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Metamorphic Rocks

Under intense pressures and temperatures deep within the earth, the minerals in a rock can be altered, without melting, into new minerals. Combinations of these new minerals form metamorphic, or "changed" rocks. Metamorphic rocks can be formed throughout large regions, for example during mountain building. Or they can occur within small areas, such as the metamorphoses of rocks that surround a molten rock mass. Many metamorphic rocks are banded because of the rearrangement of new minerals. In gneiss, for example, the banded texture is caused by the segregation of dark and light minerals. These dark and light layers can often be irregular, particularly when the rock has been folded under pressure. In metamorphic rocks, such as schist, minerals are often plate-like and parallel, creating a texture called foliated.

Classification of Metamorphic Rocks

Metamorphic rocks form a small percentage of Alberta's outcrops and can be found only in isolated localities around Alberta. The Canadian Shield is the best area to find high-grade metamorphic rocks, such as gneiss, which is particularly visible at Pelican Rapids and Mountain Rapids. High-grade metamorphic rocks are those that have been subjected to extremely high pressures and temperatures during their formation. Quartzite is another common metamorphic rock in Alberta and is one of the major rocks in the Main Ranges of the Rocky Mountains. Between Waterton and Crowsnest Pass, and in the mountains near Jasper, there are numerous outcrops of low-grade metamorphic rocks. Low-grade metamorphic rocks are those that have been subjected to relatively low pressures and temperatures during their formation.

Like igneous rocks, metamorphic rocks were also strewn about the province by glaciers during the last Ice Age, and are also commonly found in gravel pits, river 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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