The following is a script from "Volcanoes" which aired on Jan. 5, 2014, and was rebroadcast on July 6, 2014 . Scott Pelley is the correspondent. Nicole Young, producer.
If you think we're living in an unstable world, well, just listen to this: only one percent of our Earth is solid rock. Most of the other 99 percent is an oozing, mass, churning beneath our feet like road tar at temperatures between 2,000 and 10,000 degrees. The Earth's crust is only 20 miles thick. When that cracks, one of the greatest forces in nature erupts. There are 1,500 active volcanoes. And, earlier this year, we first told you about three; one that caused the most recent mass disruption, another that's the biggest threat to a major city and a third, in the United States, that could wreak havoc all around the world.
We put together an expedition to be the first to reach the summit after the eruption. The volcanic landscape covered in ice isn't hospitable to life or convoys for that matter. The man in front of the truck is pointing out cracks in the glacier that would swallow us whole. We covered miles of forbidding terrain at walking speed.
When the trucks could go no further, we hiked with our guide, one of the world's leading authorities on volcanoes, Haraldur Sigurdsson.
Scott Pelley: Wow. That is astounding. Ho, look at that. Whoa.
Haraldur Sigurdsson: Yeah!
Scott Pelley: Oh, my God.
Haraldur Sigurdsson: Incredible. What a sight. You're looking right into the crater. Magnificent.
Scientists rate volcanic eruptions on a scale of zero to eight. This is a four which they call "cataclysmic."
Scott Pelley: Tell me what you`re seeing.
Haraldur Sigurdsson: It's an explosive eruption and the explosions are producing big clouds of ash that are moving up straight up into the atmosphere at the velocity of a few hundred feet per second. And throwing out huge rocks.
Scott Pelley: How big are these pieces that we see flying?
Haraldur Sigurdsson: Some of these are the size of cars.
Scott Pelley: And how high are they going up? Must be a thousand feet.
Haraldur Sigurdsson: At least a thousand feet. But they're still red hot. Maybe 2,000 degrees Fahrenheit.
Scott Pelley: What's causing these stupendous explosions that we're hearing?
Haraldur Sigurdsson: Well, the big booms that we're hearing are huge gas bubbles in the magma that are popping open. They may be 100 feet in diameter. And when they get closer to the surface, the pressure inside these gas bubbles is so great, that they blow off the magma that is ahead of them, and then they release the gas. And that's a big sonic boom.
Scott Pelley: Look at the Earth just erupting up into the sky. Unbelievable.
This is a great place to explain exactly where volcanoes happen on the Earth. The crust of the Earth, of course, is fractured like a broken mirror. And it's fractured into about 15 major plates called tectonic plates. Volcanoes happen all around the edges where the Earth's crust is fractured. And here in Iceland a major line runs right through the middle of the island. And the two plates are breaking apart. And that's exactly what you can see behind me now.
Tomlinson: No, no you don't see...
Scott Pelley: Or not.
What ruined our view was steam--it was exploding everywhere that the lava hit the ice. The ancient glacier was melting in a flash flood--carving canyons into the mountain.
The thermal shock also lofted a fine black ash that covered farms for miles. They call it ash but it really feels like sand. In Iceland, volunteers come out from the cities to help farmers dig out. These were bankers who brought their shovels from the capital, Reykjavik.
Haraldur Sigurdsson: And the best example of that occurred in 1815, when there was an eruption in Tambora volcano, in Indonesia. And a big, explosive eruption sent out an ash cloud up to about 30 miles. And it dispersed very widely. And also, a lot of sulfur came out of this volcano. And that led to global cooling. And produced what is known as the year without a summer in New England, in North America.
Scott Pelley: The year without a summer?
Haraldur Sigurdsson: Year without a summer, in 1816.
Scott Pelley: Because of this one volcanic eruption?
Haraldur Sigurdsson: On the other side of the Earth, yeah. And that type of event will occur again that eruption also led to big migrations out of Central Europe into Russia and great disturbances worldwide.
Scott Pelley: Which volcano on Earth would you say is most dangerous to people?
