Identifying Silicic and Intermediate Igneous Rocks

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crystal grain size to tell you the rate of cooling

mineral composition and colour for silica content

igneous textures for origin

Silicic Igneous Rocks

High silica magma contains 66 to 75% silica and is mainly generated by melting of the Earth's crust at convergent plate margins. Rocks formed from high-silica magma tend to have a lot of quartz and other high-silica minerals such as plagioclase and potash feldspar and muscovite and biotite micas. The colour should be light. Silica rich magmas are very viscous, much stiffer than toothpaste, so little magma reaches the surface and granite is the most common silicic rock. These magmas solidify at relatively low temperatures, typically 600-900 degrees.

Then look at the crystal grain size:

Glassy, glassy with conchoidal fracture - obsidian

Glassy shards and fine crystals, vesicular texture - pumice

Fine crystals, flow banded - rhyolite

Coarse crystals, porphyritic or equigranular - granite or granodiorite

Intermediate Igneous Rocks

Intermediate magma contains 52 to 66% silica. The most common igneous rock formed from this type of magma is andesite, a fine grained volcanic rock. The coarse grained plutonic rock is diorite. They usually contain abundant plagioclase feldspar, together with hornblende, but little or no quartz. The magma is less viscous than a silicic magma, but still very sticky. It solidifies at a higher temperature than a silicic magma. Andesites are common at convergent plate margins.

fine, vesicular, amygdaloidal, porphyritic or equigranular - andesite

coarse, equigranular or porphyritic - diorite

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