The Earth's Crust

The crust of the earth is extremely thick.

The Earth’s crust is the outer most surface of the Earth which covers the earth’s mantle. The crust is much thinner than the other layers. The Earth’s crust floats above the denser, softer mantle. It is about 25 miles thick beneath the continents and about 6.5 miles thick beneath the oceans. It is about less than one percent of the entire depth of the Earth. The Earth’s crust is distinguished into two portions: oceanic crust and the continental crust. The continental crust is much different from the oceanic crust. One of the biggest different between the two is their difference in density. The oceanic crust (3.0 g/cm3) has a higher average density than the continental crust (2.6 g/cm3). This density difference is what allows the continents to float on the upper mantle for billions of years. The oceanic crust on the other hand, can barely float on the mantle. As the oceanic crust gets older, it builds up a heavy under layer of cooled mantle rock which results in a two-layer structure causing it to eventually sink into the mantle from its own weight. As it sinks, it is melted down and recycled. Due to this recycling process, the age of the oceanic crust is never older than 200 million years. Another difference is in their composition. The oceanic crust is composed of denser rocks such as basalt. Basalt rocks are formed out of liquid lava which tends to cool off rapidly. On the other hand, the continental crust is composed mostly of lighter rocks such as granite, which are less dense than basalt.

The region just below the crust is known as mantle. When compared to the crust, the mantle is relatively flexible thus allowing it to flow rather than fracturing. The mantle is about 1800 miles thick and about 5400 degrees Fahrenheit. Although the temperature is very high, because of its high pressures, the rocks remain solid. The inner most part of the Earth, which is beneath the mantle is the core. It is consisted of two different parts: inner and outer. The inner core is solid and the outer core is liquid because of the temperature and pressures. The Earth’s source of magnetic field comes from the outer core, which contains iron.

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