Saturday, January 21, 2012

Volcanoes - Where ARE they?

Volcanoes fascinate everyone - even volcanologists who spend their lives mapping and monitoring them are just itching to get back into one. At the Cascades Volcano Observatory, interns working for next to nothing are knocking at our electronic doors all the time. People who see restive volcanoes all the time, however, are definitely more circumspect. It's interesting to me that the Volcano Disaster Assistance Program (VDAP) international emergency response guys go out of their way to maintain a healthy distance. In conversations, when you delve a bit into it, something stands out. Virtually every volcanologist at CVO knows someone, once a friend, who is now dead because of a volcano.

Volcanoes kill. They have killed far more people than any earthquake. I'm not talking only about something fiery and explosive here. In 1984, the city or Armero, Colombia, was destroyed in just a few minutes by mud. A Lahar killed about 23,000 people just that quickly. A Lahar is a river of volcanic mud set loose by glaciers, water, and volcanic debris. Think of a 5-meter-high wall of wet concrete coming at you at 50 kilometers per hour. Another killer is a pyroclastic flow: a red hot cloud of gas and debris is a relatively common occurrence on the flanks of stratocone volcanoes, and nothing can stand in their way. In 1902, Mount Pelee on the Island of Martinique sent a fiery cloud - in French this was called a nuee ardent - that killed over 30,000 people almost instantly. These have great reach. In May, 1980, Mount St Helens destroyed about 140 square kilometers of forest in southwestern Washington State. The initial blast following the triggering earthquake may have actually reached supersonic speeds. Beyond the zone of meter-thick trees flattened, scattered, and shredded like match-sticks, there was a zone of trees, kilometers wide, that turned red-brown and died - just from the scorching heat.

So where are volcanoes located? In surprisingly predictable places. Stratocone volcanoes - the generally dangerous, explosive ones - are found all around the Rim of Fire. Wherever there is a continent being rammed up over an oceanic plate by tectonics - you will find volcanoes just inside the continent margins. The down-going slab of oceanic plate undergoes something called partial melting. This is material mixed with volatiles like water and CO2 from the former seafloor, that melts and floats up through the Earths crust just inside the continental margin. The Cascades volcanoes in the Pacific Northwest are classic examples. Virtually ALL of Japan is another aggregate-volcano example.

Where continents are splitting apart is another place where you will find active volcanoes - east Africa and the middle of the Red Sea are loaded with them, for instance. Iceland is merely an above-water part of a 25,000 kilometer-long oceanic spreading center that tracks the length of the Atlantic Ocean and winds all the way around the world. That's a 25,000-kilometer-long volcano.

Another kind of volcano is the "MORB" kind - Mid-Ocean-Ridge Basalt. This you find creating islands on top of oceanic plate that is drifting over a mantle hotspot, like Hawai'i in the middle of the Pacific ocean, or Reunion Island off the coast of Madagascar.  In fact, the largest mountain on Earth is not Mt Everest - but Mauna Loa on the Big Island of Hawai'i. It is 13,800 feet tall, standing in 25,000 feet of water, and its immense weight has depressed the ocean floor by yet another 7,000 feet - this adds up to 14,000 meters in all. Mauna Loa and Mauna Kea don't look at all like a stratocone volcano - instead, they look like the laid-down shield of a Greek or Roman soldier: a long, broad hill that just happens to be bigger than any ordinary hill you will ever see in your life.

Volcanoes are grand, mesmerizing monsters - but at least their eruptions are moderately predictable (more on that later).


I was wondering what type of lava/pyroclastic events are common of composite/strato-volcanoes? I thought it was Aa lava because of its high viscosity. Would it be more common for bombs or cinders to erupt from the volcano? I am not sure about this and was hoping that I could get some help.
Andrew B.

The short answer is all of that and a lot more. Most volcanoes erupt a wide range of compositions over their life-spans. The compositions range from very dark-looking basalt to very light-colored rhyolite, and that range of compositions is partly defined by the silica content (and this silica content is what primarily controls the types of feature you see), and partly by the other minerals in the extrusion. Water, in the form of rain or mountain-peak glaciers, and dissolved gases in the magma itself also figure prominently in that form that you will eventually see. A good place to roam around and research this sort of stuff is


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