3.4.1 Three Factors for Igneous Textures: Cooling Rate, Silica in the Magma, and Gases in the Magma

The term texture, when applied to an igneous rock, is used to describe the overall appearance of the rock based on the size, shape, and arrangement of its interlocking crystals. Texture is an important characteristic because it reveals a great deal about the environment in which the rock formed. This fact allows geologists to make inferences about a rocks origin while working in the field where sophisticated equipment is not available.

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Three factors contribute to the textures of igneous rocks. 1) the rate at which magma cools; 2) the amount of silica present or mineral composition of the magma; and 3) the amount of dissolved gases in the magma. Of these, the rate of cooling is the dominant factor, but like all generalizations, this one has exceptions.

As a magma body loses heat to its surroundings, the mobility of its ions decreases. A very large magma body located at great depths will cool over a period of perhaps tens or hundreds of thousands of years. Initially, relatively few crystal nuclei form. Slow cooling permit ions to migrate freely until they eventually join one of the existing crystalline structures. Consequently, slow cooling promotes the growth of fewer but larger crystals.

On the other hand, when cooling occurs more rapidly, for example in a thin lava flow, the ions quickly lose their mobility and readily combine to form crystals. This results in the development of numerous embryonic nuclei, all of which compete for the available ions. The result is a solid mass of a small intergrown crystals.

When molten material is quenched quickly, there may not be sufficient time for the ions to arrange into an ordered crystalline network. Rocks that consist of unordered ions are referred to as glass.

The effect of cooling on rock textures is fairly straightforward. Slow cooling promotes the growth of large crystals, whereas rapid cooling tends to generate smaller crystals. We will consider the other two factors affecting crystal growth as we examine the major textural types.