The Earth is composed of several layers. These include the crust, the mantle, and the core. The layers of Earth are divided into the lithosphere and the asthenosphere—the boundary between these two regions is sometimes called the “Mohorovičić discontinuity” (Moho). The lithosphere consists of rigid skin identified as seven large tectonic plates (North America, South America, Africa, Antarctica, Eurasia, Australia, and the Pacific).
Each has a different chemical composition and a different function within the planet’s interior.
The crust is relatively thin and brittle, not more than 50 miles thick. The uppermost layer of it is called continental crust because it forms the landmasses on Earth. Continental crust underlies nearly all landmasses on our planet but can also be found as small fragments scattered across other plates. Oceanic plates are much thicker than continental ones- averaging about 3-6 miles thick- because they are so old and have been recycled back into Earth’s mantle multiple times over hundreds of millions of years from collisions with other tectonic plates.
The mantle is the second-deepest layer of Earth after the core. It is about 1,800 km thick at the base, where it conceals the core some 30–90 km inside it. The lower part of the mantle is called the crust and consists of about 40% to 60% olivine, a magnesium iron-rich mineral with a characteristic shiny green color. Above that comes a transition from crust to the upper mantle, which again increases in composition from 40% to 50% olivine and rises through the upper mantle, gabbro, and finally upper mantle’s inner section, which includes peridotite at about 80–130 km depth.
The mantle can be divided into several layers depending on its composition. The first layer starts at a depth of around 100 km and ends at approximately 50 km below the crust. It consists mainly of peridotite, a form of olivine that may have originated from the oceanic crust or Earth’s core. Above this first layer, about 85% to 90% of the mantle consists of a “hot” mantle instead of a “cold” mantle.
At a depth of about 30–50 km below the mantle, the core is an amorphous alloy of mostly silicon with some iron and nickel, along with traces of oxygen, carbon, hydrogen, helium, and trace amounts of other elements. The inner core is solid, and the outer core is liquid. The inner core has a temperature of around 6000 K and a pressure of 3.6 million atmospheres. The outer core is molten and fluid. The viscosity of the outer core is similar to that of honey at 2750 K, which is why it does not convect under the effect of gravity (just like water does not boil under its own weight).