The pressure that allows the diamonds to form in the mantle is due to rocks weight which is present in the crust and upper mantle. Thus option (D) is correct.
What is mantle?
There are three layers of the Earth which are crust, mantle and core. Mantle is the second layer of the earth.
Diamonds are created in the mantle, where they require high temperatures and pressures to do so. Volcanic eruptions cause these to rise to higher layers.
The downward pressure from the rocks in the crust is what generates the pressure. The variation in layer thickness causes the pressure to be unevenly distributed. Diamonds are therefore not very frequent. Thus option (D) is correct.
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