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Delvo
2009-Mar-22, 10:01 PM
Are there any forecasts of where the continents will have moved to by any number of years from now? Has anybody predicted how quickly any present mountain range will be flattened or how quickly any new ones will appear? I don't mean little things like that the northern Atlantic Ocean will be a few feet wider in so many decades, but the kind of big radical changes that would make the world so different that it would be practically unrecognizable. If you were watching a movie that introduced the setting by showing pictures of places with captions naming the locations, and it showed an absurd scene-&-name combination like one of these below, how far in the future would you think the movie had to be set, and what other worldwide geological differences from the present would expect to be concurrent? (Or would you argue that it was wrong because, although some big changes will happen, those particular ones aren't the right ones?)

novaderrik
2009-Mar-22, 10:44 PM
i don't know, but no matter what happens, the future New York City will have either the head or torch from the Statue of Liberty either laying on or poking out of the ground.

korjik
2009-Mar-23, 12:07 AM
paleomap has some projections IIRC

geonuc
2009-Mar-23, 01:13 PM
I would think that any prediction looking more than a million years or so in the future would be highly suspect. While it might be possible to show how the continents would be arranged given their present movements, it is impossible to predict with any useable degree of certainty what tectonic regime changes will occur. In the geology thread, for example, we are discussing a possible new subduction zone off the North American east cost. When will that come about? A miilion years? Ten? Fifty? How will it be characterized and what effect will it have on the plates?

We don't know the answer and cannot know with our present state of knowledge.

mugaliens
2009-Mar-23, 01:30 PM
Well, following current trands...

First, it's important to look at both where the new crust is being formed at the oceanic ridges, where opposing magma circulations draw magma up to the seafloor, erupting as lava, filling the gap between the diverging plates. Second, we must also look at where old crust is disappearing (subduction zone). As the mantle circulates due to heating from the Earth's outer core and the cooling below the Earth's crust, it pulls the Earth's crust along with it, from the ridges to the subduction zones.

The Mid-Atlantic Ridge is the divergent tectonic plate boundary separating the Eurasian Plate from the North American Plate in the North Atlantic, as well as the African Plate from the South American Plate in the South Atlantic. All four of the aforementioned plates are being pulled along with it. The question is, where are those plates headed?

There are twelve other oceanic ridges, each of which has an effect on tectonic plate movement. The predominant effect on most of the Earth's major land masses at this time, however, comes from the Mid-Atlantic Ridge. As those four main plates near subduction zones, however, the effects of the other oceanic ridges may predominate.

So far, Pangaea hasn't really spread far enough apart that we're all that certain where the individual land masses will be.

Here's the kicker - Pangaea isn't the first! It appeared only about 250 million years ago. The process appears to be cyclical, with its predecessor, Rodinia, having formed about 1.1 billion years ago, and lasting until 700 to 750 million years ago. Before all of them, it is believed the first supercontinent was Vaalbara (http://en.wikipedia.org/wiki/Vaalbara), which existed around 3.3 billion years ago. After that came Kenorland, existing between 2.7 billion and 2.5 billion years ago. Then Columbia, between 1.8 and 1.5 billion years ago, before Rodinia, and finally Pangaea.

Back to Pangaea - unfortunately, we're not really sure if the separated continents are carried all the way across by their plates to the subduction zones.

For all we know, in 2012...

Scratch that.

Some time in the distant future the current continents may rejoin at the subduction zones into a new supercontinant (Aquarius?), which will change the heating/cooling locations, thereby reversing the magma circulation, resulting in a new separation. Alternatively, if the current continents go substantially more than half-way, the magma circulation may change enough to cause them to sink and give rise to new land masses. Evidence of ancient sea life atop the world's tallest peaks indicates this has happened in the past.

cran
2009-Mar-24, 12:32 PM
... Alternatively, if the current continents go substantially more than half-way, the magma circulation may change enough to cause them to sink and give rise to new land masses. Evidence of ancient sea life atop the world's tallest peaks indicates this has happened in the past.
(my bold)

huh? you wanna run that by me again?

ancient sea life atop the tallest peaks are evidence of convergent uplift -
how is that evidence of continents sinking?

------------

as geonuc points out, you can extrapolate future positions from present motions with confidence, but you cannot predict with the same level of confidence how or when the underlying drivers will change ...

as mugaliens points out, the process is apparently cyclic - I described it as a pseudo-cycle in one of my field assignment papers - that, by virtue of being in motion on the surface of a sphere (well, geoid) the continental masses eventually converge into supercontinents, and then rift apart again in new configurations ... as continental plates accrete more material, the continents tend to cover more surface area, increasing the probability of convergences ...

Swift
2009-Mar-24, 01:04 PM
Are there any forecasts of where the continents will have moved to by any number of years from now?
Here (http://www.scotese.com/future.htm) is one projection, 50 million years in the future.

Here (http://apod.gsfc.nasa.gov/apod/ap001002.html) is an APOD picture of a projection 250 million years in the future.

Here (http://www.classzone.com/books/earth_science/terc/content/visualizations/es0807/es0807page01.cfm?chapter_no=visualization) is a little animation you can run yourself, out to +100 million years.

Noclevername
2009-Mar-25, 03:19 AM
Alternatively, if the current continents go substantially more than half-way, the magma circulation may change enough to cause them to sink and give rise to new land masses. Evidence of ancient sea life atop the world's tallest peaks indicates this has happened in the past.

Er, no. The continents are continents because they're made of materials too light to sink in magma. The tall peaks are generally where tectonic plates press together-- the edges of plates, which were once coastlines, not ocean floors. Think of oil floating on boiling water; it changes shape and merges and breaks up, but at no point does it get heavier and dive to the bottom of the pot.

cran
2009-Mar-25, 03:39 PM
Er, no. The continents are continents because they're made of materials too light to sink in magma. The tall peaks are generally where tectonic plates press together-- the edges of plates, which were once coastlines, not ocean floors. Think of oil floating on boiling water; it changes shape and merges and breaks up, but at no point does it get heavier and dive to the bottom of the pot.
(my bold)
sometimes bits get caught up in the mix - we call them ophiolites ...

jlhredshift
2009-Mar-25, 03:50 PM
(my bold)
sometimes bits get caught up in the mix - we call them ophiolites ...

