By Robert Hazen, Ph.D., George Mason University
A volcano is a mountain, or some other feature, that spews out molten rock, and therefore causes new material at the surface of the Earth. Volcanic eruptions are dramatic and violent events, and volcanoes are a great example of how small, incremental changes can lead to big transformations over long periods of time.
How Volcanic Eruptions Occur
Volcanoes can occur either on the land surface or at the bottom of the ocean, on the ocean floor. All volcanoes require a plumbing system; you have to somehow bring a molten rock from the interior up to the surface.
If you can imagine beneath a volcano, you have pipes and conduits through the rock. You may have a magma chamber, or you may have a system of cracks and fissures where that molten rock accumulates.
Then, gradually, the pressure inside the Earth is large enough that it forces that molten rock up and out onto the surface. That process may be facilitated by the fact that magmas generally dissolve and incorporate water (H2O) and CO2, which are volatiles and gases; and these help make the melt more fluid.
Also, as the material comes near the surface, it causes more of a violent eruption, more of an explosive kind of behavior, as gas bubbles burst and explode, much like a Coke bottle. If you shake it up and open it, it’ll foam all over the place. Volcanoes can do the same sort of thing.
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Various Types of Magma
Many magmas, like the black basalts that flow down the sides of Hawaii’s volcanoes, are often quite fluid. They can often flow down the sides and the flanks of the mountain for miles, and they cause damage. However, they very rarely cause death because they flow in a stately, slow fashion. Photographers can move right up to them, and take a picture, and walk away safely. Even though those are dramatic volcanoes, they’re not particularly dangerous.
But there are other magmas: those especially rich in silicon and oxygen. Silicon-oxygen bonds are very strong, and they form stiff arrangements. They are such stiff and hard magmas that even though they’re in a molten state, they come welling up in a very viscous, very rigid mass.
They can plug the top of the volcano, and then when pressure builds up, the volcanic eruption is not a smooth lava flow, but rather an epic explosion that spews out hot gases and incandescent debris that can flow down the mountain at almost supersonic speeds, killing everything in its path; much, much more dangerous.
Learn more about the Earth’s topography.
Most Dangerous Volcanic Mountains
It’s a sobering fact that, even in North America, there are hundreds of thousands of people who live near and around the shadows of volcanoes. The most dramatic of these volcanic activities occur, of course, in Hawaii, where the very slow flow of lava destroys many homes on an annual basis; but nevertheless, does not take a lot of lives.
Much more dangerous are the kinds of volcanoes that occur near Washington state and Oregon state; the type of volcano near Mount St. Helens, or mountains like Mount Rainier, could, in fact, erupt in a very violent, very sudden event and cause a much larger loss of life and property.
When Mount Pele erupted in the Caribbean island of Martinique in 1902, 29,000 people were killed instantly.
Are Volcanic Eruptions Predictable?
As dangerous as volcanoes are, it is possible to make some predictions about when they might erupt, and much effort is going into volcano monitoring of this type of volcano. Around Mount St. Helens, for example, before its big eruption, most geologists knew that there was going to be an eruption sometime soon, and they knew this for several reasons.
For example, they could see a gradual swelling of the top of the mountain. If you measure the altitude or just the shape of the top of the mountain; you can see it bulging out. You can do infrared measurements; you can measure the heat flow coming out of the top of the mountain, and as magma comes closer to the surface, the top of the mountain actually begins to get hotter.
You can also make seismic measurements because as the magma is flowing towards the surface, you have small earthquakes, small shakes of the ground, which cause the whole mountain to jiggle as the magma works its way up.
Learn more about the three major types of rock.
Volcanoes, the Remote Phenomena
Volcanoes seem like remote phenomena; they seem like they’re not very common in everyday life, but it might surprise you to know that they have really played a major role in shaping the planet. In fact, 80 percent of the Earth’s crust includes volcanic rock as part of the surface.
This is because volcanoes, even though they erupt only occasionally and only in a few places, over geological time, as you add up this material, over millions of years, you start creating a layer, a mantle of volcanic rocks over the entire surface of the Earth.
Common Questions about Volcanic Eruptions
Volcanic eruptions occur when molten rock accumulated underground comes out to the surface due to pressure. As the material comes near the surface, it causes a violent eruption, almost like an explosion.
There are magmas that are not very dangerous, like the black basalts that flow down the sides of Hawaii’s volcanoes. They are very fluid. They do cause damage, but they very rarely cause death because they flow in a slow fashion.
Then there are other magmas: those especially rich in silicon and oxygen. They are such stiff and hard magmas that even though they’re in a molten state, they come welling up in a very viscous, very rigid mass. They can flow down the mountain at almost supersonic speeds, killing everything in its path, and are much more dangerous.
It is usually possible to predict a volcanic eruption. The top of the mountain can be seen bulging out before an eruption. One can also do infrared measurements; as magma comes closer to the surface, the top of the mountain begins to get hotter. One can also make seismic measurements because as the magma flows towards the surface, there are small earthquakes which cause the whole mountain to jiggle.