Plate Tectonics and Volcanoes

Three ways to melt rock

The Earth’s mantle is almost entirely solid, even though it is hot enough to glow. Making magma takes a specific trigger, and there are only three common ones. Each corresponds to a tectonic setting, and together they account for nearly every volcano on the map.

• Adding water at a subduction zone — water lowers the melting point of hot mantle rock, the way salt melts ice.
• Releasing pressure at a rift — when mantle rises as plates pull apart, the drop in pressure lets it melt without getting any hotter.
• Adding heat at a hotspot — an unusually hot plume of mantle rising from deep in the Earth melts as it nears the surface.

Subduction zones and volcanic arcs

Where an oceanic plate bends down and slides beneath another plate, it carries seawater locked in its rocks. As the slab heats up, that water is squeezed out and rises into the mantle wedge above, which begins to melt. The magma rises to the surface in a line that parallels the plate boundary about 100–150 km inland of the trench — a volcanic arc.

This is by far the most common setting for volcanoes on land, and the most dangerous. The magma is rich in silica, which makes it stiff and sticky, so dissolved gas cannot escape gently — it builds up until the magma tears itself apart. The result is a steep stratovolcano capable of violent, ash-rich eruptions: the Andes, the Cascades, Japan, Indonesia, the Aleutians, the Mediterranean arcs. Around the Pacific these arcs join into the Ring of Fire, which holds roughly three-quarters of the world’s volcanoes above sea level.

Rifts and mid-ocean ridges

Where plates pull apart, mantle rises to fill the gap and melts simply because the pressure on it drops. This decompression melting produces runny, low-silica basalt that erupts relatively gently, building broad shield volcanoes and lava plains rather than steep cones.

Most of this happens unseen along the 60,000 km of mid-ocean ridges on the deep sea floor — the largest volcanic system on the planet, producing about three-quarters of all erupted magma by volume. Where a ridge rises above the sea, as at Iceland, or where a continent is splitting, as along the East African Rift, the same process is visible on land.

Hotspots

A few volcanoes sit in the middle of a plate, far from any boundary. These are fed by hotspots — narrow columns of hot mantle, thought in many cases to rise from near the core–mantle boundary. As a plate drifts over a fixed hotspot, the volcano is carried away and goes extinct while a new one forms behind it, leaving a chain that records the plate’s motion.

The clearest example is the Hawaiian–Emperor chain: islands and drowned seamounts that get older to the northwest, with a sharp bend recording a change in Pacific plate direction about 47 million years ago. Yellowstone sits at the young end of a hotspot track that runs across Idaho. Iceland is a hotspot sitting on top of a mid-ocean ridge, which is why it is so productive.

Why eruption style varies

The single biggest control on how a volcano erupts is the silica content of its magma, which sets how thick and gas-retentive it is:

• Basalt (~50% silica, hot, runny) — gas bubbles out easily. Lava fountains and flows; shield volcanoes. Hawaii, Iceland, Nyiragongo.
• Andesite (~60% silica) — the typical subduction-arc magma. Explosive eruptions with ash columns and pyroclastic flows; stratovolcanoes. Fuji, Merapi, most of the Andes.
• Rhyolite / dacite (~65–75% silica, cooler, extremely viscous) — the most explosive. Lava domes that collapse, and caldera-forming eruptions. Pinatubo, Mount St. Helens, Toba.