magmatic activity
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Magmatism is the emplacement of
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
within and at the surface of the outer layers of a
terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, ...
, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and
extrusion Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex ...
of
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
or
lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...
.
Volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...
is the surface expression of magmatism. Magmatism is one of the main processes responsible for
mountain formation Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the Earth's crust (List of tectonic plates, tectonic plates). Fold (geology), Folding, ...
. The nature of magmatism depends on the tectonic setting. For example,
andesitic Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomin ...
magmatism is associated with the formation of
island arc Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle alon ...
s at convergent plate boundaries while
basaltic Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
magmatism is found at
mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diver ...
s during sea-floor spreading at divergent plate boundaries. On Earth, magma forms by
partial melting Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the soli ...
of silicate rocks either in the mantle,
continental Continental may refer to: Places * Continent, the major landmasses of Earth * Continental, Arizona, a small community in Pima County, Arizona, US * Continental, Ohio, a small town in Putnam County, US Arts and entertainment * ''Continental'' ( ...
or
oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic ...
. Evidence for magmatic activity is usually found in the form of igneous rocks formed from magma.


Convergent boundaries

Magmatism is associated with all stages of the development of convergent plate boundaries, from the initiation of subduction through to continental collision and its immediate aftermath.


Subduction-related

The subduction of oceanic crust, whether beneath oceanic or continental crust, is associated in almost all cases with
partial melting Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the soli ...
of the overlying asthenosphere due to the addition of volatiles (especially water) expelled from the downgoing slab. Only when the slab fails to reach sufficient depth as in the earliest stages of subduction or where there are periods of flat-slab subduction that completely pinch out the asthenosphere, is magmatism absent. The magmatism is mostly
calc-alkaline The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic m ...
in type along a well-defined curvilinear magmatic arc. Only the volcanic parts of modern arcs are exposed at the surface and the understanding of the underlying
magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it up ...
s relies on geophysical methods. Ancient arc sequences that formed on continental crust or that have become accreted to continental crust are often deeply eroded and the plutonic equivalents of the arc volcanoes become exposed.


Collision-related

Continental collisions are accompanied by major crustal thickening, leading to heating and
anatexis Anatexis (via Latin from Greek roots meaning "to melt down") is the partial melting of rocks. Traditionally, anatexis is used specifically to discuss the partial melting of crustal rocks, while the generic term "partial melting" refers to the par ...
within the crust, generally in the form of
peraluminous Peraluminous rocks are igneous rocks that have a molecular proportion of aluminium oxide higher than the combination of sodium oxide, potassium oxide and calcium oxide.Blatt, Harvey and Robert J. Tracy, ''Petrology,'' Freeman, 2nd ed., 1995, p. ...
granitic intrusions.


Post-collision

Post-collisional magmatism is a result of decompression melting associated with
isostatic rebound Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound ...
and possible extensional collapse of the thickened crust formed during the collision.
Slab detachment In plate tectonics, slab detachment or slab break-off may occur during continent-continent or arc-continent collisions. When the continental margin of the subducting plate reaches the oceanic trench of the subduction zone, the more buoyant contin ...
has also been proposed as a cause of late to post-collisional magmatism.


Divergent boundaries

The new crust that is formed at divergent boundaries within oceanic crust is almost entirely magmatic in origin.


Mid-ocean ridges

Mid-ocean ridge spreading centres are the sites of almost continuous magmatism. The basalts erupted at mid-ocean ridges are
tholeiitic The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma ...
in character and result from the partial melting of upwelling asthenosphere. The composition of Mid-Ocean Ridge Basalts (MORB) shows little variation globally as they come from a mostly homogeneous source.


Back-arc basins

Back-arc extension often leads to the formation of oceanic crust and relatively short-lived spreading centres. As the asthenosphere behind the arc has been partly affected by volatiles from the downgoing slab, the typical back-arc basin basalts are intermediate in character between MORB type basalts and Island Arc Basalts (IAB) type basalts.


Intraplate

Magmatic activity away from plate boundaries forms an important part of the magmatism on earth, including the largest magmatic events known, Large Igneous Provinces.


Hotspots

Hotspots Hotspot, Hot Spot or Hot spot may refer to: Places * Hot Spot, Kentucky, a community in the United States Arts, entertainment, and media Fictional entities * Hot Spot (comics), a name for the DC Comics character Isaiah Crockett * Hot Spot (Tr ...
are sites of upwelling of relatively hot mantle, possibly associated with
mantle plume A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hot ...
s, that cause partial melting of the asthenosphere. This type of magmatism forms volcanic seamounts or oceanic islands when they become emergent. Over short geological timescales the hotspots appear to be fixed relative to one another, forming a reference frame against which plate motions can be measured. As
tectonic plates Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
move relative to a hotspot, the location of magmatic activity on the plate shifts, causing the development of time-progressive chains of volcanoes such as the
Hawaiian–Emperor seamount chain The Hawaiian–Emperor seamount chain is a mostly undersea mountain range in the Pacific Ocean that reaches above sea level in Hawaii. It is composed of the Hawaiian ridge, consisting of the islands of the Hawaiian chain northwest to Kure Atoll, ...
. The main product of hotspot volcanoes are Ocean Island Basalts (OIB), which are distinct from MORB and IAB type basalts. Where hotspots are developed beneath the continents the products are different, as the mantle-derived magmas cause melting of the continental crust, forming granitic magmas that reach the surface as rhyolites. The
Yellowstone hotspot The Yellowstone hotspot is a volcanic hotspot in the United States responsible for large scale volcanism in Idaho, Montana, Nevada, Oregon, and Wyoming, formed as the North American tectonic plate moved over it. It formed the eastern Snake Riv ...
is an example of continental hotspot magmatism, which also displays time-progressive shifts in magmatic activity.


Rifts

Many continental rift zones are associated with magmatism due to upwelling of the asthenosphere as the lithosphere is thinned, which leads to decompression melting. The magmatism is often bimodal in character as the mantle-derived basaltic magmas cause partial melting of the continental crust.


Large Igneous Provinces

Large Igneous Provinces (LIPs) are defined as "mainly mafic (+ ultramafic) magmatic provinces with an areal extent >0.1 Mkm2 and igneous volume >0.1Mkm3, that have intraplate characteristics, and are emplaced in a short duration pulse or multiple pulses (less than 1–5 Ma) with a maximum duration of


Intruded v. extruded magma

The relative volumes of extruded versus intruded magmas has been estimated for the various tectonic settings during the Cenozoic. Overall the global total for volcanism is in the range 3.7–4.1 km3, compared to 22.1–29.5 km3 for intrusions.


References

{{Reflist Geological processes Igneous rocks