What can you tell me about this place? (Africa)
I am something of an amateur geologist. By that, I mean that I like to pick up interesting looking rocks whenever I find myself in a new places and give them fanciful names ending in "-ite" or "-ine."
Yesterday, I was reading about Lesotho and the wikipedia article mentions the importance of the Orange River to the economy, ecology, and geography of Lesotho. So I started reading about the Orange River and in that article there was this aerial or satellite photo.
When I first saw the photo I had only a few questions:
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Why is there such a marked difference in the landforms on the east side of the river compared to the land on the west? It seems the land on the right side experienced greater weathering than on the left.
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Why is there a whitish line around the darker reddish-brown areas to the east? Is that tuff?
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Why does the landscape on the east appear so much darker? Is that due to the carbon content?
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What is that spine-shaped reddish-brown formation? Zooming into it as far as possible the "lumpy" texture to that terrain is very reminiscent of coral beds (pillow lava?)
While I suspect that the answer to all my questions will basically be volcanism, I can't help but wonder why a lot of the landscape seems pretty alien to me (perhaps unique is a better term.)
Intrigued by the photo, I decided to "look around" the area and found the location here on Google Maps. It appears that most of the area is the "Aussenkehr Nature Reserve" but Googling for "Assenkehr Reserve geology" gave me a whole lot of nothing.
After looking at the area on a map, the number of questions I had exploded:
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About 30 miles away from the river to the east is large white area. Is that a salt bed from a lake/inland sea? If so, when did it evaporate and how long did it exist prior?
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Just beyond the reddish-brown spine-shaped formation, the land is much smoother. The sharp jagged terrain of the reddish-brown part gives way to region of smooth hills that appear, to my untrained eye, to be more reminiscent of an underwater landscape than anything else. You can see just how sudden the change from one type of landscape to another is here. What is the cause of this sudden transformation in the landscape?
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Just to the north of the area in the photo we see this example of folded mountains here. Why do the folded layers appear to stop just past the river to the south west? What is the process that resulted in that amazing landscape (there appears to be a fold roughly perpendicular to the one that created the mountain)?
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Immediately before the folded mountain(s) to the north there are smaller foothills here that are much smoother and have "tails" (I'm not sure of the real term for that.) They run in a north-south line and the tails extend to the east. What caused the tails formations and why are the tails darker? If you zoom into the hills they have a very finely lined texture (more visible in the hills to the north here) What is the origin of those lines?
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To the south is a huge easily delineated area of rock that is somewhat darker than it's surroundings. It has multicolored layers and is weathered in such a way that it is somewhat like the grain pattern of fine wood veneers. What causes the multicolored layers there but nowhere else?
I'll just stop there. I can probably ask another hundred questions about this area but I don't want to make this post so long that no one will read it. Hmm, I might have already crossed that line!
Thanks in advance for any information you can give me.
Posted Jun 25, 2014, 3:36 PM UTC
Hmm… to be honest, this sounds a lot like a school project.
Since you have an intellectual curiosity about you, I'll pose some questions whereby you can seek out the answers...
A couple things: weathering- what causes it? how do landscapes form? This is a pretty broad, yet intuitive problem.
what evidence do you have for volcanism? Color of rocks? Something else?
As for the folded ridge next to the river- that's a unique structure. How do rocks fold? Could this rock be uplifted? or could something else, aside from uplift, create this contrasting feature? If the rock experienced vertical displacement from the surrounding units, did this occur synchronously with the folding?
White stuff: could be limestone, could be tuff, could be a rhyolite flow. Hard to say- color doesn't help much in geology. Not on a first-order at least. What's the tectonic history here that would suggest tuff? Limesone? Rhyolite? Another evaporite?
Look at the landscape here- what active features do you see? It takes some form of energy to shape a landscape- whether a subduction zone, or an aeolian dune. What are the energy sources here? Do we have an active or past regional (e.g. orogenic) component? Active or previous localized energy source component? Look for evidence of either, and then think about basic stuff- like going for a hike, or flying over mountains- we've all experienced firsthand how the landscape has been actively shaped around us- that is, if you've ever experienced the outdoors- hike, bike, car, plane, or even sat and watched the news...
I'll take that as a compliment and an indication that I am asking the (somewhat) right kind of questions. :D
I assure you, I am no student. I am a middle-aged, professional curious person. I have never even set foot inside a university and everything I know about geology comes from Wikipedia and that Iain Stewart fellow.
