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neilcreek
07-17-2005, 03:57 PM
This blue area in Chad, Africa sticks out like a sore thumb in the middle of the rest of the desert. Can anyone guess what it might be? It's not water, and it seems obviously windswept from the northeast. In fact, zooming in, there are some places where obvious sand dunes seem to shield the ground from the blue, with brown sand appearing behind the trailing edge, suggesting whatever the blue is, that it's carried on the wind. The scale of this feature is huge! Zoom out and you can clearly see it, even when viewing the entire African continent.

I'd love to know what this is!

fishbne2
07-18-2005, 04:21 AM
That's a good one. After extensive searching, I came up EMPTY! According to everything I read there's nothing there but desert.

grav8e
07-18-2005, 01:20 PM
I know that Lake Chad used to extend up into that area but has been "disappearing" over the past few decades. So I don't know if we are seeing remnants or if it is unrelated.

fishbne2
07-18-2005, 07:04 PM
that's what I thought too, however the spot is way to high to be recents remnants of Lake Chad (IMHO)

jamocorke
07-18-2005, 10:41 PM
Could this be a dried up lake leaving salt deposits.

hypeserver
07-18-2005, 11:56 PM
This blue area in Chad, Africa sticks out like a sore thumb in the middle of the rest of the desert. Can anyone guess what it might be? It's not water, and it seems obviously windswept from the northeast. In fact, zooming in, there are some places where obvious sand dunes seem to shield the ground from the blue, with brown sand appearing behind the trailing edge, suggesting whatever the blue is, that it's carried on the wind. The scale of this feature is huge! Zoom out and you can clearly see it, even when viewing the entire African continent.

I'd love to know what this is!

I learned about this in Science class. It's permafrost the driest of deserts can get it. It's the constant freezing of top soil. You have to remember that deserts may be hot by day but get to freezing by night. My best guess is its permafrost that may have drifted in the wind.

Alex_UY
07-21-2005, 05:17 PM
it appears that the blue color is the result of a band combination in the base images. Look at 29║ 13' 42.4"N - 19║ 24║ 23.8"E... in this area the low-res image has blue color but high res image not. Maybe salts accumulation?

Hope helps,

Alex

hypeserver
07-21-2005, 06:52 PM
For some reason I still think it's drifted permafrost.

punk_sandwich
07-22-2005, 08:17 AM
Don't think it is permafrost. Have a look at the definition of permafrost in wikipedia (http://en.wikipedia.org/wiki/Permafrost): continuously frozen ground for more than two years. Rather unlikely in a desert.
There might be frost in the desert overnight but we wouldn't see it in the satellite pictures as they are taken during daylight (and probably around noon to avoid long shadows). Given the daylight temperature in the desert any frost would have melted by that time.
Turning on the water layer in GE makes me think this was indeed water and are now minerals left behind (IMHO). A map of watersheds in Africa (http://earthtrends.wri.org/maps_spatial/maps_detail_static.cfm?map_select=294&theme=2)
shows a lake at the position of the blue structure.

Cheers, Mic

Rwootton
07-22-2005, 05:20 PM
these appear to be salt flats or pans (like Lake Bonneville around Great Salt lake in Utah) formed by periodic heavy rainfall filling the basin followed by evaporation, leaving the mineral residue.

grav8e
07-22-2005, 05:45 PM
these appear to be salt flats or pans (like Lake Bonneville around Great Salt lake in Utah) formed by periodic heavy rainfall filling the basin followed by evaporation, leaving the mineral residue.

Which leads me back to my guess about Lake Chad. I'm still standing by that one... ;-)

Rwootton
07-25-2005, 01:53 PM
Actually Lake Chad is 350 miles to the southwest at the border with Niger and Nigeria, You are correct though that Lake Chad is shrinking. But the salt flats are called Erg du Djourab

http://www.un.org/Depts/Cartographic/map/profile/chad.pdf) :neutral:

Arashi
07-28-2005, 01:00 PM
As Rwottoon said, Lake Chad is far to the southwest.

My Worldatlas sais this region is named Bod elÚ Djourab, that would indicate a saltflat.
The Atlas also shows that this region is a very low bassin and periodic rivers flow in there from the mountains in the north and east.
And thats the blue stuff is drifting with the wind indicates that its a rather light sediment, not water or stone.
And Salt was the Chads most important export product for long time.....

So I would say the blue colour truly results from salt and other sedimented minerals like cobalt or such stuff, brought by the rivers that dry out in summer.

Genosse Pumuckl
07-28-2005, 01:09 PM
I had this strange idea, that we're seeing the sky being reflected in the sand, or in the lower hotter air

Alf
07-28-2005, 07:08 PM
Enable Google Earth's "water-overlay-option".

Arashi
07-30-2005, 02:00 PM
Good Idea Alf :)

For sure, thats sediment from the time when these water bassins are drought.

Stadsman
07-30-2005, 08:56 PM
The waterlayer is a great tool and a good suggestion on this one. Directly south of the placemark is another blue spot, this time surrounded by plants, suggesting the presence of water in this otherwise arid region (see also attachment for exact location). I agree with the earlier suggestion that this is a dried up lake of some sort and that the blue is the reflection of the mineral crystals.

3lumi
07-31-2005, 04:52 PM
On a german university page i found out that the area above is called Bodele depression. An area where trade winds blow sand and dust up to greenland. But i cant find anything about the "blue lake". Maybe a saltlake with a special crystal structure.

kisA
07-31-2005, 05:35 PM
hmm.. n1..