Magma: Definition, Composition, Types and Formation of Magma

Magma is a mixture of molten or semi-molten rock, volatiles and solids. Besides molten rock it may contain suspended crystals and dissolved gases. The two most abundant elements in earth’s crust and mantle are oxygen and silicon which combine to make Silica i.e SiO₂.

Types of Magma

The classification of the Magmas is done primarily on the basis of Silica content. On this basis there are four types of Magmas as mentioned below:

From the above table we may note down the following observations:

• Increasing silica content is the basis of classifying the Magma from Picritic to Felsic.
• Increasing Silica content implies a lower temperature of the Magma.
• Increasing silica content implies an explosive eruption behaviour of Magma
• Increasing silica content implies an increasing viscosity of Magma.

Magma often collects in magma chambers that may feed a volcano or turn into a pluton. Magma is capable of intrusion into adjacent rocks, giving rise to Sills and Dikes, and extrusion onto the surface as lava, and explosive ejection as Tephra to form pyroclastic rocks. The Tephra is all the volcanic material such as Ash, Plumes, Volcanic Bombs, Volcanic Blocks, lapilli etc.

Gases in Magma

The gases are dissolved in magma at high pressure beneath the layers. The gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface of the Earth; very much similar to the carbonated beverages which are bottled at high pressure. Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced.  The composition of the gases in magma is:

• Mostly H₂O (water vapour) & some CO₂ (carbon dioxide)
• Minor amounts of Sulphur, Chlorine, and Fluorine gases

The amount of gas in magma is also related to the chemical composition of the magma. Rhyolitic  magmas usually have higher gas contents than basaltic  magmas. That is also the reason that the Rhyolitic Magma is more explosive than the Basaltic Magma.

Formation of Magma

Outer core is the ONLY part of earth which is liquid, but outer core is NOT the source of Magma, because it does not have the right chemical composition.  For instance, the outer core is mostly Iron, but magmas are silicate liquids. Magma originates in the lower part of the Earth’s crust and in the upper portion of the mantle. There, high temperatures and pressure cause some rocks to melt and form magma.

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. Then, magma does not occur everywhere below us. There are only some specific places where volcanoes exist. This means that Magma is formed under some special conditions, which exist in some limited area.

Another point is that in the ocean basins, magmas are not likely to come from melting of the oceanic crust, since most magmas erupted in the ocean basins are basaltic.  To produce basaltic magmas by melting of the basaltic oceanic crust would require nearly 100% melting, which can not happen. In the continents, both basaltic and rhyolitic magmas are erupted and intruded. Basaltic magmas are not likely to have come from the continental crust, since the average composition is more siliceous, but more siliceous magmas (andesitic – rhyolitic) could come from melting of the continental crust.  Basaltic magmas must come from the underlying mantle. Thus, with the exception of the continents, magmas are most likely to originate in the mantle from melting of mantle peridotite, (a rock made up of olivine, pyroxene, and garnet) — evidence comes from pieces brought up by erupting volcanoes.

Does Geothermal Gradient causes melting of Rocks?

Temperature increases with depth or pressure in the Earth along the geothermal gradient.  The normal geothermal gradient is somewhat higher beneath the oceans than beneath the continents, at least at shallow levels.  But when we observe the normal geothermal gradients, we find that under the normal conditions, the geothermal gradient is not high enough to melt rocks, that is why that with the exception of the outer core, most of the Earth is solid. Thus, the geothermal gradient is not a very substantial factor contributing in the formation of the Magma.

Does Radioactive Heat Cause Melting of Rocks?

The radioactive elements such as Uranium, Thorium etc, keep decaying below. During radioactive decay, sub-atomic particles are released by the decaying isotope and move outward until they collide with other atomic particles.  Upon collision, the kinetic energy of the moving particles is converted to heat. If this heat cannot be conducted away, then the temperature will rise.  Most of the heat within the Earth is generated by radioactive decay, and this is the general reason why temperature increases with depth in the Earth.  But again this is not enough to prove the melting of the rocks. We should know that most the radioactive isotopes are concentrated in the crust.  Although there are areas in the continental crust where high concentrations of radioactive elements have locally raised the temperature, at least high enough to cause metamorphism, but it is more unlikely that areas of high concentration develop within the mantle.  Thus, concentrations of radioactive elements are not likely to cause melting.

Does decrease in Pressure cause rock melt?

There are two things. First is that very high pressures in mantle rocks prevent atoms within minerals from breaking chemical bonds and moving freely from one another to form magma. Therefore, most rocks within the mantle do not melt even though their temperature may be greater than that necessary to melt the same rocks at the lower pressures of the Earth’s surface. However, if something occurs that the pressure on mantle rock is decreased; the atoms may move freely from one another. This would result in the partial melting of the already very hot solid rock. This process is called pressure-release melting. It is a scientifically proved theory and is found to be common along divergent plate margins, and within mantle plumes.

Does addition of Water causes melting?

The addition of small amounts water to peridotite will result in a decrease in its melting temperature. This is largely due to the electrically polarized nature of a water molecule, as there is an unequal distribution of electrons around the water molecule. The electrical polarization causes a decrease in cation-anion bond strengths within minerals, and so at very high temperatures the bonds may be broken so that atoms may move freely from one another to form a magma. This process also results in partial melting of the mantle rock. This type of melting occurs within subduction zones as water is ‘squeezed’ from the subducted oceanic lithosphere into the overlying ultramafic mantle wedge.

How Magma is finally formed?

The initial composition of the magma depends upon the composition of the source rock and the degree of partial melting.   In general, melting of a mantle source (garnet peridotite) results in mafic/basaltic magmas, while melting of crustal sources yields more siliceous magmas. In general more siliceous magmas form by low degrees of partial melting. As the degree of partial melting increases, less siliceous compositions can be generated. So, melting a mafic source thus yields a felsic or intermediate magma. Melting of ultramafic (peridotite source) yields a basaltic magma. Then, the transportation toward the surface or during storage in the crust can alter the chemical composition of the magma.  This is called magmatic differentiation and includes some processes such as assimilation, mixing, and fractional crystallization.