Why are rift zones common places for igneous rock to form?

Igneous RockBelow the hard, solid lithosphere, Earth’s interior contains hot, melted rocks in liquid form. When liquid rock cools, it solidifies to become igneous rock. The lava that flows from a volcano can cool to form the igneous rock basalt. Molten material deep within Earth can form the igneous rock granite.

Rift zones are a common place for igneous rocks to form because when the rift in the Earth happens, magma is in the crack. When this magma cools, it hardens forming Igneous rock. 

*When magma or lava cools and hardens it forms Igneous rock.

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South wales composes a lot of coal in that region.