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MORE ABOUT CLASSIFYING ROCKS

We need to know more about rocks. Bring your rock collections to school and, keeping them separate, of course, examine the collections of the other students, and of the
classroom (if your classroom has a rock collection). Examine them by looking, handling, sniffing, checking their surfaces by feel and by looks, heft them to see if they are light or heavy for their size.

a. Size can be checked as follows:

Set out some rocks ranging in size from sand to large stones. Sort the rocks by size. Small pieces of rock are sand, the next larger rocks are pebbles, the next larger are stones (actually, the rocks referred to as "stones" are usually gemstones), and the largest are boulders. Whatever size they are, they are still rocks.

b. Hardness can also be checked in several ways:

Some rocks scratch or crumble easily and some do not. Try scratching one rock on another to see which is harder. Some rocks fracture by layers, such as shale.
You can crumble them or break them with your hands, if they are not very hard. The harder rocks have to be crushed or broken with hammers or other tools.

Sort the rocks by degrees of hardness.

c. Weight is still another class into which rocks can be sorted.

Show that some rocks are heavy and sink in water, while others are light (pumice) and will float. A pumice rock will feel very light in your hand, while a piece of granite the same size will feel much heavier.

Sort the rocks by weight.

In spite of differences in size, hardness, and weight, all of the samples are called rocks.

In what ways are all the rocks alike?

Why are they all called rocks?

In deriving a definition you may be guided by such questions as:
"Do rocks rust, rot, or burn?"
"Do they move on their own?"
"Do they breathe or grow?"

How can you be sure about your answers to the guiding questions?

A number of tests can be conducted to find the answers to the questions.

These include the following:

- Place a rock and an iron nail in jars of water for several days to see which rusts; [This test is not valid, because any rock that contains iron will rust.]

- Place a rock and a piece of fruit on a piece of plastic wrap or waxed paper on a windowsill for a week to see which rots; [This test seems not good because all things decay in some way, and the fact that a rock would not behave the same as fruit does not necessarily mean it does not decay.]

- Place a rock and a piece of wood in a flame to see which burns. [This test is not valid, because coal is a rock, and coal burns.]

Perhaps rocks containing iron, rocks that degrade, and coal are all exceptions that do not invalidate the tests. If that is correct, then the tests are valid in most cases.

After such tests, ask again in what ways all rocks are the same.

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