Questions: Magma Generation: Melting Conditions and Mechanisms

Question 1

Volcanism occurs at mid-ocean ridges where plates diverge. No external heat source is supplying extra heat to the mantle at these locations. What drives melting?

• A. Friction between diverging plates generates heat that exceeds the solidus temperature
• B. Seawater infiltrates spreading center cracks and reacts chemically with the mantle to release heat
• C. Hot mantle rock rises adiabatically; as pressure drops faster than the rock cools, the actual temperature crosses above the falling solidus
• D. Radioactive decay in the shallow mantle produces enough heat to melt rock at ridge depths

Answer: (C)

Decompression melting requires no external heat source. Mantle rock is already close to its melting temperature at depth, but enormous pressure keeps it solid by raising the solidus. As plates diverge, hot rock rises adiabatically. The solidus is pressure-dependent: it drops as the rock ascends. The rock's actual temperature decreases slightly during ascent (following the adiabat), but the solidus drops faster — eventually the temperature crosses above it and partial melting begins. This is a pressure effect, not a thermal one.

Question 2

Why does water released from a subducting slab trigger melting in the overlying mantle wedge, even though the mantle wedge rock is not unusually hot?

• A. Water reacts exothermically with mantle minerals, heating the wedge above the dry solidus
• B. Water adds mass to the mantle wedge, increasing pressure and triggering compressional melting
• C. Water disrupts the silicate crystal lattice and lowers the solidus temperature, so mantle rock at normal temperatures begins to melt
• D. Water oxidizes iron in the mantle, releasing enough heat to drive melting

Answer: (C)

Water is a flux: it breaks bonds in silicate minerals and lowers the temperature at which rock begins to melt (the solidus) by several hundred degrees. The mantle wedge above a subduction zone is at normal temperatures — it would not melt under dry conditions. When hydrous fluids from the descending slab enter the wedge, the solidus drops below the ambient temperature and partial melting begins without any heating. This flux melting mechanism explains why volcanic arcs (Andes, Cascades) parallel subduction zones globally.

Question 3

At mid-ocean ridges, mantle rock can begin to melt as it ascends even without any additional heat being supplied from an external source.

• T. True
• F. False

Answer: True

Decompression melting is driven entirely by the pressure-dependence of the solidus. As mantle rock rises adiabatically, pressure falls and the solidus temperature decreases. If the rock's actual temperature follows an adiabat that crosses above the falling solidus at some depth, melting begins without any heat input. The mid-ocean ridge system — the longest volcanic feature on Earth — produces enormous volumes of basaltic magma entirely through this mechanism.

Question 4

Adding water to mantle rock raises its melting temperature, which is why subduction zones produce magma — the water heats the surrounding mantle above its normal melting point.

• T. True
• F. False

Answer: False

Water does the opposite: it lowers the solidus of mantle rock, sometimes by several hundred degrees. Subduction zones produce magma not because the mantle is unusually hot, but because water from the subducted slab makes normally-solid rock melt at lower temperatures. Wet rock has a lower melting temperature than dry rock at the same pressure — one of the most counterintuitive and commonly missed facts in igneous petrology, and the core of the flux melting mechanism.

Question 5

Describe the three mechanisms by which mantle rock can melt, and for each, explain what changes on a pressure-temperature diagram to cause melting.

Model answer: 1) Decompression melting: pressure decreases as rock ascends at a mid-ocean ridge. On a P-T diagram, the system moves to lower pressures; since the solidus slopes upward, this moves the state leftward across the solidus into the melt field — no temperature increase needed. 2) Flux melting: addition of water shifts the solidus itself to lower temperatures at constant pressure. The solidus moves toward the ambient geotherm, intersecting it and triggering melting in the subduction zone mantle wedge. 3) Hotspot melting: an anomalously hot mantle plume raises the rock's actual temperature above the solidus at depth, moving the state upward on the P-T diagram across the solidus.

In all three cases, melting occurs when the geotherm (the actual P-T state of the mantle) crosses above the solidus. The mechanisms differ in which variable shifts: decompression moves the state along the pressure axis, flux melting moves the solidus itself, and hotspot melting moves the state along the temperature axis. Recognizing which variable changes in each tectonic setting is the key to understanding the global distribution of volcanism.