Researchers identify the cause of the most intense steam outbursts in contemporary history from the Kīlauea volcano.

One of the most active volcanoes on Hawaii's Big Island, Kīlauea, saw a string of strong steam explosions in 1924. According to a recent study, the cycle of steam-driven explosions began when hot rock beneath a drained top lava lake was penetrated by groundwater.

This discovery sharpened scientists' understanding of analogous turbulence today by reframing one of the volcano's most dramatic outbursts as the result of a covert accumulation.

Ash-based indications from the Kīlauea explosion

Thin ash bands and rock fragments recorded 17 days of explosions that eyewitness testimonies never fully caught over the ground south of Halemaʻumaʻu, the summit crater within Kīlauea where lava frequently pools and erupts.

Drew T. Downs of the U.S. Geological Survey (USGS) demonstrated how more than fifty bursts developed near the peak by matching those bands. Deposits higher in the sequence contained more new magma fragments, indicating that as the days went by, magma joined the explosions more frequently.

Nevertheless, the majority of layers remained near the crater, which makes the eruption appear smaller now than it did at the time.

Pressure dropped in the crater

The water inside Halemaʻumaʻu had drained away before any ash erupted, leaving an empty crater that continued to collapse. The floor was approximately 590 feet below the rim on the first night of explosions, removing the weight that had prevented water from entering.

Magma had flowed away from the summit, leaving behind hot walls and fresh fissures, as evidenced by an earthquake swarm down the East Rift Zone. The new theory starts with that combination, which created the conditions for groundwater to slide downward.

Fingerprints from the Kīlauea explosion

Some layers surrounding Halemaʻumaʻu had small wet pellets and fine ash, suggesting that the eruption cloud transported more than just dry dust. Instead of blasts driven solely by magma, such details suggest phreatic, steam-driven explosions from heated rock and water.

Because crumbling rock might trap steam and increase pressure prior to each explosion, even the crater walls were important. Water undoubtedly contributed to the transformation of collapse into violent steam-driven bursts, but it was not the only explanation.

Timing of model tests

The story was pushed underground by modelling, where the main question was how water got hot enough to burst on rock. Only if water entered a transient conduit—the subterranean pathway for magma—during the 11 weeks prior to the initial eruption could the model function.

Groundwater was suggested as the trigger in a previous mechanism, and the current modelling finally provides a feasible clock for that theory. The interval between the lake's draining in late February and the initial explosions in mid-May is consistent with that timetable.

Magma later joined.

Higher up, fresh pieces of freshly erupted magma became more prevalent in tephra, as rock and ash were expelled. These solid bits that are hurled from the vent during an explosive event are known to scientists as clasts.

According to Downs, "juvenile clasts are more abundant higher in the tephra profiles." The fact that the strongest explosions occurred later can be explained by the fact that more blasts involved both steam and magma.

Deposits disappear quickly.

Only roughly two miles from the vent did almost all of the remaining sediments remain. Thin ash deposits can be destroyed by rain, subsequent eruptions, loose soil, and foot traffic, particularly in the vicinity of a wet summit.

This explains why many downwind traces have vanished, but geologists discovered the best record at Halemaʻumaʻu. Because they conceal previous impacts and can make minor eruptions appear innocuous, missing strata are therefore a part of the narrative.

Unevenly aimed blasts

Many explosions blasted material into preferred areas rather than distributing it uniformly over the crater. It was challenging to match one layer across the summit because of shifting vent geometry and frequent wall collapse, which probably deflected the force.

Even when adjacent ground retained only a thin covering, fine ash continued to move tens of miles farther downwind. As a result, hazard maps are unable to depict these explosions as neat circles growing from a central point.

Communities experienced repercussions

The eruption spewed ash far beyond the summit and dimmed daylight in villages downstream. Ash and dust columns are more than two miles high, according to USGS records, with Pāhala, which is roughly twenty miles away, being particularly heavily hit.

Falling rock demonstrated how quickly the danger could arise, and one photographer perished after getting too near between shots. Thin deposits on the ground can nevertheless indicate a disaster for entire populations, according to USGS historical accounts.

1924's lessons

Many still associate Kīlauea with trickling lava rather than abrupt rock and ash cascading from the peak. This more comprehensive explanation depicts magma, falling rock, and water feeding the same sequence rather than distinct types of eruptions.

This conflicting narrative also contributes to the reason why 1924 was difficult to categorise for over a century. Even before significant plumes emerge, future instability at a drained top crater might warrant further investigation.

Why this is important

The 1924 eruption may now be understood as a series of events: steam broke rock, groundwater seeped in, and a drained lava lake reduced pressure. The new publication has already restored a missing reason to a well-known eruption, but future research can evaluate how frequently that pattern occurs.