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Mantle rock melts through three mechanisms: decompression melting (pressure drop at ridges), addition of volatiles (water in subduction zones), or temperature increase (hotspots). The melting temperature of rock varies with pressure, composition, and water content; understanding these controls explains where and why magma forms.
Plot melting curves (solidi) on P-T diagrams. Compare mantle adiabat with melting curves to predict melting locations.
From your study of igneous rocks, you know that magma is molten rock that cools to form crystalline or glassy solids. From phase diagrams, you know that whether a substance is solid or liquid depends on both temperature and pressure. The crucial insight for understanding magma generation is that the solidus — the boundary between fully solid and partially molten rock on a pressure-temperature diagram — is not a fixed temperature. It shifts depending on pressure and composition, and this shift is what allows rock to melt without necessarily getting hotter.
The mantle is almost entirely solid, yet magma forms in several tectonic settings. The most voluminous mechanism is decompression melting, which occurs at mid-ocean ridges. As tectonic plates diverge, hot mantle rock rises to fill the gap. This rock is already close to its melting temperature at depth, but it stays solid because the enormous pressure at depth raises the solidus. As the rock ascends, pressure drops faster than the rock cools, and at some point the rock's actual temperature crosses above the falling solidus — partial melting begins. No external heat source is needed; the rock melts simply because it has risen to a depth where the pressure is low enough. If you recall the Clausius-Clapeyron equation from thermodynamics, the same principle applies: the slope of the solid-liquid boundary on a P-T diagram means that reducing pressure at constant temperature can cross the phase boundary into the liquid field.
The second mechanism is flux melting, dominant at subduction zones. When oceanic lithosphere descends into the mantle, it carries water locked in hydrated minerals like serpentine and amphibole. As the slab heats up at depth, these minerals break down and release water into the overlying mantle wedge. Water is a powerful flux: it disrupts the silicate crystal lattice and dramatically lowers the solidus — by several hundred degrees in some cases. The mantle wedge rock, which would otherwise be too cool to melt, partially melts because the addition of water has moved the solidus down below the ambient temperature. This is why volcanic arcs (like the Andes or the Cascades) sit directly above subduction zones — the water released from the descending slab triggers melting in a narrow zone above it.
The third mechanism is hot-spot melting, where an anomalously hot plume of mantle material rises from deep within the Earth — possibly from the core-mantle boundary. Unlike decompression melting at ridges, which taps mantle at roughly normal temperatures, plume material is genuinely hotter than its surroundings (by perhaps 100–300°C). This excess temperature means it crosses the solidus at greater depth and produces larger volumes of melt. Hawaii and Iceland are the classic examples: both sit atop mantle plumes and produce prolific volcanism far from any plate boundary. In all three mechanisms, the key to understanding where and why magma forms is the relationship between the mantle's actual temperature profile (the geotherm) and the pressure-dependent solidus — melting happens wherever the geotherm crosses above the solidus.