Canadian Rockhound: Classifying your Rocks

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A few simple tests and observations will help you to place your specimens into a category. It may not be the full proper name for your specimen, but it will put it in the right family. Once you get to know the family by name you feel a little closer to it and it becomes familiar. Soon you will begin to learn the names of some of the different members of the family.

Basic Rock Types

Rocks are made up of groups of one or more minerals and are created in different ways. They form from lava or magma and these are called igneous rocks. They form from sediment and are called sedimentary rocks. They can also form from chemical solutions or be changed and altered to form new rocks. Those formed by changing from one to another are called metamorphic.

How to Classify your Rocks

All rocks belong to one series or another. They develop by cooling, drying or shrinking and they weather and erode. The first step is to classify into one of the three major divisions: crystalline, stratified and with or without fossils.

Examine your rock closely. Use a 10X magnifying lens if you have one. See if its made up of crystals. Some, like granite, are crystals bound together. Others are not crystalline and consist of particles such as flint, clay or chalk.

Next, determine whether your rock is made up of layers or not. These layered rocks are known as stratified rocks. Other rocks like granite, marble and basalt are one mass, unstratified, not in layers.

Thirdly, try to find out if your rock contains fossils. Such rocks as chalk and coral are made up almost entirely of fossils.

You will soon realize that a rock which is crystalline is not stratified and contains no fossils. A stratified rock is almost always non-crystalline. Now you can separate your rocks into two classifications:

rocks which are crystalline and not stratified and do not contain fossils.
rocks which are non-crystalline and are stratified and may contain fossils.

Those rocks in group one are either igneous or metamorphic and those ingroup two are sedimentary.

Glossary

Igneous

Lava -- molten rock which flows from volcanic activity.

Magma -- mixture of molten and crystalline rock.

Metamorphic -- altered by heat and / or pressure.

Petrology -- the study of the origin and structure of rocks.

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Contents:

Basic Rock Types

How to Classify your Rocks

Glossary