Earth's internal structure

The interior of the planet, like the other terrestrial planets (planets whose volume is occupied mainly of rocky material), is divided into layers. The Earth has an outer shell of solidified silicates, a viscous mantle and a core with two layers, an outer semi-solid, much more fluid than the inner mantle and a solid. Many of the rocks that now form part of the crust formed less than 100 million (1 × 108) years. However, the oldest known mineral formations are 4,400 million (44 × 108) years, which indicates that, at least, the planet has a solid crust since.

Much of our knowledge about the interior of the Earth has been inferred from other observations. For example, the force of gravity is a measure of the landmass. After knowing the volume of the planet, we can calculate its density. The calculation of mass and volume of surface rocks, and water bodies, allow us to estimate the density of the outer layer. The mass is the atmosphere or in the crust must be in the inner layers.

Estructura

The structure of the earth can be established according to two different criteria. According to their chemical composition, the planet can be divided into crust, mantle and core (external and internal), according to their physical properties are defined lithosphere, asthenosphere, mesosphere and the nucleus (external and internal).

The layers are within the following depths:

Layer	Depth (km)
Lithosphere (locally varies between 5 and 200 km)	0 – 60
... Crust (locally varies between 5 and 70 km)	0 – 35
... Upper mantle	35 – 60
Mantle	35 – 2 890
Upper mantle	35 – 660
... asthenosphere	100 – 200
Lower Mantle (Mesosphere)	660 – 2 890
Outer core	2 890 – 5 100
Inner Core	5 100 – 6 378

The division of land in layers has been determined indirectly using the time it takes to travel the reflected and refracted seismic waves created by earthquakes. Shear waves (S, or secondary) can not pass through the core, because they need a viscous material or elastic to spread, while the speed of propagation is different in the other layers. Changes in that rate due to produce a Snell refraction. The reflections are caused by a large increase in seismic velocity (speed of propagation) and are similar to reflection in a mirror.