Determining whether an igneous rock is phaneritic or
aphanitic is just the first stage in giving it a name. The second
stage is determining its mineral constituents. The chemical
composition of a magma is closely related to how explosive
and hazardous a volcanic eruption will be. The variation in
the amount of silica (SiO2) in igneous rocks is used to
describe the variation in composition of igneous rocks and
the magmas that formed them. Rocks with low amounts of
silica (basalt, gabbro) are known as mafic rocks, whereas
rocks with high concentrations of silica (rhyolite, granite) are
known as silicic or felsic rocks.
Some of the variation in the nature of different types of
volcanic eruptions can be understood by examining what
causes magmas to have such a wide range in composition.
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