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Magma

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Magma is molten rock located beneath the surface of the Earth (or any
   other terrestrial planet), and which often collects in a magma chamber.
   Magma may contain suspended crystals and gas bubbles. By definition,
   all igneous rock is formed from magma.
   Hawaiian underwater magma flow
   Enlarge
   Hawaiian underwater magma flow

   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.

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 30°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

   Melting can also occur when a rock rises through the solid earth 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 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; 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
   (e.g., Watson and others, 2006)

   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)

                SiO[2] < 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; komatiite and other
                ultramafic lavas are mostly Archean were formed from a
                higher geothermal gradient and are unknown or at least
                very rare now.

          Mafic ( basaltic)

                SiO[2] < 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)

                SiO[2] ~ 60%
                Fe-Mg: ~ 3%
                Temperature: ~1000°C
                Viscosity: Intermediate
                Eruptive behaviour: explosive
                Distribution: convergent plate boundaries

          Felsic (rhyolitic)

                SiO[2] >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

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