The Volcano Heard Around the World: Literally!

01.20.2022 - By PlanetGeo: The Geology Podcast

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Join us as we interrupt our water series re-release to talk about a major current event - the eruption of Hunga Tonga-Hunga Ha'apai volcano in Tonga. It was a huge eruption in the South Pacific from a very active volcano.  Its had some smaller eruptions in the last few months, but Saturday morning took the lid off.

In fact, some instrumented Cascade volcanoes - Mt Hood, Mt Saint Helens, and Mt Ranier, detected infrasound records (essentially specialized microphones).  The second longer but lower-magnitude signal that shows up later in the plot - is the airwave passing by the station again -- having come from the other direction!  That’s right, the airwave has wrapped around the planet!  The first wave traveled a distance of ~8500 km (~5300 miles).  The second traveled ~32,500 km (20,200 miles)!  In fact, barometers at O'hare airport in Chicago picked up the compression sound waves. You may be wondering: why was there a volcano here in the first place? The answer is: plate tectonics! Tonga is one of the volcanic islands located on top of the Tonga Kermadec subduction zone! This is the convergent plate boundary where the Pacific Plate sinks below the Australian Plate. The Pacific Plate is made of old, cold, and dense rock material that sinks beneath the Australian Plate, where the subducted rocks heat up and melt as crust is recycled. The water and other volatiles rise up as the rocks are melting -- contributing to the "Big Boom" of the eruption. Why are the eruptions so violent?  Potentially?  It’s not always violent, but about every thousand years, it let’s loose with huge eruptions.  Why doesn’t the cool ocean water cool the magma?  If magma rises slowly, there will be a thin layer of steam between the water and the magma.  This will allow the outer edge of the magma to cool.  If the magma rises fast, the magma is in direct contact with the water.  The result is much like a weapons grade chemical explosion that starts a chain reaction where fresh magma is exposed to new water.  So this is a combination of magma/water interaction as well as gas charged sticky magma that has been building for the last 1000 yrs.  By studying deposits from past eruptions, we know that we might be in for several weeks (or years) of intense volcanic activity.    The violence of the blast triggered tsunamis.  About 1.2 meters (4 feet) on the country of Tonga.  Hard to get a clear picture of devastation yet because of ongoing eruptions.  I don’t think this story is done yet. Ash reached 20km into the sky and it produced a ton of lightning ~ 100 bolts/sec!
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