Igneous Rocks

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Intrusive & Extrusive Igneous rocks
Extrusive refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out onto the surface as lava or explodes violently into the atmosphere.

An intrusion is molten rock that forms under the surface of the earth. Magma from under the surface is pushed upward from deep within the Earth into any cracks or spaces it can find, pushing existing rock out of the way if need be, a process that can take millions of years.


Classifying Igneous rocks:
Felsic, Intermediate, Mafic, Ultramafic
Fine grained vs coarse grained

Examples of Igneous rocks

Obsidian- A black, volcanic glass formed as an extrusive rock. It's produced by rapidly cooling lava that has been expelled from the Earth.
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Obsidian had famous uses during the ancient Aztec civilizations, most prominently in sacrificial daggers.
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It is still used today as the premier tool in surgery as blades in scalpels, since fractured obsidian makes the cleanest and most precise cuts of any material.
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