Classifying Rocks

What are Rocks? Made of two or more different minerals that have been: cemented together squeezed and heated together melted and cooled together.

Mineral Composition and Color About 20 minerals make up most of the rocks of Earth’s Crust. These minerals are known as rock-forming minerals. A rock’s color provides clues to the rock’s mineral compostition. Granite is generally a light colored rock that has a high silica content. Basalt is a dark colored rock that is low in silica. As with minerals color alone does not provide enough information to identify a rock.

Texture Very useful in identifying a rock Look and feel of the rock’s surface Most rocks are made up of particles of other rocks – called grains Geologists use terms based on the size, shape, and pattern of the grains The picture is Rhyolite.

Grain Size Coarse Grained – large easy to see Fine Grained -- grains are so small they can only be seen under a microscope Fine Grain Coarse Grain Fine Grained rock is Slate . Coarse grained rock is quartz-diorite

Grain Shape Grains in a rock vary widely in shape The grains can be round (Conglomerate), jagged (Breccia), star-shaped, look like small seeds, etc… They can be formed from other rocks Conglomerate Breccia

Grain Pattern The grains in rocks often form patterns. Flat layers like pancakes Swirling patterns Foliated – different colored bands Random

How Rocks Form Using color, texture, and mineral composition, geologists can classify rock according to its origin. Igneous rock – forms from cooling magma or lava Sedimentary rock – forms when particles of other rocks or the remains of plants and animals are pressed and cemented together. Metamorphic rock – forms when an existing rock is changed by heat, pressure, or chemical reaction.

Classifying Rocks Review Where do rocks come from? What are Rocks? Mineral Composition Color Texture Grain – size, shape, pattern How Rocks Form?

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