Got Gas?

Not all volcanoes erupt with great intensity. Yes, some volcanoes explode violently, destroying everything in a mile radius within minutes. But, there are other volcanoes that seep out lava so slowly that you can safely walk all around them. Why is that? It turns out that the severity of the eruption depends mostly on the composition of the magma.

The erupting force of the magma generally comes from internal gas pressure. The material that forms magma contains a lot of dissolved gases -- gases that have been suspended in the magma solution. The gases are kept in this dissolved state as long as the confining pressure of the surrounding rock is greater than the vapor pressure of the gas. When this balance shifts and vapor pressure becomes greater than the confining pressure, the dissolved gas is allowed to expand, and forms small gas bubbles, called vesicles, in the magma.

The tiny gas bubbles have a much lower density than the surrounding magma, and so push out to escape. This is the same thing that happens when you open a bottle of soda, particularly after shaking it up. When you decompress the soda (by opening the bottle), the tiny gas bubbles push out and escape. If you shake the bottle up first, the bubbles are all mixed up in the soda so they push a lot of the soda out with them. This is true for volcanoes as well. As the bubbles escape, they push the magma out, causing a spewing eruption.

The nature of this eruption depends mainly on the gas content and the viscosity of the magma material. Viscosity is just the ability to resist flow -- essentially, it is the opposite of fluidity. If the magma has a high viscosity, meaning it resists flow very well, the gas bubbles will have a hard time escaping from the magma, and so will push more material up, causing a bigger eruption, known as an explosive eruption. If the magma has a lower viscosity, the gas bubbles will be able to escape from the magma more easily, so the lava won't erupt as violently. This is called an effusive eruption.

Eruptions

• Plinian eruptions can spew material as high as 30 miles in the air, at hundreds of feet per second, last hours or even days and can produce extremely fast moving lava flows that destroy everything in their path.

• Hawaiian eruptions generally aren't very destructive or explosive. Lava flows can destroy surrounding plants, but the flow is usually slow enough that people have plenty of time to get to safety. Many eruptions are named for the area where they are most commonly found -- Hawaiian eruptions are common to Hawaii's volcanoes.

• Strombolian eruptions generally aren't particularly dangerous. They thrust small amounts of lava 50 to a few hundred feet in the air, in very short bursts. These regular explosions can produce impressive booming sounds.

• Vulcanian eruptions can include many short explosions. They will launch football-sized pyroclastic bombs into the air and generally aren't associated with lava flow.

• Hydrovolcanic eruptions occur near wet areas and create something unique. The hot magma heats the water so that it becomes steam. They can cause mudslides and major flooding.

• Fissure eruptions occur when magma flows up through cracks in the ground and leaks out onto the surface creating what looks like a curtain of fire.

Volcano Structure
Most land volcanoes have the same basic structure, but shape and size varies considerably. The basic elements that these different volcano types have in common are: summit crater (the mouth of the volcano), magma chambers and a central vent that leads from the magma chamber to the summit crater.

The biggest variation in volcano structure is the edifice -- the structure surrounding the central vent that is built up from the spewing material. The three main volcano shapes are:

• Stratovolcanoes: These have a fairly symmetrical mountain edifice, which curves steeply near the relatively small summit crater at the top.

• Scoria cone volcanoes: The most common volcano type, these have steep slopes on both sides of the edifice, which lead up to a very wide summit crater.

• Shield volcanoes: These wide, relatively short volcanoes have a slightly raised center. They occur when erupted lava spreads out over a really wide surface area building up a shield-shaped dome.

WebQuest Online Exercise
Try this:
Using this Web site: http://volcano.und.nodak.edu/, find the elevation (in feet) of the following volcanoes. Arrange a list of the volcanoes in order from the shortest to the tallest. Using this list, create a bar graph of the volcanoes with the horizontal axis representing the 17 volcanoes and the vertical axis representing the height of each volcano.