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Expert volcanologist Dr. Stanley Williams answered the following kids' questions in an interview with Scholastic.

What country has the most volcanoes?
Indonesia has the most volcanoes, by far. It is really a special place because there seem to be volcanoes all around, in all directions. Merapi (which means "mountain of fire") erupted in January 1994 and killed a few hundred people.

Are there any volcanoes in the U.S. that are threatening at this time?
The most important volcano in the U.S. is probably Rainier, which is not showing signs of activity but has produced very large eruptions. The danger is many people live close to Rainier.

Is it true that there are volcanoes in the ocean?
I would estimate the ocean contains 10,000 volcanoes! We just don't have much chance to see them because they are hidden away!

How are underwater volcanoes different from volcanoes that are above sea level?
Submarine volcanoes are very different from the volcanoes that are above sea level. Water has a higher pressure than air. This higher pressure can cause an underwater explosive volcanic eruption. One famous example of an underwater explosive eruption is Surtsey, a new volcano off the south shore of Iceland. When Surtsey erupted it punched through the sea and became an island!

Which is the biggest volcano?
The biggest volcano in the world is probably Mauna Loa, in Hawaii. It rises off of the seafloor to 13,000 feet above sea level or about 29,000 feet above the seafloor. Another huge volcano is Mt. Etna on the island of Sicily, in Italy.

How old is the oldest volcano?
The oldest volcano is probably Etna and that is about 350,000 years old. Most of the active volcanoes that we know about seem to be less than 100,000 years old. Volcanoes grow because lava or ash accumulates on the volcano, adding layers and height.

What is the longest period of time a volcano has been known to be active?
This is a tough one to answer because some volcanoes seem to erupt forever! Stromboli, in the Mediterranean of Italy, has been known to be erupting for more than 2,000 years. It is the "Lighthouse of the Mediterranean."

Where does the word volcano come from?
The term volcano is an ancient one. To answer your question, I turn to a book, Volcanoes: Fire from the Earth, written by Maurice Krafft. In his book, Krafft talks about how the Romans believed in Vulcan, the god of fire. The place where they believed he lived was inside the earth beneath the island Hiera. It is now called Vulcano and the word is used when referring to any of the active volcanoes around the world. Of course, different languages have different words for volcano. For example, in Japanese it is kazan, while in Indonesian it is gunung api, and in Spanish it is volcan.

Are all volcanoes mountains, or can they be flatlands?
Some very important volcanoes are not mountains at all. They look like deep lakes because they have had huge eruptions that make the ground sag down.

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 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 that 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 meters per second. After the initial eruption, the volcanoes may release a lava flow. The lava flow usually goes slowly — a few meters per hour.

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, a 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.

Why does the lava get so hot?
It is very difficult to melt rocks, because they are so strong. The temperature of lava flow is usually about 700° to 1,250° Celsius, which is 2,000° Fahrenheit. Deep inside the earth, usually at about 150 kilometers, the temperature is hot enough that some small part of the rocks begins to melt. Once that happens, the magma (molten rock) will rise toward the surface (it floats). Deeper inside the earth, the temperatures are so hot that the entire outer core is molten. The inner core is made of different chemicals, so they are not molten.

How do you measure heat in lava?
We measure the temperature of lava by putting a probe of metal into it. Our thermocouple tells us the temperature, in degrees Celsius.

When a volcano erupts, does it ever run out of magma? How does it "refuel" so it can erupt again?
A volcano occurs where there is magma rising from the mantle and burning its way through the crust. Volcanoes do run out of magma. That usually means that they become quiet and inactive for ten to 100 years until a new batch of magma comes up from deep inside the earth. This means that people get fooled by the quiet volcano, so they make the big mistake of thinking that it might be "extinct," which really means dead for all time. For volcanoes, it is dangerous to think that — volcanoes are just waiting for new magma to cause another eruption.

How does magma form?
Magma forms because the rocks are at high temperature and pressure deep inside the earth. Actually, it is only a small percentage of the rocks that actually melts because it requires so much energy to melt rocks.