Earth's mantle

The Earth's mantle has two layers. There is a rigid upper layer and a softer layer. It is between the Earth's crust and the Earth's outer core.

 The upper mantle 

Past episodes of melting and volcanism at the outer levels of the mantle have produced a very thin crust of crystallized melt products near the surface, where we live. The gases evolved during the melting of Earth's mantle have a large effect on the composition and size of Earth's atmosphere.  

 Uppermost mantle 
The top of the mantle is the  Moho at the base of the crust. The base of the crust varies from about 10 to 70 kilometers deep. Oceanic crust is generally less than 10 kilometers thick. "Standard" continental crust is around 35 kilometers thick, and the large crustal root under the Tibetan Plateau is about 70 kilometers thick.

A thin crust, the lower part of the lithosphere, surrounds the mantle and is about 5 to 75 km thick. There are two main zones in the upper mantle. The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere, an irregular layer with a maximum thickness of perhaps 200 km, of which the uppermost mantle is 120 to 50 km thick.

 Asthenosphere 
Below the lithosphere the upper mantle becomes notably more plastic. It is called the asthenosphere, and is composed of flowing rock in the state of plasticity, about 200 km thick.

 Lower mantle 
The lower mantle is much thicker than the upper mantle. It is made of magma, under great pressure, and so is thicker (higher viscosity) and flows less easily. The lower mantles temperature has a max of about 4,000 °C (7,000 °F). 

The chemical composition of the mantle is heavily biased towards three elements: oxygen 44.8% by weight; magnesium 22.8%; silicon 21.5%. Compounds are oxides: silica SiO2 46%; magnesium oxide MgO 37.8%.

 Place with no crust 
In 2007 a team of scientists on board the RRS James Cook went to an area of the Atlantic seafloor where the mantle has no crust covering. The anomaly is mid-way between the Cape Verde Islands and the Caribbean in the Atlantic Ocean, at or near the Mid-Atlantic Ridge. It is about three kilometres under the ocean surface and covers thousands of square kilometres.

Other such 'Oceanic Core Complexes' (OCCs) were found later. Many have active hydrothermal vents.

 References 

 Other websites 
 Information on the Mohole Project 

Category:Structure of the Earth