M2 Magmatic Cycle

Topic

The magmatic cycle encompasses all the processes by which mantle and crustal material partially melts, rises toward the surface, and ultimately either erupts or crystallizes at depth. This magmatic cycle is the primary mechanism for Earth's chemical differentiation and the principal means of transporting heat and volatiles from the planet's interior to the surface and atmosphere.

At mid-ocean ridges, the adiabatic decompression of mantle material rising toward the ridge axis causes partial melting—ranging from ten to twenty-five percent—without requiring any external heat input.

In contrast, at subduction zones, water and carbon dioxide released by the dehydrating subducting slab lower the melting point of the overlying mantle wedge; this reduced melting point generates the silica-rich andesitic magmas characteristic of these zones.

Finally, at hotspots, thermal plumes rising from the deep mantle induce melting simply through the excess heat they deliver, requiring neither the decompression found at ridges nor the volatiles released during subduction.

The ascent of magma—regardless of its origin—combines buoyancy (driven by the density contrast between the magma and the surrounding country rock, the same force that opens the dikes through which the magma rises) with the magma's inherent rheology; its viscosity can vary by up to ten billion times, ranging from fluid basalt to much more viscous rhyolite. As the magma rises and resides in magma chambers, fractional crystallization progressively alters its composition by removing early-crystallizing minerals, thereby increasingly enriching the remaining magma in elements that crystallize later. The expected behavior of the complete magmatic cycle shows an inverse correlation between a magma's silica content and the frequency with which that type of magma erupts: the most basic magmas—the very ones that predominate at the mid-ocean ridges described earlier—are also the most voluminous and the most frequent of all.

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