THE MANTLE
The mantle is one of the three layers of the Earth. It lies between the core which is the innermost layer, and the crust which is the thin outermost layer.
The Earth’s mantle consists of hot, dense, semisolid rock which is about 2,900 kilometres (1,802 miles) thick.
The mantle is divided into several layers.
The layers of the mantle are lithosphere, asthenosphere, upper mantle, lower mantle.
Lithosphere is the thin outermost shell of the upper mantle similar to the crust, though cooler and more rigid. This layer is called the Earth’s lithosphere.
The lithosphere is broken up into several large pieces, or plates. They “float” on a softer mantle layer called the asthenosphere.
Their very slow motion is because of plate tectonics, a process linked with continental drift, earthquakes, volcanoes, and the formation of mountains.
The upper mantle is the layer underneath the asthenosphere it is stronger and more solid than the asthenosphere. Any layer that is below the crust down to a depth of about 670 kilometres (416 miles) is known as the upper mantle.
The rest of the mantle that is between the upper mantle and the core is known as the lower mantle. It is denser and hotter than the upper mantle.
Exploring the Deep Earth
Most of the Earth’s interior is too deep for people to explore directly. So scientists tell the mantle apart from the crust and core by measuring the spread of shock waves from earthquakes, called seismic waves. There are two types of seismic waves that pass through the Earth’s interior. The P-waves represent vertical motion and and S-waves, which represent horizontal motion. Instruments are placed around the world to measure these waves as they arrive to different points on the Earth’s surface after an earthquake.
Seismic waves travel at different speeds and strengths through different material. For example, surface waves from a powerful earthquake near Northridge, California, in 1994 took 30 minutes to reach a point about 6,700 kilometres (4,163 miles) away, but it took P-waves only 10 minutes and S-waves just under 20 minutes to travel the same distance. Near the surface, P-waves travel about 6 kilometres per second through the ground. Below a certain point—an average depth of about 35 kilometres—they travel about 8 km/s, indicating that the waves have reached denser material at that point.(National Geographic 1996-2014)
This abrupt divide between slower and faster speeds marks the boundary between the crust and the mantle. It is called the Mohorovicic discontinuity, or simply the Moho. At the base of the mantle, 2,900 kilometres (1,802 miles) below the surface, S-waves, which can’t continue in liquid, suddenly disappear, and P-waves are strongly refracted, or bent. This point, called the Gutenberg discontinuity, marks the end of the mantle and the beginning of the Earth’s liquid core. (National Geographic 1996-2014).