Introduction
Beneath the Hawaiian island of Kīlauea, rock that has been solid for millions of years is melting right now — not because the mantle suddenly got hotter, but because rising rock crossed a pressure threshold where it no longer has to be solid. That single counterintuitive idea — that rock can melt by being pushed upward rather than heated — unlocks the entire geography of volcanism on Earth.
Earth's mantle is mostly solid — and yet volcanoes erupt molten rock onto the surface every day. This apparent paradox is resolved when you understand that melting is not simply a matter of temperature. Rock can melt if: its temperature increases enough; the pressure on it decreases enough (decompression melting); or water or other volatiles are added to it (flux melting). These three mechanisms operate in different tectonic settings and produce different types of magma, different volcanic styles, and ultimately different landscapes.
Most of Earth's internal heat budget is dominated by the decay of radioactive isotopes (uranium, thorium, potassium-40) — a process that has been slowly declining in intensity over geological time as the reservoir of these isotopes is depleted. Additional heat comes from the residual heat of Earth's formation and differentiation. This internal heat is conducted very slowly through the solid mantle rock toward the surface, a process so inefficient that the mantle loses heat primarily not by conduction but by convection — slow creep of hot rock upward and cold rock downward over millions of years. It is this mantle convection that drives plate tectonics and positions the zones where magma can form.
A common misconception is that the Earth's interior is a vast ocean of magma from which volcanoes draw their supply. In reality, most of the mantle is solid, and the molten rock erupted by volcanoes is generated in the mantle only locally, under conditions that drive the mantle past its melting temperature. Understanding these conditions — the three melting mechanisms — is the key to understanding why volcanoes form where they do.
Key Terms
Molten or partially molten rock within the Earth, including any dissolved gases (volatiles). When magma reaches the surface, it is called lava. Magma is not a uniform substance — its composition (primarily silica content, from ~45% in basalt to ~75% in rhyolite), temperature (700–1,300°C (2372°F)), crystal content, and dissolved volatile content all vary and determine its behaviour.
Melting triggered by a decrease in pressure on mantle rock without significant temperature change. As rock rises (in mantle plumes or mid-ocean ridge upwelling), pressure decreases; if temperature stays approximately constant, the rock may cross the solidus (melting point curve) and begin to melt. The primary mechanism at mid-ocean ridges and mantle hotspots.
Melting of mantle rock caused by the addition of water (and CO₂ and other volatiles), which lowers the melting temperature (solidus) of the rock. The primary mechanism at subduction zones: water released from the subducting oceanic slab rises into the overlying mantle wedge, dramatically lowering its solidus and triggering partial melting.
The melting of some minerals in a rock while others remain solid. Since different minerals have different melting temperatures, a rock heated toward its solidus will melt progressively, with low-melting-point minerals melting first. The proportion of melt produced (melt fraction) determines the composition of the resulting magma. Partial melting of the mantle typically produces basaltic melt from a peridotite source rock.
The temperature (at a given pressure) below which a rock is entirely solid. Above the solidus, some melt exists (partial melting); above the liquidus, the rock is entirely molten. The solidus of peridotite (the primary mantle rock) at ~100 km (62 mi) depth is approximately 1,300°C (2372°F), but is lowered to ~1,000°C (1832°F) by the addition of water, explaining flux melting at subduction zones.