Little bit of news coming out of Alaska this weekend: the Alaska Volcano Observatory raised the aviation alert status at Cleveland to Orange after a sequence of explosive eruptions were detected. At first, there has been no visual confirmation as seeing conditions didn't allow for direct observation, so how did we know there was an eruption (or several, in this case)? They were detected by infrasound — that is, sounds made by a volcanoes that are very low frequency, typically less than 20 hertz. This is well below anything a person can hear and are generated by magma motion and explosions during an eruption. Infrasound detectors can "hear" these sounds from thousands of kilometers away due to the ability of infrasound to travel easily through water, land and air, so they can propagate around the globe. Infrasound has been used numerous times to detect both near and far eruptions from Fairbanks, Alaska, where a number of infrasound receivers are located. Although infrasound technology has been around for decades, only recently has it been brought in as a key piece of volcano monitoring, as it can detect explosions like what occurred at Cleveland when both visibility is poor and seismometer coverage isn't available.
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