When molten lava collides with water, a phenomenon called molten fuel interaction occurs, resulting in huge explosions of sharp, glossy rock. Such was the case earlier this year when a “lava bomb” launched from the location of where Hawaii’s Kilauea volcano poured into the ocean, seriously injuring 22 tourists sightseeing from a boat. This lesser studied but common interaction could pose serious risks to communities living near active volcanoes, yet the basic physics of how water and lava interact with one another is poorly understood.
To find out why such explosions occur, researchers cooked up 45-liter (10-gallon) batches of basaltic rock for around four hours until it reached a red-hot 1,315°C (2,400°F), at which point they poured the molten rock into an insulated steel box and shot it with two or three streams of water. They then hammered a plunger into the mix in order to simulate an explosion. The entire thing was captured by high-speed cameras at a large-scale facility that allows scientists to simulate volcanic processes and control conditions not possible at a real volcano, such as changing the shape of the lava’s column or measuring the speed at which water injects into it.
“If you think about a volcanic eruption, there are powerful forces at work, and it’s not a gentle thing,” said lead investigator Ingo Sonder in a statement. “Our experiments are looking at the basic physics of what happens when water gets trapped inside molten rock.”
They found that greater box heights and water injections at higher speeds resulted in bigger explosions.
“The system response to water injection varied from mild, evaporation-dominated processes, in which only a little melt was ejected from the container alongside some steam, to stronger reactions with visible steam jets, and with melt domains ejected to several meters height,” the researchers wrote in the Journal of Geophysical Research (JGR): Solid Earth, adding that a bigger box height and greater water injection speed resulted in bigger explosions. Why this happens isn’t clear, but the team has a working theory.
When water is trapped by a hotter material such as lava, its outer edges vaporize in order to form a protective film – a process known as the Leidenfrost effect. This vapor film can hold when water is injected slowly, but when water, which is three times lighter than lava, is shot rapidly into a tall column of molten rock it speeds upwards and mixes quickly, potentially causing the vapor film to destabilize, expand rapidly, and ultimately explode.
Though the number of tests so far is small, the researchers say conducting more experiments will help to draw firmer conclusions.
“The research is still in the very early stages, so we have several years of work ahead of us before we’ll able to look at the whole range and combination of factors that influence what happens when lava or magma encounters water,” said study co-author Greg Valentine.