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Tectonic plates meeting (convergent plates) form mountains. But if those plates change direction (which I am just assuming they can over time, I have no idea if it's true) and start pulling apart (divergent plates), will the mountains automatically crumble away as the plates split, or can some mountains become part of one or the other plate, like clay sticking to your hands when you remove them?

Feel free to explain any prerequisite knowledge you feel I lack. This might be an important question to me.

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    $\begingroup$ Mountains are always crumbling. And if the crumbling outpaces the building, then there will never be more than some low rolling hills. As for reversing subduction, there's simply no such thing; the rug doesn't just get pulled back out from under the furniture, as it were. The rug itself is more like a sheet of clay: when pulled, it will thin and tear. $\endgroup$ – Knob Scratcher Feb 15 '17 at 16:53
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Quite the opposite. Divergent boundaries can cause mountain (well, actually volcano) building, because upwelling magma is part of the rifting process.

First and most importantly, the Mid-Ocean Ridge can be considered the longest, most massive mountain range in the world.

It's more obvious, of course, on continental rifts like the East African Rift. Mt. Kilimanjaro, a product of rift volcanism, is the highest mountain in Africa.

Another famous locale of volcanism at a divergent boundary is Iceland, although a hot spot may play a role there.

shows rift volcanism

Source: USGS (via usu.edu)

But BUT BUT it seems you want to know what happens to existing mountains. Some others have answered that only erosion destroys mountains, and that's mostly right, but only because it's a matter of timing.

It's often difficult for us humans to comprehend just how big geological time scales are. Most of the time, a mountain range created by a continental collision is eroded away completely by the time the next rift comes along.

This was the case along the eastern seaboard of the United States. The mountains created by the collosion of Africa and North America in the Carboniferous (the Alleghenian Orogeny) were almost completely gone by the Triassic, when the Atlantic Ocean began to form from a rift along about the same line.

However, we have an example that might result in a qualified "yes" to your question, a little closer to the present. This is the way the Yellowstone Hotspot moved across what would become the northwestern United States. One of the things that caused geologists to recognize the Yellowstone Caldera was the 80km gap it melted in a mountain range it passed under, separating the Gallatin Range in the north from the Red Range in the south.

The hotspot's track might just be the genesis of a new rift that breaks North America apart, given enough time and the right stresses.

![Yellowstone

(Source: USGS)

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  • $\begingroup$ But what about mountains created by converging plates, then? When the plates pull apart, do the mountains survive? $\endgroup$ – Henry Stone Feb 14 '17 at 7:12
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    $\begingroup$ What happens when a bulldozer pulls away from a pile of dirt? Does the pile just flatten it self out? No. There is no "reversing" mountain building. Only erosion is responsible for destroying mountains (in the conventional sense, as erosion plays a big part in creating mountains). $\endgroup$ – Knob Scratcher Feb 15 '17 at 16:57
  • $\begingroup$ @KnobScratcher I think you need to direct that at OP $\endgroup$ – Spencer Feb 15 '17 at 23:38
  • $\begingroup$ @Henry Stone I added a bit about what happens to existing mountains, does this answer your question? $\endgroup$ – Spencer Feb 19 '17 at 4:05
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    $\begingroup$ @HenryStone No. The hotspot (not a "rift" although someday there may be one there ) destroyed pre-existing mountains, millions of years after they were formed. And it is not a "standard" by any stretch of the imagination; it is an extremely rare occurrence. It is a matter of timing and coincidence, and it is unlikely that one of these things will come up under mountains before they are eroded away. $\endgroup$ – Spencer Mar 4 '17 at 13:08
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I want to add some nice examples of how divergent plate movement can actually create mountains, not destroy them. Here are two photographs I took several years ago from the Red Sea rift near Eilat, Israel:

enter image description here

enter image description here

enter image description here

These mountains exist because of the plate movement. Divergent plate boundaries cause rifting, and the geomorphological expression of a rift is a valley. Valleys are, by definition, surrounded by mountains. The mountains that you see in the photographs could not have been there if there hasn't been a divergent boundary.