Noclevername, you can search ophiolites and Eldridge Moore for some definitive work.

Argos
2009-Mar-25, 04:28 PM
So, to answer Delvo´s question, considering the maps Swift presented:

Fig 1 [Iowa]: That won´t happen in the next 250 million years.
Fig 2 [Antarctica]: 50 million years.

Amber Robot
2009-Mar-25, 05:02 PM
Here (http://www.scotese.com/future.htm) is one projection, 50 million years in the future.

Here (http://apod.gsfc.nasa.gov/apod/ap001002.html) is an APOD picture of a projection 250 million years in the future.

Here (http://www.classzone.com/books/earth_science/terc/content/visualizations/es0807/es0807page01.cfm?chapter_no=visualization) is a little animation you can run yourself, out to +100 million years.

It seems difficult to reconcile the middle picture with the other two.

cran
2009-Mar-25, 11:24 PM
Has anybody predicted how quickly any present mountain range will be flattened ... ?

yes - though the ideas (~90% within 60 Ma; completely eroded within 300Ma) have been called into question ...

there's some discussion of mountain range erosion rates (focussed mostly on the Appalachians) arising from Big Don's similar question, and loosely related to the projected continent shifts that geonuc referred to earlier as discussed in the Geology Discussion thread - notes from a published study which discuss the longevity/erosion of mountains can be found in this post:
http://www.bautforum.com/1459577-post354.html

HenrikOlsen
2009-Mar-28, 08:06 AM
It seems difficult to reconcile the middle picture with the other two.
From the comment on the animation (the third link):
The model does not account for continental collisions, subduction, or sea-floor spreading. Ignore that one for predictions, it has India in the middle of eastern Europe.

Delvo
2009-Mar-29, 03:06 AM
... as continental plates accrete more material, the continents tend to cover more surface area, increasing the probability of convergences ...8^o I never heard of that before! It seems related to the fact that Paleomap (which I didn't know about before this thread) shows outlines of the continents under water in the oldest maps, indicating that much more of the world was covered in water back then...


Here (http://www.scotese.com/future.htm) is one projection, 50 million years in the future.

Here (http://apod.gsfc.nasa.gov/apod/ap001002.html) is an APOD picture of a projection 250 million years in the future.

Here (http://www.classzone.com/books/earth_science/terc/content/visualizations/es0807/es0807page01.cfm?chapter_no=visualization) is a little animation you can run yourself, out to +100 million years.
From the comment on the animation (the third link):
The model does not account for continental collisions, subduction, or sea-floor spreading. Ignore that one for predictions, it has India in the middle of eastern Europe.Ignoring pretty much every major tectonic process is a pretty odd way to project the tectonic future. The fact that it shows India becoming superimposed with Asia is one oddity about it, but I also notice that it has Australia moving northeast across the equator, while Paleomap has it lingering in the southern temperate zone or puttering southwest just a bit, and bumping into Antarctica, although the two models agree with each other about other things.


So, to answer Delvo´s question, considering the maps Swift presented:

Fig 1 [Iowa]: That won´t happen in the next 250 million years.
Fig 2 [Antarctica]: 50 million years. The maps show tectonic movement but don't say anything about mountains growing or shrinking, at least not directly, so a statement about mountains growing or shrinking must be using information from elsewhere. How did you derive the conclusion about mountains in Iowa from the maps? Because Iowa is in the middle of a plate and mountains aren't expected to form in the middle of a plate? (But if that's the case, what about rifts forming new plate edges or old inactive plate edges reactivating, such as the New Madrid fault through Missouri and Illinois?)

hhEb09'1
2009-Mar-29, 06:08 AM
8^o I never heard of that before! It seems related to the fact that Paleomap (which I didn't know about before this thread) shows outlines of the continents under water in the oldest maps, indicating that much more of the world was covered in water back then.../*/)You must be talking about the shallow inland seas. That's why we find dinosaurs in Wyoming and Colorado.

cran
2009-Mar-30, 02:58 PM
8^o I never heard of that before! It seems related to the fact that Paleomap (which I didn't know about before this thread) shows outlines of the continents under water in the oldest maps, indicating that much more of the world was covered in water back then...
that's right ...


You must be talking about the shallow inland seas. That's why we find dinosaurs in Wyoming and Colorado.
If, by "oldest maps", Delvo is referring to the Precambrian and Palaeozoic maps on the Scotese site, then not necessarily ...

Delvo
2009-Mar-30, 10:21 PM
I was indeed talking about Paleomap's maps showing the world long before the dinosaurs. There, you have not just continents with epieric seas over parts of them, but almost entire continents' outlines simply under the single worldwide ocean. Land sticking up out of the water back then is what's uncommon and small, so in a way it's the opposite of a little bit of water flooding part of a large and otherwise dry land mass.

What's the mechanism for continental growth over the years? Is it just that a bunch of low-density stuff got trapped below long ago and has been continually coming out since then, or high-density stuff got trapped at or near the surface long ago and has been working its way down since then? Have uplift sites and spreading ridges simply been more efficient or worked faster than subduction zones and erosion? How much longer might it continue that way?

The prediction that was made above, that as far as we can foresee mountains will never grow in Iowa, made me start pondering mid-plate structures and behaviors. Not far from my absurdly fictional "Iowa" site in the picture & caption are the New Madrid fault and the Ozark highlands. (BTW, the emphasis goes on "mad", not "rid".) The fault is an example that faults can sometimes sprout up in the middle of a plate, essentially splitting a plate into two or three, and can also die and mostly cease to act like faults, essentially fusing two plates into one. (And apparently get covered thereafter by MILES of sediment and lava flows?!)

But the Ozark highlands predate the fault. They're one of the oldest structures we still have around to see, and were among the earliest few bits of dry land in a mostly water-covered world. And they're in the middle of a plate, rather than near a collision front. How did that happen? Can the middle of a plate rise? Could the Ozarks have originally been created by a collision of two plates, along a fault even more ancient than New Madrid, so ancient that they then fused so thoroughly that not even a New-Madrid-like ghost of the fault is detectable anymore? Can a single plate split and spread, then turn around and collide, and if so are they more likely to fuse into a single plate than a pair of "unrelated" plates would be, and/or leave less of a trace of a fault? Could the really really ancient world with very little or no dry land have had significantly larger hot spots than today's hot spots, which formed part of the continents we have now?

cran
2009-Mar-31, 12:49 AM
...What's the mechanism for continental growth over the years? it's a collection of mechanisms, but the primary driver is described in a geological theory called Plate Tectonics ...


