Island of lava helps clear the air on volcanoes

Réunion, a French island in the western Indian Ocean, is a jigsaw of two massive shield volcanoes. The younger, Piton de la Fournaise, or “peak of the furnace,” is a furious factory of lava, erupting every eight months on average over the past four decades.

That hellish environment makes it an ideal real-world laboratory for studying the internal viscera of volcanoes, about which scientists know surprisingly little.

The more they map out, the better they grow to understand why, how and when volcanoes all over the world will next erupt.

In a study published last month in Scientific Reports, volcanologists reported using a novel technique to map out 150 square kilometres of Piton de la Fournaise’s shadowy underworld. Their survey revealed a 3D view of its insides, from the plumbing network of superheated hydrothermal fluids to scores of faults that allow magma to sneak up to the surface during eruptions.

The success of the technique on Réunion means that it could be deployed elsewhere, said Marc Dumont, a geophysicist at the Sorbonne University in Paris and the lead author of the study, from lava-effusing mountains like Hawaii’s Kilauea to the more explosive peaks in the volcanic spine running up the U.S. Pacific Northwest.

Piton de la Fournaise is a byzantine volcano, comprehensively monitored by scientists as it is regularly modified by eruptions. Spidery tendrils of magma escape through lines of weakness. When molten material meets the groundwater cycling through the volcano’s uppermost segments, powerful explosions can happen without warning, much like the lethal detonations that recently rocked New Zealand’s White Island. Old faults can suddenly slip and cause parts of the volcano to catastrophically collapse.

These features control how future eruptions manifest, so finding out where they are is of paramount importance.

One way to locate these subterranean features is to use instruments to see how well the rocks below conduct electricity. Scorching, circulating water is highly conductive. Old volcanic rock that has been degraded by it has water inside its Swiss cheese-like holes, making it relatively conductive. Newly cooled, structurally sound lava flows are much more electrically resistant.

Deploying electrical resistivity-detecting instruments on an active volcano can be both dangerous and time-consuming. Often, expeditions must choose between a high-resolution underground map of a small area or a low-resolution map of a larger space.

Scientists had previously traipsed across the Piton de la Fournaise by foot, deploying equipment to reveal parts of its internal structure. To speed things up, they took to a helicopter.