Sections of the Earth

The Earth is a fickle character, forever changing through spurts of hot and cold fits. This is due to three distinct layers of varying temperatures: crust, mantle and core. The structure of these layers can be compared to a boiled egg. The crust, the outermost or top layer, is thin and hard. But like the shell of an egg, it is brittle and can break easily. Most of the Earth's crust beneath the oceans is only about 5 km/3 mi deep. Crust beneath continents, however, averages about 30 km/19 mi in thickness.

Below the crust is the mantle, a dense, hot layer of semi solid rock. Temperature and pressure inside the Earth increase with depth, making the mantle much hotter and denser than the crust. The mantle is relatively flexible, so it flows rather than fractures. In this case, the mantle is the white of a boiled egg.

Beneath the mantle lies the core. This layer is twice as dense as the mantle because it is an iron-nickel alloy, rather than a stony composition. Unlike the yolk of an egg, however, the core is made up of a fluid outer core and a solid inner core. As the Earth rotates, the metallic liquid outer core spins, creating the Earth's magnetic field.

Because the crust and upper part of the mantle are cooler and more rigid than the deep mantle, these form a layer of rock called the lithosphere. Pressure over time has caused the lithosphere to break up into moving plates, which sit directly below the Earth's continents and oceans. The size and position of these plates change over time. When the edges of these plates move against each other, intense geologic activity can occur, such as earthquakes, volcanoes, and mountain building. This is called plate tectonics. Below the lithospheric plates, the mantle is partially molten and can flow. This semi solid layer softens as temperature and pressure rise below the Earth's surface. The rigid lithosphere "floats" or moves around on this softened layer.