Is it just that a bunch of low-density stuff got trapped below long ago and has been continually coming out since then, or high-density stuff got trapped at or near the surface long ago and has been working its way down since then? crudely put ... but yes, that's part of it ...


Have uplift sites and spreading ridges simply been more efficient or worked faster than subduction zones and erosion? no* ...
most of the growth of continental plates around the early cratons can be attributed to convergences - subduction zones and continental collisions ...

*unless you're a subscriber to one of the non-mainstream "Expanding Earth" ideas ...


How much longer might it continue that way? which? Plate tectonics? Continent-building?
well, for as long as there's sufficient internal heat and pressure to maintain a partial melt and plastic flow, and surface water to facilitate subduction and lower melt temperatures ...


The prediction that was made above, that as far as we can foresee mountains will never grow in Iowa, made me start pondering mid-plate structures and behaviors. Not far from my absurdly fictional "Iowa" site in the picture & caption are the New Madrid fault and the Ozark highlands. (BTW, the emphasis goes on "mad", not "rid".) The fault is an example that faults can sometimes sprout up in the middle of a plate, essentially splitting a plate into two or three, and can also die and mostly cease to act like faults, essentially fusing two plates into one. (And apparently get covered thereafter by MILES of sediment and lava flows?!)not all faults are rifts, nor plate boundaries - most are local to regional in extent; ie, they do not "split" plates, merely disrupt some upper (brittle) layers - the lower (ductile) layers do not tend to fault ...
regions, even in the middle of continental plates, can experience tremors, faulting, uplift or downwarping, as a response to the net stresses acting upon them ...


But the Ozark highlands predate the fault. They're one of the oldest structures we still have around to see, and were among the earliest few bits of dry land in a mostly water-covered world. And they're in the middle of a plate, rather than near a collision front. How did that happen? were they in the middle of a plate when they first formed?
what was happening there, and in neighbouring regions at the time of formation?


Can the middle of a plate rise? yes ...


Could the Ozarks have originally been created by a collision of two plates, ...yes ...


... along a fault even more ancient than New Madrid, so ancient that they then fused so thoroughly that not even a New-Madrid-like ghost of the fault is detectable anymore? no ...


Can a single plate split and spread, ... yes ...


... then turn around and collide, and if so are they more likely to fuse into a single plate than a pair of "unrelated" plates would be, and/or leave less of a trace of a fault? I'm not sure what you mean by "turn around", but no - not more likely ...


Could the really really ancient world with very little or no dry land have had significantly larger hot spots than today's hot spots, which formed part of the continents we have now?larger? possibly ...

"significantly" larger? that depends on what you'd consider "significant" ...

an alternative probability is more rather than larger - more plates (called microplates), more boundaries, more tectonism ...

geonuc
2009-Mar-31, 09:07 AM
Could the Ozarks have originally been created by a collision of two plates, along a fault even more ancient than New Madrid, so ancient that they then fused so thoroughly that not even a New-Madrid-like ghost of the fault is detectable anymore?
I'll add to cran's excellent analysis of your post that you seem to be confusing spreading centers (rifts) and convergent zones (subduction), calling both simply 'faults'.

jlhredshift
2009-Mar-31, 12:37 PM
If memory serves, the Ozarks are part of the tail end of the Appalachians.

Delvo
2009-Mar-31, 03:50 PM
it's a collection of mechanisms, but the primary driver is described in a geological theory called Plate Tectonics ... I've seen and heard about plate tectonics many times, although not necessarily in much detail (mostly just as the explanation of continental drift, crumple mountains, and most vulcanism), but it's normally described as if it's in an overall balance, not in terms that make it apparent that there'd be a general trend over the eons.


most of the growth of continental plates around the early cratons can be attributed to convergences - subduction zones and continental collisions ...OK, I was thinking of subduction zones as places where crust gets pushed down (so the "destruction" of crust was balanced with the "creation" of new crust at spreading sites), but the key here seems to be that that's only when it's oceanic crust colliding with continental crust... and that when both colliding plates are oceanic (a situation I never pondered before because it's not among the examples used in most basic explanations of tectonics), it's different. Then, they can both crumple (like two pieces of colliding continental crust can), so that, although the overall area of the plates doesn't increase, the area of dry land can increase if the crumple zone gets thick enough to reach above water.

Is that how it works? If so, then I missed it simply by never considering a collision between two plates that are both not continents, or thinking of crumple zones that start under water. The usual presentation of plate tectonics treats the continental masses as neither shrinking & getting destroyed nor growing & getting created, but as just moving around between the lines where crust under the oceans comes and goes.



How much longer might it continue that way?which? Plate tectonics? Continent-building?Continent-building; the long-term net increase in continental area and decrease in oceanic area. I didn't know it was inherent to the nature of plate tectonics, so thought we might get that result under some circumstances and not others.


not all faults are rifts, nor plate boundaries - most are local to regional in extent; ie, they do not "split" plates, merely disrupt some upper (brittle) layers - the lower (ductile) layers do not tend to faultThe New Madrid one is not one of those. It's always described as a failed rift.


regions, even in the middle of continental plates, can experience tremors, faulting, uplift or downwarping, as a response to the net stresses acting upon themHow? (Plate tectonics is usually presented, at least to non-geologists, as just being about interactions of two plates at their edges...)


were they in the middle of a plate when they first formed?
what was happening there, and in neighbouring regions at the time of formation?That's what I'm asking. All I see elsewhere online so far is that their origin is volcanic, but that doesn't tell me whether that was subduction-related vulcanism, some kind of middle-of-plate phenomenon, or something else.



Can the middle of a plate rise?yes ...That was a weird question for me to ask. I knew of examples! (For example, I grew up in northern Missouri, knowing that our area had been under the sea at the time of the dinosaurs but was now several hundred feet above sea level.) I guess I asked because I was wondering what would make the middle of a plate act like that. Compression between two crumple zones at the edges? Upward flow under it that isn't forceful or focused enough to punch a hot spot through it?



