The phaneritic appearance of granite is a clue to
where granite forms. It takes hundreds of thousands of years for
silicate minerals to grow to the sizes they are in granite. This means
that the minerals cooled slowly. This can only take place deep inside
the crust of the Earth. Here, the rocks surrounding the magma act like a
blanket, keeping it warm and keeping the magma chamber hot, allowing the
magma to cool slowly. This means that if you see granite exposed on the
Earth’s surface, a lot of erosion must have taken place. All of the
rock which once covered the granite has been removed.
The minerals in granite can be many different
sizes. Some crystals can be quite large. This variety is caused by
differences in how fast crystals grow, and in the availability of the
elements to make certain minerals. For example, if the magma contains
very little iron, then few iron-rich minerals will form.