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Glossary of Igneous Rocks pg2

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What are factors that control melting temperature?
Pressure, water, and mixed minerals
Pressure
It increases the melting point and reduces in pressure may cause melting
Water
An increase in water decreases melting point. Water under pressure (water vapor entrapped in magma) lowers the melting point
Mixed Minerals
Different minerals have various melting temperatures. Some combos may lower the overall melting temp.
Identification of Igneous Rocks
Texture and color
Texture
(size and arrangement of mineral grains) refers to the size, shape, and boundary relations between adjacent minerals in a rock
What determines a rocks texture?
They develop in response to the rate cooling of the magma and its viscosity
Intrusive (Plutonic) Igneous Rocks
Magmas deep within Earth's crust cool slowly
Coarse Grained
(visible crystals) phaneritic, individual minerals are large enough to be plainly visible
Very Coarse Grained
Pegmatites: consist of large crystalsm measured in cm/m (usually granitic)
Extrusive(Volcanic) Igneous Rocks
a magma extruded out onto Earth's surface cools rapidly and the crystals have only a short time to grow.
Fine Grained
(aphanitic) Can't be seen without microscope. Appear massive and structureless
Glassy
Extremly rapid cooling stops crystal growth

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