Krakatau, or Krakatoa, a mountainous island in Indonesia, self-destructed in August 1883. Episodic volcanic eruptions culminated in an explosion that hurled debris 80 kilometers into the air and engulfed 800,000 square kilometers of the Earth’s surface in corrosive ash. As much of the island shattered and splashed into the sea, a tsunami surged up and crashed into the neighbouring islands of Java and Sumatra, killing the majority of the 36,000 people killed in the eruption.
While Indonesia took the brunt of the destruction, the Krakatau eruption had global ramifications. Despite the fact that there appeared to be no way Krakatau’s tsunami could have leapfrogged from the Indian Ocean over continents and into other ocean basins, tiny tsunamis lapped at the shorelines of countries in both the Pacific and Atlantic seas. Scientists at the time attributed these far-flung tsunamis to coincidentally timed earthquakes since they had no other explanation.
However, geophysicists continued to puzzle with the data in future decades. A 1955 research, for example, discovered that the arrival of the pressure wave that travelled outward through the air from the eruption was connected with the arrival of distant tsunamis. The authors of the research thought that there was some form of link between the air disturbance and the water. Krakatau’s primary tsunami lagged behind minor tsunamis in areas like Hawai’i, California, and Alaska, which instead synced up with the quicker pressure wave from the blast, according to computer simulations from 2003. (A sound is a pressure wave with a frequency in the audible range.)
Scientists needed to see another version of Krakatau in real time in the current age to confirm the hypothetical concept that volcanoes’ sound or pressure waves may produce tsunamis – an inconvenient dream, to be sure.
Then, on January 15, 2022, a mostly submerged volcanic cauldron dubbed Hunga Tonga-Hunga Ha’apai in the South Pacific erupted in a terrifying scream. The archipelagic Kingdom of Tonga was destroyed by its mushroom cloud of ash and local tsunami. Despite the fact that it killed only a few people, this volcano set a number of records: It hurled debris two-thirds of the way into space, produced 200,000 lightning discharges per hour in its ash cloud, and was one of the most powerful explosions ever recorded.
The Tongan event was “essentially like Krakatoa 2” in terms of scale and intensity, according to Matthew Haney, a geophysicist at the United States Geological Survey’s Alaska Volcano Observatory. In Anchorage, where Haney is located, a gunshot-like crack could be heard. “We’re around 6,000 kilometers apart.” Is it possible that you may hear a volcanic eruption? Wow. That really boggles my mind.”
It wasn’t only the music of Hunga Tonga-Hunga Ha’apai that went across the world. A series of tens of centimeter-high tsunamis slammed against distant coastlines in several ocean basins. “That tsunami signal in the Caribbean was unexpected,” said Paul Fanelli, an oceanographer with the National Oceanic and Atmospheric Administration (NOAA).
This time, experts believe they’ve discovered the answer. When wave height gauges were linked with equivalent air pressure monitors all across the world, it became evident that the explosion’s pressure wave must have collided with the surfaces of many oceans and seas, transferring energy to the water, and causing a slew of tsunamis.
This solution also clarified a 139-year-old riddle surrounding Krakatau. “Given what was observed in 1883,” Greg Dusek, a physical oceanographer at NOAA, said, “it seems reasonable that it would have happened then as well.” “Both sets of observations appear to be in good agreement.”
However, as with any major scientific breakthroughs, new issues have arisen. When and why do the pressure waves of volcanoes dance with the waves of the ocean? Why did Tonga’s far-flung tsunamis only hit particular parts of the country? What is the potential for these tsunamis to become more powerful and destructive?
In order to create a tsunami, a large amount of mass must be pushed into a body of water. Earthquakes do this function in a basic manner. “Tsunamis from earthquakes are very, really basic,” said Emily Lane, a hydrodynamicist at the National Institute of Water and Atmospheric Research in New Zealand. “An earthquake occurs beneath the surface of the water, causing the seafloor to deform, and this deformation travels to the surface of the water, where it radiates out as a tsunami.”
Tsunamis caused by volcanic eruptions are more difficult. Ocean can be displaced by debris propelled into or splashed down into the water, the partial or total collapse of the volcano itself, and underwater explosions. In the following months, research on the seafloor at Hunga Tonga-Hunga Ha’apai will reveal which process, or combination of events, caused the region’s classic-style tsunami.
Small wave peaks landed at the Ogasawara Islands, some 1,000 kilometers south of Tokyo, about three hours before the big tsunami traversed the Pacific Ocean and hit Japan. Similar peaks appeared in the Caribbean Sea from Puerto Rico to Mexico on the same day, as well as 18,000 kilometers distant in the Mediterranean Sea.
Some scientists were reminded by these small, quick waves of a less common technique for Earth to produce tsunamis: by exploiting the atmosphere.
Storms may cause severe and long-lasting atmospheric disruptions. Joseph Proudman, a British mathematician and oceanographer, proposed in 1929 that if a disturbance moves at a specific speed over a body of water, it might trigger a phenomenon known as a Proudman resonance. His calculations revealed that the air pressure wave may transmit energy to water waves, causing them to become larger. Meteotsunamis are what happen when these heightened waves reach the coast.