Melt generation#
When the temperature of mantle rocks first exceeds their solidus, partial melting occurs. It becomes completely molten when the temperature exceeds the liquidus. The position of the solidus and liquidus depend on the composition of the rock, pressure, and the presence of volatiles such as water and carbon dioxide.
Phase diagram of aluminous lherzolite with geothermal gradient and regions where melting occurs (shaded red region). Melting begins at marked points in the image: (1) increase in temperature (2) decompression melting (3) water saturated peridotite solidus. Image: https://steemit.com/geology/@sooflauschig/how-does-the-earth-s-mantle-melt
Some surface expressions of melting#
Large igneous provinces: Formed by hot spot volcanism, where a mantle plume creates a large volume of magma that erupts over a short period of time. Examples include the Deccan Traps and Columbian River Basalts.
Continental hot spots: Formed by hot spot volcanism, where a plume produces volcanism beneath a continental lithosphere over a long period of time. Examples incluse Yellowstone.
Rift volcanism: At regions where the Earth’s crust is pulled apart, the underlying mantle melts due to decompression melting generating surface volcanism along the line of rifting. Examples include East African rift and Mid Atlantic rift system.
Ocean island basalts: Formed by hot spot volcanism, where a mantle plume creates a volcanic island as it rises through the mantle and melts. Examples include Hawaii and Iceland.
Volcanic arcs: Formed by subduction zone volcanism, where one tectonic plate is forced beneath another, causing the mantle to melt and create magma that rises to the surface. Examples include the Andes and the Cascade Range.
Some surface expressions of melting and volcanism. Images from Wikipedia and NASA Science.
Types of melting#
Decompression melting: Melting of a hot ascending mantle rock due to rapid decrease in pressure. Temperature may remain constant but solidus at those low depths (or pressures) can generate melt. For example, at mid-ocean ridges where mantle material rises as the plates move apart.
Flux melting: Melting due to the addition of volatiles (e.g., water, CO2) which lower the solidus temperature of the rock. Flux-melted magmas may produce volcanoes. Such melting is observed in volcanic arcs above subduction zones, where oceanic crust carries water into the mantle.
Heat-induced melting: Melting that occurs when hot magma intrudes into cooler surrounding rock, transferring heat and causing partial melting of the surrounding rock. This process occurs in hot spots.
Melting in different tectonic settings#
Melt generation in different tectonic settings and the corresponding temperature profiles. Image from Johnson, Affolter, Inkenbrandt, and Mosher under Wikimedia Commons. Accessed February 2026.
References#
Turcotte, D. L., & Schubert, G. (2002). Geodynamics. Cambridge University Press.
