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February 13, 2011


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So, I went to Google maps to look more closely at the area, and wondered what the oval shapes just south of the falling dune are. On the satellite view they look like depressions, but I imagine they are rocks. I guess I just answered my own question. Anyway, I'm looking at the desert! From space! So thanks for posting this stuff about Bagnold.

Yes, rocks - isolated outcrops. But there are, in the region, all kinds of circular and oval features, often depressions, that have been the subject of much speculation with respect to meteorite impacts. A number of these features are volcanic craters with raised rims, it's been suggested that some are of hydrothermal origin, but some are definitely of impact origin. Quite recently one of these has been located to the South of the Gilf, close to the Sudan border - see http://www.sciencedaily.com/releases/2010/09/100923081902.htm. This can be seen on Google Earth if you zoom in on 22 degrees 01' 06.34" N, 26 degrees 05' 15.50" E.

Then if you head west from there towards the Libyan border and look around 22 degrees 18' N, 25 degrees 30' E, you'll see some of the volcanic craters with some spectacular associated dike swarms. I have some photos - indeed, the one in the "about" section of the sidebar of this blog was taken there, but I have geological ones too...

I have to say that the Google Earth folk have done an amazing job of stitching together the imagery from this are into an almost seamless whole - a few years ago it was just a patchwork of different resolutions and displays. The trouble is that, once you start poking around, it's difficult to stop!

Thanks for the comment!

It IS really difficult to stop. I, too, have been very grateful for the more even resolution on Google Earth, although some of the places I look (the mountains of Tajikistan, for example) are still pretty fuzzy. Thanks for the coordinates. I'll be looking at them presently.

Whoa. The resolution on those volcanic craters in unbelievable. What, if I may ask, is a dike swarm?

Oops - sorry about that - fell instantly into geo-jargon!

A volcanic dike forms when molten magma finds it way up to the surface through a more or less vertical crack in the rocks, and, if it reaches the surface, erupts (many of the Icelandic eruptions are like this, often called "fissure eruptions"). When everything has cooled off and solidified, the volcanic rock filling the crack is generally harder than the surrounding rock aand so, when erosion strips away the surface, the dike is left as a kind of topographic wall (Shiprock in New Mexico is a famous example). There's quite a good explanation at http://www.geology.um.maine.edu/geodynamics/AnalogWebsite/UndergradProjects2008/Nate%27s_website/Web%20Page/HTML%20Files/Introduction%20page.html

Dikes rarely occur singly but rather in "swarms", lots of them often radiating out from a volcanic center (again as at Shiprock). A good example of the ones I was referring to in Egypt can be seen if you zoom in around 22 degrees 12' N, 25 degrees 07' East - and tilting the Google Earth view shows the "walls" quite effectively. The odd thing about the volcanic features in this area is that they are a long way from where the continents are splitting apart (the Red Sea) and are situated essentially in the middle of old, stable, crust; but no-one seems to have done much work on them.

Thank you for the explanation. I did not know the proper term for the Shiprock formation, though I did drive through that area with a geologist last September, and learned a ton.

The photos were great and attractive; these are one of the best sceneries I've ever seen. I enjoy reading your blog, and it is relevant to the daily life of the people now. I am looking forward to read more thoughts and ideas here in your site.

Glorious photos, thanks. Speaking of Google Earth, did I get this from you, or am I telling you about it, or neither?

Richard - welcome back!

I'm not sure of the answer to your question - I certainly have much enjoyed this great series of images, but exactly what the link itinerary was is lost in the mists of time (never mind my feeble brain).

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