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Igneous Rocks

There are two groups of igneous rocks: those that hardened beneath the earth's surface are called intrusive or plutonic; those that hardened on the surface are called extrusive or volcanic. All igneous rocks are classified by the types of minerals present and by the size of their crystals. While the minerals reflect the chemistry of the original magma, the size and shapes of the crystals indicate how long it took for the magma to cool. Plutonic rocks, such as granite, have crystals large enough to be seen with the naked eye, indicating a slowly cooling magma within the earths crust. Volcanic rocks, however, usually have microscopic crystals because the magma cooled very quickly when it was exposed to cool air on the surface. Basalt is the most abundant type of volcanic rock. 

Most of Alberta's exposed bedrock is sedimentary but there are some outcrops of igneous rocks. Volcanic rocks can be found in the mountains of Waterton Lakes National Park and north to the Crowsnest Pass. Intrusive rocks, such as granites, form extensive parts of the Canadian Shield in northeastern Alberta. They can be seen particularly well at Fort Chipewyan and the Slave River Rapids. There are also small outcrops of such rocks south of the Milk River.

Even though outcrops of igneous rocks are uncommon in Alberta, the advancing continental glaciers during the last Ice Age plucked blocks of both igneous and metamorphic rocks from the Shield and scattered them across most of the province. Thus, such rocks are abundant in gravel pits, gravel bars, and rock piles in farmer's fields.

Reprinted from A Traveller's Guide to Geological Wonders in Alberta by Ron Mussieux and Marilyn Nelson with the kind permission of the authors and the Provincial Museum of Alberta.

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