From lava fountains to exploding gas slugs, how volcanoes choose their style
Volcanologists name eruption styles after the mountains that made them famous. Hawaii's volcanoes ooze runny basalt in fountains and curtains of fire, Stromboli pops like a burst soap bubble every few minutes, and the most extreme ultra-Plinian blasts throw columns more than 30 km high.
Three engines drive eruptions. In magmatic ones, gas escaping from decompressing magma pushes it out. Phreatic eruptions are steam explosions heated by nearby magma that release no fresh melt, only pulverised existing rock. Phreatomagmatic eruptions happen when magma meets water directly and shatters as it chills. Strength is measured on the Volcanic Explosivity Index, which runs from 0 to 8 and, like the Richter scale, climbs tenfold with each step. Most eruptions score between 0 and 2, and a single volcano can switch styles within one active spell.
Hawaiian eruptions sit at the gentle end. Fluid, gas-poor basalt often starts from a line of vents along a crack, the so-called curtain of fire, then settles into fountains that can rise hundreds of metres. Steady outpourings build broad shield volcanoes, and they can last: the cone Puʻu ʻŌʻō on Kilauea erupted from 3 January 1983 until 2018. The record fountain, though, came from Etna in Italy on 23 November 2013, holding around 2,500 metres for 18 minutes and briefly touching 3,400. Wind stretches flying droplets into glassy Pele's tears and threads called Pele's hair, and frothy basalt can form reticulite, the least dense rock on the planet.
The lava itself comes in two textures. Pahoehoe is smooth, ropey or billowy, while a'a is thicker and slower, its crust breaking into rubble that insulates a molten core; the front steepens, snaps off and the mass lurches forward. Pahoehoe can turn into a'a, never the reverse.
Stromboli, active almost nonstop for centuries, lends its name to eruptions powered by gas slugs, giant bubbles that rise through the magma and burst at the surface, flinging bombs and cinders in arcs that pile into cinder cones. Old ideas held that magma mixed in chambers for thousands of years before rising, but researchers found Costa Rica's Irazú eruption of 1963 was likely fed by magma that shot up from the mantle in a few months.
Source: Volcanic eruption
