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Earth's Mantle Help

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By — McGraw-Hill Professional
Updated on Aug 30, 2011

Earth's Mantle

The mantle is the next layer down in the Earth’s crust. It is located just below the lithosphere. The mantle makes up 70% of the Earth’s mass. It is estimated to be about 2900km thick. The mantle is not the same all the way through. It is divided into two layers, the upper mantle or asthenosphere ( asthenes is Greek for “weak”) and the lower mantle. Figure 1-8 shows how the upper and lower mantle layers are separated. These layers are not the same. They contain rock of different density and makeup.

Planet Earth Ridges And Trenches

 

Fig. 1-8. The mantle contains upper and lower layers of different rock types.

The highest level of the mantle is called the asthenosphere or upper mantle. It is located just below the lithosphere.

Except for the zone known as the asthenosphere, the mantle is solid, and its density, increasing with depth, ranges from 3.3 to 6 g/cm 3 . The upper mantle is made up of iron and magnesium silicates. The lower part may consist of a mixture of oxides of magnesium, silicon, and iron. This layer is made up of mostly 11 elements: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium, titanium, hydrogen, and phosphorous. These 11 elements combine with different compounds to form minerals.

The upper mantle is a lot thinner compared to the lower mantle. It can be found between 10 and 300km below the surface of the Earth and is thought to be formed of two different layers. The bottom layer is tough semisolid rock and probably consists of silicates of iron and magnesium. The temperature of this layer is 1400–3000°C and the density is between 3.4 and 4.3 g/cm 3 . The upper layer of the outer mantle is made up of the same material, but is harder because of its lower temperature.

The upper mantle is solid, but can reach much greater depths than the overlying lithosphere. Compared to the crust, this layer is much hotter, closer to the melting point of rock.

Heat and pressure allows malleability within the mantle. Mantle material moves within this moldable, under layer. Movement is a very slow process, more of a creeping than an actual flowing movement.

Creep is the extremely slow atom by atom movement and bending of rock under pressure within the mantle.

The heated materials of the asthenosphere become less dense and rise, while cooler material sinks. This works very much like it did when the planet was originally formed. Dense matter sank to form a core, while lighter materials moved eventually upward.

The lower part of the mantle or mesosphere is measured from the Earth’s core to the bottom of the asthenosphere, at roughly 660km. Although the average temperature is 3000°C, the rock is solid because of the high pressures. The inner mantle is mostly made up of silicon and magnesium sulfides and oxides. The density is between 4.3 and 5.4 g/cm 3 .

The mesosphere is the lower layer of the mantle that borders the Earth’s molten core.

The different amounts of heating in the upper and lower parts of the mantle allow solid rock to creep one atom at a time in a certain flow direction. When solids move like this, it is known as plasticity . As plasticity occurs in the mantle, slow currents are formed. The continental and oceanic crusts are subducted into the mantle and moved depending on the direction of this deep movement.

Practice problems of this concept can be found at: Planet Earth Practice Test

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