Magma is molten or partially molten rock found beneath the Earth’s surface, commonly reaching temperatures between \(700^{\circ}\text{C}\) and \(1300^{\circ}\text{C}\). Generating this melt typically requires either extreme heat or a significant decrease in pressure to overcome the strong molecular bonds of solid rock. The central geological puzzle is how large volumes of magma are created in deep environments where the ambient temperature is hot, but still insufficient for dry rock to melt. Water provides the answer, acting not as a source of heat, but as a chemical agent that fundamentally changes the conditions required for melting.
Water’s Presence in Deep Earth Minerals
The water involved in generating deep-seated magma is not free-flowing liquid. Instead, it is chemically locked within the crystal structures of specific rock-forming minerals, known as hydrous minerals. These minerals, such as serpentine, amphibole, and mica, form primarily through hydration when seawater circulates through the oceanic crust, often at mid-ocean ridges.
Water, carbon dioxide, and sulfur are classified as volatile substances. Under the immense pressure of the deep Earth, these volatiles remain dissolved or chemically bound within the rock. Tectonic plates act as a conveyor belt, transporting this water-laden oceanic crust deep into the mantle, delivering the necessary fluid ingredients for melting.
The Mechanism of Flux Melting
The core process driving magma formation in water-rich, high-pressure environments is called flux melting. This mechanism relies on introducing a volatile substance to dramatically lower the melting temperature of the surrounding rock, known as the solidus. Water is exceptionally effective as a flux because it chemically compromises the structural integrity of the solid rock, similar to how salt lowers the freezing point of water.
Water molecules penetrate the crystal lattice structure of the silicate minerals. When water is introduced, its hydrogen and oxygen atoms interact with and partially break the strong chemical bonds holding the mineral framework together. This disruption weakens the rigid structure, requiring significantly less thermal energy for the rock to transition from a solid to a liquid phase.
Experimental petrology demonstrates the impact of water flux on peridotite, the mantle’s main rock type. Dry peridotite at 100 kilometers depth typically requires temperatures exceeding \(1500^{\circ}\text{C}\) to begin melting. When water is introduced, the same rock can begin to melt near \(800^{\circ}\text{C}\). This substantial lowering of the solidus is the physical consequence of water chemically interfering with the silicate framework.
The introduction of water shifts the rock’s solidus curve closer to the geothermal gradient, the natural increase in temperature with depth. The existing temperature of the mantle wedge, which is hot but below the dry melting point, becomes sufficient to intersect the lowered melting curve. The resulting melt is less dense than the surrounding solid rock and begins its buoyant ascent toward the surface. Even a small concentration of water, sometimes less than one percent by weight, can initiate partial melting and generate large volumes of magma.
Subduction Zones: The Primary Melting Environment
The specific geological setting that provides the necessary combination of depth, pressure, and hydrated material for flux melting is the subduction zone. This is where one tectonic plate, typically oceanic, slides beneath another and descends into the Earth’s mantle. This environment is responsible for the vast majority of magma that forms the planet’s volcanic arcs, such as the Pacific Ring of Fire.
As the cold oceanic slab descends, it is subjected to increasing pressure and begins to heat up. The critical step that initiates melting occurs through dehydration. As the subducting slab reaches depths typically between 80 and 120 kilometers, the pressure and temperature cause the hydrous minerals locked within the slab to become unstable.
These minerals, such as serpentine and lawsonite, break down through metamorphic reactions, releasing their stored water and other volatiles. This released water, now a hot, buoyant, and chemically active fluid, rises from the subducting slab into the overlying mantle wedge. The mantle wedge is a triangular block of hot peridotite rock situated between the trench and the volcanic arc.
The volatile fluid acts as the melting agent, immediately triggering the flux melting mechanism in the overlying peridotite. The resulting magma is typically basaltic in composition and is the source of the volcanic arcs that parallel the subduction zone. This cycle of hydration at the surface and subsequent dehydration at depth drives plate tectonics and arc volcanism.