(Mountain - the essential element - is erosion)
Fig.1. Mountains. (a) Somewhere in the Canadian Rockies. Not a so-called 'fold mountain', like the one on the right (b) in Spitsbergen, ... but one which graphically illustrates the essential dynamic of mountain formation - in this case the downward gravitational removal of the rock' by the exploitation of fractures and microfractures (which the said mountain is manifestly riddled with) - otherwise known as , ...'erosion'.
"...Yeah? But what if we were to consider the gravitational effect on the ductile elements of the rock, and not the brittle elements."
"What ductile elements?"
"The more incompetent layers in the sequence."
"What about them?"
"Well, ... what if *they* collapsed.? What if instead of collapsing along a whole network of brittle fractures, ...you know, ...frittering away by erosion, ...the whole lot collapsed along ductile layers?"
"Yeah, ..well, ...but anyone can see they're not. They're crumpled up. Those rocks were once below the sea and now they're above it. They'be been crumpled - UP."
"But the other ones were below the sea too and they're up now as well. And they're not crumpled."
"That's right. They're not."
"But they're up. They're both UP."
"S'right. Two sorts of up. One's crumpled up, the other's not. Look, obviously you need to read a book. To get crumpling mountains you need colliding plates - with continents on top. Here, these quotes from the USGS and NASA will help you":-
|Understanding plate motions
[This Dynamic Earth, USGS]
"The Himalayan mountain range dramatically demonstrates one of the most visible and spectacular consequences of plate tectonics. When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. The collision of India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override the Indian Plate."
|"...Sometimes, when there
is a convergent boundary between two continental plates, subduction cannot
occur. Since continental crust is more bouyant, or less dense, than oceanic
crust, one plate does not easily override the other. Instead, the plates
crumple as they plow into one another, and a very high mountain range is
created. This is a special type of convergent boundary called a collisional
boundary. The Himalayas in India are the result of two continental plates
(the Indo- Australian and Eurasian plates) colliding head on.
"Gee, thanks, ..but the one with the crumpled folds is on the continental shelf - where there are no colliding plates. And the other one without any folds is the Canadian Rockes where there are colliding plates, ..but no crumpling."
"Yeah, well, ..but that doesn't change what I'm saying."
"What are you saying, .. exactly?"
"I'm saying exactly what the USGS and NASA geologists are saying, and they're studying other planets. Surely you don't think that they're going to be studying other planets if they haven't already studied this one, now do you?"
(...To be continued) ...
"But they're both forming by ersoion. Erosion is taking stuff away and leaving what's left - mountains."
"That one's being eroded and leaving a mountain, but the other one is a mountain because it's folded. The folding gave the mountain. Colliding plates crumpled it up and gave the mountain. Obviously it has to erode if it got pushed up. "
"But there are no colliding plates."
"Yeah , ..but there might have been one once. You don't know. There might have been any number of colliding plates that have been subducted underneath that mountain and you don't know. In fact the folds are proof that there were plates and that they collided. In fact I'll bet that fold there and the one over there belonged to separate plates and they're now crashed and crumpled together. I'll bet you'll find a suture in between."
"But it's all the same sequence. That unit correlates with that one and that one correlates with that one."
"What? Do you mean to say you can't have two the same sequences riding on different plates? Of course you can!"
"Well, how far apart do you think those sequences were before they started colliding?"
"Any distance at all. They could even have been exactly in the same place, then ripped apart a whole world away, and then crashed together again. "
"You mean like Wilson Cycles?"
"Yeah of course, .. An' I'll bet if you mapped carefully beneath that mountain you'd find a subduction zone somewhere. In fact we could virtually dot one in on the map."
"But, ... But... "
Obviously you don't know what science is about. Science is about making hypotheses, and falsifying them. Now you prove to me there is no suducting plate underneath all that rubble. Go on, .. Be scientific. Prove it."
"But both are eroding plateaus. They're both eroding plateaus. Both were flat before they started being eroded like you see now. All the folds on that one were planed down flat - you know, ..peneplained. They're both eroding a peneplain. That's why they're mountains now, it's what's left from once being high."
