Last week a volcano on the eastern Caribbean island of St. Vincent erupted after hours of spewing ash and steam. The volcano,La Soufriere, which means “sulfur outlet” in French, hasn’t erupted since 1979. The first explosion shot a column of ash ten kilometers into the sky and, according to reports from local authorities, was followed by two subsequent explosions several hours later. Heavy ashfall was reported in communities around the volcano, with lighter ashfall observed on the nearby islands of Barbados, St. Lucia, and Grenada. Despite the explosive ash, seismologists reported that this was what’s called an effusive eruption, meaning lava steadily flows out of the volcano onto the ground.

As you probably know, this isn’t the only volcano that is erupting right now; there is also Fagradalsfjall in Iceland. However, these eruptions are quite different. The Icelandic volcanoes have lava that is flowing out the ground from several openings and filling up the valley they’re located in; so far, there haven’t been large ash clouds like there have been with La Soufriere.

These are two very different types of volcanoes. Iceland is over a hot spot and a mid-ocean ridge, so the magma welling up is hotter and less full of solid minerals. It flows faster. Meanwhile, La Soufriere is created in a subduction arc of volcanoes, similar to Japan and Alaska’s Aleutian Islands. These volcanoes are created when plates are pushed down toward the mantle, heating up and melting the rock. Add in some water to lower the melting point, and you have magma that is cooler, very viscous, and full of crystals. Think of honey that has hardened a little. You have to warm it up to get it flowing again.

So while Icelandic volcanoes tend to be flowing and free, volcanic arc volcanoes have explosive eruptions as the partially solid magma backs up in the volcanic cones until the pressure behind the solid rock becomes high enough, due to all the escaping gas in the rising magma, and boom. Ash, rock, and lava all burst out, creating massive hazards for the surrounding communities. And that is why tourists are flocking to watch the lava flow in Iceland and being evacuated on St. Vincent. You can roast a hot dog over a gentle lava flow (do not try this at home despite what you may see on YouTube) but get far away from a mountain due to explode.

We hope that the communities on St. Vincent remain safe and receive the assistance they need to survive this latest eruption.

More Information

La Soufrière info page (Smithsonian)

Fagradalsfjall info page (Volcano Discovery)