This page last updated on 23-Jan-2012
Prof. Stephen A. Nelson
Magmas and Igneous Rocks
Magma and Igneous Rocks
Igneous Rocks are
formed by crystallization from a liquid, or magma. They include two types
igneous rocks form when the magma cools and crystallizes on the
surface of the Earth
igneous rocks wherein the magma crystallizes at depth in the Earth.
is a mixture of liquid rock, crystals, and gas. Characterized by a wide range of
chemical compositions, with high temperature, and
properties of a liquid.
Magmas are less dense than surrounding rocks, and will therefore move upward. If magma
makes it to the surface it will erupt and later crystallize to form an
If it crystallizes before it reaches the surface it will form an igneous rock at depth called a
intrusive igneous rock
Types of Magma
Chemical composition of magma is controlled by the abundance of elements in the Earth. Si,
Al, Fe, Ca, Mg, K, Na, H, and O make up 99.9%. Since oxygen is so abundant, chemical
analyses are usually given in terms of oxides. SiO
is the most abundant oxide.
1. Mafic or Basaltic
45-55 wt%, high in Fe, Mg, Ca, low in K, Na
2. Intermediate or Andesitic
55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K
3. Felsic or Rhyolitic
65-75%, low in Fe, Mg, Ca, high in K, Na.
At depth in the Earth nearly all magmas contain gas.
Gas gives magmas their
explosive character, because the gas expands as pressure is reduced.
O with some CO
Minor amounts of Sulfur, Cl , and F
Felsic magmas usually have higher gas contents than mafic magmas.
Temperature of Magmas
Mafic/Basaltic - 1000-1200
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