How to Classify Rocks

Studying rocks is like uncovering a mystery. At first glance, you might not be able to tell much about a rock, but as you study its different characteristics, you can learn how it was made, what minerals it is composed of, and where that type of rock is generally located. By learning about the three different classes of rock, sedimentary, igneous and metamorphic, you can learn to name and identify those interesting rocks you come across.

Things You'll Need

• Rock identification guide
• Magnifying glass (optional)
• Rocks

Instructions

1. Know how the three different types of rocks are formed and where they are formed to begin to classify them. Sedimentary rocks are made from layers of sand, minerals and rock particles that have been compacted together. You can find these rocks near bodies of water, deserts, high mountains or where bodies of water have once been. Igneous rocks are formed when hot magma from the Earth's lithosphere solidifies. The two types of igneous rock are intrusive and extrusive--intrusive rocks are solidified underground; extrusive rocks form at the earth's surface. You can find igneous rocks near current or past volcanic activity. Metamorphic rocks are formed from existing rocks that have gone through change by heat and pressure, or heat alone. These rocks are found deep within the earth.

2. Learn the physical characteristics of the three types of rocks. Sedimentary rocks can have layers, and there may be evidence of weathering by wind, snow or rain on the rock. Rocks that contain fossils are most likely sedimentary rocks. They are usually brown or gray in color. Igneous rocks are made from interlocking crystals in random formations. The colors that usually make up igneous rocks are black, white and gray. They can be glossy or coarse grained. Metamorphic rocks can have layers that have a "wavy" or "foliated" appearance. The crystals in metamorphic rocks are often organized into patterns, unlike igneous rocks. These rocks can vary in color.

3. Take a good look at the grain size. A large grain can be seen with the naked eye, while smaller grains are microscopic. You can also get a more accurate classification if you can learn to identify the different minerals and particles that form a particular rock. Sedimentary rocks are composed of particles of sand, mud and small pebbles. There are three main groups of sedimentary rocks: clastic, organic and chemical rocks. Clastic rocks are formed when different particles are squeezed together. Shale and sandstone are types of clastic rocks. Organic rocks are formed from decaying plants and animals that have been cemented together. Coal and limestone are types of organic rock. Chemical rocks are formed as minerals precipitate from water. Igneous rocks are classified by grain size and by the amount of silica in the mineral content. Obsidian is an igneous rock that has small grains and high silica. Granite also has high silica, but it has large grains. Metamorphic rocks can be classified by the arrangement of their grains. Some grains form layers, such as in slate. In other metamorphic rocks, such as granite, the grains are arranged randomly.

4. Write down the identifying characteristics of a rock you wish to identify. These include color, texture/grain size, where it was found, recognizable minerals and formation of the particles.

5. Take the information of your rock and compare it to the information found in your rock identification guide. While there can be many variables and techniques to rock classification, you can learn to identify rocks by studying more about them (see Resources below).

Resources

• Rock Identification Chart