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This page last updated on 23-Jan-
Magmas and Igneous Rocks
Magma and Igneous Rocks
Igneous Rocks are formed by crystallization from a liquid, or magma. They include two types z Volcanic or extrusive igneous rocks form when the magma cools and crystallizes on the surface of the Earth
z Intrusive or plutonic igneous rocks wherein the magma crystallizes at depth in the Earth.
Magma 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 extrusive or volcanic rock. If it crystallizes before it reaches the surface it will form an igneous rock at depth called a plutonic or 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 2 is the most abundant oxide.
1. Mafic or Basaltic -- SiO 2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na 2. Intermediate or Andesitic -- SiO 2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K 3. Felsic or Rhyolitic -- SiO 2 65-75%, low in Fe, Mg, Ca, high in K, Na.
Gases - At depth in the Earth nearly all magmas contain gas. Gas gives magmas their explosive character, because the gas expands as pressure is reduced. z Mostly H 2 O with some CO 2 z Minor amounts of Sulfur, Cl , and F z Felsic magmas usually have higher gas contents than mafic magmas.
Temperature of Magmas
z Mafic/Basaltic - 1000-1200 oC z Intermediate/Andesitic - 800-1000oC z Felsic/Rhyolitic - 650-800oC.
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Viscosity of Magmas
Viscosity is the resistance to flow (opposite of fluidity). Depends on composition, temperature, & gas content.
z Higher SiO 2 content magmas have higher viscosity than lower SiO 2 content magmas z Lower Temperature magmas have higher viscosity than higher temperature magmas.
Summary Table
Magma Type
Solidified Volcanic Rock
Solidified Plutonic Rock
Chemical Composition
Temperature Viscosity Gas Content
Mafic or Basaltic
Basalt Gabbro
45-55 SiO 2 %, high in Fe, Mg, Ca, low in K, Na
1000 - 1200 o^ C Low^ Low
Intermediate or Andesitic
Andesite Diorite
55-65 SiO 2 %, intermediate in Fe, Mg, Ca, Na, K
800 - 1000 oC Intermediate Intermediate
Felsic or Rhyolitic
Rhyolite Granite
65-75 SiO 2 %, low in Fe, Mg, Ca, high in K, Na
650 - 800 o^ C High^ High
Origin of Magma
As we have seen the only part of the earth that is liquid is the outer core. But the core is not likely to be the source of magmas because it does not have the right chemical composition. The outer core is mostly Iron, but magmas are silicate liquids. Thus magmas DO NOT COME FROM THE MOLTEN OUTER CORE OF THE EARTH. Thus, since the rest of the earth is solid, in order for magmas to form, some part of the earth must get hot enough to melt the rocks present. We know that temperature increases with depth in the earth along the geothermal gradient. The earth is hot inside due to heat left over from the original accretion process, due to heat released by sinking of materials to form the core, and due to heat released by the decay of radioactive elements in the earth. Under normal conditions, the geothermal gradient is not high enough to melt rocks, and thus with the exception of the outer core, most of the Earth is solid. Thus, magmas form only under special circumstances. To understand this we must first look at how rocks and mineral melt.
As pressure increases in the Earth, the melting temperature changes as well. For pure minerals, there are two general cases.
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z Melting of rocks containing water or carbon dioxide is similar to melting of wet minerals, melting temperatures initially decrease with increasing pressure, except there is a range of temperature over which there exists a partial melt.
Three ways to Generate Magmas
From the above we can conclude that in order to generate a magma in the solid part of the earth either the geothermal gradient must be raised in some way or the melting temperature of the rocks must be lowered in some way. The geothermal gradient can be raised by upwelling of hot material from below either by uprise solid material (decompression melting) or by intrusion of magma (heat transfer). Lowering the melting temperature can be achieved by adding water or Carbon Dioxide (flux melting).
Decompression Melting - Under normal conditions the temperature in the Earth, shown by the geothermal gradient, is lower than the beginning of melting of the mantle. Thus in order for the mantle to melt there has to be a mechanism to raise the geothermal gradient. Once such mechanism is convection, wherein hot mantle material rises to lower pressure or depth, carrying its heat with it.
If the raised geothermal gradient becomes higher than the initial melting temperature at any pressure, then a partial melt will form. Liquid from this partial melt can be separated from the remaining crystals because, in general, liquids have a lower density than solids. Basaltic magmas appear to originate in this way.
Upwelling mantle appears to occur beneath oceanic ridges, at hot spots, and beneath continental rift valleys. Thus, generation of magma in these three environments is likely caused by decompression melting.
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Transfer of Heat - When magmas that were generated by some other mechanism intrude into cold crust, they bring with them heat. Upon solidification they lose this heat and transfer it to the surrounding crust. Repeated intrusions can transfer enough heat to increase the local geothermal gradient and cause melting of the surrounding rock to generate new magmas.
Transfer of heat by this mechanism may be responsible for generating some magmas in continental rift valleys, hot spots, and subduction related environments.
Flux Melting - As we saw above, if water or carbon dioxide are added to rock, the melting temperature is lowered. If the addition of water or carbon dioxide takes place deep in the earth where the temperature is already high, the lowering of melting temperature could cause the rock to partially melt to generate magma. One place where water could be introduced is at subduction zones. Here, water present in the pore spaces of the subducting sea floor or water present in minerals like hornblende, biotite, or clay minerals would be released by the rising temperature and then move in to the overlying mantle. Introduction of this water in the mantle would then lower the melting temperature of the mantle to generate partial melts, which could then separate from the solid mantle and rise toward the surface.
Chemical Variability of Magmas
The chemical composition of magma can vary depending on the rock that initially melts (the source rock), and process that occur during partial melting and transport.
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