How can an island be growing in lava lake atop Hawaii volcano? Scientists have ideas

One of the most intensely watched elements of the ongoing eruption of Hawaii’s Kilauea volcano is an island that mysteriously formed early on in the crater’s fiery lava lake.

It’s about seven acres in size and drifting like a raft.

The U.S. Geological Survey has been monitoring this deadly oasis since the start of the eruption on Dec. 20, noting its growth and mystifying movements. On Thursday, the island measured 820 feet long and 440 feet wide and stood about 25 feet above the lava lake’s surface, officials wrote on Facebook.

This defies logic for most people, who understand molten lava to be something that melts, well, just about everything.

And that’s not wrong. The USGS says the magma still squirting from a fissure in Kilauea’s crater wall is somewhere between 1,830 and 2,200 degrees Fahrenheit.

So how can an island of rock survive in a 643-foot deep lake of lava?

How did the island form?

The lava lake in Kilauea’s massive crater (called the Halemaumau crater) currently covers 69 acres, or about seven football fields, officials say.

It’s getting deeper, too, as lava continues to pour and “spatter” from vents on the west side of the crater.

Experts believe the island is made of “material erupted early in this eruption that accumulated at the base of Halemaumau crater.”

“Photographs, webcam imagery, and eyewitness observations indicate that it formed through a combination of lava interacting with the lake water, early lava flows, and tephra erupted from the early highest fountains,” USGS officials posted Jan 7.

Ten or so smaller islands have also formed at the east end of the lake, officials said.

On New Year’s Day, the big island began moving “more rapidly to the west, as if it were seeking out the west lava source filling the lake,” the USGS reported. It eventually stopped directly in front of the spewing fissure around midnight Friday, officials said.

Scientists did not offer explanations, but noted the smaller islands stayed put.

“The apparent buoyancy changes of the island may be due to a density increase in the lava lake as gases escape or sloughing off of island material from the subsurface,” USGS officials said Jan. 6.

Photos shared on Facebook show the largest island to be forbidding turf: scorched black, full of cracks and sporting wisps of white smoke. The atmosphere on its surface is toxic, with a mix of water vapor, carbon dioxide and sulfur dioxide, experts say.

The temperature on the island is 200 to 400 degrees, the USGS says.

Why doesn’t it melt?

On Jan. 6, the USGS addressed the question many people were asking about the island on social media.

“It seems counter-intuitive that solid lava can float within molten lava but islands have been observed in Kilauea lava lakes for more than 100 years — some float and some don’t,” the USGS posted on Facebook.

“Why doesn’t it melt more quickly? Solidified lava is a very good thermal insulator and would melt very slowly. During the past 10 days or so, the island has gotten smaller, from pieces calving off and maybe some melting. It has also started to float a little higher in the lake.”

Its ever changing location confirms the island is floating freely, and not the tip of something hidden below the surface, officials said.

Experts offer theories

Among the first published studies of lava lake islands was a 1913 report in the American Journal of Science. It referred to multiple sightings of islands “floating tranquilly and apparently unharmed, on the seething bosom of the glowing lava lake.” Experts have a series of theories for what causes the islands and how they stay afloat.

In some cases, the islands are bits of lava that somehow found a way to cool amid the inferno, experts say. In other cases, they are rocks that fell into lava lakes and were too big to melt all at once, the journal reported.

The islands can be a result of the current anomalies, the report says.

“Amid the swirling eddies of the surface lava, a certain area balanced between opposing forces remains, for a considerable time, at rest. There results the formation of a crust which gradually thickens by accretion from below and may also be built up ... until a true island is formed,” the report said.

“Why does not the island sink and why does it not melt? The answer is that it does sink and it does melt, but not all at once. It may be a matter of months rather than of moments.”

As for how such islands stay afloat, they may be buoyed atop a rising current of lava from the lake’s floor, or even a stream of gas bubbles, the report says.

This story was originally published January 1, 2021 at 8:36 AM with the headline "How can an island be growing in lava lake atop Hawaii volcano? Scientists have ideas."