The Layers of the Earth

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Undergraduate Geology - Part 1 (Earth Structure) Note on The Layers of the Earth, created by siobhan.quirk on 05/11/2013.

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Inner Core: the inner core is believed to be made of a solid material, due to extreme pressure which is estimated at 3,600,000 bars compared to 1 bar at the surface. Both P and S waves move through the inner core, with S waves being generated by P waves at the boundary. This provides good evidence of a solid layer. The composition of the core is possibly a mixture of iron and nickel which is sometimes referred to as Nife (nickel and iron) based on meteorite evidence. It is believed that the whole solar system formed at the same time from a nebula cloud. Nearly all the material coalesced to make the planets, but some remained to form meteorites and asteroids. The meteorites ar either iron and nickel or silicate (stony). Iron Nickel is a material with roughly the correct density 12+gm/cm3. There must be dense material at the centre of the earth because the average density of the earth is 5.5gm/cm3 and the average density of the surface rocks is 2.8gm/cm3.

The inner core is from 6371km to approximately 5000km

Lehmann BoundaryThe Lehmann boundary is at a depth of approximately 5000km. it is a phase boundary between materials of the same composition but in different states so it is not a distinct boundary. it is actually a zone of about 100km where the rocks change from all liquid through a liquid-solid mix to all solid. Even though it is hotter at the inner core it is solid, but it is a liquid above the Lehmann boundary as the pressure is reduced.

The Outer Core the outer core is made of liquid seismic or earthquake waves provide clear evidence because S waves cannmt pass through liquid and they do not pass through the outer core P waves slow down due to the reduction in rigidity it causes the earthquake shadow zone the heat and pressure in the outer core is less than that on the inner core the outer core is made of iron nickel because the density graph shows the same pattern as that of the inner cre and the same meteorite evidence is true the earth's magnetic field is probably generated by the convection of the liquid iron The outer core is from 5000km to 2900km

The Gutenberg Boundary - 2900km this is a very distinct and clear boundary and is the most distinct boundary in the Earth. this is because it is at a change of material from metallic to a stony type of material it also changes state as it goes from liquid to solid at this boundary the P wave velocity decreases S waves stop altogether

The Lower Mantle the mantle is solid because S waves travel through it P waves increase in velocity steadily as the increasing pressure causes the rocks to become more rigid - less compressible it is made of the same type of silicate material as the stony meteorites in the lower mantle the pressure is high, therefore the elements push together to form tight bonds, creating dense silicate minerals - perovskite The lower mantle is from 2900km to 700km.

The Upper Mantle the upper mantle also consists of solid silicates but they are less dense the main rock type is peridotite olivine is a common mineral in the upper part, but changes to spinel when the pressure increases with depth Rarely lumps of rock are brought up by volcanic action from the upper mantle providing the first direct evidence of the interior of the Earth. The main source is kimberlite pipes which go down as far as 250km into the upper mantle.

The Asthenosphere the asthenosphere is a layer within the Upper Mantle where there is 5% partial melting as a result of the intersection of the geothermal gradient and the melting curve of peridotite as a result both P and S waves slow down hence the name Low Velocity Layer it is a rheid - a solid material that can flow very slowly as a plastic and may allow convection the layer acts as a lubricant which allows the layer above - the Lithosphere - to move across the asthenosphere the asthenosphere is composed of peridotite It goes from approximately 100km to 250km.

The Lithosphere This consists of the crust and the part of the upper mantle above the asthenosphere a little bit of the upper mantle forms the lowest part of the lithosphere and is made of peridotite

The Mohovovicic DiscontinuityThe MOHO boundary is different from the other discontinuities becuase it is not straight and its shape mirrors the surface of the earth. The boundary rises under the sea and falls under the mountains therefore its depth varies from 5-7km under the sea to 15-90km under the continents,We can detect the MOHO by the fact that earthquake waves, both P and S can be reflected by the MOHO. This is because there is a difference in the density of the rock above and below the MOHO.

The CrustDivided into two layers: oceanic and continentalContinental: 0-65km thick and has an average thickness of 35km rich in silica and aluminium granitic on average, bt a variety of sedimentary, metamorphic and igneous 2.7gm/cm3 between 0-3900 million years old deformed structure

Oceanic: thickness of 5-10km with an average of 7km rich in silica but lower than continental crust and rich in magnesium basaltic on average and nearly all igenous 2.9gm/cm3 fairly new maximum of 200 million years old in layers

Inner Core to Upper Mantle

Asthenosphere to the Crust

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