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  • Subject area(s): Science
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  • Published on: 15th October 2019
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Today, we are exploring something a little different. We are taking a trip into the Earth rather, than going to a country. I want to go over the features of the Earth itself. I think that it is really important that we familiarize ourselves with what is going on in the inside of the world than just the outer part of it. Today we take a trip into the Earth’s crust. The Earth's crust is a really thin layer of rock that makes up the outermost solid shell of our planet. In comparative relations, it's width is like that of the skin of an apple. It amounts to less than half of one percent of Earth’s total mass but plays a huge role in most of Earth's cycles. The crust can be thicker than 80 kilometers in some spots and less than one-kilometer-thick in others. Underneath lies the mantle, a layer of silicate rock that is approximately 2700 kilometers thick. The mantle reasons for the bulk of the Earth. The crust is composed of many different types of rocks that fall within three main categories. Those are igneous, metamorphic and sedimentary. However, most of those rocks were granite or basalt. The mantle beneath is made of peridotite. Bridgmanite, is the most common mineral on Earth, is found in the deep mantle.

Not much was known about Earth having a crust until the early 1900s. Then along came seismology, which then brought a new type of evidence; seismic velocity. Seismic velocity measures the speed at which earthquake waves propagate through the different materials and example of that is rocks beneath the surface. With a few exceptions, seismic velocity within the Earth tends to increase with depth. There was a change in seismic velocity. A break of some sort about 50 kilometers deep in the Earth. Seismic waves bounce off it which reflect and bend which also means to refract as they go through it, the same way that light behaves at the discontinuity between water and air. That discontinuity, named the "Moho" is the accepted boundary between the crust and mantle.

The crust and tectonic plates are not the same thing. Plates are denser than the crust and consist of the crust and the shallow mantle beneath it. This rigid and hard two-layered combination is called the lithosphere. The lithospheric plates lie on a layer of softer, more plastic mantle rock called the asthenosphere. The asthenosphere allows the plates to move slowly over it. We know that the Earth's outer layer is made of two grand categories of rocks. Those are basaltic and granitic. Basaltic rocks motivate the seafloors and granitic rocks make up the continents. We know that the seismic speeds of these rock types match those seen in the crust down as far as the Moho. Therefore, we're confident that the Moho marks a real change in rock chemistry. The Moho isn't a perfect boundary only because some crustal rocks and mantle rocks can pretense as the other. However, everyone who talks about the crust, whether in seismological or petrological terms, fortunately, means the same thing. In general, then, there are two kinds of crust which are the oceanic crust (basaltic) and continental crust (granitic).

Oceanic crust covers about 60 percent of the Earth's surface. Oceanic crust is thin and new no more than about 20 km thick and no older than about 180 million years. Everything older has been pulled underneath the continents by subduction. Oceanic crust is born at the mid ocean ridges, where plates are pulled apart. As that happens, the pressure upon the underlying mantle is released and the peridotite there responds by starting to melt. The fraction that melts becomes basaltic lava, which rises and erupts while the remaining peridotite becomes depleted. The mid ocean ridges migrate over the Earth like Roombas, extracting this basaltic component from the peridotite of the mantle as they go. This works like a chemical refining process. Basaltic rocks contain more silicon and aluminum than the peridotite left behind, which has more iron and magnesium. Basaltic rocks are also less dense. In terms of minerals, basalt has more feldspar and amphibole, less olivine and pyroxene, than peridotite. In geologist's shorthand, oceanic crust is mafic while oceanic mantle is ultramafic. Oceanic crust, being so thin, is a very small fraction of the Earth only about 0.1 percent but, its life cycle serves to separate the contents of the upper mantle into a heavy residue and a lighter set of basaltic rocks. It also extracts the so called incompatible elements, which don't fit into mantle minerals and move into the liquid melt. These, in turn, move into the continental crust as plate tectonics proceeds. Meanwhile, the oceanic crust reacts with seawater and carries some of it down into the mantle. Continental crust is thick and old on average about 50 km thick and about 2 billion years old. It covers about 40 percent of the planet. While almost all of the oceanic crust is underwater, most of the continental crust is exposed to the air. The continents slowly grow over geologic time as oceanic crust and seafloor sediments are pulled beneath them by subduction. The descending basalts have the water and incompatible elements squeezed out of them, and this material rises to trigger more melting in the so called subduction factory. The continental crust is made of granitic rocks, which have even more silicon and aluminum than the basaltic oceanic crust. They also have more oxygen thanks to the atmosphere. Granitic rocks are even less dense than basalt. In terms of minerals, granite has even more feldspar and less amphibole than basalt and almost no pyroxene or olivine. It also has abundant quartz. In geologist's shorthand, continental crust is felsic. Continental crust makes up less than 0.4 percent of the Earth, but it represents the product of a double refining process, first at mid-ocean ridges and second at subduction zones. The total amount of continental crust is slowly growing.

The incompatible elements that end up in the continents are important because they include the major radioactive elements uranium, thorium and potassium. These create heat, which makes the continental crust act like an electric blanket on top of the mantle. The heat also softens thick places in the crust, like the Tibetan Plateau, and makes them spread sideways.

Continental crust is too buoyant to return to the mantle. That's why it is, on average, so old. When continents collide, the crust can thicken to almost 100 km, but that is temporary because it soon spreads out again. The relatively thin skin of limestones and other sedimentary rocks tend to stay on the continents, or in the ocean, rather than return to the mantle. Even the sand and clay that is washed off into the sea returns to the continents on the conveyor belt of the oceanic crust. Continents are truly permanent, self-sustaining features of the Earth's surface.

The crust is a thin but important zone where dry, hot rock from the deep Earth reacts with the water and oxygen of the surface, making new kinds of minerals and rocks. It's also where plate tectonic activity mixes and scrambles these new rocks and injects them with chemically active fluids. Finally, the crust is the home of life, which exerts strong effects on rock chemistry and has its own systems of mineral recycling. All of the interesting and valuable variety in geology, from metal ores to thick beds of clay and stone, finds its home in the crust and nowhere else. It should be noted that the Earth isn't the only planetary body with a crust. Venus, Mercury, Mars and the Earth's Moon have one as well.

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