Rock Cycle - Rocks (4A)
Pre Lab 

  • Developing criteria to distinguish rocks
  • Comparing rock characteristics through reference material.
  • igneous
  • metamorphic
  • sedimentary

Students compare library book information on rocks and minerals.

Thin section of granite.


The Rock Cycle is a way to conceptually understand the creation and destruction of igneous, sedimentary, and metamorphic rocks on and within the crust of the Earth. Rocks form in many environments.

Identification of rocks can be difficult for beginners. Many people try to match a rock sample with a picture. This rarely works, as the same type of rock can have a great variety of appearances. Geologists classify rocks using two basic features: mineral composition and texture (appearance).

All igneous rocks began as magma (molten rock) which cooled and crystallized into minerals. Igneous rocks may look different because of two factors: (1) they may have cooled at different rates and (2) the "mother" magma (original melted rock) was of a different composition. Variations in these two factors have created many different types of igneous rocks. When the magma cools at different rates, it creates different sized minerals. Quick cooling magmas have small minerals (with the exception of obsidian, which is actually composed of silica, but has no crystalline structure). Basalt, for example, has small minerals, most of which can only be seen under a microscope. Quick cooling magmas are generally the ones which are erupted onto the earth’s surface; they are thus called volcanic rocks. Magma that cools slowly creates rocks like granite, which have large minerals that can be seen with the naked eye. These igneous rocks cool inside the earth, and are called plutonic igneous rocks. Geologists classify igneous rocks based on both their crystal size and composition.

Sedimentary rocks form at the Earth’s surface in two main ways: (1) from clastic material (pieces of other rocks or fragments of skeletons) which have become cemented together, and (2) by chemical mechanisms including precipitation and evaporation. Sedimentary rocks are usually associated with liquid water, which facilitates erosion, transportation, deposition, and cementation. However, sedimentary rocks may also form in dry, desert environments or in association with glaciers. Geologists classify sedimentary rocks using the composition of their components and their appearance.

Metamorphic rocks are formed from igneous, sedimentary, or preexisting metamorphic rocks that have been changed by great pressures and temperatures within the crust and upper mantle of the Earth. The temperatures were not enough to melt the rock, otherwise, an igneous rock would have formed. The pressures were much greater than those required to simply break the rocks into pieces. They were high enough to change the chemical make up of the rock by forcing the elements in it to "exchange partners." Different grades of temperature and pressure will cause the same original rock to form very different metamorphic rocks. Slate, which forms from the sedimentary rock shale, is very dense, smooth and does not contain visible minerals. However, if more pressure and temperature are applied to a slate, it could turn into schist, which has visible layers of minerals. If yet higher temperature and pressure are applied, the schist could turn into gneiss, which shows visible bands of minerals. Metamorphic rocks are named using their physical appearance and mineral composition. These characteristics are quite variable, depending on the pre-existing rocks and the range of temperature and pressure.


Gathering information about rocks is sometimes difficult for students. Children love rocks, but good information is difficult to find. In this activity, students are asked to find and critique reference materials.

  1. Provide books on rocks and minerals for the students to read. You may want them to go to the local library and find a book on rocks and minerals. Since there may not be many sources, you may want the students to work in teams. Let them compare and contrast the style of the books for content and readability. If you have internet access, you can search for sites on rocks. Note that on a search engine "rock" will more often than not, bring up sites on "rock and roll." It may help to search for specific types of rocks.
  2. Ask students the following questions to see which books or sites they would recommend to someone who wants to learn more about rocks and minerals. The answers to these questions are subjective, since each student will probably have a different opinion.
    1. Which book explains that minerals make up rocks?
    2. Which book explains clearly that there are three different types of rocks, sedimentary, igneous, and metamorphic?
    3. Does one book make it very clear that rocks are formed on the crust of the earth by different processes?
    4. Which book is easy to read?
    5. Which book has the most information?
    6. Which book has ideas for home activities?
    7. Which book would you recommend to a friend who wants to learn more about rocks and minerals?
    8. Which book has the prettiest pictures?
  3. Go over the rock cycle as illustrated below. You may want to use the worksheet for students to follow along with your cycle. You may want the students to try and figure out the "blanks" before you give them the answer.

[Back to Rock Cycle Grid]   [Back to Rocks (4)]