Rock Topics

Classification

The classification of igneous rocks is based on texture and composition.  Texture is the description of the size, shape, and arrangement of the mineral grains in a rock.  Composition refers to the types of minerals that make up the rock.

Texture (Part 1)

Texture is determined mainly by the cooling rate of magma.

• Slow cooling occurs deep within the earth, and produces a phaneritic texture.  Phaneritic means that mineral grains are large enough to be seen with the naked eye.

Diorite Phaneritic texture in diorite (Photograph by Parvinder Sethi)

Phaneritic rocks, like this diorite, have mineral grains large enough to be seen with the naked eye.

• Rapid cooling occurs near or on the earth’s surface, and produces aphanitic or glassy texture.  Aphanitic means that mineral grains are too small to be seen with the naked eye.   

Andesite and Basalt(Left photograph by Phyllis Newbill, right photograph by Parvinder Sethi)

Aphanitic rocks, like this andesite (left) and basalt (right) have mineral grains too small to be seen with the naked eye.