Earth, Our Environment - Class Notes
Chapter 4, The Cornerstones of Geology: Rocks - 04
Igneous Rock Classification

Igneous Rock Texture

There are several general types of igneous rock texture. The texture results from the cooling history of the rock.

Mineral crystals take time to grow - if magma cools slowly, large crystals can form. Lava generally cools faster, so the minerals are generally small in extrusive rocks.

Phaneritic (coarse-grained) rocks contain minerals that are easy to see. They are intrusives that cool slowly inside Earth. The Name comes from "phanes" which means "appearing".

Aphanitic (fine-grained) rocks no distinct crystals are visible because the rocks cools quickly at Earth's surface.

Porphyritic rocks are a combination of the two above. These rocks contain a fine matrix with no discernible crystals and embedded phenocrysts, large isolated crystals that are easy to spot.

Pyroclastic rocks are formed by the explosive ejection of magma from volcanoes and rapid cooling. Such rocks often have vesicles and bubble-like features.

Glassy rocks also form when magma is quickly quenched. They have no crystalline structure and break in sharp edges.

Igneous Rock Composition

The composition of a rock is based on its mineral or chemical composition.

The minerals most common in igneous rocks are used to distinguish between igneous rock types. The composition variations are continuos and a few names are used to classify composition ranges. The common minerals used to classify are:

[Study Figure 4.2 on page 73 of the text]

Identification of rocks can be difficult and we often use microscopic analyses of thin sections to aid in rock classification or chemical analyses of crushed rock samples to directly estimate the content of a rock.

Rocks with different textures can have the same chemical composition - thus a crushed rhyolite is chemically equivalent to a crushed granite.

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Prepared by: Charles J. Ammon
January 1997