Identify and describe the six major igneous textures.
The term igneous exture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of its mineral grains—not how it feels to touch.
Texture is an important property because it reveals a great deal about the environment in which the rock formed (Figure 1).
Geologists can make inferences about a rock’s origin based on careful observations of grain size and other characteristics of the rock.
Three factors influence the textures of igneous rocks:
• The rate at which molten rock cools
• The amount of silica present in the magma
• The amount of dissolved gases in the magma
Among these, the rate of cooling tends to be the dominant factor. A very large magma body located many kilometers beneath Earth’s surface remains insulated from lower surface temperatures by the surrounding rock and thus will cool very slowly over a period of perhaps tens of thousands to millions of years.
Initially, relatively few crystal nuclei form. Slow cooling permits ions to migrate freely until they eventually join one of the existing crystals. Consequently, slow cooling promotes the growth of fewer but larger crystals.