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Classifying Rocks. After this lesson you will be able to explain how geologists classify rocks. Vocabulary . Rock-forming mineral Granite Basalt Grain Texture Igneous rock Sedimentary rock Metamorphic rock. How do geologists classify rocks?.
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Classifying Rocks After this lesson you will be able to explain how geologists classify rocks.
Vocabulary • Rock-forming mineral • Granite • Basalt • Grain • Texture • Igneous rock • Sedimentary rock • Metamorphic rock
How do geologists classify rocks? • If you were a geologists, how would you examine a rock for the first time? You might look at the surfaces. But you would also probably use a hammer to break open a small sample of the rock and look inside. • To study a rock sample, geologists observe the rock’s mineral composition, color, and texture.
Mineral composition and color • Rocks are made of mixtures of minerals and other materials. Some rocks contain only a single mineral. Other rocks contain several minerals. • Granite, for example, is made up of quartz, feldspar, mica, and hornblende. • About 20 minerals make up most of the rocks of Earth’s crust. These minerals are known as rock-forming minerals. The minerals that make up granite are rock-forming minerals.
Mineral composition and color • A rock’s color provides clues to the rock’s mineral composition. For example, granite is generally a light-colored rock that has high silica content. • That is, it is rich in the elements silicon and oxygen. Basalt is a dark-colored rock that has a lower silica content than granite has. • But unlike granite, basalt has mineral crystals that are too small to be seen with the naked eye. As with minerals, color alone does not provide enough information to identify a rock.
Texture • Most rocks are made up of particles of minerals or other rocks, which geologists call grains. • Grains give rock its texture. Texture is the look and feel of a rock’s surface. • To describe the texture of a rock, geologists use terms that are based on the size, shape, and pattern of the grains.
Grain size, grain shape, and grain pattern • Rocks with grains that are large and easy to see are said to be coarse grained. Fine-grained rocks have grains that are so small they can be seen only with a microscope. • In some rocks, grain shape results from the shape of the mineral crystals that form the rock. Other rocks have a grain shape that results from rounded or jagged bits of several rocks. • In banded rocks, grains can lie in a pattern of flat layers or can form swirls or colored bands. Nonbanded rocks have grains that do not lie in any visible pattern.
Using the characteristics of color, texture, and mineral composition, geologists can classify a rock according to its origin. A rock’s origin is the way that the rock formed. Geologists have classified rocks into three major groups: igneous rock, sedimentary rock, and metamorphic rock. • Each of the groups of rocks formed in a different way. Igneous rock forms from the cooling of magma or lava. The magma hardens underground to form rock. The lava erupts, cools, and hardens to form rock on Earth’s surface.
Origin • Most sedimentary rock forms when small particles of rocks or the remains of plants and animals are pressed and cemented together. Sedimentary rock forms in layers that are buried below the surface. Metamorphic rock forms when a rock is changed by heat or pressure, or by chemical reactions. Most metamorphic rock forms deep underground. • ????’s
Assignment • Compare and contrast how igneous, sedimentary, and metamorphic rocks form using a three ring Venn diagram. • Lab zone-How do rocks compare and the rocks around us
