Scientists are continuing to paint a picture of just how strong the undersea volcanic eruption that happened Jan. 15 near the island nation of Tonga was. In short, many, many, many multiples of the nuclear bomb dropped on Hiroshima. NBC News has a statement from NASA that puts the preliminary estimate of the amount of energy released by the eruption at between 4 to 18 megatons of TNT; the force of the blast that was unleashed on Hiroshima measured about 15 kilotons of TNT. That makes the Tonga blast "hundreds of times the equivalent mechanical energy of the Hiroshima nuclear explosion," per NASA. But it's not a record: "Scientists estimate Mount St. Helens exploded in 1980 with 24 megatons and Krakatoa burst in 1883 with 200 megatons of energy," NASA added.
That would make it one of the loudest eruptions in more than a century, with NPR reporting it was heard in places as distant as Alaska. It's also one that scientists say may be deserving of a new name. NPR explains that "in late 2014 and early 2015 ... volcanic activity built a platform that rose up out of the sea, creating a new island. Layers of steam and ash eventually connected the island, known as Hunga Tonga-Hunga Ha'apai, to two much older islands [that are part of Tonga] on either side of it." Gizmodo reports the "small but intermittent explosions" that built up the island are called Surtseyan eruptions, and they happen when seawater enters a vent and meets with the hot materials there.
What was different this time, per NASA scientist Jim Garvin, who has been studying Hunga Tonga-Hunga Ha'apai: "We don’t have any seismometers on Hunga Tonga-Hunga Ha'apai—but something must have weakened the hard rock in the foundation and caused a partial collapse of the caldera's northern rim," he tells Gizmodo. "Think of that as the bottom of the pan dropping out, allowing huge amounts of water [at about 68 degrees F] to rush into an underground magma chamber" whose magma measures upwards of 1,832 degrees F. Perhaps because of all that water, the explosion was far more powerful than normal. Garvin and his peers have been referencing the resulting explosion as an "ultra Surtseyan" eruption—and one that wiped Hunga Tonga-Hunga Ha'apai off the map. (Read more volcano stories.)