Earlier this week, we talked briefly about the volcanic eruptions occurring in the Kingdom of Tonga last weekend, and we promised you an in-depth look at the situation. Scientists are continuing to analyze a wealth of data, with more coming in every day, but here is what we know so far.

Several weeks ago, on December 20, 2021, the submarine volcano Hunga Tonga-Hunga Ha’apai began to erupt. That initial set of eruptions was barely noted in international media, with most of the coverage remaining local because nothing much was going on. The visible portion of the volcano, or rather, the island produced at the surface, grew based on an analysis of satellite images. But the volcano quieted back down, and by January 11, 2022, it had been declared dormant.

And then, at 0420 UTC on Friday, January 14 (5:20 pm local time on Saturday, January 15), a much larger eruption occurred that sent a mushroom cloud of ash, steam, and gas up into the atmosphere to a height of nearly twenty kilometers, although later analysis of sulfur dioxide plumes in the stratosphere suggests the height was closer to thirty kilometers. Geologists observing from an ocean vessel nearby noted that the base of the mushroom cloud was about five kilometers wide.

Incredibly, initial reports on Twitter had people hearing the explosion as far away as Australia, Samoa, and Fiji, but further analysis in the past few days has led to the discovery that the explosion was heard as far away as Anchorage, Alaska. I personally checked in with a colleague to ascertain if the very loud boom we heard here at my house on Friday night could have been the eruption as well, and she confirmed that yes, it absolutely could be and probably was.

Ashfall from the eruption coated the Tongan islands, and a tsunami warning was issued for both the Kingdom of Tonga and much of the Pacific Ocean. The initial wave height was about twenty centimeters, which may not sound like much, but later waves were almost two meters high. From images released earlier this week by the Tongan government, several villages were wiped out, numerous other buildings were damaged or destroyed, and there was a lot of minor damage nearly everywhere on every island.

Additionally, the nation was cut off from the rest of the world due to an underwater cable being severed during the massive eruption, which made coordinating rescue and relief efforts difficult and also challenged the government in its ability to assess all the damage. Hence why the assessment was only released a couple of days ago. Plus, with all the ashfall, the airports in the nation were closed because runway markings were not visible. Communications between islands were limited to patrol boats.

At least three people in Tonga were killed due to the tsunami, with more injured and missing, although due to the tsunami warning, those numbers seem to have been kept very low.

Damage was not limited to Tonga, however. The tsunami rolled through the Pacific Ocean, where waves reached Japan that were eighty centimeters in height and caused damage to harbors and boats. Similar effects were seen along the coast of North America, with waves between twenty and 100 centimeters in Alaska, and about 25 centimeters in British Columbia. Harbors in California also saw increased wave activity and harbor damage. In Peru, two people were killed as a result of the tsunami, and news sources there also reported flooding and damage.

One of the more fascinating side effects of this eruption is the huge amount of electrical activity in the ash cloud itself, which generated hundreds of thousands of lightning discharges. Remember from earlier in the show that volcanoes can produce lightning, and this volcano produced more of it than any eruption recorded to date. The lightning strikes were detected by Vaisala’s GLD360 network, which basically “hears” lightning as radio waves. There was a point during the major eruption when the ash cloud produced over 200,000 discharges in one hour.

And the GLD360 network wasn’t the only system keeping an eye on this volcano and revealing a wealth of information. 

In the weeks leading up to the eruptions of the Hunga Tonga-Hunga Ha’apai volcano in Tonga, a fleet of spacecraft from governments and countries were monitoring it from space. These included the commercial satellites from Planet Labs and the Sentinel constellation run by the European Space Agency. Still other spacecraft observed the volcano from high orbit, including NASA’s GOES-17 spacecraft.

Planet Labs runs a constellation called PlanetScope with two different types of spacecraft. The smallest and most numerous are the Doves, CubeSats with a small telescope. Planet Labs also uses larger satellites called Skysat. These satellites are bigger, about 100 kilograms, and capable of taking pictures with a ground resolution of fifty centimeters. They can also take stereo images, showing an object in three dimensions. Many of the images of the volcano during its early eruptions were taken by Planet Lab’s Skysats.

Another satellite that took images of the volcano in Tonga was ESA’s Sentinel 2 optical imaging satellite, providing images of the remains of the island after the ash cloud had cleared. It has a much lower resolution than Skysat, at only ten meters per pixel.

The GOES-17 spacecraft run by NOAA is in geostationary orbit and covers the western half of the United States and the Pacific Ocean. Gifs on social media from GOES-17 data showed the 15 kilometers tall and 300 kilometers in diameter ash cloud, and more interestingly, the shockwave, spread from the island and around the world.

In between the major eruption and the ash cloud dissipating to show the aftermath, the optical satellites were unable to take any images. So, radar satellites stepped in. Radar satellites can see through ash and cloud cover. ESA’s Sentinel 1 satellite took pictures showing the first signs of the devastation before optical satellites could get images again.

The eruption of Hunga Tonga-Hunga Ha’apai is considered to be the largest eruption of this century, although I think that doesn’t take much since we’re only 20ish years in, but it’s also the largest eruption since the 1991 eruption of Mt. Pinatubo in the Philippines. That eruption caused a slight drop in global temperatures for a few years. And while we could use a global temperature drop right now, we would prefer it not be at the expense of people’s lives and their communities.

So why was this eruption so big? To understand that, you have to look at the structure of the Earth’s crust. The Pacific Ocean is surrounded by what is known as the Ring of Fire, with numerous volcanoes erupting at any given point in time. In the case of Tonga, the Pacific tectonic plate is subducting or diving down under the Australian plate. As the descending plate moves into the mantle, the rocks are heated and begin to melt. The water from the ocean gets added to the mix, which actually lowers the melting point. But these rocks don’t melt all the way into that nice, liquid magma we see from Hawaiian-type eruptions. No, this stuff is sticky, thick, and viscous; more like peanut butter than honey. So when it tries to rise up through the surrounding cooler rocks, crystals continue to grow, the water has evaporated into gas, and everything has been set up for a very explosive eruption.

Hunga Tonga-Hunga Ha’apai itself is more than nineteen kilometers wide at its base. It’s huge. And there’s a massive caldera in there that’s about five kilometers across. The volcano has been erupting since at least as far back as 1912, and during eruptions in 2014 and 2015, the island that had been appearing and disappearing for years became stable. It even had plants and some animals on it. Once things quiet down again, more plants and animals will find their way to the island.

And in one final piece of good news, aid planes and ships finally began to arrive from New Zealand and Australia just yesterday, January 19. Phone lines have also been partially restored, so information is beginning to flow from the islands again. We hope the death toll continues to remain very low and that Tonga gets the help it needs to rebuild.

More Information

Tonga volcano: New images reveal scale of damage after tsunami (BBC News)

Tonga volcano felt around the world (EarthSky)

The Tonga eruption explained, from tsunami warnings to sonic booms (National Geographic)

Report on Hunga Tonga-Hunga Ha’apai (Tonga) — 12 January-18 January 2022 (Smithsonian Institution)

Expanding Islands In The South Pacific (Planet Labs)

Hunga Tonga-Hunga Ha‘apai Erupts (NASA Earth Observatory)