But I will play along in the hopes that I will learn something (or, even better, lots of somethings!)
Out of curiosity: Is this a region that is often studied as part of an education in geology?
Well, from my limited knowledge of the subject, rock weathering is primarily caused by three things: mechanical, biological, and chemical (damn, oxygen why are you so clingy?)
I sort-of, kind-of ruled out much biological weathering since that area is pretty barren. I know that geology is concerned with such long time spans, that statement is quite silly but my "gut" isn't much of a scientist. :D
I must admit that I don't know much of anything about the chemical weathering processes or what minerals are even suseptable to this form of weathering. Or the timescales on which it occurs. Or the signs to look for. Or..
So that leaves mechanical. Which I understand is mostly a process of some sort of particle-containing fluid scouring away the rock over eons.
Landscapes are lumps that haven't been smoothed (to sea-level) yet.
Wait, let me say that a bit more clearly: Landscapes are the result of a complex interaction between a variety of forces constantly making entire earth's surface an unsmooth sphere and a variety of counter forces trying to make the surface of the earth smoothly spherical again.
The reason for the complex landforms we see is a consequence of different rates of rock weathering, the way fluids naturally hate traveling in straight lines, and the uneven action of plate tectonics raising and lowering different parts of the land. I am sure there is some biological component - such as grass covered sand dunes not experience rapid erosion - but I don't know if that really matters all that much on long time-scales. Hmm, I guess things like coal beds and limestone would argue that biological components can be pretty significant, huh?
To be honest, this was something of a joke. It just seems like every time I read about something cool in Africa, it ultimately has to do with volcanism at some point in it's incredible geological history!
Until recently, most of my readings have been about the Eastern African Rift zone. I am something of a coffee connoisseur and, in my opinion, the best coffee's in the world come from that region (and Indonesia) so I was recently reading a good bit about the about the geology and geography of that area. So the huge role that volcanism and tectonics plays in Africa was fresh on my mind when I wrote that.
BUT there are things that make me think volcanoes - there are several flat-topped and dark-colored mountains that stick out of the pale colored plane (mostly to the east of the "salt lake" though).
Rocks fold as a result of the forces generated at the collision between two plates. As a result of the the massive amount of energy involved and the long time-frame over which it happens, rock undergoes plastic deformation.
Hmm. Well, it could have just been there for a very long time and erosion has finally gotten the chance to expose it, no? Honestly, I assumed it was uplift though I guess subsidence could also cause this tilting of the strata.
Given the proximity to the continental border my gut feeling (there he is again!) is that this might be related to a time when Africa was colliding with another continent or, perhaps, when later began to break away. Also the way the fold has a "hook" at the end, suggests that there might have been something going on that is a little more complex going on but I can't even begin to imagine what that is.
What else could have created this feature?
LOL! This is one of those things that I've always wondered about. How can you even tell?
I mean with the exception of a few small spots it doesn't appear that the surrounding rock layers are at the same angle as the main feature. Then again most of the surrounding area is that pale area that appears to be the remains of something massive that eroded away. Hmm.
Ooo! Ooo! I never even thought to look at the techonic history. [Slaps self on forehead.] Be right back!
Ok, that took some detective work but I think I am close to getting my answers!
First, I had to find out what I should be searching for to read about the history of this area. I found these soil maps: left half with legend and right half with the region of interest in the very lower left.
Going by that, most of the area is actually part of the "Karro Supergroup" - notably the Ecca and Dywka formations. The smoother, hilly areas are the newer (and softer?) shales of the Ecca series while the reddish brown belts demarcate the border between it and the older Dwyka tillite. I am still now sure which group that reddish belt belongs to or what it is made out of though.
The white layer seems to be roughly between the Dwyka and Ecca formations. During Dwyka and Ecca periods, the area was underneath a large sea. I am guessing that the deposit is some sort of carbonate rock? My gut tells me it isn't an evaporite because the sea must have continued to exist here for a long while to deposit the large amounts of Ecca shales over top of that white layer.
Mostly just the river? I'm not sure what an active feature is, exactly.
These formations are related to the formation and breakup of Gondwana and are associated with the cape fold belt.
Not sure what you mean by this at all!
Thanks again for taking the time to read my questions. Oh, and thanks so much for getting me on the right track!