Tuesday, March 19, 2013

Geo 365: March 19, Day 78: Hole-In-The-Ground

Dana admires Hole-in-the-Ground, a classic maar on the tableland to the west-northwest of Fort Rock. There's a foot path to the bottom from this point- you can see it to Dana's left. However, most geology people probably are aware of the problem of hikes that start downhill: the easy part is when you're fresh, the hard part comes when you're not. So I used to tromp down with kids, but didn't on this trip. (It's a serious slog, coming back up that slope, roughly 400-500 feet  (correction- closer to 200 meters or more, looking more carefully at the guide linked in the next paragraph, so maybe a bit less than) 700 feet vertical, and relentlessly steep.)

The reason I chose this photo as an introduction to Hole-in-the-Ground can be seen above that little tree past and to the left of the central playa, on the opposite wall. The offset in that layer of basalt marks a fault which bisects this crater, and it's thought that this was the weak feature that allowed the basaltic magma to migrate upwards at this spot. More detailed information on HIG can be found in USGS Circular 838, at mile 25.1 in this road log.

In one of those nice little bits of synchronicity, tweeted yesterday, "What is this little perfect round hole in Africa? Likely in Ethiopia near 7.99N, 37.19E. " I replied," Given location- not too far off axis of rift- & water, guessing maar. Funny, just looking at another: " And a few minutes later, I was able to pin it down in FlashEarth: " Yep, definitely looks like a maar- explosive interaction between magma and groundwater, steam blasts hole open." and " location, oriented ~same, scale ~same, as your photo Maar def.: " However, my comments got no reply, so I don't know if he saw them.

Photo unmodified. August 20, 2011. FlashEarth Location. (You can see the fault on the crater wall in that view, almost directly across from the center cross hairs.)


Cujo359 said...

For me, Hole In The Ground provided on object lesson about thinking, in league with another crater. There's a panorama of the crater at that link, BTW. Feel free to borrow it if you want.

And thanks for showing it to me.

Lockwood said...

Thanks for the link- as it happens, I didn't think to get a nice set of photos suitable for a panorama, a fact over which I've been kicking myself ever since.

The mode of formation is different from Crater Lake- that is a collapse feature, a caldera, while this is an explosive feature, a true crater. (Though looking at the cross-section in that road log, it definitely looks like subsidence was involved.)

Still, nice panorama and discussion- I'll link it tomorrow.

My pleasure, and I've been sort of blown away by just how much great stuff we saw during just one day- I've been mining August 20, 2011 for over a month now!