Friday, March 13, 2015

More About Volcanoes

Here are some more questions from the precocious Cadee, this time about volcanoes.

Q: I am in 6th grade, and we are learning about volcanoes. I've been assigned to ask you some questions about volcanoes. 
Question: How many estimated volcanoes are in the world?
- Cadee C

A: ​The Smithsonian Institution Global Volcanism Program maintains a database that currently contains 1,562 volcanoes with eruptions during the Holocene period (approximately the last 10,000 years) or which are currently exhibiting any kind of unrest. There are many, MANY more volcanoes on the planet than that - they are just older. I have mapped volcanoes in Mexico that are at least 100 million years old, and I have mapped volcanoes in Venezuela that are at least 3 BILLION years old. In my office I have a piece of Mount St Helens volcano that was semi-liquid and oozing as red-hot magma just 9 years ago.

Making matters even more complicated is the question of what IS a volcano? When I served a 5-year assignment as the chief scientist for volcano hazards of the US Geological Survey, I asked one of my senior scientists to do a compilation of the Pacific Cascades. I thought he would come up with a list of 15 - 20 volcanoes - the recognizable peaks like Mt. Hood and Mt. Rainier. His final list had over 3,500 entries - he counted every small separate cinder cone and dike.

Q: Are there warnings before a volcano erupts?

A: Yes, in almost all cases if there are an adequate number of telemetered instruments nearby. Generally we first see the mountain swelling as magma starts to come up into the edifice from below. We "see" this generally with telemetered GPS networks or with satellite radar data (InSAR). Later, as the eruption gets closer in time, we start detecting magmatic gases like H2S (smells like burnt matches), and we begin to detect small earthquakes whose signatures tells us that rocks are being broken below the volcano.​

​You can't predict an earthquake, but you CAN predict a volcanic eruption if you have enough instrumentation in place on or near it.​

Q: Where do volcanoes usually form?

A: ​The vast majority of volcanoes can be found near and along tectonic plate margins - like the so-called Ring of Fire around the Pacific Basin. Some are harder to explain, except as hot spots that tectonic plates are drifting over, such as the Hawai'i chain. ​Some countries exist on Island Arcs - the entire land is made up of active volcanoes, like Iceland and Indonesia.

Q: How do some volcanoes change over time?

A: If magma moves up into a reservoir well below the ground and stays there for a long while, it can differentiate. By this I mean that crystals start to form, and the heavier ones drift to the bottom of the magma chamber. When the volcano erupts, the less dense material (the lighter-colored stuff with higher silica content, such as rhyolite) tends to come out first, from the top of the reservoir. Later the material from lower down in the stratified reservoir comes out - this tends to be darker and flows more easily (basalt). ​This ends up on top of the lighter, more silica-rich flows. 

This sort of process works until the percentage of crystals rises to the point of the magma becoming a crystal "mush". Then this sort of "flow easily stuff" does NOT flow more easily.

​Some volcanoes change over time because they get different magma feeds from the mantle as continents move to different places on the mantle with plate tectonics. An example like this would be some volcanoes in the western United States that rather dramatically change in character over long periods of time. ​

Q: Are there patterns in volcanic eruptions from the same volcano?

A: Yes, and we count on these patterns to roughly predict the future behavior of a volcano. This pattern recognition requires very careful geologic mapping and age-dating of the rocks, as you might imagine, to see what was erupted and when ​- and how. This is a rough art still, and volcanologists are always getting surprises - like when Four Peaks Volcano erupted in the Alaska Peninsula a few years ago.  ​No one had any idea that this volcano had ever erupted at all within the previous 10,000 years. ​We had no instruments close to it, so we had very little warning.

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