Despite how it looks, Hawaii’s volcanic eruption is small potatoes

If you knew there was, say, a 0.5% risk that your dream home would burn up in any given year, would you still build it? 

That, volcanologists suggest, is the type of decision residents on the southeast side of Hawaii’s Big Island have always had to face. 

The climate there is about as perfect as it can get. At an elevation of 1,150 metres, for example, the average high temperature is 19 to 22 degrees Celsius, year-round, with an average low of 10 to 13. Not too hot; not too cold. 

There’s just one problem: a volcano named Kilauea. 

The youngest and most active of Hawaii’s volcanoes, Kilauea covers less than 10% of the Big Island’s surface, but is no slouch at spewing lava. Overall, says Michael Poland, a geophysicist at the Cascades Volcano Observatory in Washington State, US, 90% of Kilauea’s surface is less than 1000 years old. 

Thus, there is a near certainty that any given home-site will be hit at least once every millennium — and probably more often. In fact, Leilani Estates, the 10.5 square-kilometre subdivision where the current eruption began, has seen lava erupt within it three times in the past 200 years: in 1840, 1955, and now. A 1961 eruption came close, but missed. 

“We’ve mapped out lava-flow hazard zones,” Poland says. “Zone One is a place where lava could come out of the ground” — as opposed to other places, such as the city of Hilo, where lava flows might snake down from above. “Leilani Estates is built on Hazard Zone One.” 

In fact, it’s built right near the crest of a geologic region known as the East Rift Zone, which extends 60 kilometres eastward from the crest of Kilauea Volcano, creating a peninsula on the southeast side of the island. There, it enters the sea and extends another 100 kilometres offshore.

“I was based in Hawaii for 10 years,” Poland says. “I told people that Leilani is right on the rift zone. ‘It is possible,’ [I told them], ‘that one day lava could erupt in your front yard’.” 

At the time, he says, he was thinking, “‘It’s going to happen one day.’ I didn’t think ‘one day’ would be May 3.”

That said, Kilauea and other Hawaiian volcanoes are more a risk to property than to lives. Largely, that’s because Hawaiian magmas contain less water and other gases than do those from such places as Indonesia or America’s Cascades, both of which have produced titanic explosions. 

Furthermore, says Adam Kent, a geologist at Oregon State University, Corvallis, US, Hawaiian lavas are unusually hot and fluid — averaging about 1,200 degrees Celsius, compared to 800 to 900 for lavas in the US Cascades. 

“They tend to be more runny, [so] any gas that’s there can get out relatively easily,” he explains. 

The difference may stem in part from the fact that Hawaiian volcanoes are powered by heat rising from a blowtorch-like hot spot deep in the Earth’s interior. Indonesian and Cascade volcanoes are fueled by offshore subduction zones, where seabed rocks are dragged into the Earth’s interior, melt, and rise back to the surface in chains of volcanoes. 

“Those magmas tend to have higher water content because there’s water involved in that initial process,” Kent says.

Not that this is any consolation for those who live in the wrong spot. “If you have a house, there’s really nothing you can do if there’s a lava flow coming toward you,” Kent says. 

But it does mean that Hawaiian volcanoes are relatively non-deadly, because people can walk away, even if their homes cannot. In the past 120 years, Kilauea has only directly killed one person — a photographer who in a 1924 eruption ventured too close and got hit by a flying rock. A few others have died from toxic fumes and from venturing onto unstable lava ledges that collapsed and dumped them into the sea, Poland says, but again, the primary culprit was curiosity, not the volcano itself. 

Even recent explosions and ash clouds spewing from the summit caldera aren’t all that large by volcanic standards. 

They are produced, Poland says, because the outflow of lava at Leilani Estates drained magma from higher elevations, lowering the magma level in the summit caldera. That allowed groundwater to flow in, producing steam explosions that, while powerful, are still small potatoes compared to what other volcanoes can produce.

In fact, much as Kilauea continues to hypnotise the world, it is relatively insignificant in the pantheon of what other volcanoes are capable of doing. 

In 1815, for example, an Indonesian volcano known as Tambora blew 41 cubic kilometres of ash into the air, killing perhaps 100,000 people directly and devastating climate worldwide. (In much of the Northern Hemisphere, the ensuing year became known as “the year without a summer” or, more humorously, as “1800 and froze to death.”)

More recently, in 1985, a 5300-metre Colombian volcano called Nevado del Ruiz had a relatively small eruption that dumped hot ash on summit glaciers and snow. The result was a mudflow that sped downslope at 60 kilometres per hour and killed 23,000 people — including three-quarters of the residents of the downstream town of Amero. 

Such mudflows can occur even on volcanoes devoid of snow and ice. The term for them, “lahars”, originates in Java, Indonesia, where there are no summit glaciers. All it takes, says Kent, is a major weather event, striking before loose ash from a recent eruption has time to consolidate. “That happens a lot on tropical volcanoes,” he notes.

That said, this is another hazard from which Hawaii is generally spared, because its runny lavas produce volcanoes with relatively gentle slopes. “Lahars are more common on steeper-sided volcanoes,” Kent explains.

Even more deadly than Nevado Del Ruiz was Mount Pelée, a 1400 metre peak on the Caribbean island of Martinique. In 1902, it destroyed an entire town of 30,000 to 40,000 people, leaving only three survivors (one of whom was a prisoner in an underground cell and another of whom was a young girl who took to a boat and was later found adrift, three kilometres offshore). 

That time, the culprit was a pyroclastic flow, a phenomenon similar to the glowing cloud of ash that roared down on the Roman City of Pompeii in 79 CE. There are a number of mechanisms that can cause this to happen, but one is when a giant plume of superheated ash is ejected into the sky — vastly larger than anything Kilauea’s current explosions are capable of producing. That ash then cools enough to fall back — not as individual particles but as an incandescent mass that produces what is known as a density flow of tightly packed, searing ash. 

At the moment, Kilauea is not producing ash columns dense enough to allow such an event to occur, nor is anything like them expected. Rather, the ash being produced is likely to fall back gently to the surface and float away, Kent says. 

But this may not always have been the case. “Even though it’s not known for killing people [today], Kilauea has killed more people than any other volcano in the US in the past few hundred years,” Poland says.

That’s because it appears to cycle between relatively gentle phases, like today, and more explosive ones. “We think it’s tied to the amount of magma being supplied to the volcano,” he says. 

When there’s not a lot of magma, the volcano is more explosive than today, probably because magma levels drop in the caldera, allowing more groundwater to enter and mix with it. “The little magma that’s supplied is more explosive,” Poland says. 

When there’s more magma, as is currently the case, the summit builds up higher, making it harder for significant amounts of water to intrude. At these times, Poland says, “water and magma don’t generally mix, or don’t mix in a really explosive way.”

The last time Kilauea went through a more explosive phase was from 1590 to 1800, Poland says. “In 1790, there was an explosion that killed several dozen Hawaiian warriors and their families,” he says. 

The victims were part of a war party traveling across the island, who happened to be near the summit when Kilauea erupted. “Many were killed,” he says. “There are reports of anywhere from a few dozen to thousands. I think most scholars think it was on the order of a few hundred.”

That said, people have been living with volcanic hazards on the Hawaiian Islands for at least 1500 years. “There are risks with any place you would chose to live,” Poland says. “It’s a matter of what you’re willing to live with.”