Magma vs Lava: What's the Difference?

Magma is molten rock stored beneath Earth's surface. It is not pure liquid — most magma is a mush of liquid, solid crystals, and dissolved gases (mostly water vapour and carbon dioxide). The liquid portion is called the melt; the suspended crystals are minerals that have already started to solidify. When magma reaches the surface, we call it lava.

The difference is purely location. Magma crystallises underground to make intrusive (plutonic) igneous rocks. Lava cools above ground to make extrusive (volcanic) igneous rocks. The chemistry can be identical; only the cooling environment changes.

Most of the mantle and crust are solid, so melting requires a special combination of temperature, pressure, and composition. Geologists recognise three main mechanisms, each tied to a tectonic setting:

• Decompression melting — lowering the pressure on hot mantle rock so it crosses its melting point.
• Flux melting — adding water or other volatiles, which lower the melting temperature of rock.
• Heat-transfer melting — hot magma rising from deep in the mantle transfers enough heat to melt the surrounding crustal rock.

At divergent boundaries, tectonic plates pull apart and the hot mantle rises to fill the gap. As the mantle rock rises, the pressure on it drops (decompresses). Because melting point depends on pressure, the rock can now melt without getting any hotter. This is decompression melting, and it produces the basaltic magma that builds new oceanic crust at mid-ocean ridges and fills rift valleys on continents.

The magma here is typically low in silica (~45–55 % SiO₂), low in dissolved gases, and low in viscosity — it flows easily.

At convergent boundaries, an oceanic plate sinks (subducts) into the mantle. The subducting slab carries water-rich sediments and altered oceanic crust down with it. As the slab descends, heat and pressure drive water out of the rock. This water rises into the hot mantle wedge above the slab and acts as a flux — it lowers the melting temperature of the mantle rock, much like salt lowers the freezing point of water on a winter road.

The resulting magma is generally richer in silica (~55–65 % SiO₂) and dissolved gases than ridge basalt. It is more viscous and more prone to explosive eruptions. This is the magma that feeds the volcanic arcs of the Pacific Ring of Fire.

At hotspots, a plume of unusually hot mantle rock rises from deep within Earth — possibly from the core–mantle boundary. Like the mantle beneath a mid-ocean ridge, the rising plume decompresses as it ascends, and decompression melting dominates, generating basaltic magma. This is why Hawaiian hotspot lavas are basaltic (~45–52 % SiO₂) even though they erupt in the middle of a plate, far from any ridge.

A secondary process — heat-transfer melting — kicks in wherever hot, mantle-derived basaltic magma stalls and ponds at the base of the crust. The intruding magma transfers heat to the surrounding solid country rock, melting it (this is also called contact melting). If that country rock is thick continental crust, the new melt is far richer in silica — which is why Yellowstone's hotspot produces rhyolitic magma rather than basalt. Heat-transfer melting is distinct from assimilation: heat-transfer melting melts the country rock, whereas assimilation is the magma physically digesting and incorporating that melted material into itself.

The two most important compositional controls on magma behaviour are:

• Silica (SiO₂) content — silica forms long, chain-like molecules in molten rock. High-silica magma is thick and sticky (high viscosity); low-silica magma is thin and runny (low viscosity).
• Dissolved gases — mainly H₂O and CO₂. Gases stay dissolved under high pressure underground, but as magma rises and pressure drops, bubbles form. The more gas and the thicker the magma, the more explosive the eruption.

These two properties — viscosity and gas content — are what determine whether a volcano will ooze quiet lava flows or blast ash kilometres into the sky.