The topics covered in this chapter can be summarized as follows:

Section Summary
4.1 Magma and Magma Formation Magma is molten rock, and in most cases, it forms from partial melting of existing rock. The two main processes of magma formation are decompression melting and flux melting. Magmas range in composition from ultramafic to felsic. Mafic rocks are rich in iron, magnesium, and calcium, and have around 50% silica. Felsic rocks are rich in silica (~75%) and have lower levels of iron, magnesium, and calcium and higher levels of sodium and potassium than mafic rocks.  Intermediate rocks have compositions between felsic and mafic.
4.2 Crystallization of Magma As a body of magma starts to cool, the first process to take place is the polymerization of silica tetrahedra into chains. This increases the magma’s viscosity (makes it thicker) and because felsic magmas have more silica than mafic magmas, they tend to be more viscous. The Bowen reaction series allows us to predict the order of crystallization of magma as it cools.
4.3 Classification of Igneous Rock Igneous rocks are classified based on their mineral composition and texture. Felsic igneous rocks have less than 20% ferromagnesian silicates (amphibole and/or biotite) plus varying amounts of quartz and both potassium and plagioclase feldspars. Mafic igneous rocks have more than 50% ferromagnesian silicates (primarily pyroxene) plus plagioclase feldspar. Most intrusive igneous rocks are phaneritic (crystals are visible to the naked eye), whereas most extrusive (volcanic) rocks are aphanitic (crystals are too small to be seen with the naked eye). If there were two stages of cooling (slow then fast), the texture may be porphyritic (large crystals in a matrix of smaller crystals). Gas bubbles “frozen” in an igneous rock are called vesicles, and the textural term for a rock with vesicles is vesicular.
Lab 4 Exercises The best way to learn rock identification is to practice by examining the samples in your Rock Kit 1 and 2. Igneous rocks are classified according to their mineral composition (or colour, in the case of aphanitic rocks), and texture (size of the crystals). Knowing the diagnostic properties of the minerals within an igneous rock help you identify its composition as mafic, intermediate, or felsic. Just as with mineral samples, different samples of the same rock may not always look exactly the same (e.g., pink versus white granite), but they can always be identified by closely examining the mineral composition and texture.


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A Practical Guide to Introductory Geology Copyright © 2020 by Siobhan McGoldrick is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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