Iceland, located on top of the Mid-Atlantic Ridge, is one of the most geologically active places on Earth. Its Katla volcano—buried deep under a huge glacier—is particularly large and active: It tends to explosively erupt every 50 or 60 years, although its most recent eruption was back in 1918. One type of near-by seismic activity is so-called long-period (Ip) earthquakes, which have shallow origins and magnitudes of less than 3.3. Swarms of those quakes are sometimes thought to indicate an imminent eruption; after more than 900 Ip events took place in October 2002, local authorities developed evacuation plans. But Kristín Jónsdóttir (shown here making field measurements) and her colleagues at Uppsala University in Sweden conclude that the glacier, not the volcano, is the culprit. As the glacier gradually flows down a long valley, part of it reaches a cliff where gargantuan 80-meter-thick ice sheets break off and drop 100 meters,...

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