Conventional wisdom holds that shield volcanoes like Mauna Loa and Kilauea don’t blow their tops like Mount St. Helens.

Rather, they erupt in slow-moving flows like the one plaguing Puna. Or they shoot lava up in fountains, as Kilauea did in the 1980s.

But new research is bolstering the little-known fact that Kilauea can explode and did so to deadly effect as recently as 1924.

And it will certainly do so again, although probably not soon, says Don Swanson, a geologist with the Hawaiian Volcano Observatory.

Such explosions represent Kilauea Volcano’s “most dangerous and least predictable hazard,” Swanson and colleagues report on the website for the Geological Society of America Bulletin. His co-authors are University of Hawaii geologists Saman­tha Weaver and Bruce Houghton.

Scores of people died when Kilauea erupted spectacularly in 1790, sending up “everything from flour-sized particles to baseballs, and a lot of steam,” Swanson said in a telephone interview Saturday.

“We don’t really know for sure what the cloud looked like,” he said. “It certainly wasn’t ink black, but was probably gray, and I would imagine people would have difficulty seeing very far. It was hot and dusty, and when they inhaled that, their air passages were probably scorched by the ash.”

Historically, the estimates for how many people were killed by the explosion and ash cloud vary from 80 to 5,000, the latter “absurdly high,” Swanson said.

More likely it was in the hundreds, he said, and included warriors and their families.

Many walked through ash fields and left footprints that still exist.

“In the footprints we can see evidence that there were indeed women and children, by the size of their feet — children below adolescence,” he said.

The November 1790 explosion resulted from a violent interaction of groundwater and hot rocks, Swanson and colleagues report.

Another such explosion will happen when the summit caldera collapses down to the water table, about 2,000 feet below, they say.

The groundwater then will turn to steam, which, if trapped by rocks, would build intense pressure. When the pressure ultimately is released, it will carry a lot of ash and rocks with it, he says.

“Water can’t enter the magma conduit when it is full,” Swanson elaborated by email. “But when the conduit empties below the water table, then water can pour in. It will generally simply boil off up the conduit, but if parts of the wall collapse and plug the conduit above the steam, then pressure can build and an explosion can take place. That kind of thing happened in 1924, when the floor of Hale­mau­mau dropped to below the water table, producing lots of steam.”

One person was killed in that blast, a Pahala resident named Truman Taylor.

A caldera collapse could happen any time, theoretically, Swanson said. But when it does, it will provide a few days of warning before a blast — time to make any necessary evacuations.

“Something like what happened in 1790 is almost inevitable, actually,” Swanson said. “I don’t see any evidence of anything in the near future, meaning months to a few years, but conditions can change quickly.”

Kilauea’s size and shape mean that an explosive eruption will never match that of Mount St. Helens, which killed 57 people in May 1980.

But once it explodes, it could continue doing so for centuries, says Swanson.

In fact, he says, the 1790 blast culminated about 300 years of frequent explosive eruptions.

“Hawaiians were used to explosive eruptions, but the 1790 activity was probably the largest they had encountered,” Swanson’s article says.

Caught in the blast was a party of warriors and their families, led by a chieftain named Keoua. They were marching from Hilo to South Point to do battle with Keoua’s rival and cousin, Kame­ha­meha, for supremacy of the island, according to historical accounts.

“The event was a milestone in the struggle, convincing many island residents that Pele favored Kame­ha­meha, even though Keoua himself survived the eruption,” the scientists report.

The Rev. William Ellis, who led the first group of Westerners to visit Kilauea in 1823, was told about the eruption by his guides.

After a “violent percussion,” Ellis later wrote, “immense rocks in a state of ignition (were) thrown into the air.”

He added, “A volley of smaller stones, thrown with much greater velocity and force, instantly followed the larger ones. … Many of Keoua’s people were killed by the falling fragments of rock, and many actually were buried beneath the overwhelming mass of ashes.”

One witness recounted seeing a “pillar” of rocks and ash, and Swanson and colleagues agree with estimates that it reached an altitude of 30,000 feet.