Haraldur Sigurdsson: The volcano that, where there is a very large population adjacent to it and on it, and that's Vesuvius, in Italy.
Vesuvius is our second stop. You might think if anyone knew better than to live by a volcano it would be the people around the most infamous mountain of all time. But today, in southern Italy, a metropolis spreads within striking distance of Vesuvius.
Mike Sheridan: Nobody wants to believe that the area that they live in could kill them.
We went for a look, up close, with American volcanologist Mike Sheridan. We flew over the cinder cone on the helicopters of the Guardia di Finanza a police force that helps keep watch the mountain.
Vesuvius is Sheridan's life's work. And he has warned the government it can't count on evacuating the number of people in harm's way.
Scott Pelley: And what is that number?
Mike Sheridan: Well, it depends on the type of explosion. If there's one like the last big eruption that occurred in 1631 there would be about 600,000 people. But if it is an eruption like the 2,000 year ago eruption that destroyed Pompeii the number could be up to 3.5 million.
"Nobody wants to believe that the area that they live in could kill them."
Pompeii, as it was Aug. 24, 79 AD, the moment it was preserved under more than 10 feet of ash and rock. The boulevards, the homes, the mosaics are the volcano's contribution to history. 'Round here they do a lively business in the dead. Citizens of Pompeii are frozen in timeless agony. About 16,000 were killed, sculpted where they fell. Scientists have a good idea of what these people saw after studying the evidence of what remains. Witnesses described the mountain rumbling for days before it launched a column of ash and rock 12 miles high which fell back as hell on Earth.
Today, from this control room, volcanologist Giuseppe Mastrolorenzo monitors the instruments that will provide Italy's early warning. He showed us that satellite view of those three and a half million people that all crowd around the cinder cone. One day it may be up to him to sound the alarm.
Scott Pelley: How much time will you have?
Giuseppe Mastrolorenzo: Probably just a few days. We can just hope that we will have weeks or months, but we cannot make a contract with a volcano.
Scott Pelley: So your friends say, "Look, it hasn't erupted in hundreds of years." And you must say, "That's the problem."
Giuseppe Mastrolorenzo: That's the problem. Yeah. I'm trying to convince the people that this quiet mountain can be a killer.
At the base of the "quiet mountain," the peaceful piazza of Torre del Greco is wiped out, on average, about once every 100 years, give or take. The bell tower survived the eruption of 1794. Today, old men rest their feet on rock solid evidence of what's coming next. Michael Sheridan told us Vesuvius has a long life ahead of it.
Mike Sheridan: It has different personalities and the last personality was rather benign but it's got some mean personalities down there that we don't want to experience in our lifetime.
There are bigger personalities among volcanoes which scientists call super volcanoes. Remember our eruption in Iceland was a four on a scale of eight?
Well, an "eight" would change life on Earth. Haraldur Sigurdsson told us there is a name for one of the places where that is likely to happen. It's called Yellowstone National Park--our third volcano. Old Faithful's here for a reason. In the northwest corner of Wyoming, the caldera is about 50 miles wide, so big you that can't see it from the ground. Below, is what science calls a "hotspot" where a vase plume of magma has pushed into the crust.
"It has different personalities and the last personality was rather benign but it's got some mean personalities down there that we don't want to experience in our lifetime."
Haraldur Sigurdsson: The floor of the volcano is breathing like an animal. It's rising, and moving up and down. Because of magma inside the volcano.
Scott Pelley: What's the history of eruption of the hot spot in Yellowstone?
Haraldur Sigurdsson: The last eruption was about 400,000 years ago, the last big one. That was a devastating, explosive eruption. The Yellowstone-size eruption will occur. Of course, we have no idea when. It's being monitored very, very closely. So there is no chance of it occurring without any warning. But it's a devastating event.
Devastating to aviation, communications and agriculture, volcanoes can change the course of history. Never before have so many people lived within striking distance -- 200 million worldwide. Science is good at warning of eruptions that are weeks away but, beyond that, it's impossible to predict which one is next or how big it will be.