Could the Ozarks have originally been created... along a fault even more ancient than New Madrid, so ancient that they then fused so thoroughly that not even a New-Madrid-like ghost of the fault is detectable anymore?no ...Why not? Do plates just never get stuck together and act like one plate thereafter without leaving a mark where the edges once were? Or is it just that that can happen and the Ozarks are not an example of it?


I'm not sure what you mean by "turn around", but no - not more likely ...I meant reverse direction. Does a spreading site ever reverse direction and become a compression or subduction site? I guess there's no particular law against it but it just doesn't happen anyway because it would require pretty weird circumstances. I only thought of it because I was trying to come up with as many ways as I could for mountains to end up in the middle of a plate.



Could the really really ancient world with very little or no dry land have had significantly larger hot spots than today's hot spots, which formed part of the continents we have now?larger? possibly ...

"significantly" larger? that depends on what you'd consider "significant"Large enough to explain the origin of something the size of the Ozarks.

cran
2009-Mar-31, 08:23 PM
I've seen and heard about plate tectonics many times, although not necessarily in much detail (mostly just as the explanation of continental drift, crumple mountains, and most vulcanism), but it's normally described as if it's in an overall balance, not in terms that make it apparent that there'd be a general trend over the eons.
perhaps that's why it takes most people 3 years or more of university studies to get a degree in geology?

and why, in my reference library, I have two books (De Sitter 1964, and Badgley 1965) devoted solely to structural geology and tectonics), and two more just about volcanoes ...

as with most things, the devil is in the details ...

like, for instance, the key to why continents don't subduct didn't come from plate tectonics, but predates it by roughly 30 years (first published in 1928) - and the author is cited in just about every geology textbook or course ...


OK, I was thinking of subduction zones as places where crust gets pushed down (so the "destruction" of crust was balanced with the "creation" of new crust at spreading sites), but the key here seems to be that that's only when it's oceanic crust colliding with continental crust... and that when both colliding plates are oceanic (a situation I never pondered before because it's not among the examples used in most basic explanations of tectonics), it's different. Then, they can both crumple (like two pieces of colliding continental crust can), so that, although the overall area of the plates doesn't increase, the area of dry land can increase if the crumple zone gets thick enough to reach above water.

Is that how it works? If so, then I missed it simply by never considering a collision between two plates that are both not continents, or thinking of crumple zones that start under water. no ... that's not how it works ...

subduction zones form at convergent marine boundaries - whether it's ocean-continent or ocean-ocean ... ocean-ocean convergences result in the formation and growth of volcanic island arcs - eg, Indonesia, the Caribbean islands, the Scotia Ridge Islands, the Aleutians, Japan, New Zealand ... the main difference between these and ocean-continent subduction-related volcanoes is the characteristic depth and geology of the associated back-arc basin ... another difference is the chemistry of the volcanic products ...

there are two main sources of material that go into land-building associated with subduction zones, and both are supplied (if indirectly) by the subducting slab ...


The usual presentation of plate tectonics treats the continental masses as neither shrinking & getting destroyed nor growing & getting created, but as just moving around between the lines where crust under the oceans comes and goes. that sounds like the media-reconstituted pablum that all too often is passed off as "science reporting" ...


Continent-building; the long-term net increase in continental area and decrease in oceanic area. I didn't know it was inherent to the nature of plate tectonics, so thought we might get that result under some circumstances and not others. rest assured, continent-building, and island-building, are pretty well entrenched in geology as a natural result of plate tectonic processes ...


The New Madrid one is not one of those. It's always described as a failed rift. that's interesting - then it should have some features characteristic of failed rifts - sure enough-
NEW MADRID FAULT GEOMETRY
CARNEY, Mike, INCA Engineers INC:
http://gsa.confex.com/gsa/2003NC/finalprogram/abstract_49585.htm

... The sharp discontinuities between the age of the Ozark Mountains and the abutting Mississippi embayment suggests a tear edge. The Appalachian Mountains share a similar age, and appearance to the Ozarks, suggesting a similar origin. The Appalachians appear to descend into embayment sediment terminating abruptly in Wash County, Mississipi, suggesting a mate to the east end of the Ozark Mountains. Tension tears on the outside of the rotating Appalachians appear in Cambell County Tennessee and Buchann County, Kentucky. Since the motion occurs in a continuous crustal surface, an opening in one location requires closing in another area. This is proposed as an explanation for seismic activity in Charleston occurring simultaneously with New Madrid activity...
How? (Plate tectonics is usually presented, at least to non-geologists, as just being about interactions of two plates at their edges...)

well, probably because that's where the most visually interesting and exciting stuff happens ...
but it doesn't just happen in isolation - neighbouring regions are also affected -

eg, India getting up close and personal with China didn't just convert some Tethyan seafloor into a growing mountain range; it's also a major contributor to the uplift of a neighbouring chunk of crust ... the Tibetan Plateau ... similar stuff happened in North America at different times ...

African volcanoes are rift-related ... Iceland seems to be both rift and hotspot related, and it's growing as we speak ...

Italy, and the Central American Isthmus, formed between converging continental plates ...

in some cases, regional mid-plate or passive uplift is diapiric - caused by less dense material rising up ...

mid-plate faults can occur simply because of the stresses in plate motion ...

but, as for the Ozarks? They first formed a long time ago (Precambrian) during the convergence of smaller continental plates ... in other words, they resulted primarily from subduction zone volcanism and continent-building ...


