(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

This telephoto image shows dark fragments of molten and semi-solid lava being blasted upward and outward during a hydrovolcanic explosion at the Waikupanaha ocean entry west of Kalapana in April 2008. Similar explosions are occurring at KÄ«lauea Volcano’s current lower East Rift Zone ocean entry. For more information, please see “Littoral hydrovolcanic explosions: a case study of lava–seawater interaction at Kilauea Volcano” (https://volcanoes.usgs.gov/vsc/file_mngr/file-186/Mattox and Mangan_hydrovolcanic explosions.pdf). USGS photo by M. Patrick.

Since May 3, 2018, Kīlauea Volcano’s lower East Rift Zone eruption has destroyed more than 700 structures, covered more than 32 sq km (12.4 sq mi) of land with black lava, and added about 700 acres of new land to the island. Yet, remarkably, injuries had been few.

But then, on July 16, a large underwater explosion sent lava bombs (solid or semi-solid lava fragments) the size of basketballs through the roof of a tour boat, injuring 23 volcano watchers. The detailed cause of the explosion is uncertain, but we are fairly certain that it resulted from the heating of seawater by molten lava.

Underwater explosions are enigmatic and can be deadly. They have also fascinated scientists since a transformational event in 1963. That year, at first dawn on November 13, Icelandic fishermen stared with awe as, beyond the bow of their boat, jets of black tephra launched out of the sea in long, arcing trajectories that resembled rooster tails. The jagged front of each tail was tipped by finger-like projections, each led by a large flying block.

A new volcanic vent, Surtsey, had emerged days earlier from the 130-m (almost 430-ft) deep ocean bottom. By that fateful morning it had shallowed to several meters (yards) depth, and explosions were beginning to breach the water surface. From 1963 through 1967, the world gaped at magazine and television images of the activity.

As the vent emerged, “intermittent explosions” with cock’s-tail plumes and finger jets, alternated with continuous jetting of wet ash and steam. Some intermittent explosions shot tephra columns upward, which then collapsed back to the water surface and moved radially outward as ring-shaped clouds. Only after the vent fully emerged from the sea did the explosive behavior give way to normal basaltic lava fountains and flows.

The eruption of Surtsey, more than any other, demonstrated the explosive effect of water in volcanism. Terms like “cock’s-tail plume,” “finger jet,” “Surtseyan eruption,” and “hydrovolcanism” entered the volcanic lexicon.

Scientists noted similarities to other phenomena. Ring-shaped clouds called “base surges,” for example, rolled outward during the 1946 atomic blast at Bikini atoll. Similar violent melt-water explosions had plagued aluminum foundries and steel mills when those metals inadvertently mixed with water.

In the 1970s and 1980s, many nuclear power plants were under construction, and engineers were spooked by the prospect of nuclear fuel melting through the plants’ floors into groundwater and exploding. Melt-water explosions acquired a name—fuel-coolant interactions—but not a clear explanation. Experiments pouring metals, such as tin, into water found that explosions were devilishly hard to reproduce.

By the 1990s, scientists had found a complicated sequence that required (1) dispersing melt blobs in water, and then (2) passing a shock wave through the mixture that stripped off the steam jackets that insulated them. Whether this sequence causes Surtseyan explosions has been controversial.

German experiments using magma (rather than metals) found success using a similar two-step process, but with step one involving water bubbles entrapped in melt. Laboratory experiments, however, are not good at capturing the dynamics and complexity of real lava-water mixing. So, here we see the value of KÄ«lauea as a natural laboratory.

Kīlauea lava flows have entered the ocean for decades. During most of the Puʻu ʻŌʻō eruption, molten lava flowed beneath the waves with minor splashing but no real jetting. In a few cases, though, black tephra jets and steam were violently thrown tens of meters (yards) into the air.

These occasions were of great concern, and found to occur under two conditions: (1) when a lava delta collapses into the sea, exposing a severed tube and molten lava to ocean waves; or (2) when a submarine lava tube ruptures, allowing water entry, and is then partly blocked. The confined water heats like that in a pressure cooker and then explodes when the tube walls rupture.

Explosions of this type have occurred periodically throughout Kīlauea’s East Rift Zone eruption. Last week’s explosions suggest that tubes lie below the waves at the current ocean entry, and that more explosions are possible.

The explosions at Kīlauea’s ocean entry are smaller than the big Surtseyan cock’s-tail jets. The latter occurred above an active vent, whereas the former are fed by surface flows. But Kīlauea’s explosions are the best observed of any hydrovolcanic explosions—and they offer the best insight into how they form.