The following questions were answered by expert volcanologist Dr. Stanley Williams in February 1995.

How are volcanoes formed?
Volcanoes are really mountains that build taller and taller, with time, as they erupt. That means that molten rock, magma, comes from within the earth and erupts onto the surface. The volcano might be explosive and produce ashes or be effusive and produce lava. The explosions are usually first because there are lots of gases inside the magma. When you have a bottle of soda pop, you do not see any bubbles of gas, but when you open it, bubbles form almost instantly. Once the gas bubbles have all escaped, the soda is flat. Once the magma is flat, a lava flow comes out. Most of the volcanoes from around the Pacific Ocean are composite, which means that there are layers of ashes and lava. Most volcanoes are 10,000 to 100,000 years old — it takes time for them to grow big.

What gases do volcanoes emit?
Inside the crater of a volcano there is nothing alive and many small fumaroles (holes) release nasty gases. There are many colorful minerals being deposited from the gases as they cool. The most important gas is water, and then carbon dioxide. These two important gases are not poisonous. Sulfur dioxide, hydrogen chloride, and hydrogen fluoride are emitted, as well. They are strong poisons and cause pollution problems.

What kinds of rocks do volcanoes make?
Volcanoes make many different types of rocks. For example, black shiny rocks with only a few crystals are usually basalt. The opposite — white shiny rock with many crystals and often many bubble holes inside the rock is rhyolite. In between are andesites, which are light gray and usually have large box-shaped crystals called plagioclase. They come from the Andes Mountains, which is a chain of volcanoes in South America. The Hawaiian islands are mostly made up of basalts, so they are famous for their beautiful black-sand beaches.

How many volcanoes have been identified in the world?
We know of at least 1,500 active volcanoes around the world. That is a big increase from the number that we used to think was correct. It means that more people are searching the earth for them. A graph of the number of volcanoes of the world shows that it goes up just about as fast as the number of people on the earth does.

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.

How many volcanoes are there in the United States?
The lower 48 states in the U.S. have about 40 volcanoes that we think have had very recent activity, so they must be considered as active volcanoes. In Alaska, the number is more like 60. When we talk about whether a volcano is active or potentially a threat, it is important to look at the past ten years. Most of the important eruptions and disasters have happened at mountains that were not even recognized as being volcanoes, for example Pinatubo (Philippines, 1991), El Chichon (Mexico, 1982), Arenal (Costa Rica, 1968).

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?
On the surface of the earth, we know of at least 1,500 active volcanoes. 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.

Do people live near volcanoes?
About 500 million people live close to active volcanoes! Most of them do not realize that the earth is very much alive and that they need to pay attention to what it is up to. More people need to learn about volcanoes, like you are doing.