That's what I'm asking. All I see elsewhere online so far is that their origin is volcanic, but that doesn't tell me whether that was subduction-related vulcanism, some kind of middle-of-plate phenomenon, or something else. USGS: http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_missouri.html
simple, bite-sized explanation, with links to follow up various aspects ...

something with a bit more chew - USDA Forest Service:
http://www.fs.fed.us/r8/ouachita/natural-resources/minerals/aquatic.shtml

Structural Geology of the Ozark Plateaus

The Ozark Plateaus Province is underlain by a structural dome formed by a series of uplifts that have gradually occurred since Pre-Cambrian time. The total uplift is approximately 5,000 ft (McCracken 1967). The dome is asymmetrical with the dip of sedimentary rocks greater to the east-southeast than to the south, west, or north (McCracken 1967). Regional dip east of the St. Francois Mountains is 150 ft/mi (Tikrity 1968) whereas regional dip in southwestern Missouri is about 10 ft/mi. The dip to the south increases to 200 ft/mi in the southern flank of the Boston Mountains as a result of faulting in the area (Frezon and Glick 1959).
Extensive fracturing, jointing, and faulting of the rocks have resulted from the uplifting in northwestern Arkansas. Fractures generally trend northwest, northeast, and east-west (Ogden 1980, Adamski 1987, Leidy and Morris 1990). Vertically oriented joints (fractures in rock that do not displace the rock) are present in many of the Paleozoic-age rocks and trend east to west, north to south, northwest to southeast, and northeast to southwest (McCracken 1971). Major faults in the Ozark Plateaus have a northwest trend (McCracken 1967). Displacement can be as much as 1,000 ft. Some of the major faults form escarpments that are visible for several miles (Beveridge and Vineyard 1990).

That was a weird question for me to ask. I knew of examples! (For example, I grew up in northern Missouri, knowing that our area had been under the sea at the time of the dinosaurs but was now several hundred feet above sea level.) I guess I asked because I was wondering what would make the middle of a plate act like that. Compression between two crumple zones at the edges? Upward flow under it that isn't forceful or focused enough to punch a hot spot through it?

Why not? Do plates just never get stuck together and act like one plate thereafter without leaving a mark where the edges once were? Or is it just that that can happen and the Ozarks are not an example of it?
oh, plates get stuck together plenty, and act like one plate ...
but they always leave a mark ... or five ...

the most obvious sign of continental plates joining?
a dirty great mountain range - the suture ... the orogenic zone ... the volcanic evidence can be lost beneath/behind the uplifted sedimentary sections - modern example? the Himalayas ... old example? the Urals ...


I meant reverse direction. Does a spreading site ever reverse direction and become a compression or subduction site? I guess there's no particular law against it but it just doesn't happen anyway because it would require pretty weird circumstances. I only thought of it because I was trying to come up with as many ways as I could for mountains to end up in the middle of a plate. weird circumstances - yes ... it would require a reversal of the mantle convection in that part of the world ...



Large enough to explain the origin of something the size of the Ozarks.not necessary ... and the Ozark region is way too young for that, anyway ...

hhEb09'1
2009-Apr-01, 05:26 PM
I guess I asked because I was wondering what would make the middle of a plate act like that. Compression between two crumple zones at the edges? Upward flow under it that isn't forceful or focused enough to punch a hot spot through it?Part of North America has been quickly uplifted, creating the spectacular relief in the Grand Canyon. Just west, is the Basin and Range, a region extended to produce horst and graben.

Oceanic crust sinks, mostly, and when two oceanic crusts meet, they both sink, producing the deep trenches, I think. The continents in those early paleomaps are reconstructions of where the modern contintental parts fit, rather than their actual positions at the time, right? They've grown since then.

cran
2009-Apr-02, 08:14 AM
Part of North America has been quickly uplifted, creating the spectacular relief in the Grand Canyon. Just west, is the Basin and Range, a region extended to produce horst and graben.
yep ...


Oceanic crust sinks, mostly, and when two oceanic crusts meet, they both sink, producing the deep trenches, I think. not as far as I'm aware - do you have a reference for that?

I think you'll find that one slab (plate edge) will subduct, the other will override - you can tell which is which because the overriding plate hosts the volcanic island arc ...

in general, it seems that the larger oceanic plate will subduct in these convergences, and at least some of the smaller can eventually be uplifted onto the leading edge of a continental plate -
whether that is because the larger plate has moved further from source ... is therefore cooler and denser and carries more sedimentary overburden ... or it's just bored with touring the surface and looking for something different, the outcome is the same ...

the eastern and western Pacific regions have some lovely complex interactions between large and small oceanic plates - around The Philippines, there's a plate where part of the western leading edge is overriding its neighbour, right next to where it is subducting - ie, it's being torn as it progresses ...

ETA: Trench depth correlates with plate motion and subduction velocity - faster means deeper ...


The continents in those early paleomaps are reconstructions of where the modern contintental parts fit, rather than their actual positions at the time, right? They've grown since then.yes ...

mostly, what the earliest paleaomaps show is the reconstruction of what are now the cratons - there is some "artistic licence" involved, because ancilliary landscapes are inferred (by ancient - but younger - mountain chains and basins) but much more difficult to map (being all scrunched up, eroded, etc) ...
the modern continental outlines show where the remains of those early landscapes can now be found ...

cran
2009-Apr-02, 09:10 AM
not sure if this belongs more here, or in the other discussion about mountain erosion and uplift - sort of fits both ...

I found this abstract note while sorting through my files for GOE stuff:

from - http://adsabs.harvard.edu/abs/1996JGR...10117747A



Title:
Erosion as a driving mechanism of intracontinental mountain growth
Authors:
Avouac, J. P.; Burov, E. B.
Publication:
Journal of Geophysical Research, Volume 101, Issue B8, p. 17747-17770 (JGR Homepage)
Publication Date:
08/1996

Abstract
In nature, mountains can grow and remain as localized tectonic features over long periods of time (>10 m.y.). By contrast, according to current knowledge of lithospheric rheology and neglecting surface processes, any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years. For example, assuming a quartz-dominated crustal rheology, the relief of a range initially 3 km high and 300-400 km wide is reduced by half in about 15 m.y. as a result of lateral spreading of its crustal root. We suggest that surface processes might actually prevent such a ``subsurface collapse.'' Removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny. We performed a set of numerical experiments in order to validate this hypothesis. A section of a lithosphere, with a brittle-elasto-ductile rheology, initially loaded by a mountain range is submitted to horizontal shortening and to surface processes. If erosion is intense, material is removed more rapidly than it can be supplied by crustal thickening below the range, and the topography is rapidly smoothed. For example, a feature 3 km high and 300-400 km wide is halved in height in about 15 m.y. for an erosion coefficient k=103 m2/yr (the erosion rate is of the order of a few 0.1 mm/yr). This regime might be called ``erosional collapse.'' If erosion is not active enough, the crustal root spreads out laterally and ``subsurface collapse'' occurs. In the third intermediate regime, removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow. In this ``mountain growth'' regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law. The erosion rate may be high (e.g., 0.5-0.9 mm/yr), close to the rate of tectonic uplift (e.g., 0.7-1.1 mm/yr), and few times higher than the rate of topographic uplift (0.15- 0.2 mm/yr). These experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain. Surface processes must therefore be taken into account in the interpretation and modeling of long-term deformation of continental lithosphere. Conversely, the mechanical response of the lithosphere must be accounted for when large-scale topographic features are interpreted and modeled in terms of geomorphologic processes.

