The asthenosphere lies below the lithosphere, from about 100-200 km to about 670 km deep. The crust and outer mantle moves together as a unit, so they are combined together into the lithosphere. The lithosphere consists of the crust and the cool, rigid, outer 80-100 km of the mantle. Basalt also forms from cooling magmas, but exposure to air or water can cause them to cool faster, which does not allow time for large crystals to form.īased on physical characteristics, we can also divide the outermost layers of Earth into the lithosphere and asthenosphere. 2.7 g/cm 3.) Granite also contains larger crystals than basalt, as underground or surface continental magmas can cool slowly, which allows time for crystal structures to form before the rocks solidify. Basalt contains a higher proportion of heavier elements like iron and magnesium, making oceanic crust denser than continental crust (3 g/cm 3 vs. Continental crust is made largely of granite, while oceanic crust is mostly composed of basalt. Finally, the two types of crust differ in their composition. Continental crust is also older than oceanic crust the oldest rocks in continental crust are about 4.4 billion years old, while the oldest oceanic crust only goes back about 180 million years. Continental crust is thicker than oceanic crust, averaging 20-70 km thick, compared to 5-10 km for oceanic crust. There are two main types of crust continental crust and oceanic crust that differ in a number of ways. The crust averages 15-20 km thick, but in some places, such as under mountains, the crust can reach thicknesses of up to 100 km.

The outermost layer is the crust, which forms the solid, rocky surface of the Earth. Magma that rises to the surface through volcanoes originates in the mantle. The uppermost layer of the mantle is more rigid, while the deeper regions are fluid, and it is the motion of fluid materials in the mantle that is responsible for plate tectonics (see section 4.3). The mantle has a density of 4.5 g/cm 3, and temperatures in the range of 1000-1500 oC. The mantle consists of iron and magnesium silicates and magnesium oxides, so it is more similar to the rocks of Earth’s surface than to the materials in the core. It is 2900 km thick, and contains about 80% of the Earth’s volume. The mantle extends from the outer core to just under Earth’s surface.

The outer core is 2300 km thick, and has a density of 12 g/cm 3. It is the movement of the fluid iron in the outer core that creates Earth’s magnetic field (see section 4.2). The temperature is 4000-6000 oC, and the metals remain in the liquid state because the pressure is not as great as in the inner core.

It has the same composition as the inner core, but it exists as a fluid, rather than a solid. The outer core sits outside of the inner core. The solid metals also make the inner core very dense, at about 17 g/cm 3, giving the inner core about one-third of the Earth’s total mass. Despite the high temperature that should melt these metals, the extreme pressure (from literally the weight of the world) keeps the inner core in the solid phase. The temperature in the inner core is about 6000 oC (10,800 oF), which is roughly the temperature of the surface of the sun ( section 3.1 explains the sources of this intense heat). It is composed primarily of iron alloys and nickel, with about 10% comprised of oxygen, sulfur or hydrogen. The inner core lies at the center of the Earth, and is about 1200 km thick. The traditional view based on chemical composition recognizes four distinct layers: Figure 3.2.1 Interior structure of Earth (By Kelvinsong (Own work), via Wikimedia Commons).

Liquid iron and nickel