Decompression melting is a primary geological mechanism that transforms solid rock deep within the Earth into magma. This process is defined by the reduction of pressure on hot mantle material, causing it to melt without an increase in temperature. It is one of the main ways magma is generated beneath the Earth’s surface, driving volcanism and plate tectonics. The process relies on the physical state of the Earth’s mantle, where melting is suppressed by immense overlying weight.
The State of Mantle Rock
The Earth’s mantle, extending hundreds of kilometers beneath the crust, is composed predominantly of solid silicate rock, mainly peridotite. Although temperatures reach thousands of degrees Celsius, the mantle remains mostly solid due to the enormous confining pressure from overlying layers. This pressure stabilizes the rock’s atomic structure, significantly raising the temperature required for it to become liquid.
Geologists use the geotherm to describe the actual temperature profile within the Earth, showing how temperature increases with depth. This is contrasted with the solidus, the curve indicating the minimum temperature required for a rock to begin melting at a given pressure. In the deep mantle, the geotherm lies below the solidus on a pressure-temperature diagram. This means the rock’s actual temperature is below its melting point, explaining why the super-hot interior remains largely solid.
The Physics of Pressure Release
Decompression melting begins when hot, solid mantle rock rises toward the surface in a process called adiabatic ascent. Adiabatic means the rock rises quickly enough that it does not exchange significant heat with its surroundings, so its temperature remains nearly constant. As the rock ascends, the immense pressure stabilizing its solid structure is progressively removed.
The removal of pressure dramatically lowers the rock’s melting point, shifting the solidus curve to lower temperatures. Since the rising rock maintains its high temperature, its temperature curve eventually crosses the lowered solidus line. At this shallower depth, the rock’s actual temperature exceeds its melting point, and partial melting spontaneously begins. Even a small pressure drop, such as 1 GPa, can generate a significant amount of melt, potentially up to 20% of the rock’s volume.
Geological Settings That Facilitate Melting
The necessary upward movement of mantle material occurs primarily in two tectonic settings: divergent plate boundaries and mantle plumes. Divergent boundaries, such as mid-ocean ridges, are areas where tectonic plates pull apart. As the plates separate, pressure is released on the underlying mantle, allowing hot asthenospheric material to well up and fill the gap.
Mantle plumes, commonly called hot spots, also cause decompression melting. These plumes are columns of unusually hot rock rising rapidly from deep within the mantle, such as beneath Hawaii. The rapid ascent of this hot material causes a significant reduction in pressure, leading to extensive melting. This vertical movement of rock is the mechanism that triggers the pressure drop in both settings.
Magma Formation and Resulting Structures
The melting process in the mantle is typically partial melting, meaning only a portion of the source rock turns into liquid magma. Materials with lower melting temperatures melt first, resulting in a magma composition chemically different from the original solid rock. Magma generated by decompression melting of the ultramafic mantle rock is overwhelmingly mafic. This means it is rich in magnesium and iron, and low in silica. This melt is known as basaltic magma.
Decompression melting generates the largest volume of magma on Earth, fundamentally shaping the planet’s surface. The buoyant basaltic magma rises toward the surface, where it cools and crystallizes to form the vast oceanic crust. This continuous process at mid-ocean ridges produces new lithosphere and is the primary method of crustal formation. Mantle plumes create large-scale volcanic features like oceanic islands and extensive flood basalt.