so, whether a mid-plate mountain or plateau persists might depend on the weather ... :lol:

hhEb09'1
2009-Apr-02, 12:48 PM
Oceanic crust sinks, mostly, and when two oceanic crusts meet, they both sink, producing the deep trenches, I think. not as far as I'm aware - do you have a reference for that?

I think you'll find that one slab (plate edge) will subduct, the other will override - you can tell which is which because the overriding plate hosts the volcanic island arc ... Sorry, that made it sound like the two trenches would be subducting together, which I guess would be essentially straight down. I don't think there's any example of that, but that example you give in the Phillipines is a good one of how complicated it can get.

But I can't think of any example where oceanic crust over-rides oceanic crust and it goes up, or stays up. They sink, as near as I can tell, because they're both heavier. The back-arc volcanism is lighter material though, and it can be "scraped off" and accumulated, and then might end up agglomerating to a continental crust.

jlhredshift
2009-Apr-02, 12:54 PM
Oceanic crust sinks, mostly, and when two oceanic crusts meet, they both sink, producing the deep trenches, I think. The continents in those early paleomaps are reconstructions of where the modern contintental parts fit, rather than their actual positions at the time, right? They've grown since then.

Hayes and Ewing, 1970, plotted 35 topographical deep sea Pacific trenches and showed that the characteristic shape was one wherein the subducting trench curved at a larger radius under the sharply almost vertical edge of the over riding plate. The shape is almost the same as the V Bulletin avatar such as is found when you click the "insert link" button.

cran
2009-Apr-02, 04:26 PM
Sorry, that made it sound like the two trenches would be subducting together, which I guess would be essentially straight down. I don't think there's any example of that, but that example you give in the Phillipines is a good one of how complicated it can get.

But I can't think of any example where oceanic crust over-rides oceanic crust and it goes up, or stays up. They sink, as near as I can tell, because they're both heavier. The back-arc volcanism is lighter material though, and it can be "scraped off" and accumulated, and then might end up agglomerating to a continental crust.

again, can you cite a reference for your extraordinary ATM claims?

can you point to an example where both plates at a convergent boundary have sunk, or are sinking?

as volcanoes form the island arc, what exactly is "back-arc" volcanism?

from USGS:
http://pubs.usgs.gov/gip/dynamic/understanding.html



Oceanic-oceanic convergence

As with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the other, and in the process a trench is formed...

Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved...

Magmas that form island arcs are produced by the partial melting of the descending plate and/or the overlying oceanic lithosphere. The descending plate also provides a source of stress as the two plates interact, leading to frequent moderate to strong earthquakes. ------------

But I can't think of any example where oceanic crust over-rides oceanic crust and it goes up, or stays up.

from - Oregon State:
http://volcano.oregonstate.edu/education/facts/ophiolites.html


Ophiolites are pieces of oceanic plate that have been thrusted (obducted) onto the edge of continental plates. They provide models for processes at mid-ocean ridges.

Ophiolites are an assemblage of mafic and ultramafic lavas and hypabyssal rocks found in association with sedimentary rocks like greywackes and cherts. They are found in areas that have complex structure. Cross-sections simplified from R.C. Coleman, 1981, Journal of Geophysical Research, v. 86, p. 2497-2508.

Ophiolites have been found in Cyprus, New Guinea, Newfoundland, California, and Oman. The Samail ophiolite in southeastern Oman has probably been studied in the greatest detail. The rocks probably formed in the Cretaceous not far from the what is now the Persian Gulf. The rocks were later thrust (pushed uphill at a low angle) westward onto the Arabian shield. now, can you guess how those pieces of oceanic plate were in a position to ride up over sedimentary rocks?

if you can come up with an alternative explanation that fits the geological evidence and ongoing observations, then your name will end up in the text books ...

cran
2009-Apr-02, 04:31 PM
Hayes and Ewing, 1970, plotted 35 topographical deep sea Pacific trenches and showed that the characteristic shape was one wherein the subducting trench curved at a larger radius under the sharply almost vertical edge of the over riding plate. The shape is almost the same as the V Bulletin avatar such as is found when you click the "insert link" button.
that's right - and it's another way to tell which plate is which ...

hhEb09'1
2009-Apr-13, 08:50 AM
again, can you cite a reference for your extraordinary ATM claims?I'm not making ATM claims. :)

I was just relating my memory of the situation to the best of my (limited) ability. I tried to clarify a few posts back that I was not talking about two oceanic crusts subducting together (or, subducting underneath each other) but I can see that you still think that that's what I said:

PS - should I ask how you're going with that "both marine plates subduct" idea?
I just meant, that oceanic crust is cooling and getting heavier, and tends to sink, rather than rise up. Even with ophiolites, aren't those generally attributed to oceanic/continental crust collisions, where the (less dense) continental crust has been entrained on the subducting slab?

geonuc
2009-Apr-13, 10:46 AM
I just meant, that oceanic crust is cooling and getting heavier, and tends to sink, rather than rise up. Even with ophiolites, aren't those generally attributed to oceanic/continental crust collisions, where the (less dense) continental crust has been entrained on the subducting slab?
Just the opposite, if I read you right. Ophiolites are portions of oceanic crust that got plastered onto the non-subducting continental crust.

hhEb09'1
2009-Apr-13, 05:53 PM
Just the opposite, if I read you right. Ophiolites are portions of oceanic crust that got plastered onto the non-subducting continental crust.I meant where the lighter continental crust is on the subducting slab and gets wedged under the oceanic crust--thereby raising the opposing ocean crust.

