Identifying Rocks

The following was taken from the 1980 version of Holt's Elementary Science (the information is still useful). I got the pictures from various of the websites found in my Earth Science Rocks and Minerals Links section.

Knowing what minerals are in a rock provides us with just one clue to the rock's identity. When trying to identify most igneous, sedimentary, and metamorphic rocks, other or additional clues are needed. Let's find out what these clues are.

Many igneous rocks can be identified by the position of their minerals. The minerals usually fit together like pieces of a jigsaw puzzle. Specific igneous rocks can usually be identified by the different minerals they contain. Granite, for example often contains the minerals quartz, feldspar, and mica.

See how the minerals in Granite have a definite pattern?

Rhyolite's minerals are harder to see but there is a pattern to it all.


Certain igneous rocks can also be identified by the size of their crystals. Granite and rhyolite are both igneous rocks made of the same minerals. Granite has large crystals and rhyolite has small crystals.

Granite has larger crystals.

Rhyolite's crystals are much smaller.

You learned that sedimentary rocks are composed of different-sized particles that have been cemented together. These various-sized particles make many sedimentary rocks easy for us to identify. Because of these differently sized particles, the minerals in sedimentary rocks do NOT fit together like pieces of a jigsaw puzzle. They are not arranged in any order. Sedimentary rocks also contain the remains of animals that lived long ago.

Here is a conglomerate, obviously sedimentary my dear Watson.

This Atherfield rock has the fossil of a lobster, proof positive that it's sedimentary.

The position of the minerals in metamorphic rocks is often the most important clue in identifying such rocks. As you know, they are arranged in bands, or layers. Another clue that helps identify metamorphic rocks is the fact that they usually do not contain any remains or of past life. Such evidence was probably destroyed by the heat and pressure under which these rocks were formed.

Bands and layers very clear in this Gneiss.

Check out these bands and layers.

Minerals play an important part in identifying rocks. Color, luster, texture, and hardness are some properties of minerals. The minerals in igneous rock usually fit together like pieces of a jigsaw puzzle. The minerals in sedimentary rock are not arranged in any order. A metamorphic rock's minerals are arranged in bands, or layers.

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