Eruptions

Coulées!

As a volcanologist, I am partial to impressive lava flows, especially in volcanoes that erupt material that you’d think wouldn’t produce big flows. For example, there are quite a few volcanoes in the Chilean Andes that erupt dacite lavas, which are relatively viscous (sticky), so you might expect it to erupt explosively. However, you can get large dacitic to rhyolitic lava flows, quite commonly, and these large flows are called coulées (a “volcanic dome flow”).

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Lllullaillaco volcano on the Chile/Argentina border. Note the very prominent coulée with flow levees. Click on the image to see a larger version. Image courtesy of the NASA EO.

I bring this up because the NASA Earth Observatory posted a great shot of a volcano on the Chilean-Argentine border – Llullaillaco – where you can see the gorgeous coulée that came down the side of the volcano (see above). The flow has a flow front that is likely tens of meters tall with impressive flow levees on each side, where lava that was erupted first was pushed out of the way by subsequent lava (like a bulldozer).


Chao flow in Chile. Image courtesy of Volcano World.

You see these types of flows commonly in the Andes, most famous in the biggest dacite lava flow on the planet, the Chao Dacite (see above). This flow is over 14 km long, has obvious flow features like pressure ridges and a flow front that approaches 500 meters tall! There is some pumice associated with the flow that might have come from the clearing of the vent before the eruption, but it appears to be dominantly effusive – you can see on the image that there isn’t even an obvious crater from where the flow erupted.

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Lava flows on the south flank of Aucanquilcha, Chile. Image by Erik Klemetti, November 2000. Click on the image to see a larger version.

Another Chilean volcano with impressive coulees is Aucanquilcha (see above), near the Chilean-Bolivian border. I have a special fondness for Aucanquilcha because I wrote my dissertation on the volcano (and even have a paper on its volcanic history). The edifice itself is a series of long dacite lava flows, some of which reach 5-7 km in length, with very little preserved evidence for explosive volcanism going with it. Some of the flows (see above) have prominent flow levees and steep flow front. The volcano as a whole is really four stacked domes (see image at the GVP – taken by me!) of these coulée-style dacite flows.

Comments

  1. #1 Randall Nix
    March 31, 2010

    Erik, It’s basaltic but at 75km how about the Carrizozo lava flow in New Mexico:
    nmnaturalhistory.org/volcano/carrizozo.html
    nmnaturalhistory.org/volcano/sb1_large.jpg

  2. #2 Diane
    March 31, 2010

    Thank you for posting such beautiful picturs, Erik. I now know what a coulee is. I had heard of the Grand Coulee and I think we saw the Grand Coulee Dam when I was about 9. Now I want to take a look at what they are calling the Grand Coulee in Montana. This is exactly what I was asking about when I inquired about rhyolitic lava flows only I didn’t know what they were called. I figured they existed somewhere and now I know more about the subject.

    Hat tip to you, Erik.

  3. #3 Diane
    March 31, 2010

    @Randall, thanks for your post and the info about Carrizozo. I checked it out and I would like to go there some day, but that is not possible right now. Oh well…

  4. #4 Passerby
    March 31, 2010

    In Eastern WA State,(Columbia Basin Flood basalts)..

    We KNOW coulees!

    http://www.bentler.us/eastern-washington/east-state-map.aspx

  5. #5 Randall Nix
    March 31, 2010

    Diane If you ever do go that way be sure to check out the Owl Bar in San Antonio, NM…best chili cheeseburgers and chili cheese fries in the world….An order of those and a cold beer is the best way to finish off a prospecting day;) Also be sure to see Pinos Altos, it’s just above Silver City, there are some great places to pan for gold there on BLM land which are easy access from the road:) There is some great volcanic landscape all around the Gila National Forest area just North and Northwest of Silver City….I hope one day you will be able to visit there.

  6. #6 Anne Jefferson
    March 31, 2010

    Careful, commenters! The coulees of Washington’s scablands region, or western Wisconsin’s Driftless Area, or Louisiana’s floodways are NOT lava flows.

    The term coulee is a colloquial geomorphological term that is usually applied to drainages or valleys. The wikipedia article is actually somewhat useful here.

    I suspect that term coulee applied to lava flows has to do with the levees on the sides of the flows, creating a sort of valley within the flow, even though the whole thing is higher than the surrounding landscape.

  7. #7 Doug C.
    March 31, 2010

    Erik et al.
    Is there any data that explains why the Andes have such wonderful examples of viscous flows. Does the elevation (ASL) have anything to do with depletion of gasses in magma bodies making their products less explosive? Just curious.

  8. #8 Diane
    March 31, 2010

    @Passerby, my mistake. I thought the Grand Coulee was in Montana. Oops!

  9. #9 EKoh
    March 31, 2010

    C’mon Erik, I though you would’ve gone with “Coulees are cool” for a title.

  10. #10 Passerby
    March 31, 2010

    Geological definition
    http://www.webref.org/geology/c/coulee.htm

    The Central WA coulees are basalt, but are flood excised. It does meet technical criteria, def 1b.

    b. A term applied in the Northwestern United States to a dry or intermittent stream valley, gulch, or wash of considerable extent; esp. a long, steep-walled, trenchlike gorge or valley

    Detailed map of basin coulees including the Grand one, for Diane.

  11. #12 Henrik
    April 1, 2010

    Erik – “you can see on the image that there isn’t even an obvious crater from where the flow erupted”

    What of the upside-down horseshoe shaped feature just below and to the left of the “A” (not the “A” in the top righthand corner)?

  12. #13 Erik
    April 1, 2010

    Henrik – I believe that is just a scarp on an adjacent hill. The vent itself is though to be beneath the high point on the dome (just to the lower left of “A”).

  13. #14 Henrik
    April 2, 2010

    Erik , there are at least three features close to the “A” that could be interpreted as broken volcanic cones – at 7 o’clock (the one you referred to?), at 9 o’clock and at 10.30. Judging the relative height of features from a height is tricky (we had a 2hr lecture on photo-recconaissance during the bat. cdr. course at MHS), but to my untrained eye it would seem all of these are at a lower altitude. Also, the flow patterns seem to converge at the horseshoe shaped feature. Presumably, “the call” has been made by professionals investigating the flow on foot, which just goes to show how tricky aerial recognition can be!

  14. #15 Diane
    April 2, 2010

    Thanks for the map, Passerby. That area looks not too far from where some friends of mine live.

  15. #16 Mu
    April 5, 2010

    I try the owl bar about once every five years hoping it got better and I just hit a bad day last time. Was there again two weeks ago, same stuff as before, decent green chili, but nothing to write about. I agree it’s probably the best burger for 100 miles around, but that’s because there is nothing for that far ;).

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