Underplating
Encyclopedia
Underplating is the accumulaton of partial melts at the base the crust
where an ocean plate is subducting under continental crust
. Underplating is the result of partial melts being produced in the mantle wedge
above a subducting plate. The partial melting is induced by a lowering of the solidus
by the input of water and other volatiles supplied by phase changes in the subducting slab
. When the partial melt rises upwards through the mantle
, it will usually stall at the base of the crust
and pond there. This is because the crust is usually less dense than the underplating magma
, and this is the point at which the ascending magma reaches a level of neutral buoyancy.
The melt will remain here until it fractionates/evolves enough (through MASH processes) so that the remaining melt is less dense than the surrounding rock and the melt will then continue up into the crust, leaving behind the heavier mafic
minerals which were crystallized during fractional crystallization
. The assemblage of minerals remaining behind are typically mafic or ultramafic, and are responsible for the observed seismic anomaly which indicates underplated material.
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
where an ocean plate is subducting under continental crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
. Underplating is the result of partial melts being produced in the mantle wedge
Mantle wedge
A mantle wedge is a zone of mantle, triangular in cross–section, that lies between subducting and overriding tectonic plates. Flow inside the wedge is aligned with plate motions. Flux melting of material within wedge due to the release of volatiles from the underlying subducting slab results in...
above a subducting plate. The partial melting is induced by a lowering of the solidus
Solidus (chemistry)
In chemistry, materials science, and physics, the solidus is the locus of temperatures below which a given substance is completely solid...
by the input of water and other volatiles supplied by phase changes in the subducting slab
Slab (geology)
In geology, a slab is the portion of a tectonic plate that is being subducted.Slabs constitute an important part of the global plate tectonic system. They drive plate tectonics both by pulling along the lithosphere to which they are attached in a processes known as slab pull and by inciting...
. When the partial melt rises upwards through the mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
, it will usually stall at the base of the crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
and pond there. This is because the crust is usually less dense than the underplating magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
, and this is the point at which the ascending magma reaches a level of neutral buoyancy.
The melt will remain here until it fractionates/evolves enough (through MASH processes) so that the remaining melt is less dense than the surrounding rock and the melt will then continue up into the crust, leaving behind the heavier mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
minerals which were crystallized during fractional crystallization
Fractional crystallization (geology)
Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. Fractional crystallization is the removal and segregation from a melt of mineral precipitates; except in special cases, removal of the crystals changes the...
. The assemblage of minerals remaining behind are typically mafic or ultramafic, and are responsible for the observed seismic anomaly which indicates underplated material.