Igneous Rock Help (page 3)

By — McGraw-Hill Professional
Updated on Aug 31, 2011

Rock Texture

In the late 1700s, while working in a field near his home in Scotland, James Hutton noticed coarse-grained granites cutting across and between layers of sedimentary rocks. Wondering how they penetrated the smooth fine sediments, Hutton thought they might have been forced into and between cracks as liquid magma.

The rock’s texture provided Hutton with clues that the different rock types came from a different beginning.

As Hutton studied more and more about granites, he concentrated on sedimentary rocks that shared a border with the coarse granites, compared to sedimentary rock where no granite was present.

Hutton thought that the physical changes he saw in bordering sedimentary rocks must have come from an earlier exposure to high heat. This gave him the idea that molten magma from deep within the Earth had squeezed into areas of sedimentary rock and crystallized.

Grain size and color are the two main ways that geologists describe rock textures.

The size of the minerals or crystals that make up a rock’s texture is called grain size . Color can change depending on lighting, mineral content, and other factors, so it is thought to be less dependable when describing a specific rock.

When a rock’s grains can be easily seen with the eye, roughly a few millimeters across, they are classified as coarse grain . When individual grains are not visible, the texture is considered to be fine .

Mineral grains or crystals have an assortment of different shapes and textures. They may be flat, parallel, needle-like, or equal in every direction like spheres or cubes. The shapes that crystals take, along with their grain size, combines to make rock samples unique. Think of it in terms of people and cultures of the world. Just as the combination of genetic inheritance and environment makes people individual and unique, the same thing happens with rocks!

Granites have a coarse grain size compared to obsidian with a very fine grain size. Granites are used for building materials because of their larger grain size and decorative pink or gray color. Obsidians are used for jewelry and art.

Practice problems of this concept can be found at: Igneous Rock Practice Test

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