Earth's Layers

Earth's layers are the four main parts that make up our planet from the surface to the center. Scientists divide Earth into the crust, the mantle, the outer core, and the inner core. Each layer has a different temperature, a different makeup, and a different way of behaving. Together they stack up to about 3,960 miles from the ground beneath your feet to the very middle of the planet.

The crust is the thin rocky layer we live on. Under the oceans it is only about 5 miles thick. Under the continents it is thicker, around 25 miles deep on average. Compared to the rest of the planet, the crust is very thin. If Earth were the size of an apple, the crust would be no thicker than the apple's skin. The crust is broken into giant pieces called tectonic plates. These plates move slowly, and their bumping and grinding causes earthquakes and builds mountains.

Below the crust is the mantle. The mantle is the thickest layer, about 1,800 miles deep. It is made of hot rock, but the rock is not all liquid. Most of the mantle is solid rock that flows very slowly, like soft clay being squeezed. This slow flow is what pushes the tectonic plates around above it. When mantle rock melts and pushes up through the crust, we get volcanoes.

Next comes the outer core. The outer core is about 1,400 miles thick and made mostly of liquid iron and nickel. It is incredibly hot, around 8,000 degrees Fahrenheit. As this liquid metal swirls around, it creates electric currents. Those currents make Earth's magnetic field, the invisible force that pulls a compass needle north and protects the planet from harmful particles streaming off the Sun.

At the very center is the inner core. It is a ball of solid iron and nickel about 1,500 miles across, roughly the size of the Moon. The inner core is the hottest part of Earth, about 9,000 to 10,000 degrees Fahrenheit, as hot as the surface of the Sun. You might wonder how metal that hot can be solid. The answer is pressure. The weight of the whole planet pressing down squeezes the iron so hard that it cannot melt.

Nobody has ever seen these layers directly. We know about them mostly from earthquakes. When an earthquake shakes the planet, scientists track how the waves bend and bounce as they pass through different materials. Those wave patterns let us map the inside of a world we cannot visit.