EXPLAINER: Why Tonga eruption was so big and what's next

WELLINGTON, New Zealand (AP) — People around the world looked on in awe at the spectacular satellite images of an undersea volcano erupting in a giant mushroom cloud in the Pacific. Many wondered why the blast was so big, how the resulting tsunami traveled so far, and what will happen next. New Zealand scientists 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, help explain.

EXPLOSIVE BUT BRIEF

The eruption on Saturday was incredibly explosive but also relatively brief. The plume rose into the air more than 30 kilometers (19 miles) but the eruption lasted only about 10 minutes, unlike some big eruptions that can continue for hours. Cronin said the power of the eruption of the Hunga Tonga Hunga Ha’apai volcano ranks among the world's biggest over the past 30 years, and the height of the plume of ash, steam and gas was comparable with the huge 1991 eruption of Mount Pinatubo in the Philippines, which killed several hundred people.

WHY SO BIG?

The magma inside the volcano was under enormous pressure and had gasses trapped within it. A fracture in the rock likely induced a sudden drop in pressure, allowing the gas to expand and blast the magma apart. Cronin said the crater was sitting about 200 meters (650 feet) below the sea surface, a kind of Goldilocks depth for a big explosion in which seawater pours into the volcano and turns instantly into steam, adding to the rapid 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 that a single eruption could produce a Pacific-wide tsunami of about 1 meter (3 feet) that smashed boats in New...