Volcanoes: Nature's ticking time bombs

There are about 1,500 active volcanoes that scientists know of on Earth, but ask them when they might erupt and the answer is anytime -- they don't know.  Nor can they predict the severity of the eruptions. Scott Pelley speaks to volcanologists about three volcanoes around the world for a 60 Minutes story to be broadcast Sunday, Jan. 5 at 7 p.m. ET/PT.                

In Iceland, he witnesses firsthand the spectacular eruption of Eyjafjallajokull in 2010 -- an event scientists deemed just a four on a severity scale of eight.  Pelley then visits perhaps the most famous active volcano on the planet, Italy's Mount Vesuvius, which sits ominously close to millions of people. But he also learns about the volcano on Earth most likely to produce an eruption that could be an eight -- a globe-changing event --  and it's right here in the U.S. under Yellowstone National Park's "Old Faithful" geyser.