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
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
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
- 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.
- 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.
- Which book explains that minerals make up
- Which book explains clearly that there are three
different types of rocks, sedimentary, igneous, and metamorphic?
- Does one book make it very clear that rocks are
formed on the crust of the earth by different processes?
- Which book is easy to read?
- Which book has the most information?
- Which book has ideas for home activities?
- Which book would you recommend to a friend who
wants to learn more about rocks and minerals?
- Which book has the prettiest pictures?
- 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.