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Volatile Sakurajima volcano is a lightning laboratory

By Dr. Jeffrey Johnson
May 01, 2016, 3:35:13 AM EDT


Japan is a country of volcanoes, and Sakurajima is one of its most infamous. Its notoriety stems from its poor behavior in 1914, when powerful explosions and pyroclastic flows forced the evacuation of the small volcanic island. Shortly after the explosions stopped, extensive lava eruptions began. The amount of lava that erupted was enough to span Kagoshima Bay, connecting the volcano to Kyushu's mainland. For much of the next forty years, the volcano was relatively quiet.

But Sakurajima has been exploding intermittently since 1955. And although it no longer is an island, it is still nearly surrounded by water, and its 7,000 residents are exposed to volcanic hazards including ash fall, lahars, and the potential for lava bombs. Today, schoolchildren commute wearing hard hats just in case rocks start to rain down from the sky. Because ashfall often grays the landscape, everyone wears masks to avoid breathing in the tiny particles of volcanic glass. While downwind areas on the island are more vulnerable to the ashfall, no corner of the island remains completely unaffected, since any location is less than 4 miles from the exploding crater.

Although explosions are a worrisome inconvenience to the local population, the phenomenon draws volcano scientists to the region like moths to a flame: Sakurajima's reliable, frequent and powerful explosions provide researchers with an unparalleled laboratory for studying eruptions that are categorized as quintessentially "vulcanian."

On a typical day, a few vulcanian blasts can be expected to erupt out of the Showa Crater like canon shots. Ten seconds later, a concussion sound wave — often exceeding 100 pascals in pressure, akin to the sound pressure levels on an aircraft carrier deck — reaches the Kurokami Observatory 2 miles away. Most of this sound energy is subsonic, but if it were audible, it would be deafening: the equivalent of 140 decibels. To put it another way, it would exert a force of approximately 100 lbs. on a (well-sealed) window.

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