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Many of the facing stones used for shop and building fronts in Oxford City Centre are igneous rocks, and commonly are types of granite. An igneous rock is a rock that has solidified from molten rock, called magma. Granite is a type of intrusive igneous rock because it solidifies within the earth. Intrusive igneous rocks cool well below the surface, and can take millions of years to solidify. Geologists can therefore never actually see intrusive igneous rocks forming, and can only study them when they appear on the surface after the rock above them has been worn away.
An example of how granite forms.
Because these rocks form underground, the magma cools much more slowly than if it had been erupted from a volcano. The slow cooling time gives the crystals a longer time to grow, and this means that the crystals in granite are relatively large. It is possible to see three different minerals in the granite. The pink or white crystals are feldspar, the clear crystals are quartz, and the black crystals are biotite mica.
Polished granite
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