Small families are more common in countries. island poor developing developed

Geothermic gradient is the rate of increasing temperature with respect to increasing the depth in the earths interior. away from tech tonic plates boundaries, it is about 25-40 c/km (72-87 degrees f/mi) of depth near the surface in most of the world.

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The name may be in importnce  to them

A. it is created from magma. the agents of a metamorphic rock is heat and pressure. (heat from intrusion of magma or burial)