The following questions were answered by expert volcanologist Dr. Stanley Williams. Dr. Williams was online with Scholastic Network in February 1995.

How long does it take for a volcano to cool?
Volcanoes usually have a life of many thousands of years. Once a volcano has begun to erupt, it usually takes about ten years before that particular eruption comes to an end. Sometimes the eruption lasts for hundreds of years.

How does a volcano build up the pressure to erupt?
The magma (molten rock) which is erupted from a volcano comes from deep inside the earth — usually from about 150 kilometers deep. The pressure there is enormous. The pressure forces the magma to rise through the crust of solid rocks, creating a volcanic eruption.

Do volcanoes spew ash or lava first?
Volcanoes release ash at the beginning of an eruption because the amount of gas is very high and it drives the explosions. After that time, the lava may come out but it usually has very little energy, so it is not very dangerous.

How can you tell if a volcanic eruption might occur?
Volcanoes that are approaching an eruption will usually have unusual earthquakes and emit very different gases. Some of the volcanoes even change shape — like the "bulge" of Mount St. Helens that moved toward Seattle at about four meters per day. Forecasting volcanoes is still very hard because we don't usually have the right measurements before it happens. In Mexico in 1993, we measured a really high release of sulfur dioxide, one of the volcanic gases, and warned the Mexican government of the possibility of an eruption. It finally erupted just before Christmas. Forecasting an eruption is the main reason for our research and it is really very difficult. What we need, as scientists, is more eruptions! We need more chances to test our ideas or hypotheses.

How many volcanoes erupt every day?
The Smithsonian Institution has the Global Volcanism Network and a monthly bulletin about eruptions. About 50 to 60 eruptions happen each month. Some volcanoes are in constant activity — Stromboli, Kilauea, or Sakurajima, for example. There are many examples of volcanoes which show some sign of renewed danger and then erupt within an hour, though more commonly, within one day. Most eruptions last hours but some continue for weeks and months.

How many times does the average volcano erupt each century?
I would say that the average volcano erupts about one time every 100 years, but that varies from one volcano to the next. My favorite, Masaya, has been very active every 25 years, but has not had any important eruptions for about 200 years.

At what speed does a volcano erupt?
There are many different speeds that we try to measure or estimate with erupting volcanoes. When volcanoes erupt explosively, they throw rocks at velocities of 200-300 m/sec. After the initial eruption, the volcanoes may release a lava flow. The lava flow usually goes slowly — a few m/hour.

Can volcanoes blow up out the sides, rather than top?
Mount St. Helens is a good example of a volcano blowing out of the side. That is not an unusual thing.

What happens to the sea animals that live in areas around the volcanoes when they erupt?
I have not studied underwater volcanoes extensively, so I could not tell you exactly what happens to the sea animals during an eruption. I have heard that oceanic volcanoes can release strong acid water, which is harmful to sea life. This is happening at Fernandina volcano in the Galápagos. However, I think that most animals in the sea would be able to move away from the eruption fast enough so that they are not seriously injured.

What was the largest volcanic eruption?
Approximately 75,000 years ago, the volcano Toba, in Indonesia, had the largest volcanic eruption ever known.

What was the worst explosion in modern times?
The worst eruption was that of Tambora, in Indonesia. In 1815, this eruption killed about 90,000 people.

Where and when did the first volcano erupt?
The first volcanic eruption happened before the first human existed. The earth has had erupting volcanoes since just about the first years that it existed — about four and a half billion years ago. In fact, some of the best places for finding the fossils of ancient humans are in east Africa because they are buried in ashes of ancient volcanoes that erupted around them. Of course, the other planets in our solar system have erupting volcanoes on them, as well.