Tectonics and the Formation of Magmas

Island arc
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These are called plutonic rocks. Igneous rocks are found where plates diverge, as lava rises and fills the gap between the plates.

Plate Tectonics and Volcanism – Physical Geology

Igneous rocks also form where plates converge. The subducting plate melts as it sinks into the crust of the Earth, and the melt rises into the overriding plate forming volcanoes. Metamorphic rocks are formed mainly in the lithosphere, wherever there is high pressure and high temperature.

If the pressure and temperature are too high, metamorphic rock will melt and become magma. Sedimentary rocks form only on the surface of the Earth. Sedimentary rocks form in two main ways: 1 from clastic material pieces of other rocks or fragments of skeletons become cemented together, and 2 by chemical mechanisms including precipitation and evaporation.

Anywhere there is a rising convection current, hotter material at depth will rise carrying its heat with it. As it rises to lower pressure decompression it will cool somewhat, but will still have a temperature higher than its surroundings.

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Thus, decompression will result in raising the local geothermal gradient. If this new geothermal gradient reaches temperatures greater than the peridotite solidus, partial melting and the generation of magma can occur.

This mechanism is referred to as decompression melting. As we saw in our discussion of phase diagrams, mixtures of components begin melting at a lower temperature than the pure components. In a two component system addition of a third component reduces both the solidus and liquidus temperatures.

This suggests that if something can be added to the mantle, it could cause the solidus and liquidus temperatures to be lowered to the extent that the solidus could become lower than the geothermal gradient and result in partial melting, without having to raise the geothermal gradient. Such a melting mechanism is referred to as flux melting. It's difficult to imagine how solid components could be added to the mantle. But volatile components, for example H 2 O and CO 2 , because of their high mobility, could be added to the mantle, particularly at subduction zones. Oceanic crust is in contact with sea water, thus water could be in oceanic crust both due to weathering, which produces hydrous minerals like clay minerals, and could be in the pore spaces in the rock.

Oceanic sediments eventually cover the basaltic oceanic crust produced at oceanic ridges. Much of this sediment consists of clay minerals which contain water and carbonate minerals which contain carbon dioxide. As the oceanic lithosphere descends into the mantle at a subduction zone, it will be taken to increasingly higher temperatures as it gets deeper. This will result in metamorphism of both the basalt and the sediment.

As we will see later in our discussion of metamorphism, metamorphism is essentially a series of dehydration and decarbonation reactions, i. Addition of this fluid phase, either to the subducted lithosphere or the mantle overlying the subducted lithosphere could lower the solidus and liquidus temperatures enough to cause partial melting.

Crustal Anatexis In the continental crust, it is not expected that the normal geothermal gradient will be high enough to cause melting despite the fact that hydrous and carbonate minerals occur in many continental rocks. Furthermore, because continental rocks are at low temperature and have a very high viscosity, convective decompression is not likely to occur. Yet, as we will see, there is evidence that continental crustal rocks sometimes melt. This is called crustal anatexis.

Birth of an oceanic spreading center at a magma-poor rift system

The following scenario is one mechanism by which crustal anatexis could occur. Basaltic magmas, generated in the mantle, by flux melting, decompression melting or frictional heat, rise into the crust, carrying heat with them. Because basaltic liquids have a higher density than crust, they may not make it all the way to the surface, but instead intrude and cool slowly at depth.

Upon cooling the basaltic magmas release heat into the crust, raising the geothermal gradient increasing the local temperature.

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Successive intrusions of mantle-derived mantle into the same area of the crust may cause further increases in temperature, and eventually cause the geothermal gradient to become higher than the wet solidus of the crustal material, resulting in a partial melt of the crust. From the discussion above it should be obvious that magmatism is closely related to plate tectonics. The diagram below summarizes melting mechanisms that occur as a result of plate tectonics and may be responsible for the generation of magmas in a variety of plate tectonic settings, such as oceanic ridges, near subduction zones, and at rift valleys.

Diverging plate boundaries are where plates move away from each other. These include oceanic ridges or spreading centers, and rift valleys. Oceanic Ridges are areas where mantle appears to ascend due to rising convection currents. Decompression melting could result, generating magmas that intrude and erupt at the oceanic ridges to create new oceanic crust.

Iceland is one of the few areas where the resulting magmatism has been voluminous enough to built the oceanic ridge above sea level.

Subduction Zones

Converging plate boundaries are where plates run into each other. The most common type are where oceanic lithosphere subducts.

Several mechanisms could contribute to the generation of magmas in this environment see diagram at top of this section. If an oceanic lithospheric plate subducts beneath another oceanic lithospheric plate, we find island arcs on the surface above the subduction zone. If an oceanic plate subducts beneath a plate composed of continental lithosphere, we find continental margin arcs. If magma generated near the subduction zone intrudes and cools in the crust, it could induce crustal anatexis.

In areas where two continental lithospheric plates converge fold-thrust mountain ranges develop as the result of compression. If water-bearing crustal rocks are pushed to deeper levels where temperatures are higher, crustal anatexis may result.

These areas occur in the middle of plates, usually far from the plate boundaries. This phenomenon is referred to as intraplate magmatism. Intraplate magmatism is thought to be caused by hot spots formed when thin plumes of mantle material rise along narrow zones from deep within the mantle. The hot spot remains stationary in the mantle while the plate moves over the hot spot. Decompression melting caused by the upwelling plume produces magmas that form a volcano on the sea floor above the hot spot.

The volcano remains active while it is over the vicinity of the hot spot, but eventually plate motion results in the volcano moving away from the plume and the volcano becomes extinct and begins to erode. As the lithosphere stretches and thins, the aesthenosphere gets closer to the surface, and pressure is reduced, in turn causing partial melting.

Again, basalts are typically produced which, at mid-ocean ridges mostly erupt as pillow lavas on the sea floor. In continental rifts, magma rising through the thicker continental crust is much-modified, resulting in a wide variety of volcanic activity.

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They form where a slab of old, cold, dense oceanic lithosphere sinks back into the mantle.