GEOL1002 Lecture Notes - Lecture 8: Andesite, Aphanite, Plagioclase
25 Jun 2018
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Magma and Igneous Rocks
Igneous rocks
Crystalline or glassy rocks that form from the cooling of magma or lava.
Magma
Molten rock derived from the partial melting of pre-existing rock in the mantle or crust.
Magmas may contain crystals of various minerals and contain a gas phase.
When molten rock exists below the Earth’s surface it is termed magma and when it is above
the surface it is termed lava.
Freezing rocks
Any rock formed from the solidification (freezing) of a melt is an igneous rock. Igneous rocks
freeze at temperatures typically between 600C and 1200C depending on the chemical
makeup.
Intrusive rocks: form from the solidification of magma in contact with air or water.
Extrusive rocks: form by the solidification of melt underground after it has intruded into pre-
existing wall rocks.
The classification of igneous rocks
We need to understand:
The composition of the original melt:
Colour
Texture
Mineralogy
Chemistry
How the melt crystallised (level of emplacement in the crust)
Depth
Relationship with other rock units
Depth of emplacement
As the depth increases grain-size also increases as rocks at depth take longer to cool,
allowing crystals to grow larger.
Volcanic: magma reaches the surface and is extruded as lava.
Hypabyssal: magma reaches shallow depths in the crust as sheets such as dykes and sills.
Plutonic: large volumes of magma stay at depth as plutons like batholiths.
Making room for rocks
Magma rises because it’s buoyant. The rate of melt movement is affected by viscosity,
which in turn, depends on composition and temperature. When molten rock enters a
cooler environment, it freezes. The rate of cooling depends on the environment and the
shape of the magma body.
Igneous chemistry
Igneous rocks are comprised of major elements, volatiles and trace and rare earth
elements.
Major elements are reported and measured as their oxides.
10 main oxides:
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Crystalline or glassy rocks that form from the cooling of magma or lava. Molten rock derived from the partial melting of pre-existing rock in the mantle or crust. Magmas may contain crystals of various minerals and contain a gas phase. When molten rock exists below the earth"s surface it is termed magma and when it is above the surface it is termed lava. Any rock formed from the solidification (freezing) of a melt is an igneous rock. Igneous rocks freeze at temperatures typically between 600 c and 1200 c depending on the chemical makeup. Intrusive rocks: form from the solidification of magma in contact with air or water. Extrusive rocks: form by the solidification of melt underground after it has intruded into pre- existing wall rocks. How the melt crystallised (level of emplacement in the crust) As the depth increases grain-size also increases as rocks at depth take longer to cool, allowing crystals to grow larger.
