Why do rocks have to be in the mantle to change to a metamorphic rock?

Answer 1

The short answer is that they don't! For metamorphism to occur rocks require either heat, pressure or a combination of the two.

Both of these conditions occur together in the mantle, however they can occur in other places to. Such as in the rocks around igneous intrusions (high temperatures and low to moderate pressures) which cause contact metamorphism.

It can also occur in zones of high tectonic stress (such as continental collision zones where mountains are being built where there is a relatively low temperature by mantle standards but very high pressure) which causes regional metamorphism.

Finally in zones characterised by very high strains (e.g. the rocks in very close proximity to fault zones) metamorphism can occur. The type depends on the strain rate and the depth (which affects the pressure and temperature). This type of metamorphism is known as dynamic metamorphism).

All the above conditions (and a number of others) can cause metamorphism to occur in sections of the earth outside the mantle.

The differing types of metamorphism at differing temperatures and pressures create differing assemblages of minerals. These are known as metamorphic facies.

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Answer 2

Metamorphic rocks form due to high pressure and temperature conditions deep within the Earth's crust or mantle. These conditions cause the minerals in existing rocks to recrystallize and rearrange, resulting in the formation of a metamorphic rock. This process typically occurs at depths where the temperatures and pressures required for metamorphism are found, which is commonly in the mantle or deep within the crust.