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USGS CMG InfoBank: Intrusion

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Comment: 04:17 - 05:46 (01:29)
Source: Annenberg/CPB Resources - Earth Revealed - 14. Intrusive Igneous Rocks
Keywords: "James Hutton", granite, plutonic, Scotland, mafic, gabbro, felsic, granite, silica, aluminum, diorite, volcanism, cooling, crystallization, magma, crystal, igneous

Our transcription: James Hutton studied granite because it is among the common plutonic rocks of Scotland, but worldwide there are, in fact, many types of plutonic rocks.
Those that contain abundant iron and magnesium are called "mafic rocks" by geologists.
Gabbro is an example.
"Felsic rocks", such as granite, contain abundant silica and aluminum.
Finally, many igneous rocks are essentially mixtures of mafic and felsic compositions.
These are known as the "intermediate igneous rocks".
Diorite is one of the most abundant examples.
Each of these plutonic rocks has a compositionally identical counterpart in volcanic rocks, but although they may be made of exactly the same material, their textures are very different.
This can only be due to the fact that each cools and hardens under different conditions.
Laboratory studies indicate that if the molten rock cools too quickly.
Crystals are unable to form since the individual atoms in the melt don't have enough time to build crystal lattices.
If the molten rock cools slowly however, crystals will ultimately form throughout the magma; and the more slowly the magma cools, the larger the crystal will grow.

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