When Hunga Tonga-Hunga Ha'apai unexpectedly exploded unlike any volcano is decades, the only information we had was from satellites. The tsunami and eruption knocked out most communication between Tonga and the outside world as undersea cables were damaged, airports were covered in ash and infrastructure destroyed. We are now beginning to get a better sense of what happened and how big this blast was.
Images of the volcano after the eruption show that little remains of the main island. Instead, two small promontories stick out of the Pacific. In an image from Planet (above), large biege pumice rafts can be seen -- products of this massive eruption. Other images from UNOSAT using Pléiades images captured the destruction of the tsunami waves that ran up the shore as high as 50 feet (15 meters).
First off, a word of warning: estimating the size and power of volcanic eruptions is tricky. You need to measure ash deposits, look at changes in the volcano's shape, collect all the data you can about the height of the ash column and the composition of the material that came out. All of this can't be done quickly, especially when the volcano is mostly under the surface of the ocean. It might be years before we fully map out what happened at Hunga Tonga-Hunga Ha'apai with sonar mapping or ROV exploration of the caldera.
Just How Big Was It?
However, we can make some initial estimates about the eruption ... and it was a doozy:
The ash plume reached as high as 130,000 feet (40 kilometers). That is likely the tallest ash plume since the 1991 eruption of Pinatubo in the Philippines. OMPS satellite data show the ash column puncturing the atmosphere.
The first estimates of how much stuff was erupted seem to suggest it was fairly small, only about 0.1 cubic miles (0.5 cubic kilometers). That puts it well below the volume of volcanic debris erupted at Pinatubo, El Chichón or Chaitén (other large explosive eruption from the last few decades).
Similarly, the sulfur dioxide output of the eruption was only about 1.1 million tons (0.4 Tg). Pinatubo released 15 Tg, or about 38 times more sulfur dioxide. Why is that important? Sulfur dioxide in the stratosphere is what could cool the planet (or at least a hemisphere).
NASA estimates that the blast released the equivalent of 5-10 megatons of energy. For perspective, that is about 300-600 times the energy released by the bomb dropped on Hiroshima.
The sound of the blast might have been the loudest noise on Earth since the 1883 eruption of Krakatau in Indonesia. The pressure wave from the Tongan blast was recorded circling the globe multiple times in the days after the eruption.
Although we are still far from final numbers, the death toll (three) has been very low for such a large blast that also generated a tsunami. Now, reports of damage and casualties are still coming in, with some islands seeing most of their buildings damaged or destroyed by the tsunami or ash fall. Yet, Tonga weathered the disaster remarkably well. If you want some uplifting news, read about the fellow who was in the ocean for 24 hours after being swept away by the tsunami.
That being said, the UN says that over 80% of Tonga's population has been "severely" impacted by the disaster. That is over 80,000 people, which is why the supply planes and boats bringing water and other necessities are so vital right now. By some estimates, almost all of the agriculture in Tonga has been "ruined".
The Biggest Questions
Probably the biggest question facing volcanologists right now is why the eruption from Hunga Tonga-Hunga Ha'apai was so large and violent. The ability for water to enhance explosivity of eruptions is well known, but we've never seen an explosion of this magnitude caused by interactions with water alone. In fact, this could be the first time we've seen such an eruption: high explosivity but not a lot of magma behind it.
The other big question is exactly what happened at the volcano to cause the tsunami. Most speculation right now focuses on the size of the blast as the trigger for the tsunami, but there could be other events like landslides or the collapse of the island. Once mapping can happen, we might be able to answer those questions.
However, right now the most important issue is helping the people in Tonga. The eruption and tsunami has left the island nation on precarious footing, with need for food and water as well as extensive repairs to almost all infrastructure. The time for understanding the science will come, but right now the humanitarian crisis is the most immediate need.