4.3: Magma Generation

Magma and lava contain three components: melt, solids, and volatiles. The melt is made of ions from minerals that have liquefied. The solids are made of crystallized minerals floating in the liquid melt. Volatiles are gaseous components—such as water vapor, and carbon dioxide—dissolved in the magma [6]. The presence and amount of these three components affect the physical behavior of the magma.

Geothermal Gradient

Figure \(\PageIndex{1}\): Geothermal gradient

Below the surface, the temperature of the Earth rises. This heat is caused by residual heat left from the formation of Earth and ongoing radioactive decay. The rate at which temperature increases with depth is called the geothermal gradient. For every kilometer of depth, the temperature increases by about 25°C.

An important concept to know is pressure affects the temperature at which a material melts. If a material is solid at a high temperature and high pressure, you can either raise the temperature to get it to melt OR lower the pressure. The lower pressure allows the ions in the minerals to vibrate and the material will melt. This is called decompression melting.

There are three principal ways rock converts to molten magma: 1) decompression melting caused by lowering the pressure, 2) fluid-induced melting caused by adding volatiles (see more below), and 3) heat-induced melting caused by increasing the temperature. The Bowen’s Reaction Series shows that minerals melt at different temperatures. Since magma is a mixture of different minerals, the temperature at which it cools is more of a fuzzy zone rather than a well-defined line; some minerals are melted and some remain solid. This type of rock behavior is called partial melting and represents real-world magmas, which typically contain solid, liquid, and volatile components.

Decompression Melting

Figure \(\PageIndex{1}\): Progression from rift to the mid-ocean ridge, the divergent boundary types. Note the rising material in the center.

Magma is created at mid-ocean ridges via decompression melting. Strong convection currents cause the solid asthenosphere to slowly flow beneath the lithosphere. The upper part of the lithosphere (crust) is a poor heat conductor, so the temperature remains about the same throughout the underlying mantle material. Where the convection currents cause mantle material to rise, the pressure decreases, which causes the melting point to drop, and partial melting starts. As this magma continues to rise, it cools and crystallizes to form new lithospheric crust.

Flux Melting

Figure \(\PageIndex{1}\): Diagram of ocean-continent subduction. Note water vapor driven out of hydrated minerals in the descending oceanic slab.

Fluid-induced melting occurs in island arcs and subduction zones when volatile gases are added to mantle material. Fluid-induced melting magma produces many of the volcanoes in the circum-Pacific subduction zones, also known as the Ring of Fire. The subducting slab contains oceanic lithosphere and hydrated minerals. As covered in Chapter 2, these hydrated forms are created when water ions bond with the crystal structure of silicate minerals. As the slab descends into the hot mantle, the increased temperature causes the hydrated minerals to emit water vapor and other volatile gases, which are expelled from the slab-like water being squeezed out of a sponge. The volatiles dissolve into the overlying asthenospheric mantle and decrease its melting point. In this situation the applied pressure and temperature have not changed, the mantle’s melting point has been lowered by the addition of volatile substances. This is analogous to adding salt to an icy roadway. The salt lowers the freezing temperature of the solid ice so it turns into liquid water.

Heat-Induced Melting

Figure \(\PageIndex{1}\): Migmatite is a partially molten metamorphic rock. (Source: Peter Davis)

Heat-induced melting, transforming solid mantle into liquid magma by simply applying heat, is the least common process for generating magma. Heat-induced melting occurs at the mantle plumes or hotspots. The rock surrounding the plume is exposed to higher temperatures, and the rock begins to melt. The mantle plume includes rising mantle material, meaning the pressure drops and some decompression melting is occurring as well.

References

6. Wallace, P. J. Volatiles in subduction zone magmas: concentrations and fluxes based on melt inclusion and volcanic gas data. Journal of Volcanology and Geothermal Research 140, 217–240 (2005).

7. Petrini & Podladchikov. Lithospheric pressure–depth relationship in compressive regions of thickened crust. Journal of Metamorphic Geology 18, 67–77 (2000).