On Saturday, January 15th, a powerful undersea volcano erupted near Tonga, an archipelago in the Pacific Ocean, covering the group of islands in a thick layer of ash. A 4-foot wave swept ashore in the Tongan capital, leaving behind flooded homes and structural damage. The blast was heard as far away as Alaska! It also triggered tsunami warnings along the West Coast of the United States, with waves reaching the likes of Japan and Peru.
There are nearly 1400 active volcanos around the world, with many located along the Pacific “Ring of Fire”. When it comes to undersea volcanos, most are extinct – but about three-quarters of all volcanic activity actually occurs underwater. The Hunga Tonga-Hunga Ha’apai volcano last erupted in 2014 and, in recent days, had showed signs of renewed activity in the form of small puffs of ash. When the superheated magma met with the comparatively cool seawater, it caused an instantaneous and massive explosion, with the displacement of water during the eruption leading to the formation of tsunami waves.
Imagine dropping a big rock in a children’s pool filled with water – the waves generated will reflect off the sides of the pool and slosh back and forth. In an ocean basin, tsunami waves also slosh back and forth as a result of reflecting off coastlines. You can’t think of a tsunami as a single wave and go to the beach after the first wave hits. There will be more to come. Not only that, tsunamis race across the deep ocean at jet speed. When they reach shore, they slow considerably but they are still fast: 10 – 20 mph (16 – 32 kph). Depending on the shape and slope of the shoreline, tsunamis can reach pretty far inland too: up to 10 miles (16 km). If you’re ever on a beach and the ocean starts to look or sound strange, head for the highest elevation around immediately. You can’t outrun nature: if you see a large rising wave rushing to shore, it’s already too late.
Image: “Tsunami evacuation sign” by epugachev is licensed under CC BY 2.0