Magma

This article is about the type of molten rock. For other meanings of magma, see Magma (disambiguation).

Magma is molten rock often located inside a magma chamber beneath the surface of the Earth. Magma is a complex high-temperature silicate solution that is ancestral to all igneous rocks. It is capable of intrusion into adjacent crustal rocks or extrusion onto the surface. Magma exists between 650 and 1200 °C. Magma is under high pressure and sometimes emerges through volcanic vents in the form of flowing lava (melt as it exists above the Earth's surface) and pyroclastic ejecta. These products of a volcanic eruption usually contain liquids, crystals and dissolved gases which have never before reached the planet's surface. Magma collects in many separate magma chambers within the Earth's crust, and will have slightly different compositions in different places, which can occur at either a subduction zone, a rift zone or mid-oceanic ridge, or above a mantle plume hot spot. Magma's formation only takes place under specific conditions in the Earth's asthenosphere.

Formation

A sudden decrease in pressure can cause what is known as decompression melting. This may occur due to tectonic adjustments or as molten rock displaces and fractures adjacent rock during its rise to a shallow depth in the Earth's crust. The geothermal gradient averages about 25°C/km with a wide range from a low of 5-10°C/km within oceanic trenches and subduction zones to 30-50°C/km under mid-ocean ridges and volcanic arc environments. A combination of high temperature and low pressure near surface environments are most conducive to melting due to pressure reduction.

Magma can also be formed due to the addition of volatiles to heated rock. Volatiles (water and gases) are released from a descending slab of oceanic crust as it is subducted, these volatiles move into the overlying crustal material and initiate melting. Volatiles can break up the mineral bonds within the melting rock and cause its melting point to decrease, allowing for magma formation.

Magma formation also results due to the melting of crustal rock by pre-existing magma whose temperature is so great that it melts the crust as it rises, creating even more magma.

Magma rises primarily because a melt is less dense than its source rock, it is propelled upward through the lithosphere by the buoyancy that its lower density creates (the way less dense wood is pushed up and floats in denser water). This results in the formation of magma chambers and eventually volcanoes, magma being pushed all the way to the Earth's surface results in a volcanic eruption.

Composition

The composition of magma will change depending on the make-up of the overlying rocks that it melts as it penetrates the Earth's crust to erupt in the form of lava. There are three basic types of magma: mafic, andesitic (or intermediate), and felsic. Magma is composed mainly of silica; alkalis (sodium, potassium, and calcium); and magnesium and iron. Generally speaking, the more mafic the magma is, the gentler the eruption will be. This is because high levels of silica cause volatiles to build-up and can create an explosive eruption which is seen in composite volcanoes.

Characteristics of different magmas are as follows:

Mafic (basaltic)

SiO₂ < 50%

Fe-Mg ~ 4%

Temperature: up to 1500°C

Viscosity: Low

Eruptive behavior: gentle

Distribution: divergent plate boundries, hot spots, convergent plate boundries; basaltic magma is typically found in areas where oceanic crust is being melted, oceanic crust contains high levels of iron.

Intermediate (andesitic)

SiO₂ ~ 60%

Fe-Mg: ~ 3%

Temperature: ~1000°C

Viscosity: Intermediate

Eruptive behavior: explosive

Distribution: convergent plate boundaries

Felsic (rhyolitic)

SiO₂ >70%

Fe-Mg: ~ 2%

Temp: 700°C

Viscosity: High

Eruptive behavior: explosive

Distribution: hot spots in continental crust (Yellowstone National Park); this type of magma occurs mainly where continental crust, which contains large amounts of silica, is being melted, causing the explosive behavior.