Rock Cycle - Rocks (3A)

  • Learning certain characteristics of rocks.
  • Comparing and contrasting sedimentary, metamorphic, and igneous rocks.
  • dense
  • fossil
  • glassy
  • igneous
  • metamorphic
  • mineral
  • sedimentary

Students describe the three types of rocks.

Obsidian at Newberry Crater, Oregon


Students have learned that there are three different types of rocks, each of which possesses different characteristics. However, rocks are difficult to identify because there are great variations in their appearance and composition. For example, the layers of sedimentary rocks are easily confused with the squished bands of minerals in metamorphic rocks. Likewise, fine-grained (volcanic) igneous rocks often resemble well-cemented sedimentary rocks.

Some features, however, are consistently useful for rock identification. For example, rocks that have a gritty feel (like sand paper) are usually sedimentary. Many igneous rocks show randomly oriented large minerals, especially those that have been cooled very slowly. Rocks that look "squished" are usually metamorphic.

The ambiguities in rock identification make the process fun. It is like a mystery. If students bring you rocks, which you cannot identify, have them ask several people until someone can identify it. The fun is in the research!

  1. Read the worksheet with the students so they understand the meaning of each sentence. Have them then use the hints to try and match the rocks to the questions. If available, have students use a hand lens to observe the specimens. Encourage them to use other descriptive terms to help identify their samples. The key objective is for students to develop observational techniques that force them to think logically about selecting a rock that fits each description.
  2. After students have worked through the lab sheet, go over each specimen and make sure the students understand the correct answers.
  3. In conclusion, reinforce that there are only 3 major types of rocks (igneous, sedimentary, and metamorphic), but that there are hundreds of specific rock names. Each rock has its own characteristics. The students will learn to recognize various rock types by going through the process of formal identification of rock samples. Review how the different types of rocks are formed.


    OBSIDIAN - Also known as volcanic glass. It is very hard, but more importantly it breaks into sharp edges that easily cut through many materials. Note that broken obsidian looks like broken glass. Obsidian occurs in almost any color, depending on what trace elements are present in it. Black and brown obsidian are most common. Obsidian is an amorphous solid; that, it is a solid rock composed of silicon dioxide, but this material lacks crystalline structures. It is one of very few exceptions to the rule that rocks are made of minerals.

    PUMICE - Students will immediately notice that pumice is spongy or "full of holes." This characteristic makes pumice extremely lightweight; it even floats in water (you may wish to show this to your students). It is commonly light gray to blackish-gray in color. It is easily broken and has sharp edges. Like obsidian, pumice is volcanic glass; it thus looks glassy (especially with a magnifying glass) and lacks visible minerals.

    SCORIA - Scoria is composed of volcanic glass and preexisting rock fragments that became incorporated into the magma as it erupted. The volcanic glass looks similar to pumice, but is reddish in color, because it contains more iron than pumice. Scoria lacks large visible minerals; small ones may be visible with a magnifying glass. Scoria is often sold as "lava rock" for use as a landscaping material.

    GRANITE - Granite is composed of visible minerals, most commonly quartz, mica and feldspar. Quartz looks clear and glassy, mica is black and flaky, and the feldspars (commonly two or more different types are present) are either pale pink/orange or white in color. The relatively large size of the minerals indicates that the magma that formed the granite cooled slowly. This took place deep inside the earth, not on the surface, like pumice or scoria; it is a plutonic rock.


    SANDSTONE - The gritty feel of the surface of sandstone hints that this rock was once sand that has been cemented together. Sandstones have quite varied compositions; some are composed entirely of quartz, and others are mixtures of rocks, crystals and fossils. Almost any combination is possible. Sandstones thus come in a wide array of colors. By definition, the grains in a sandstone are "sand-sized"; most students will recognize this if you demonstrate "sand size" by showing them a bag of sand.

    SHALE - Shale is composed of very small particles of mud, which have been compacted and cemented together. Individual mud grains are very small; they will rarely be visible. Shales are quite variable in color.

    MUDSTONE WITH FOSSIL SHELLS - Mudstone is a variety of shale that is more massive. The samples in the kit contain marine fossils, indicating that these rocks formed in the ocean.


    MARBLE - Marble is composed exclusively of large commonly visible crystals of calcite. The gray/white bands in some of the samples are due to impurities within the calcite. Marble actually comes in a variety of colors, including black, gray, white, and pink. Marble, like all rocks that have calcite in them, fizz if you put a weak acid on it (usually 10% solution of hydrochloric acid). Marble forms when a rock containing calcite in it (such as limestone) was put under high temperature and pressure conditions. Marble has been used throughout history because it is easy to break and to carve.

    SERPENTINITE - Serpentinite has a smooth, soapy feel, a green mottled color, and a somewhat flaky texture. It is composed mainly of the mineral serpentine. Serpentinite is so named because of its mottled color, which resembles the back of a sea-serpent. The geologic origin of serpentinite is still debated, but many scientists agree that it formed from a rock like basalt that was put under high temperature and pressure. Serpentinite is the state rock of California.

    SCHIST - Schist is composed of visible minerals, mostly micas. Schists form under moderately high pressure conditions; this causes the naturally platy mica crystals to line up, giving the rock a platy look. This is a good example for illustrating the characteristic "squished" look of metamorphic rocks to your students.  

  4. Please note that rocks with the same name can vary in appearance. Geologists use other information besides appearance in order to identify rocks. For example, mineral compositions are key in determining the names of many rocks.

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