E5 Magmatism and Igneous Petrology

Topic

Magmatism is the complete process of the generation, ascent, and emplacement of molten rock within the Earth's crust. Partial melting of the mantle or crust occurs when the rock's temperature exceeds the threshold at which melting begins; this threshold can be surpassed in three distinct ways: when hot material rises and undergoes decompression, as happens at mid-ocean ridges and mantle plumes; when water is added to the rock, as occurs in subduction zones; or simply through conductive heating from a nearby heat source.

The degree of melting a rock undergoes directly determines the composition of the resulting magma: low degrees of melting generate magmas richer in silica and incompatible elements, whereas high degrees of melting produce more primitive magmas rich in iron and magnesium.

A well-established mathematical relationship models how trace elements are partitioned between the molten magma and the unmelted minerals; this relationship is essential for tracing the petrogenetic processes a magma underwent before reaching the surface.

Magma ascent is driven by the same principle of buoyancy that causes any material less dense than its surroundings to rise, as magma is less dense than the solid rock enclosing it. The rate of magma flow through a dike depends heavily on both the dike's radius—raised to the fourth power—and the magma's viscosity; consequently, a dike only slightly wider than another can allow a vastly greater volume of magma to flow through. Magma viscosity is, in fact, the most critical parameter of all: it varies by a factor of up to a billion between Hawaiian basalts—which flow with a viscosity of between one and ten pascal-seconds—and silica-rich rhyolites, which reach viscosities of between a million and a billion pascal-seconds; it is precisely this viscosity that determines whether a magma eruption will be effusive and calm or explosive and violent, thereby linking the magma generation described at the beginning with the ultimate fate of that same magma upon reaching the surface.

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