"Ha! Now you just said it - "peneplains". That means they were once low, ..you know, ..what is it you say? - "Surfaces of zero erosion potential". That means they were virtually down at sea-level."
"Yes, ..but .... well, not exactly, ... but I know what you mean."
"And then got uplifted"
"Dunno, ..but they did."
"Well, .. why not colliding plates?"
"Because ... If colliding plates uplifted that peneplain then it wouldn't be just folds that were crumpled, the peneplain itself would be all crumpled. And it's not. It's.an eroding, flat, ... peneplain. And that peneplain was formed low down, .. and it's now high up. "
"Yeah, well, .. How?"
"Dunno, .. but it is. An' it certainly isn't crumpling pushing it up - cos it's not crumpled. That peneplain's still all at the same height."
"Do you want to know what I think?"
"Yeah, tell us, .. Tell us what you think <smart-ass>."
"I don't think it ever got pushed up at all. I think it's just falling down."
"What? Woargghhh, ..run that past us again. How can it once have been flat as a tack and low down, and then flat as a tack and high up without getting pushed up? Does it just levitate or something?"
"Yeah well, sort of, .. in a way.. .. You got to think of it this way. Those folds that you're looking at, .. they're not sideways crumpling things, they're part of a pattern of gravitational collapse. Well at least it's the gravitational collapsing that's making them look like they got crumpled sideways. People used to think they were sideways, but now they've got it mapped out right they don't. They're all falling-down, gravitational-collapse things. At least the ones they've mapped right, like the Alps and the Middle East and the Himalayas. There are some backward places where they still think they're made by sideways crumpling."
"America. parts of America."
"High bits of anything collapse when they're gravitationally unstable. I saw a picture of San Francisco once that...
"But surely you're not suggesting that plateau there in the picture is..."
"... gravitationally unstable, are you? It's as solid as a rock."
"Of course it's gravitationally unstable It's being reduced to rubble isn't it?"
"Yeah, ..but that's weather, chemical erosion - you know, .. snow and ice, .. Earth Wind and Fire and all that."
"And gravity. What do you think is causing all that gravel to fall down?"
"Yeah, ..well, .. But its not a plateau collapsing that's giving you folds."
"Yes it is. Depends what you mean by plateau though, doesn't it?"
"And what do you mean by a plateau?"
"Well, .. A plateau is like you said, ."..an elevated surface of zero erosional potential"
"OOooOhh ...fancy that..! You mean it's flat."
"Yeah, ...flat as a tack. And high."
"And what are you saying? - Flat ground collapses?"
"Yeah, ..if it's high."
"Yeah? And how high does it have to be then, before it collapses."
"Well, .. Not much high at all, ..just high enough to ... Look, ..here, ..pass your beer mat, I'll sketch it for you."
<... sketching ..>
"But that's not high.. That's sort of ... off-balance."
"'Sright. ..It's off-balance. That bit's off-balance, this bit's not."
"Why, .. if it's all moving up, ..out from the centre of the Earth, ..why does it get off-balance?"
"Because it's brittle. Obviously it doesn't curvature-correct all at the same time."
"Obviously? why obviously?"
"Because the rocks are all different."
"Yeah, ..like in the figure above. Some are brittle, some are not."
"Well, ..that one's brittle, ..but the other one's not"
"Yes it is. Didn't used to be brittle. But it is now."
"Yeah, ... brittle..."
(..Rock, ..differentially ductile and brittle, ...when gravitationally unstable, ... collapses, ..giving folds and thrusts..)
Fig.3. Glacier eroding a plateau (/peneplain). Here, ductile gravitational collapse of the rock known as ice. Compare fan with detritus shedding from fold mountain above. The flat surface that existed prior to erosion (to form mountains) is apparent in the uniform height of the peaks
Fig.4. Detail of Fig.3 Showing the flat-lying bedding in the rock etched by snow (foreground left of centre and middle distance). The peneplain surface being eroded *is* the bedding surface - flat. This surface from which the mountains are forming was never crumpled.
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