Magma

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 (molten rock 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 hotspot. Magma's formation only takes place under specific conditions in the Earth's asthenosphere.

Melting of solid rock

In order to produce a magma, it is first necessary to melt a formerly solid rock.

Melting of the solid earth is controlled by three physical conditions; temperature, pressure and composition of the rock which is undergoing melting.

Temperature

At any given pressure and for any given composition of rock, a rise in temperature past the solidus will cause melting. Within the solid earth, the temperature of a rock is controlled by the geothermal gradient and the radioactive decay within the rock.

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.

Pressure

Melting can also occur when a rock of a given temperature and composition rises through the solid earth. This causes melting because on for each compositional possibility of rock at a constant temperature the solidus position will change due to the influence of pressure.

A sudden decrease in pressure can cause what is known as decompression melting. This may occur due to tectonic adjustments or from the rise of a volume of rock to a shallow depth in the Earth's crust.

Composition

It is usually very difficult to change the bulk composition of a large mass of rock, so composition is the basic control on whether a rock will melt at any given temperature and pressure. The composition of a rock may also be considered to include volatile phases such as water and carbon dioxide.

The presence of volatile phases in a rock acts similar to a solvent, assisting in the break-down of the strong silicate bonds in the minerals of a rock. This is a very important process for generating melts, as the presence of even 1% water may reduce the temperature of melting by as much as 100°C. Conversely, the loss of water and volatiles from a magma may cause it to essentially freeze or soldify.

Partial melting

When rocks melt they do so incrementally and gradually. It is very rare to find a rock which has melted completely because, firstly it is difficult to focus energy into a rock mass so efficiently, and secondly, rocks which melt to a certain degree become mobile.

This is because as a rock melts, its volume changes. When enough rock is melted, the small globules of melted rck (generally occurring in between mineral grains) link up and soften the rock. Under the great confining pressures deep within the Earth's mantle, as little as 5% partial melting may be sufficient to cause this melted rock to be squeezed from its source.

Melts can stay in place long enough to melt to 20% or even 35%, but are rarely melted in excess of 50%, because eventually the melted rock mass becomes a crystal and melt mush typically known as magma. This crystal-liquid mush can then ascend en masse as a diapir, which may then cause further decompression melting. This is considered one possible mechanism for generating mantle plumes and large igneous provinces.

Primary melts

When a rock melts it melts to form a liquid, the liquid is known as a primary melt. Primary melts have not undergone any differentiation and represent the starting composition of a magma. In nature it is rare to find primary melts. The leucosomes of migmatites are examples of primary melts. Primary melts derived from the mantle are especially important, and are known as primitive melts or primitive magmas. By finding the primitive magma composition of a magma series it is possible to model the composition of the mantle from which a melt was formed, which is important because science has little direct evidence of the mantle.

Parental melts

Where it is impossible to find the primitive or primary magma composition, it is often useful to attempt to identify a parental melt. A parental melt is a magma composition from which the observed range of magma chemistries has been derived by the processes of igneous differentiation. It need not be a primitive melt.

For instance, a series of basalt flows are assumed to be related to one another. A composition from which they could reasonably be produced by fractional crystallization is termed a parental melt. To prove this, fractional crystallization models would be produced to test the hypothesis that they share a common parental melt.

Geochemical implications of partial melting

The degree of partial melting is critical for determining what type of magma is produced. This is because melting begins with the minerals which are least stable under the melting conditions. These minerals, in turn, expel from their matrix those elements which are least stable. These are known as the incompatible elements potassium, barium, caesium, rubidium.

Next to enter the melt are some transition elements, aluminium, calcium, sodium, magnesium and some iron. The residual part of the melted rock is usually enriched in compatible elements such as zirconium, hafnium, titanium, etcetera.

Thus, the degree of partial melting required to form a melt can be estimated by considering the relative enrichment of incompatible elements versus compatible elements.

Rock types produced by small degrees of partial melting, which usually occurs at great depths in the mantle, are typically alkaline (Ca, Na), potassic (K) and/or peralkaline (high aluminium to silica ratio). Typically, primitive melts of this composition form lamprophyre, lamproite, kimberlite and sometimes nepheline-bearing mafic rocks such as alkali basalts and essexite gabbros or even carbonatite.

Small degrees of partial melting of the continental crust produce alkaline silica-undersaturated rocks such as nepheline syenite, foidolite, phonolite, tephrite and rocks of the melilite association.

Rocks produced by moderate degrees of partial melting of the mantle include calc-alkaline basalts, picrites, some forms of lamprophyre and other potassic rocks such as ankaramite, shoshonite, etcetera, trending toward tholeiite basalt at higher degrees of partial melting.

Moderate degrees of partial melting of the crust produces S-type and I-type granite in all its forms, including volcanic equivalents such as andesite, rhyolite, dacite and so forth.

At high degrees of partial melting of the mantle, komatiite, boninite and picrite are produced. At high degrees of partial melting of the crust, granitoids such as tonalite, granodiorite and diorite to monzonite can be produced, although this is usually uncommon.

Tectonic environments of magma generation

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

There are three basic types of magma: mafic, andesitic (or intermediate), and felsic. Magma is composed mainly of silica; alkalis (sodium, potassium, calcium, 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.

The composition of magma will change depending on the make-up of the rocks that it melts as it penetrates the Earth's crust to erupt in the form of lava. Fractional crystallization, contamination and magma mixing are some of the processes by which a primary melt can change composition enroute to the surface.

Characteristics of different magmas are as follows:

Ultramafic ( picritic)

SiO₂ < 45%

Fe-Mg >8% up to 32%MgO

Temperature: up to 1500°C

Viscosity: Low to Very Low

Eruptive behaviour: gentle

Distribution: divergent plate boundaries, hot spots, convergent plate boundaries; picrites and boninites are typically found in back arc areas where oceanic crust is being melted in a water saturated environment; ancient komatiite and other ultramafic lavas were formed from a higher geothermal gradient and are unknown now.

Mafic ( basaltic)

SiO₂ < 50%

Fe-Mg ~ 4%

Temperature: up to ~1200°C

Viscosity: Low

Eruptive behaviour: gentle

Distribution: divergent plate boundaries, hot spots, convergent plate boundaries; 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 behaviour: explosive

Distribution: convergent plate boundaries

Felsic (rhyolitic)

SiO₂ >70%

Fe-Mg: ~ 2%

Temp: 700°C

Viscosity: High

Eruptive behaviour: 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 behaviour.