cran
2009-Apr-13, 06:12 PM
I'm not making ATM claims. :)

I was just relating my memory of the situation to the best of my (limited) ability. I tried to clarify a few posts back that I was not talking about two oceanic crusts subducting together (or, subducting underneath each other) but I can see that you still think that that's what I said:
I just meant, that oceanic crust is cooling and getting heavier, and tends to sink, rather than rise up.
OK - I misconstrued what you meant by
when two oceanic crusts meet, they both sink ...
which you later clarified with
that made it sound like the two trenches would be subducting together, which I guess would be essentially straight down. I don't think there's any example of that ...But I can't think of any example where oceanic crust over-rides oceanic crust and it goes up, or stays up. They sink, as near as I can tell, because they're both heavier. -
as maintaining your claim that both plates sink in oceanic-oceanic convergence, just not straight down ... in my experience, that is as about as ATM as it gets ...

I think the issue here is the relative properties - they're both heavier -
yes, but only one is heavier (cooler, denser) than the other at the time of convergence ... the other is heavier than continental crust, and heavier than it was at formation, but not heavier than the subducting plate ...
and as subduction progresses, the leading edge of over-riding plate is uplifted (relative to its pre-convergence height), and pieces are broken off and forced back onto the plate as part of the accretionary wedge ...
when the oceanic-oceanic convergence evolves into an oceanic-continental convergence, or a continental-continental convergence, the melange (wedge) is further uplifted onto a continental plate - that's why we find chunks of old oceanic plates (ophiolites) sitting on top of continental-style materials ...

The back-arc volcanism is still not explained - the mainstream view of the back-arc is that it begins as a depositional basin behind the volcanic arc; it's a region of relative downwarping caused by the relative uplift of the leading plate edge, and maintained thus as arc formation develops from increasingly less mafic materials ... where the back-arc basin adjoins a continental mass, it will undergo both infilling and relative uplift over time, forming a substantial highland or plateau connecting the volcanic chain to the continent ... so, back-arc volcanism also sounds ATM ...

Even with ophiolites, aren't those generally attributed to oceanic/continental crust collisions, their emplacement on continents usually are, but not their initial creation ...


where the (less dense) continental crust has been entrained on the subducting slab?no - don't go there ...

cran
2009-Apr-14, 09:02 AM
I meant where the lighter continental crust is on the subducting slab and gets wedged under the oceanic crust--thereby raising the opposing ocean crust.

OK - I think I see what you're aiming at - but no, the ophiolites are already emplaced by this stage ...

by the time an entrained continent reaches the trench (and remember that there was a fore-trench anticlinal upwarping), the leading edge of the over-riding plate has lost its original rocks - they've either been broken off and pushed back (ophiolites), or abraded into the trench, or reworked by partial melting, physical disruption, and metamorphism ... in other words, by this stage, the leading edge of the over-riding plate is a volcanic chain attached to a continent ... the leading edge of the approaching continent was, up to this stage, a passive margin ... well dressed in the finest sediments - in other words, it's Crunch Time! ...

the nature of subduction and convergence changes from oceanic-continental to continental-continental ... the upper sedimentary layers of the approaching continent are peeled from the igneous-metamorphic base and provide the visible front of the growing orogeny ... the base of the approaching continent - because of its greater relative density - is forced under the leading edge of the over-riding plate ... however, its density is not sufficient to subduct in the same way as the preceding oceanic crust - instead, it follows a shallower path underplating and uplifting more of the over-riding plate ...

BigDon
2009-Apr-14, 12:58 PM
So then what is happening off of where I live (California)? I have a stationary continental plate edge and then an offshore plate edge moving northward. Is the offshore plate rotating clockwise or is the whole plate moving northward?

Plus we are due for our twenty year 7 to 8 point earthquake pretty soon, since it's been that long since the Loma Prieta shake. (I was in the process of being electricuted when, thank God, the power finally failed. Always an E-ticket ride, to be sure.)

dgavin
2009-Apr-14, 02:04 PM
Slip zones like California's still have a lot of presure associate with them.

Even thought the plates do slide, over time they are also pushed into each other due to the shape and motion of the plates. This is whats makes the hills and mountian prevelent in those regions, known as Fault Block mountains.

This can also happen in interior fault zones. Check out the Steens Mountains http://en.wikipedia.org/wiki/Steens_Mountain for a good example of rather impressive fault block mountains.

hhEb09'1
2009-Apr-14, 03:29 PM
The back-arc volcanism is still not explained - the mainstream view of the back-arc is that it begins as a depositional basin behind the volcanic arc; it's a region of relative downwarping caused by the relative uplift of the leading plate edge, and maintained thus as arc formation develops from increasingly less mafic materials ... where the back-arc basin adjoins a continental mass, it will undergo both infilling and relative uplift over time, forming a substantial highland or plateau connecting the volcanic chain to the continent ... so, back-arc volcanism also sounds ATM ...
Barc-arc volcanism is ATM? There are plenty of examples of back-arc volcanism.

Looking back, my remark would have applied to arc volcanism as well, so that's probably what I should have said.

hhEb09'1
2009-Apr-14, 03:33 PM
Even thought the plates do slide, over time they are also pushed into each other due to the shape and motion of the plates. This is whats makes the hills and mountian prevelent in those regions, known as Fault Block mountains.

This can also happen in interior fault zones. Check out the Steens Mountains http://en.wikipedia.org/wiki/Steens_Mountain for a good example of rather impressive fault block mountains.Fault block mountains are like the norst and graben I mentioned earlier, and are usually the result of extension, the lands being pulled apart, rather than being pushed into each other. See the fault block mountain link at that wiki page.

cran
2009-Apr-14, 11:00 PM
Barc-arc volcanism is ATM? There are plenty of examples of back-arc volcanism.
then you shouldn't have any problem explaining how it* works, and providing some of the examples - enlighten me ...

*it, being:

The back-arc volcanism is lighter material though, and it can be "scraped off" and accumulated, and then might end up agglomerating to a continental crust.
what "scrapes off" the back-arc volcanism?


