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BASIC IGNEOUS ROCK

Rock collections are such fun! Almost everybody collects rocks at some time, either as a child or as an adult. Rocks can be good reminders of places we have been. There are some areas where it is unlawful to pick up rocks. Before collecting, each visitor to a "natural" area should check to see if it is okay to collect a few rocks. (It is NEVER okay to collect wildflowers.)

If your teacher wants to invite you to do so, those of you who have rock collections can bring them to class. Perhaps you have already identified and/or classified them, or your teacher and classmates can help you do that.

In this activity, we will learn some things about igneous rock. (Pronounce that "IGG-nee-us".) Some kinds of igneous rock are granite (GRAN-itt), basalt (baa-SALT), obsidian (obb-SIDD-ee-un), and pumice (PUMM-iss). Each of these rocks was formed from magma, which is the reason that some people who are not scientists refer to igneous rocks as "fire rocks." While it is good to know the common terms for things, it is better to know (and be able to use) the scientific terms, because scientific terms are more specific.

Now, looking at the assortment of igneous rocks we have, get out your notebooks and make notes describing the characteristics of each type of rock. Include descriptions
of their shapes, colors, textures, and how heavy they seem relative to their size.

Taking proper precautions, such as wearing safety goggles, and placing the rock into a bag, break pieces of each kind of igneous rock with a rock hammer (provided by the school), for closer examination with a hand lens and to see if each breaks in a regular way. (Geologists don't say "break;" they say "fracture." The manner in which rock fractures can tell the geologist many things about the rock, its approximate age, its composition, and, sometimes, the area from which it came!)

Some of the following information on each type of igneous rock might be helpful:

a. Granite.

Granite forms when magma cools and hardens in spaces between layers of rock under the ground. Note that granite is coarse and contains large crystals. Granite
varies in shading from white to gray and is sometimes pinkish in color.

If possible, look at examples of the use of granite in making statues, monuments, and buildings.

b. Basalt.

Basalt forms when magma pours out of the earth slowly and cools and hardens slowly. Some pieces of basalt have holes made by escaping gases. (Would this kind be
Pahoehoe or Aa?)

Note that basalt contains very small crystals. Basalt is usually a dark greenish-gray color.

c. Obsidian.

Obsidian forms when magma pours out of the earth slowly, like tar, but cools quickly. Note that obsidian contains no visible crystals. Obsidian has a black, glassy
appearance. Small pieces of obsidian are sometimes called "Apache's tears." See if you can find out about the legend which gave obsidian that name.

Fracture a piece of obsidian with a rock hammer, and note what pattern the chips make. Hold pieces of obsidian up to the light to see if light shines through them. If
possible, look at examples of Indian arrowheads made of obsidian.

d. Pumice.

Pumice forms when magma spills out of the earth so quickly that it is foamy, and it cools very quickly. The "foamy" consistency of pumice is caused by gases. It is
actually a mixture of ash, glass dust, and gas. (You might shake and squirt the foam from a bottle of soda pop to illustrate how magma sometimes spills out in a way
that would make pumice.)

Pumice is a grayish or beige color. Note that pumice is very light weight, and it is porous because the gas in it leaves little pockets. It can be floated in a container
of water.

There might be a block of pumice in your homes. It is used in areas where the water is "hard," for cleaning toilet bowls and other porcelain fixtures.

Dentists used to buy a paste made of pumice, and would use it to clean people's teeth! You might ask your dentist about it the next time you have an appointment.

 
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