Magma

Magma (Plurals: magmas and magmata) is molten rock that sometimes forms beneath the surface of the earth (or any other terrestrial planet) that often collects in a magma chamber. Magma may contain suspended crystals and gas bubbles. By definition, all igneous rock is formed from magma.

Magma is a complex high-temperature fluid substance. Temperatures of most magmas are in the range 700°C to 1300°C, but very rare carbonatite melts may be as cool as 600°C, and komatiite melts may have been as hot at 1600°C. Most are silicate solutions.

It is capable of intrusion into adjacent rocks or of extrusion onto the surface as lava or ejected explosively as tephra to form pyroclastic rock.

Environments of magma formation and compositions are commonly correlated. Environments include subduction zones, continental rift zones, mid-oceanic ridges, and hotspots, some of which are interpreted as mantle plumes. Environments are discussed in the entry on igneous rock. Magma compositions may evolve after formation by fractional crystallization, contamination, and magma mixing.

Contrary to some impressions, the bulk of the Earth's crust and mantle is not molten. Rather, the bulk of the Earth takes the form of a rheid, a form of solid that can move or deform under pressure. Magma, as liquid, preferentally forms in high temperature, low pressure environments within several kilometers of the Earth's surface.

Melting of solid rock

Melting of solid rock to form magma is controlled by three physical parameters: its temperature, pressure, and composition. Mechanisms are discussed in the entry for igneous rock.

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 30-80°C/km under mid-ocean ridges and volcanic arc environments.

Pressure

As magma buoyantly rises it will cross the solidus-liquidus and its temperature will reduce by adiabatic cooling. At this point it will liquify and if erupted onto the surface will form lava. Melting can also occur due to a reduction in pressure by a process known as decompression melting.

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 under pressure can stabilize a melt fraction. The presence of even 0.8% 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 solidify.

Partial melting

When rocks melt they do so incrementally and gradually; most rocks are made of several minerals, all of which have different melting points, and the phase diagrams that control melting commonly are complex. As a rock melts, its volume changes. When enough rock is melted, the small globules of melt (generally occurring in between mineral grains) link up and soften the rock. Under pressure within the earth, as little as a fraction of a percent partial melting may be sufficient to cause melt to be squeezed from its source.

Melts can stay in place long enough to melt to 20% or even 35%, but rocks are rarely melted in excess of 50%, because eventually the melted rock mass becomes a crystal and melt mush that can then ascend en masse as a diapir, which may then cause further decompression melting.

Primary melts

When a rock melts, 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 in understanding evolution 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. 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. The degree of partial melting required to form a melt can be estimated by considering the relative enrichment of incompatible elements versus compatible elements. Incompatible elements commonly include potassium, barium, caesium, rubidium.

Rock types produced by small degrees of partial melting in the Earth's 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.

Pegmatite may be produced by low degrees of partial melting of the crust. Some granite-composition magmas are eutectic (or cotectic) melts, and they may be produced by low to high degrees of partial melting of the crust, as well as by fractional crystallization. At high degrees of partial melting of the crust, granitoids such as tonalite, granodiorite and monzonite can be produced, but other mechanisms are typically important in producing them.

At high degrees of partial melting of the mantle, komatiite and picrite are produced.

Composition and melt structure and properties

Silicate melts are composed mainly of silicon, oxygen, aluminium, alkalis (sodium, potassium, calcium), magnesium and iron. Silicon atoms are in tetrahedral coordination with oxygen, as in almost all silicate minerals, but in melts atomic order is preserved only over short distances. The physical behaviours of melts depend upon their atomic structures as well as upon temperature and pressure and composition.

Viscosity is a key melt property in understanding the behaviour of magmas. More silica-rich melts are typically more polymerized, with more linkage of silica tetrahedra, and so are more viscous. Dissolution of water drastically reduces melt viscosity. Higher-temperature melts are less viscous.

Generally speaking, more mafic magmas, such as those that form basalt, are hotter and less viscous than more silica-rich magmas, such as those that form rhyolite. Low viscosity leads to gentler, less explosive eruptions.

Characteristics of several different magma types are as follows:

Ultramafic ( picritic)

SiO₂ < 45%

Fe-Mg >8% up to 32%MgO

Temperature: up to 1500°C

Viscosity: Very Low

Eruptive behaviour: gentle or very explosive (kimberilites)

Distribution: divergent plate boundaries, hot spots, convergent plate boundaries; komatiite and other ultramafic lavas are mostly Archean and were formed from a higher geothermal gradient and are unknown in the present

Mafic (basaltic)

SiO₂ < 50%

FeO and MgO typically < 10 wt%

Temperature: up to ~1300°C

Viscosity: Low

Eruptive behaviour: gentle

Distribution: divergent plate boundaries, hot spots, convergent plate boundaries

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: < 900°C

Viscosity: High

Eruptive behaviour: explosive

Distribution: hot spots in continental crust (Yellowstone National Park), continental rifts, island arcs