Top: Science: Geology: Rocks and Minerals: Igneous



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Overview

Igneous rocks form when molten rock (magma) originating from deep within the Earth solidifies. The chemical composition of the magma and its cooling rate determine the final igneous rock type.


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Intrusive OR Plutonic

Forms below ground. Intrusive igneous rocks are formed from magma that cools and solidifies deep beneath the Earth’s surface. The insulating effect of the surrounding rock allows the magma to solidify very slowly. Slow cooling means the individual mineral grains have a long time to grow, so they grow to a relatively large size. Intrusive rocks have a characteristically coarse grain size.


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Extrusive OR Volcanic

Forms above ground. Extrusive igneous rocks are formed from magma that cools and solidifies at or near the Earth’s surface. Exposure to the relatively cool temperature of the atmosphere or water makes the erupted magma solidify very quickly. Rapid cooling means the individual mineral grains have only a short time to grow, so their final size is very tiny, or fine-grained Sometimes the magma is quenched so rapidly that individual minerals have no time to grow. This is how volcanic glass forms.


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Rocks and Minerals

Name Description
Granite Intrusive.
Dorite Intrusive.
Gabbro Intrusive.
Basalt Extrusive. Low in silica. Very fluidy/runny.
Scoria Extrusive. Similar to Basalt. Full of gas holes.
Andesite Extrusive. Medium amounts of silica. Not very fluid. Formed from explosive eruptions.
Rhyolite Extrusive. Very viscous/not fluid. High in silica. Highly explosive volcanoes.
Obsidian Extrusive. 



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