but hekla could be far more disruptive. 3,000 years ago, hekla produced 75 times more ash. the concern is she might be about to do it again. thordarson: the pattern that we have detected is suggesting that right now it is at the end of a phase with eruptions diminishing in size, indicating that we might be entering a new phase which usually starts with a big, explosive eruption. so it's possible that hekla might produce a very big, explosive eruption in the near future. narrator: what chaos might we expect if this happens? claire witham has been asking the same question. she's part of a rapid response team at the london volcanic ash advisory center at the uk's met office. it's the first line of defense against volcanic ash for europe's aviation industry. and when eyja erupted, it put the team through their paces. we hadn't seen anything that had really blown an ash cloud from iceland to our area in recent history, so it hadn't really got into people's minds that that might happen. the eyjafjallajokull eruption went on for about 39 days. people were working around the clock

hekla-- a menacing peak almost 5,000 feet high.historically she has erupted violently, without warning belching vast amounts of ash. and katla-- a giant volcano 18 miles across hidden under 2,000 feet of ice. katla is decades overdue for an eruption. she's currently rumbling so volcanologists are most worried about her. so, why does iceland have so many threatening volcanoes packed into such a small area? it's a result of its unique location. iceland lies in the north atlantic, between north america, greenland and europe. here, two of the earth's crustal plates are being pulled apart and iceland straddles their boundaries. below is a hot spot-- a huge upwelling of hot rock from deep within the earth. as the rock rises, it produces magma. the magma forces through cracks in the crust until it erupts at the surface forming active volcanoes. thordarson: one of the reasons why it is difficult to predict volcanic eruptions is that we're trying to predict the behavior of something that we can't see. we can't see the magma that actually is