Structure of Earth

Earth’s interior consists mainly of the mantle and the core. The mantle and core make up by far the largest part of Earth’s mass. The distance from the base of the crust to the center of the core is about 6,400 km.

Scientists have learned about Earth’s interior by studying rocks that formed in the interior and rose to the surface. The study of meteorites, which are believed to be made of the same material that formed the Earth and its interior, has also offered clues about Earth’s interior. Finally, seismic waves generated by earthquakes provide geophysicists with information about the composition of the interior. The sudden movement of rocks during an earthquake causes vibrations that transmit energy through the Earth in the form of waves. The way these waves travel through the interior of Earth reveals the nature of materials inside the planet.

The mantle consists of three parts: the lower part of the lithosphere, the region below it known as the asthenosphere, and the region below the asthenosphere called the lower mantle. The entire mantle extends from the base of the crust to a depth of about 2,900 km. Scientists believe the asthenosphere is made up of mushy plastic-like rock with pockets of molten rock.

The term asthenosphere is derived from Greek and means “weak layer.” The asthenosphere’s soft, plastic quality allows plates in the lithosphere above it to shift and slide on top of the asthenosphere. This shifting of the lithosphere’s plates is the source of most tectonic activity. The asthenosphere is also the source of the basaltic magma that makes up much of the oceanic crust and rises through volcanic vents on the ocean floor.

The mantle consists of mostly solid iron-magnesium silicate rock mixed with many other minor components including radioactive elements. However, even this solid rock can flow like a “sticky” liquid when it is subjected to enough heat and pressure. The core is divided into two parts, the outer core and the inner core. The outer core is about 2,260 km thick. The outer core is a liquid region composed mostly of iron, with smaller amounts of nickel and sulfur in liquid form. The inner core is about 1,220 km thick. The inner core is solid and is composed of iron, nickel, and sulfur in solid form. The inner core and the outer core also contain a small percentage of radioactive material. The existence of radioactive material is one of the sources of heat in Earth’s interior because as radioactive material decays, it gives off heat. Temperatures in the inner core may be as high as 6650°C.

Interior of Earth
Interior of Earth (c) Encarta

Earth’s Crust

Crust is the outermost layer of Earth. The crust is solid and relatively thin, and it lies below both landmasses and oceans. The dry land of Earth’s surface is called the continental crust. It is about 15 to 75 km thick. The oceanic crust is thinner than the continental crust. Its average thickness is 5 to 10 km. The crust is very thin in relation to the rest of Earth.
If a trip to the center of Earth at 100 km/h were possible, it would take 64 hours, of which only the first 15 to 45 minutes would be in the crust.

Upper Mantle

Earth’s Upper mantle is about 650 km thick and features two distinct layers. Directly beneath the crust is a solid layer that, combined with the crust, forms the lithosphere, which makes up the earth’s plates. Beneath this layer is the asthenosphere, where semi-molten rock flows slowly like hot tar. It is believed that convection currents, which move within this area like boiling water, drive the overlying plates.

Lower Mantle

Earth’s lower mantle is about 2300 km thick. Even though temperatures are higher here, this part of mantle is solid. Tremendous pressures keep the rock material from melting.

Outer Core

Earth’s liquid outer core is about 2300 km thick. As a result of extremely high temperatures, this region is made up of molten iron and nickel. The liquid material helps produce Earth’s magnetic field.

Some scientists theorize that the flow of liquid iron in the outer core sets up electrical currents that produce Earth’s magnetic field. Known as the dynamo theory, this theory appears to be the best explanation yet for the origin of the magnetic field. Earth’s magnetic field operates in a region above Earth’s surface known as the magnetosphere. The magnetosphere is shaped somewhat like a teardrop with a long tail that trails away from the Earth due to the force of the solar wind.

Inner Core

Earth’s inner core, is about 1200 km thick and is made up of solid iron and nickel. Temperatures of the inner core may reach 6650°C. It is under very high pressure and has very high density.

Lithosphere and Asthenosphere

Earth's Crust and Asthenosphere

The solid, outermost section of the mantle and the solid crust together form the lithosphere. The lithosphere is approximately 65 to 100 km thick and covers the asthenosphere

Interior of Earth

Earth's Layers

The asthenosphere is approximately 100 to 350 km thick. It consists of rocky material that is softer and less rigid than that in the lithosphere. This softer, less rigid state results from higher pressures and temperatures, which cause the rocks partially to melt and become soft.