Earth’s Interior

The Zones of Earth’s Interior Much of our knowledge of the Earth’s interior comes from the study of earthquake (seismic) waves. Seismic waves seem to change speed, direction or disappear as they pass through the Earth. These changes suggest that different zones of the Earth exist due to differences in states of matter and/or density. The zones of the Earth include: the crust, mantle, outer core and inner core.

The Crust The outermost portion of the Earth is known as the crust. The crust includes the solid bedrock upon which we live, as well as, the soil and weathered/eroded sediments from bedrock. Two types of crust exist: continental crust and oceanic crust. Continental crust makes up the large continents that exist. It is thicker and made up of largely light-colored, less dense granitic rock. Oceanic crust makes up the crust under the oceans. It is thinner and made up of largely dark-colored, more dense basaltic rock. The crust is broken into large pieces called plates. The crust and upper part of the mantle are known as the lithosphere.

The Mantle The largest portion of the Earth is known as the mantle. The mantle is located beneath the crust. The mantle is composed of many sub- layers. Just below the crust is the Moho – a thin interface between the crust and mantle. Just below the Moho is the Upper Mantle – a very dense section of rock. These layers along with the crust make up the lithosphere. Another portion of the upper mantle is known as the asthenosphere. The asthenosphere is believed to be “plastic-like” or partially molten. Convection currents within the asthenosphere allow the crustal plates to move. The Lower Mantle (stiffer mantle) is located below the asthenosphere and is more rigid.

The Outer Core The Outer Core of the Earth is located beneath the mantle. The dimensions of the outer core have been determined by the behavior of seismic waves that pass through it. Some seismic waves slow down and are refracted when they move through the outer core (P-waves). Other seismic waves are absorbed when they pass through the outer core (S- waves). Due to these observations, it is theorized that the outer core is liquid. It is theorized that the outer core is composed of iron and nickel.

The Inner Core The Inner Core is located beneath the outer core. The inner core is believed to be a solid due to tremendous pressure surrounding it. Some seismic waves increase in velocity when passing through the inner core. The inner core has the highest density of all zones within the Earth. It is theorized that the inner core is composed of iron and nickel.