About 80% of Earth’s volcanism occurs underwater. Out of sight and largely out of reach in the deep ocean, submarine volcanoes are hard to study, and direct observations of their eruptions are extremely rare. So for guidance into how they work, geologists have had to rely principally on the rich data from land-based volcanoes that erupt into the atmosphere.
Water’s influence on the behavior of submarine volcanism can be significant, though. Dissolved gases in the magma tend to remain there when it erupts at depth, under pressure, as lava. Fewer bubbles form, and the eruptions tend to be less explosive than they would be on land, with lava flows that are less frothy. And when the lava spills from fissures during an eruption, the crust formed by rapid cooling as molten rock contacts near-freezing seawater is denser and more insulating than the crust of lava exposed to air. Counterintuitively, deep-sea...
