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Tectonic Colonic

When Tungurahua erupted earlier this month, three hundred local people were forced to evacuate. Tungurahua is the most active volcano in Ecuador, which is itself a volcanically active region.

This image, captured by NASA’s Terra satellite on October 25, shows Tungurahua mid-eruption; a plume of dark, rather than white, volcanic ash blows westward from the summit, indicating that this blast was composed more of ash than steam.

Eruptions like this serve as a reminder that geologic time includes now.

More below the fold.

The Andes were formed by eruptions like this one. The Andes lie at the junction of two tectonic plates, one underlying the Pacific Ocean, and one underlying the South American continent. Where the two plates meet, the relatively denser oceanic plate sinks under the more buoyant continental plate. As the oceanic plate sinks towards the mantle, it melts, and plumes of melted rock burrow upward through the overriding continent. Eventually, the magma reaches the surface and and reveals itself–often forcefully–as blasts of magma, ash and steam.

This USGS site has a map of historical seismicity in the Andes. Bonus points to the (non-geologists) who can explain why earthquakes get deeper, moving eastward across the range…


  1. #1 Thomas Palm
    October 30, 2006

    It seems to me as if the plume splits into two forks in the middle of the picture, which is an interesting puzzle for non-meteorologists.

  2. #2 kevin v
    October 30, 2006

    here’s another for the non-atm people: why is the plume moving to the west instead of to the east? if this were a Cascades volcano the plume would be moving eastward….

  3. #3 Sarah Dasher
    October 30, 2006

    Ooh! Good question, Kevin. I don’t know much about this, but I’ve got some guesses. First, I think the tradewinds (which blow southeast to northwest) dominate in Ecuador, whereas the Cascades have prevailing westerlies. Second, is it possible that while air travels over the Cascades (dumping precipiation in the process), the Andes are too tall, and air gets looped back on itself instead?

  4. #4 kevin v
    October 31, 2006

    what you said first. the continental US is in the midlatitude westerlies zone while Ecuador is in the (easterly) Trade Winds zone.