and the lighter ones towards the surface. The mantle is 83% of the volume of the earth. The mantle is dynamic. Above the mantle is the crust. The crust is the rigid outermost layer of the planet. It is composed of minerals. There are two types of crusts. There is continental crust, which we’re standing upon. It is mostly silicon and aluminum. It is thick. Then, there is oceanic crust. It is thin, but dense. It is primarily iron and magnesium. This crust that we’re standing on, that forms the floor of ocean basins is sitting on top of a thin layer of the mantle. The crust + the thin layer of mantle right below are together called the lithosphere. The lithosphere is sitting on top of the asthenosphere. All of the outer crust is flowing/moving. The crust of the planet is moving. It is broken into a series of plates, which are composed of either continental crust, or oceanic crust, or both...and these plates are moving...but they’re not moving in the same direction. This process is called “plate tectonics”. Plate tectonics is responsible for the origin of life on Earth. Amongst other things, plate tectonics is a recycling mechanism (for the destruction and creation of new plates). Plate tectonics is the formation of new plates and crust and the destruction of old plates and crust. It is a cycle. Plates are moving around over the mantle. The reason these plates move is because they’re being driven around b/c of heat transfer at the center of the planet. As they move around, the plates are breaking, converging, diverging away from each other...basically, they’re moving! When plates converge, a process called “subduction” takes place. Subduction is the process by which heavier plates dip under lighter plates. It is almost always the case that the heavier, denser, oceanic crust subducts under the lighter, less dense continental crust. When this happens, the oceanic crust descends back down into the mantle, where it decomposes and simply reduces back to its elemental composition as part of the mantle. So, plates subduct under each other. However, keep in mind that this is what happens at one end of the plate. If a plate is moving towards one plate, then it is moving away from another. When two plates move away from each other, you have a rift/vent between these plates. As a result, magma comes up from the upper mantle, cools down, and forms new crust. Again, the general pattern is that heavier, denser, oceanic crust subducts under lighter, less dense continental crust. It is destroyed at one end and more is created at the other end. Now, moving into further detail, as a piece of oceanic crust is descending, a couple of things are happening. Its minerals are breaking down. Some of the minerals have water in them, which lowers the temperature . These start to rise up, carving a way through the continental crust, and manifest in the form of volcanoes. When you have two plates moving against another plate, you get earthquakes. You have subduction at one end, and the formation of new crust at the other. The formation of new crust at a mid-ocean ridge is called seafloor spreading. What this shows is that there is a recycling pattern of oceanic crust. Continental crust is generally much older and it generally is not subducted (i.e., continental crust is lighter and is rarely pushed below the heavier oceanic crust). It is generally destroyed by erosion. It weathers away. How do we get these basins? What happens is that the heat energy underneath continental crust starts to pull apart the continental crust. As it separates, it will start to form a series of rifts. Eventually, the rifting will become so great that magma will rise and form new crust. When this happens, the magma forms oceanic crust, not continental crust. However, because these are topographically low areas, water floods in. If you look at the red sea, it is one of these r
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