What happens when lava meets water? One might be tempted to think that lava just pours forth, quickly cools down and solidifies. But it is a lot more complicated -- and sometimes more explosive -- than that. Indeed, water plays many roles in volcanology, and the Kilauea eruption that has transfixed us all for weeks showcases them quite clearly.
First of all, water drives "lava fountaining" -- the sustained emission of fluid lava hundreds of feet into the air that has riveted so many people to Kilauea's "lava cam." Magma ("lava" that has not yet surfaced) stored underneath the volcano in its feeding system -- in the Earth's crust -- contains just under 1% of water by weight. As magma rises toward the surface of the Earth, the pressure it is under decreases, making it impossible to keep the water dissolved.
Just think about opening a bottle of soda -- by unscrewing the cap, you release the pressure and all of a sudden lots of bubbles (carbon dioxide in that case) suddenly "appear," or, rather, separate from the liquid and become visible. In volcanology, we say that they "nucleate."
Once they have nucleated, those light, buoyant bubbles rise toward the surface faster and faster. As they do, they keep growing bigger and bigger, expanding as surrounding pressure decreases. As bubbles rise, they drag magma along. By the time they get to the surface, they are moving so fast that they overshoot the crater and head toward the sky, forming a lava fountain.
Lava fountains then feed lava flows. After journeying through Kilauea's East Rift Zone, lava meets water again as the flow enters the ocean. Underwater, lava changes its appearance, forming lava pillows. This "pillow lava" -- spherical blobs of lava that cool from the outside in -- piles up and forms a steep underwater slope called a lava delta. As they are made up of loose blocks of lava amassed chaotically, lava deltas are very unstable and prone to collapse suddenly.
Still at a temperature exceeding 2000 -F, lava turns the surrounding ocean water into a rising plume of white steam, often acidic -- what is referred to as "laze." For the most part, this activity is toxic but nonviolent.
However, water can become trapped among the pillow lava and cause violent explosions at the shore. The process driving them is called Molten Fuel Coolant Interaction (MFCI). It is caused by the sudden contact of a hot molten fuel (lava) and a cold volatile fluid (water).
As the lava enters the ocean, a thin film of steam rapidly develops at the interface between molten lava and liquid water, keeping them separated. This ultra-thin, vapor buffer layer is very unstable, and can collapse in seconds. When it does, lava and water come into contact, triggering a chain reaction of growing intensity that causes a big underwater explosion. Incandescent shards of glassy lava blast in all directions, blanketing the surroundings in translucent, sharp fragments of newly forged Earth.
Meanwhile, up at the summit, water is causing phreatic eruptions inside Halemaumau crater. Phreatic eruptions occur when water flashes to steam. Water from the water table, which sits just over 50m above sea level, gets trapped inside the crater, where it heats to the point of quickly turning to steam, leading to violent explosions in which some of the crater walls turns to ash. In a phreatic eruption, magma doesn't come into play at all.
All eyes are on the lava these days, including my own, but as you watch summit explosions, lava fountains and ocean entries do not forget -- the driving force behind it, in one form or another, is water.