WELLINGTON, New Zealand — Looking on with in awe at the satellite images of an undersea volcano erupting in a giant mushroom cloud in the Pacific Ocean, it was hard not to wonder why the Tonga blast was so big.
Shane Cronin, a volcanology professor at the University of Auckland, and Emily Lane, a tsunami expert at the National Institute of Water and Atmospheric Research, offer these explanations.
EXPLOSIVE BUT BRIEF
The eruption was incredibly explosive but relatively brief. The plume rose more than 19 milesr, but the eruption lasted only about 10 minutes, unlike some eruptions that can last hours. Cronin said the power of the Hunga Tonga Hunga Ha’apai volcanic eruption ranks among the world’s biggest in 30 years, and the height of the ash, steam and gas plume was comparable with the 1991 Mount Pinatubo eruption in the Philippines, which killed several hundred people.
WHY SO BIG?
The magma in the volcano was under enormous pressure, with gases trapped inside. A fracture in the rock likely induced a sudden drop in pressure, allowing gas to expand and blast the magma apart. Cronin said the crater was about 650 feet below the sea surface, a kind of Goldilocks depth for a big explosion in which seawater pours into the volcano and instantly turns to steam, adding to the expansion and energy of the explosion. Any deeper, and the extra pressure of the water would have helped contain the eruption.
FARFLUNG TSUNAMI
Many scientists were surprised an eruption could produce a Pacific-wide tsunami of about three feet that smashed boats in New Zealand and caused an oil spill and two drownings in Peru. Lane said oceanwide tsunamis usually are triggered by earthquakes that extend across huge areas rather than from a single volcano. She said other factors might have been at play, such as an underwater flank of the volcano collapsing, displacing water.
NO GLOBAL COOLING
Huge eruptions can cause temporary global cooling as sulfur dioxide is pumped into the stratosphere. But satellite measurements indicated the amount of sulfur dioxide released in the Tonga eruption would have only a tiny effect of perhaps 0.02 Fahrenheit global average cooling, said Alan Robock, a Rutgers University professor.
WHAT’S NEXT?
Cronin envisions two main scenarios. The first: that the volcano has exhausted itself for now and will go quiet for 10 to 20 years. The second: that new magma rises quickly, which might mean ongoing eruptions. He thinks the cracks and rifts caused by the explosion will allow more gas to escape, and subsequent eruptions won’t be as big.
