The two processes that most commonly generate magma are decompression melting and flux melting. Both work by pushing mantle rock past its melting point, but they do it in opposite ways: decompression melting drops the pressure on hot rock until it liquefies, while flux melting adds water to lower the temperature at which rock begins to melt. Together, these two mechanisms account for nearly all magma production on Earth.
How Decompression Melting Works
Rock deep in the Earth’s mantle is extremely hot, often hot enough to melt at the surface. It stays solid because the enormous pressure from the rock above it raises its melting point. Decompression melting happens when that rock rises toward the surface faster than it can cool down. The pressure drops, the melting point falls below the rock’s actual temperature, and part of the rock turns to liquid. No extra heat is needed. The rock was always hot enough to melt; it just needed the pressure to get out of the way.
The classic setting for this is a mid-ocean ridge, where two tectonic plates pull apart. As they separate, mantle rock wells up to fill the gap. That upward movement reduces pressure rapidly, triggering partial melting. The magma produced rises into the gap between the plates and solidifies into new oceanic crust. This is happening right now along the Mid-Atlantic Ridge and the East Pacific Rise, continuously building the ocean floor.
Decompression melting also occurs during continental rifting, where a continent is being stretched and thinned, and in mantle plumes. A mantle plume is a column of unusually hot rock rising from deep in the Earth. As it ascends, it decompresses and partially melts, producing the magma that feeds hotspot volcanoes like those in Hawaii and Iceland. Even small upward displacements of mantle rock that sits near its melting temperature can trigger this process, because the melting point of mantle rock increases with depth more steeply than the rock’s actual temperature drops along its path upward.
How Flux Melting Works
Flux melting takes a different approach. Instead of reducing pressure, it introduces water or other volatile compounds into hot mantle rock. Water acts as a flux, meaning it disrupts the chemical bonds in the rock’s mineral structure, which lowers the temperature required for melting. The mantle rock doesn’t need to move. It just needs to get wet.
This process dominates at subduction zones, where one tectonic plate dives beneath another. As the subducting plate descends, the heat and pressure drive water out of its minerals. That water rises into the wedge of mantle rock sitting above the subducting plate. The mantle wedge is already extremely hot but not quite at its melting point. The addition of water drops that melting point enough to trigger partial melting. The resulting magma is what feeds the chains of volcanoes that line subduction zones around the Pacific Rim, the Andes, Japan, and Indonesia.
Because flux melting involves water-rich magma, the eruptions it produces tend to be more explosive. Water dissolved in magma expands violently as the magma reaches the surface and pressure drops, producing the kind of explosive eruptions associated with stratovolcanoes like Mount St. Helens and Mount Pinatubo.
Only a Tiny Fraction of Rock Actually Melts
One of the more surprising facts about magma production is how little rock actually turns to liquid. In both decompression and flux melting, the process is partial melting. Not the whole rock mass liquefies. Different minerals within the rock have different melting points, so the minerals with the lowest melting points go first, producing a liquid that has a different chemical composition than the original rock.
Away from mid-ocean ridges, the amount of melt present in the upper mantle is remarkably small. Seismic studies and lab experiments suggest melt fractions as low as 0.1 to 0.5% in the low-velocity zone beneath oceanic plates. Even these tiny amounts of liquid between mineral grains are enough to change how seismic waves travel through the region, which is how scientists detect them. At mid-ocean ridges and subduction zones, the melt fraction is higher, but it’s still a small percentage of the total rock volume. That small fraction collects, rises through cracks and channels, and eventually accumulates into the magma chambers that feed volcanoes.
Different Processes, Different Magma
The two melting processes produce chemically distinct magmas. Decompression melting at mid-ocean ridges generates relatively “dry” magma with low water content. This magma tends to be basaltic, with lower silica content (roughly 45 to 50% silica by weight), and it erupts as fluid lava flows rather than explosive blasts. The ocean floor is built almost entirely from this type of magma.
Flux melting at subduction zones produces magma with significantly more dissolved water. This higher water content changes the way the magma evolves as it rises and cools. Minerals crystallize in a different sequence, and the resulting rocks tend to be more silica-rich. Research on ancient arc sequences, like the Kohistan arc preserved in Pakistan, shows that these two melting regimes leave distinct chemical fingerprints in the rocks they create. The hydrous magmas from flux melting follow one chemical evolution path, while the drier magmas from decompression melting follow another, ultimately producing different types of crustal rock.
Where Each Process Operates
The type of plate boundary determines which melting process dominates. At divergent boundaries, where plates spread apart, decompression melting is the primary mechanism. At convergent boundaries, where one plate subducts beneath another, flux melting takes over. Hotspot volcanoes sitting in the middle of plates rely on decompression melting driven by rising mantle plumes.
There is some overlap. Subduction zones can experience both flux melting in the mantle wedge and decompression melting where mantle flow patterns create upwelling. Continental rift zones may involve decompression melting with some contribution from volatiles stored in the continental crust. But as a general framework, the pairing holds: divergent boundaries and hotspots produce magma through decompression, while subduction zones produce magma through flux melting. These two processes together are responsible for virtually all volcanic activity on Earth.