Crystals used to classify volcanic eruptions

A new study using crystals as a time capsule of sorts for information about the early formation of magma is giving scientists a better understanding as to what causes different eruption types at Kilauea volcano.

The study found that the source rock forming the magma largely signals whether the eruption will be violent or ooze out of the earth, said Bruce Houghton, a professor in volcanology at the University of Hawaii.

Houghton published the study last month in the peer-reviewed journal Nature Geoscience along with Hawaiian Volcano Observatory geologist Don Swanson and researchers from the University of Cambridge.

For the study, the scientists collected tiny olivine crystals from 25 Kilauea eruption sites up to 600 years old.

The crystals form inside molten rock beneath the volcano and capture liquid surrounding them in tiny bubbles — much like air captured in bubbles of ice — freezing the early composition of the molten rock long before an eruption.

"The crystals themselves lock in these little pockets of the molten rock, which then sits undisturbed until after the eruption," Houghton said. "They give us a very early window into particularly how much gas is in the magma."

Because those bubbles are extremely tiny — about a hundredth of an inch across — the Hawaii researchers received help from the Cambridge researchers, who have access to one of the few labs in the world that can analyze such minuscule specks of material.

After examining the "frozen" samples, the scientists found magma with more gas was involved in the more explosive eruptions.

"The release of that gas is what turns what might be a quiet eruption into an explosion," Houghton said.

He said the gas helps the magma rise quickly and power through reservoirs of magma near the surface. In contrast, less-gassy magma tends to settle in reservoirs where it releases more gas, further curbing the volatility of the eruption.

"For the weaker eruptions, the bulk of that magma gets stored for a while at relatively shallow depths," Houghton said. "The magma that comes up fast and causes the big problem is coming up from much deeper. It’s coming up from, say, 50 to 80 miles down and not lingering on its way to the surface."

The gas in the magma comes from the carbon dioxide, water and sulfur gases released when the host rock melts. The molten rock can be more or less gassy, depending on the host rock.

Scientists have long maintained that eruption types were tied to activity occurring 1,000 feet below the level at which magma reaches the surface. Houghton said his study provides evidence for a fuller picture detailing the early formation of magma.

Houghton is already working on another study to determine how fast deep magma is rising to the surface, which will help answer the question of how soon a violent eruption might take place if there is gassy magma forming deep in the earth.

"If we can work out these velocity meters, then potentially we can give them a much better estimate — narrow it down and say it will be days or you’ll have weeks or whatever," Houghton said.