When we talked about collecting rocks in the field we spoke of sedimentary rocks in particular. This time we will talk about igneous rocks. Igneous is a word that refers to fire and heat. The rocks in this classification were formed from volcanoes, lava or magma.

Magma and lava are both molten rock. The difference is that lava is molten rock that has come to the surface of the earth's crust and magma is still underground.

How igneous rocks are formed

When magma comes to the surface through vents (volcanic openings) becoming lava, it seeps through cracks and splits in the surface and hardens when it cools to form dykes which cut across layers of other rocks. Or it forms sills between layers of other rocks. It may also fill gaps to form large lumps called laccoliths. Batholiths are the largest form of igneous intrusions and may extend for hundreds of miles! An intrusion is the name they give to something that squeezes in between two other things – like lava squeezing between other rocks.

Look closely at an igneous rock and study its crystal formation with a 10x hand lens. If the crystals that make up the rock are large, the material cooled slowly giving the crystals time to grow large. These rocks are usually called plutonic.

If the crystals are very small and fine, the material must have cooled quickly. Some cooled very rapidly and look like glass. These are known as vitreous rocks, meaning glass-like.

Some lavas had a complex cooling – hot and cold. They formed rocks containing, as you'd expect, large crystals imbedded in finer crystals. These are known as porphyritic rocks. They are more common in felsic, or light-coloured rocks than in mafic, or dark-coloured rocks. The word felsic comes from feldspar-silica. The word mafic comes from magnesium-iron. The words refer to the mineral content of the rocks.

Where to look for igneous rocks

Types of igneous rocks

When larger bubbles became trapped in the hardened lava, they formed pockets and over many millions of years the pockets filled with other liquids that formed mineral crystals. The fillings are called amygdales, a Greek word that means almonds. (I don't know why they used that word, though!) The rocks are known as amygdaloidal rocks.

Banded agate forms in large gas bubble cavities. Geodes also develop in these pockets.

Some igneous rocks are:

• Dolerite – dark medium-grain rock with light feldspar crystals and dark pyroxene crystals.

• Granite – coarse-grained rock with crystals of glassy quartz, white or pink feldspar, dark mica and sometimes black horneblende.

• Pegmatite – very coarse-grained rock, usually of quartz and feldspar. They occur as dykes. They are the best source of large crystals: quartz, feldspar, mica and others. Some crystals in pegmatite may be up to a meter in length.

• Quartz porphyry – fine-grained rock with large quartz crystals scattered through it. It is found in sills, dykes and old volcanic pipes.

A very useful little book to help you with all these terms is the Longman Dictionary of Geology published by Longman Group Limited. Coles, Smithbooks or Chapters can find this book for you if you give them the special code: ISBN 0-582-55549-3. That is an international registration number for all books published. This book costs about $15.00. It has an index to make it easy to find the word you want and it has numerous small coloured diagrams to show you what the word means.

Glossary

• Feldspar – (sometimes spelled "felspar") a silicate (glass-like) class of minerals of sodium, potassium, calcium, and barium. They occur in all types of rocks.

• Geode – a hollow nodule of rock lined with crystals. When separated from the rock body by weathering, it appears as a hollow, rounded shell partly filled with crystals.

Copyright ©1998 Marilyn Fraser
E-mail: silver@tor.axxent.ca

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