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  • $\begingroup$ First: Damn, those are nice pictures! I envy you your trip. Second, though, I am aware that diverging plates can create mountains (I assume you mean by matter rising in the rift to form masses on top, then the next rift adds to it, etc., like with oceanic rifts? If not, let me know!). What confuses me is where the converging-plate mountains go when the plates no longer converge. $\endgroup$ – Henry Stone Feb 18 '17 at 11:36
  • $\begingroup$ @HenryStone rifting creates valleys. Valleys have mountains around them. Take a piece of bread and pull it apart. An ant standing in between the two pieces of bread now sees two mountains of bread. Next, the mountains don't go anywhere when the plates stop converging. They just stay there, slowly being eroded. $\endgroup$ – Gimelist Feb 18 '17 at 15:47
  • $\begingroup$ This is very similar to the tilted block faulting found in the South West US. Death Valley is a prominent example where the the plate is pulling apart, (although not necessarily a divergent plate boundary more likely to occur mid plate). See en.wikipedia.org/wiki/Tilted_block_faulting $\endgroup$ – Friddy Feb 21 '17 at 22:45
  • $\begingroup$ Michael & Friddy: But what if two plates cause mountains, slow down, stop converging, and later drift apart? What becomes of the mountains they made? $\endgroup$ – Henry Stone Feb 22 '17 at 15:35
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Do divergent tectonic plates destroy mountains?

The short answer is "no". Erosion destroys mountains. Period. Not slumping, not faulting, not "flowing back to flatness" in the absence of compression, not sticking to a wayward plate. Just plain old, basic erosion. To fully understand the physics of rocks and the processes that lead to mountain building, you'd have to at least pick up a basic college level intro to geology. There are many good ones out there with awesome illustrations and clear explanations. On the other hand, simple videos can be useful too: You can check out these short clips from the British Geological Society for a basic view of mountain building at plate margins.

To answer your question more precisely, the mountains that form at the edges of subduction zones are the result of several processes: 1) regional uplift of the overriding plate inland of the subduction boundary, 2) building of accretionary wedges from scraped materials off the subducting plate 3) Emplacement of island arc magmas as either extrusive (lavaflows) or intrusive (deep underground) plutons, and 4) Erosion, which cuts the deep channels through inherent structural weaknesses in this enormous mass of uplifted volcanic rock, exposing the sharp peaks and prominent ridges that we associate with mountains.

Does reversing this subduction effectively stop these processes? Yes...sorta. And this has happened around the planet frequently enough over 4.5 billion years with the creation of at least to supercontinents, Gondwana and Pangea. I say "sorta" because the uplift is still there and the erosion continues unabated. Does reversing the subduction effectively reverse these processes in any way? No. Reversing the direction of India's movement into Tibet will not cause the Himalaya to slump back onto the sea floor. It's now chemically altered, bullet-hard, de-watered metamorphosed accretionary wedge. Only the steady grinding of glaciers and weather are going to "destroy" those mountains; the same process already occurring.

Will reversing South American subduction cause the Andes to "stick" to the Nazca plate as it's pulled back out from under South America, or slump back into the ocean? That would impose a strength to oceanic crust that it doesn't have. If anything, in this new "extensional regime", oceanic crust would simply weaken, be uplifted by a shallow aesthenosphere, and likely be perforated by a mid-ocean ridge, or hotspots. Hope that helps!

