Magma, the molten rock beneath the Earth’s surface, is the source of all volcanic activity. Magma formation is counter-intuitive because the deep Earth is primarily solid, and immense pressure increases the melting temperature of rock. Scientists recognize three primary mechanisms that overcome these high-pressure barriers to form magma. Flux melting is one such mechanism, a chemical process that alters the melting behavior of hot rock, allowing it to liquefy without a significant rise in temperature.
Defining Flux Melting
Flux melting occurs when the introduction of a specific chemical component, known as a flux, lowers the temperature at which a rock begins to melt. This temperature is called the solidus. In geology, the flux is often a volatile substance that acts as an impurity within the solid rock structure. The addition of a flux effectively shifts the solidus curve to much lower temperatures, similar to how adding salt lowers the freezing point of ice. In flux melting, the temperature of the mantle rock often remains relatively stable, but the chemical environment changes drastically. This allows the material to cross the newly lowered solidus line, enabling the solid mantle to partially melt.
The Role of Volatiles as Flux Agents
The most common flux agents are volatiles, primarily water (\(\text{H}_2\text{O}\)) and, to a lesser extent, carbon dioxide (\(\text{CO}_2\)). These molecules interact chemically with the crystalline structure of the mantle minerals. Water is highly effective because it incorporates into the melt structure. When water molecules are introduced into hot mantle rock, they weaken the atomic bonds within the silicate minerals. This weakening requires less thermal energy for the solid framework to transition into a liquid phase. The presence of a small percentage of water can reduce the solidus temperature of mantle rock, such as peridotite, by up to several hundred degrees Celsius. The resulting partial melt is essentially a mixture of the original rock and the incorporated volatile components.
Geological Significance in Plate Tectonics
Flux melting is the dominant process for generating magma at convergent plate boundaries, specifically in subduction zones. Here, oceanic lithosphere descends beneath another plate, carrying water bound within minerals like serpentine and amphibole. As the subducting slab sinks, increasing temperature and pressure cause these water-bearing minerals to become unstable. This leads to metamorphic dewatering, a process where the minerals break down and release their stored water as a hot, buoyant fluid.
This fluid then rises into the overlying mantle wedge, the block of hot mantle material situated above the descending slab. The water-rich fluid acts as the flux, lowering the solidus of the overlying mantle wedge rock, which is already hot but solid. This causes the hot, dry peridotite of the mantle wedge to undergo partial melting, generating magma. The resulting magma is less dense than the surrounding solid rock and begins its ascent toward the surface. This continuous cycle of subduction, dehydration, and flux melting is responsible for the formation of volcanic arcs, such as the famous Ring of Fire.