PEELING THE LAYERS
Although Earth's interior is nearby in astronomical terms—its center lies less than 4,000 miles (6,400 km) below our feet—it remains frustratingly out of reach. To look inside our planet, scientists rely on earthquakes, studying how seismic waves rebound and refract as they travel through the interior. In simple terms, the picture they give of Earth resembles an onion made of rock.
At the center is the core, which extends upward to a point 1,800 miles (2,900 km) below the surface. The core is about 90 percent nickel and iron, and has two parts. The inner core is more than hot enough to be liquid—about 9,000 degrees Fahrenheit (5,000° C)—but it is kept solid by the enormous pressure, which is nearly four million times that at the surface.
In the outer core, the pressure is lower, temperatures are about 900 degrees Fahrenheit (500° C) cooler, and the iron remains liquid. This liquid nickel-iron flows in turbulent currents that give rise to the Earth's magnetic field. The field reaches up through the planet, past the surface where it controls our magnetic compasses, and out into space where it affects the flow of charged solar particles blowing past Earth.
Above the core is a layer called the mantle, composed of high-density iron and magnesium-silicate minerals. While not liquid like the outer core, the rocks of the mantle are hot enough to flow, and the energy that they carry powers all the tectonic activity we see at the surface.
The rocks that make up Earth's surface form a thin but distinct layer called the crust. We live on the continental type of crust. It is made of lightweight granitic and carbonate rocks and is between 20 and 40 miles (30 and 65 km) thick. A second kind of crust, oceanic, is made of heavier and denser basalt. It is only 3 miles (5 km) thick.
Most oceanic crust is covered by miles of sea water, so, until recently, geologists mainly studied the continental crust. However, there are a few places, such as Iceland, where oceanic crust is exposed.