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  • $\begingroup$ The OP thinks that plates 'reverse', instead of subduction, they pull apart? Does this happen even with the old continents of Gondwana and Pangea? A plate can actually reverse directions? I do think people have no idea the power of 'mass wasting'...ooohhh. You sound as if you've got great knowledge of geomorphology. Few humans understand the depth and breadth and time involved. I know little but your answer is fascinating...have you written a book perhaps? $\endgroup$ – stormy Feb 16 '17 at 19:03
  • $\begingroup$ "The OP thinks that plates 'reverse', instead of subduction, they pull apart?" No. The OP (me) thinks that two tectonic plates pushing together at one point in time will not push together forever, or plates would basically be crumpled up in a few places at our point in time. Plates push together, slide along each other, slip away, and move in very complex patterns. Two plates converging, say, 2 billion years ago may today have changed direction enough to slide along each other instead, maybe to the point of drifting apart. (cont.) $\endgroup$ – Henry Stone Feb 17 '17 at 11:56
  • $\begingroup$ (cont.) What I would like to know is what happens to the mountains they made back when converging, whther they subducted or not back then. Do the mountains get destroyed as the plates they were made of no longer support them (by converging as before), or do the mountains remain somewhere, as a part of one or the other plate, for all eternity (barring erosion)? I just stated it a bit shirter in the OP because the question is a bit hard for me to phrase clearly. $\endgroup$ – Henry Stone Feb 17 '17 at 11:56
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    $\begingroup$ Once "built" and in the absence of erosion, mountains will last forever. Want proof? Look at the moon: despite heavy cratering, many lunar uplifts are close to 4 billion years old. Earth has no uplifts anywhere near that age. As for what would happen to the Himalaya if the Indian collision into Asia stopped, the formation of the Appalachian Mountains and subsequent passive plate boundary offers a good analog. Read more here: en.wikipedia.org/wiki/Geology_of_the_Appalachians. Great questions, by the way! $\endgroup$ – Knob Scratcher Feb 17 '17 at 14:58
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    $\begingroup$ Thanks, I'm finally deep enough into this to ask the "tough" questions. And thanks for bearing with me on them :) $\endgroup$ – Henry Stone Mar 23 '17 at 14:03
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Mountains tend to fault and collapse back to flatter ground once the building forces cease. This is a very slow process and rarely leads to the complete elimination of a mountain range. So, left on their own, mountains tend to flatten, however, mountain ranges rarely completely disappear certainly not from these forces alone. Even the oldest mountain ranges tend to leave behind traces like hills and highlands. That is not to say they cannot be destroyed but it tends to require an extreme event like being trapped under an ice sheet or a second later mountain building event.

If a continental crust slips over a divergent boundary, underplating occurs. This is why most of the western USA is elevated. The forming east pacific plate is too new to sink back down so it gets welded to the bottom of the continental plate, pushing it upward.

This process can stop a spreading center. However, the underlying forces in the mantle remain, so a new spreading center may form somewhere else. The new spreading center is formed wherever the rock breaks more easily. The thicker mountainous crust is generally harder to break than normal continental crust. So the spreading center tends not to form under mountains. A new spreading center that forms under a continental plate creates a rift valley which can become more mountainous than the surrounding crust, OR, it can end up thinner and more rugged, which depends on how fast the crust can move away from the upwelling.

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  • $\begingroup$ .....This is simply incorrect. You cannot conflate the age of a mountain building event with the age of the rocks contained within: mountains purported to 3.5 billion years old are just uplifted erosional remnants of rocks that happen to be 3.5 billion years old. $\endgroup$ – Knob Scratcher Feb 16 '17 at 5:09
  • $\begingroup$ I didn't, unless you are referring to the cental NA uplift, which is hardly controversial. $\endgroup$ – John Feb 16 '17 at 11:14
  • $\begingroup$ Your statement, "However, mountain ranges may never completely disappear. Even the oldest mountain ranges leave behind hills and highlands." implies that mountains ranges are never entirely eroded, and this is wrong. For what it's worth, the blanket statement, "Mountains tend to fault and collapse back to flatter ground without compressive forces." is also wrong as there are multiple processes that lead to mountain "building". $\endgroup$ – Knob Scratcher Feb 16 '17 at 14:55
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    $\begingroup$ Your right the wording is a bit murky, I'll clean it up, and yes compressive is not the only force in play I'll change it. $\endgroup$ – John Feb 19 '17 at 2:32

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