Flying boulders and clouds of ash: The Kilauea volcano's new explosive threat

Kilauea, the volcano that's been erupting on the Island of Hawaii over the past week, has already forced the evacuation of nearly 2,000 people and the destructionof more than two dozen homes. Nowthere's a new threat on the horizon: a violentexplosion.

Kilauea which has been in a near-constant state of eruption for more than 30 years isn't a volcano that typically erupts explosively in a cloud of ash andspewing lava,the way most people may imagine. Ratherit experienceseffusive eruptions, where magma moves along rifts below the surface before rising and pushing upthrough fissures in the earth. (Once above the surface, it is called lava and itmoves slowly across ground as it expands, often flowing downhill.)

Kilauea can, however,experience more violent events as a result of something called a phreatomagmatic explosion, which occurs when searing hot magma interacts with water, sendingash and rock spewing into the sky.

Volcanologists believe these explosions happen whena lava lakea large volumeof molten lava often contained in a crater or volcanic vent dropsandgroundwater is able to seep into the resulting shaft, or lava tube.The water boils instantaneously when it comes into contact with the magma, resulting in a violent mixing of the two substances.

Meantime, if rockand other sedimenthavefallen inward, blocking the steam that's being created, intense pressure builds up. It is eventually releasedthrough an explosion, sending extremely hotsteam, rocks, gases and volcanic ash forcibly upward.

The United States Geological Survey (USGS) issued an orange alert Wednesday, warning that the "steady lowering" of the lava lake in Kilauea's Overlook Crater has "raised the potential for explosive eruptions in the coming weeks."

Officials with the agency wouldn't estimate the likelihoodof such an explosion, but called it a "distinct possibility."

Already, the volcano has been experiencing small explosions.

"What we've already been seeing is that chunks of the surrounding vents are just dropping off into the lavaand that's why we're getting these small explosions," said Jessica Johnson, a volcanologist at the University of East Anglia in the U.K.

"It's like when you drop a Mentos into a bottle of Coke:it starts making bubbles. That's what's happening at the momentbut these are just fairly small eruptions."

Kilaueahas a history of phreatomagmatic explosions. The last major one occurred in 1924 and spanned more than 2 weeks, with more than 50 explosive events. It threwboulders as heavy as 12 tonnes into the sky.

As a result of that event, the lava lake that once existed there disappeared,thoughone returned in 2009.

Warning signs

Over the past week, the Big Island has experienced more than 600 earthquakes a sign that something is occurring deep beneath the volcano.

"The earthquakes that we get at volcanoes can tell us an awful lot of what the magma is doing as it's moving through the rocks,"Johnsontold CBC News. "It's one of the best ways we can 'see' what's going on."

While Kilauea has the potential to fire rocks and boulders into the sky, no one resides in the immediate area of the summit crater,part ofHawaii Volcanoes National Park, which was set to be closed Friday.

Aslong as people stay away, they will be safe, Johnson said, noting that residents are likely well aware of the potential dangerswhile tourists may be less so.

"As much as lava itself moves slowly, there are other hazards as well, such as the gas [and]the ash," said Johnson, who previously worked at the Hawaiian Volcano Observatory at Kilauea as part of a research fellowship.

People should also be cautious aroundthe water: once lava interacts with saltwater, it produces hydrochloric acid, which is toxic.

Still mysterious

If volcanologists can better understand Kilauea and underlying reasons for its seemingly strange behaviour, Johnsonsaiditcould help them better understandother volcanoes.

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For example,she pointed to the factthere are two eruptions points around the volcano, where magma has a free path to rise to the surface. But it never seems to take the easy path, andinstead, forces itself up through rock; 15 fissures have opened since Kilauea's vents started spraying lava on May 3.

"One of the things I love about Kilauea is that it's so well-monitored and so well-studiedthe more we know, the more we realize we don't know," she said.