Looking back, my remark would have applied to arc volcanism as well, so that's probably what I should have said.
OK - what "scrapes off" the arc volcanism?

cran
2009-Apr-14, 11:22 PM
Slip zones like California's still have a lot of presure associate with them.

Even thought the plates do slide, over time they are also pushed into each other due to the shape and motion of the plates. This is whats makes the hills and mountian prevelent in those regions, known as Fault Block mountains.


You might be thinking of thrust faults or reverse faults?




A thrust fault is a type of fault, or break in the Earth's crust with resulting movement of each side against the other, in which a lower stratigraphic position is pushed up and over another. It is the result of compressional forces...

If the angle of the fault plane is low (generally less than 20 degrees from the horizontal) and the displacement of the overlying block is large (often in the kilometer range) the fault is called an overthrust...
Thrust Fault (http://www.absoluteastronomy.com/topics/Thrust_fault)

hhEb09'1
2009-Apr-14, 11:43 PM
then you shouldn't have any problem explaining how it* works, and providing some of the examples - enlighten me ...Enough problem. :)

I'm no expert, my knowledge was derived from a few geology course fifteen years ago (and one intro TA). Just googling, the Intermontane Belt in British Columbia looks like it was an island arc that accumulated on a continent.

Here are the first few "back-arc volcanism" links:
Early Cambrian back-arc volcanism in the western Taurides, Turkey (geolmag.geoscienceworld.org/cgi/content/abstract/142/5/617)
Petrology and geochemistry of cross-chains in the Izu-Bonin back arc (www.agu.org/pubs/crossref/2008/2007GC001641.shtml)
Late Mesoproterozoic Arc and Back-arc Volcanism in the Heimefrontfjella (sciencelinks.jp/j-east/article/200314/000020031403A0485618.php)

The second area, Izu-Bonin, you'd brought up earlier, I think.

cran
2009-Apr-15, 12:29 AM
Enough problem. :)

I'm no expert, my knowledge was derived from a few geology course fifteen years ago (and one intro TA).
it's OK, even for mods, to say "I've mis-remembered that bit" or "Oops";
I do it all the time ...


Just googling, the Intermontane Belt in British Columbia looks like it was an island arc that accumulated on a continent.such is the fate of most (if not all) island arcs: to become the active margin of a continent - until some other continent decides to get up close and personal - but "accumulated on" might also be a bit misleading; I would have said "attached to" ...


Here are the first few "back-arc volcanism" links:
Early Cambrian back-arc volcanism in the western Taurides, Turkey (http://geolmag.geoscienceworld.org/cgi/content/abstract/142/5/617)
Petrology and geochemistry of cross-chains in the Izu-Bonin back arc (http://www.agu.org/pubs/crossref/2008/2007GC001641.shtml)
Late Mesoproterozoic Arc and Back-arc Volcanism in the Heimefrontfjella (http://sciencelinks.jp/j-east/article/200314/000020031403A0485618.php)

OK - so you're talking about dyke emplacements during back-arc basin formation, extension of arc volcanism into the basin sub-strate caused by the deeper subducting slab, and volcanism associated with basin rifting and spreading - that's according to the first two links (the third is just a title) -
so yes, OK - back-arc volcanism; I stand (well, sit) corrected on that point -

but I don't see anything about that volcanism being "scraped off" ...


The second area, Izu-Bonin, you'd brought up earlier, I think.Did I? OK ...

dgavin
2009-Apr-15, 12:33 AM
You might be thinking of thrust faults or reverse faults?

Thrust Fault (http://www.absoluteastronomy.com/topics/Thrust_fault)

Steen's Mountain is a Fault Block formation, which is more similar to a Slide Fault hill formation scenario then a Thrust or Reverse Fault formation.

There isn't any material pushing up under one side of the fault compared to the other. If anything the Steen's are probably best described by Plate Buckling mountain building similar to the West US Coastal or the Himalayan ranges.

It is also possible that Steen's formation was related (or assisted by) to the Yellowstone/Newberry Hot-spot at one time.

And don't forget about Drop Faults like Sun Valley Idaho region. My personal favorite. Got to love a valley that just drops almost 7 feet during an earthquake, because both sides of a split fault surrounding the valley decided to go at the same time.

hhEb09'1
2009-Apr-15, 02:50 AM
such is the fate of most (if not all) island arcs: to become the active margin of a continent - until some other continent decides to get up close and personal - The fate of most oceanic crust is to sink--that island arc's plate was subducting under the continent it seems.
but "accumulated on" might also be a bit misleading; I would have said "attached to" I've sometimes viewed continents as accumulations of micro-continents
Did I? OK ...Maybe not, looking back I'm not sure what the reference was to.
Steen's Mountain is a Fault Block formation, which is more similar to a Slide Fault hill formation scenario then a Thrust or Reverse Fault formation.Yahbut, I think cran was responding to your description of it as "they are also pushed into each other". You used Steen's Mountain as an example of that, but it doesn't seem to be an example of that.

cran
2009-Apr-15, 11:35 AM
just quickly (I'm on my way to work) -

yes -- without knowing the specific geology there, I'll take your word for it ...

OK ...

OK ...

yes, that's what I saw in the general description -
Steen's seems to be a result of both extensional (evidenced in the tilt) and volcanogenic uplift, rather than compressional or transform plate motion ...

dgavin
2009-Apr-15, 02:33 PM
Yahbut, I think cran was responding to your description of it as "they are also pushed into each other". You used Steen's Mountain as an example of that, but it doesn't seem to be an example of that.

Yeah, wasn't my best choice of an example.

The point I was trying to make though was that even though the two plates slide, there is a lot of pressure laterally also as they push into each other. Hill formation is from that and from the sheering forces as they do slide.

hhEb09'1
2009-Apr-15, 02:37 PM
Yeah, wasn't my best choice of an example.

The point I was trying to make though was that even though the two plates slide, there is a lot of pressure laterally also as they push into each other. Hill formation is from that and from the sheering forces as they do slide.A lot of the western USAn hills are from extension, like Steen's, not compression.

cran
2009-Apr-25, 10:31 PM
The fate of most oceanic crust is to sink--that island arc's plate was subducting under the continent it seems.
really?
which island arc's plate were you referring to?
considering where island arcs tend to form, what makes you think it was subducting under the continent?