The structure of the earth

1. The inner core6371 km to approximately 5100 km

The inner core is believed to be made of solid material, due to the extreme pressures which is estimated at 3600000 atmospheres, compared to 1 atmosphere at the surface

Earthquake waves, both P and S waves, move through the inner core, with the s waves being generated by the p waves. This provides good evidence of a solid layer 

The composition of the core is a mixture of iron and some nickel, based on meteorite evidence and the fact that iron nickel has the correct density of more than 12g/cm3

2. The Lehmann Discontinuity at 5100 km 

A phase boundary between materials of the same composition but in different states, so it is not a district boundary. It is a zone of about 100 km where the rocks change from all liquid in the outer core through a liquid solid mix, to all solid in the inner core

3. The outer core from 5100 km to 2900 km

The outer core is liquid iron and nickel. Seismic or earthquake waves provide clear evidence that the outer core is liquid because S waves cannot pass through liquid and they do not pass through the outer core 

P waves slow down due to reduction in rigidity. The pressure in the outer core is less than in the inner core, allowing a liquid to exsist

4. The Gutenberg Discontinuity at 2900 km

This is a very distinct and clear boundary, marking a change of material from metallic iron nickel to stony silicate material. It also changes state between the liquid outer core and the solid mantle. Therefore at this boundary the P wave velocity decreases and the S waves stop altogether

5. The lower mantle from 2900 km to 700 km

The lower mantle is solid because S waves can travel through it.P waves increase in velocity steadily as the increasing pressure causes rocks to become more rigid- less compressible.It is made of the same type of silicate material as the stony meteorites 

6. The upper mantle from 700 km to an average of 35 km 

Consists of solid silicates but they are less dense than the lower mantle. The main rock is peridotite. Part of the upper mantle lies within the asthenosphere, while part is in the lithosphere

 The asthenosphere

Behaves as a solid with the ability to flow. This type of rock is known as rheid. The upper mantle is made up of a rock called peridotite. At depths between 75 and 670 km but especially between 75 and 250km the temperature is high (1300 degrees) enough for about 5% of crystals in the peridotite to partially melt. Causing a film of melted minerals to surround each solid crystal, allowing the mantle to flow more easily. 

 The crust

This upper part of the upper mantle shows plastic properties even though it is still solid. It has also lost some of its rigidity and therefore P and S waves will slow down, which is why it is sometimes called the low velocity layer

The lithosphere

Nearer the surface, the mantle is cool enough to prevent partial melting this zone is always rigid and brittle. The lithosphere cannot flow, but it can be carried by the underlying asthenosthere. It is brittle, so has broken into plates. The outermost skin of the lithosphere is the oceanic or continental crust, divided from the mantle lithosphere by the Moho.

Oceanic crust and the continental crust. The oceanic crust is rich with iron and magnesium and consists of basalt, dolerite and Gabbro. The continental crust is rich in aluminium and Silicon. It consists of granatic rocks, igneous, metamorphic and sedimentary rocks. 

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