The Earth Crust.

The earth crust is comparatively thin layer, its thickness ranges from 3 km in the oceanic ridges and 70 km in large terrestrial mountain ranges like the Andes and the Himalayas.
The bottoms of the major ocean basins are formed by the oceanic crust, with an average thickness of 7 km is composed of mafic (iron-magnesium silicate) with an average density of 3.0 g / cubic cm.
The continents are composed of continental crust, which is composed of felsic rocks (silicates of sodium, potassium and aluminum), lighter, with an average density of 2.7 g / cubic cm.
The boundary between crust and mantle is manifested in two physical phenomena. First, there is a discontinuity in seismic velocity, which is known as the Mohorovicic discontinuity or "Moho." It is believed that this phenomenon is due to a change in the composition of rocks, some containing plagioclase feldspar (located at the top) to other feldspars have not (at the bottom). Second, there is a chemical discontinuity between ultramafic and harzburgite tectonizadas clusters, which has been observed in deeper parts of oceanic crust that have been obducidas within the continental crust and preserved as ophiolite sequences.

Deep Inside Planet Earth, Interplay Of Temperature, Pressure, Chemistry

ScienceDaily (Oct. 30, 2007) — Seismologists in recent years have recast their understanding of the inner workings of Earth from a relatively benign homogeneous environment to one that is highly dynamic and chemically diverse. This new view of Earth's inner workings depicts the planet as a living organism where events that happen deep inside can affect what happens at its surface, like the rub and slip of tectonic plates and the rumble of the occasional volcano.

http://www.sciencedaily.com/releases/2007/10/071025143332.htm

Inside Planet Earth

The structure of the Planet’s interior is much more dynamic and much less precisely identified than many people learned in their science class. The moving of tectonic plates that trigger earthquakes is nothing in comparison to the continuous motion of the superheated magma. This interior layer, usually looked at as a radiant red ring of liquid, is way more intricate compared to what minor we witness of it going out from an erupted volcano.