Process

Melted rock, called magma, originates deep inside the earth. As bodies of magma rise toward the surface and cool, atoms in the magma organize into groups which form the seeds for crystals of specific minerals. The growing crystals accumulate the chemical elements which compose those minerals, changing the composition of the magma. Different minerals may then begin to crystallize. The initial composition of the magma and how fast it cools determine which minerals crystallize, the sequence in which they develop, and the size and shape of the grains.

These large crystals of feldspar crystallized as magma cooled slowly. Growing freely in a liquid, they developed straight edges and faces which reflect their internal structure. Small grains of feldspar and pyroxene surrounding the large crystals formed rapidly when the magma erupted at the surface as lava. They developed ragged, interlocking edges because they were crowded together as they grew.

Here, pyroxene formed first. Changing magma composition or physical conditions produced rims of slightly different composition on the crystals. Rapid chilling then froze the remaining magma to a fine grained aggregate.