#Fold mountains
37 messages in this thread
Hi, Kamron–
I am answering your message in the older thread as well as this.
>> The whole continental miles deep crust isn't compressing is it?
It could be, in cases of fold mountains like the Himalayas. We can measure the
gravitational differences (and I think the seismic wave distortions) caused by
the "roots" of major mountain chains where the crust extends unusually deep
into the lithosphere (shallow mantle), counterbalancing the weight of the
mountains (acting like a float to hold the mountains up).
The entire plate (crust plus lithosphere) may be compressed when continents
collide. If the lithosphere bulges into the outer mantle, I figure it simply
becomes outer mantle rock, but the crustal bulge remains as fold mountains and
their roots. I do not know if the roots are _precisely_ matched to the
mountains or just generally matched, as when paper is crumpled (no filling
needed).
Howard has told you how the craton may not be compressed, but I am looking at
the crust as a whole. As the slop (geosyncline) you have been talking about is
pushed atop the craton, I expect the craton is forced down to gain buoyancy to
support the mountains; if the collision is fierce enough, I expect the craton
to crumple and shorten also, which is my guess for the Himalayas.
Fault block mountains have roots too, though they are created differently. Over
time, I expect any mountain without a "float" underneath would simply sink into
the crust, faster than it could erode.
>> How would this transfer back hundreds of miles behind the coast leaving the
coast in many areas relatively flat?
It did not transfer back much if any; a good part of the Appalachians formed
where two coasts came together as Pangaea was assembling, but they were not
today's coasts; new coasts were cut by the later spreading that split N.
America from W. Africa. Land between the Appalachians and the east coast was
largely part of Africa before Laurentia (the predecessor of N. America) hit
Africa while Pangaea was being assembled.
–Doug
Hi Doug,
[To all really, mainly Kamron]
If one lives in California as I do, one doesn't have much problem with this
whole continental drift idea. Earthquakes are pretty convincing <G>!
Just a short thought. Bye.
Troy Britain (Printer & Amateur Naturalist)
Kamron
>>Howard has told you how th craton may not be compressed, but I am looking at
the crust as a whole. As the slop (geosyncline) you have been talking about is
pushed atop the craton, I expect the craton is forced down to gain buoyancy to
support the mountains; if the collision is fierce enough, I expect the craton
to crumple and shorten also, which is my guess for the Himalayas.<<
The Himalayas do look like they were formed by an impact by India. It is hard
to mentally simulate this type of event accurately but again it appears like
the results were caused by a rapid event rather than a slow steady process.
If there is no real speed then the inertia required to smash and compress is
not
there. If we look at the continent of india smashing at a faster pace it is
much easier to see the heat and forces necessary to accomplish the compression
and folding. A slow motion would get resisted and I still have not heard a
mechanism to produce the forces necessary to overcome the resistance that a 25
mile thick granitic block could absorb.
>>Fault block mountains have roots too, though they are created differently.
Over time, I expect any mountain without a "float" underneath would simply sink
into the crust, faster than it could erode.<<
I agree
>>It did not transfer back much if any; a good part of the Appalachians formed
where two coasts came together as Pangaea was assembling, but they were not
today's coasts; new coasts were cut by the later spreading that split N.
America from W. Africa. Land between the Appalachians and the east coast was
largely part of Africa before Laurentia (the predecessor of N. America) hit
Africa while Pangaea was being assembled.<<
Ok I can bite off on that..a bit anyway. There is an excellent match between
the east coast of N. America and the northern part of the eastern African
Coast.
What evidence is there that this eastern US coast was separated from the the
rest of the Continent?
Kamron
Hi, Kamron–
First, I will respond to your message in the "All" thread, since this is a
better place. Then I will respond to the your message in this thread.
>> I have a major problem with the continent compressing though.
I see continental collisions as resembling car collisions. A light impact just
crumples the bumper (shoves the sediments and older-rifting detritus on from
the continental margins up on top). A heavier impact crumples the whole front
end of the car (including the continental craton).
Granite is no more ultimately rigid than basalt or any other rock. Rigidity is
relative, as we have discussed before, and looks different in geological time
frames when there is enough time to push solids along cleavage planes (like
slowly but surely pushing those magnet-studded boards one space at a time). In
human time frames, there is time only to push them hard so they shatter or melt
(if you are in too much of a rush, you can only kick the boards apart or rip
the magnets loose), so granite appears utterly rigid. It is more rigid than the
deep mantle, but not absolutely rigid – if it were, there would be no granitic
gneiss.
>> It would bulge downward to match the upward motion. But why would this occur
rather than the thin ocean basin give way and buckle up?
We were talking about _continents_ colliding; how did the ocean floor get
involved? There may be a seafloor-like underlayer to the colliding continents,
which along with lithosphere may still be subducting out from under the
collision point to keep pulling the continents together even after first
impact; I gather this has been proposed in the Himalayas case. The ocean floor
that was following the colliding continents may start subducting at the outer
edges of both continents, but only after some of the impact has been absorbed.
>> Slow motion would absorb and dissipate the heat…
Is this supposed to mitigate the impact?
>> …and transmit the pressures to the other side better than a rapid event
where the enormous inertias would help create the upthrusts.
Transmitting pressures to the other side is what the collision-crumpling is all
about. Slowness might allow the pressures to be transmitted back away from the
collision zone, so the crumpling might be allowed elsewhere. Indeed, I expect
subduction and compression at the far sides of the continents in some cases.
>>it appears like the results were caused by a rapid event rather than a slow
steady process
But rapidity would not allow time for smooth deformation of solid rock – it
would all have to be brittle breakage or liquefaction. Signs of faults and
melting appear to be much too modest where the Himalayas and other fold
mountains are concerned.
>> If there is no real speed then the inertia required to smash and compress
is not there.
There is less inertia, but still a lot. And as noted above there are likely
forces still being applied while the collision is underway. Indeed, I have
heard that the Himalayas have been measured as still rising, though I cannot
recall where.
Your way has _too much_ inertia given the mind-staggering masses involved;
again, fault blocks rather than folds would be expected, if not a total
meltdown – which we do not see.
>> There is an excellent match between the east coast of N. America and the
northern part of the eastern African Coast.
As the cookie matches the dough mat it was cut from, so do the modern coasts
match because the mid-Atlantic ridge cut them from one supercontinent. The
coasts that collided to form the Appalachians may not have been such a good
fit, and may have needed a lot of jamming together.
>> What evidence is there that this eastern US coast was separated from the the
rest of the Continent?
I am not familiar with this case. I dimly remember something about some
seafloor-rock trapped in the Appalachians. If there are pre-Carbonifereous
fossils east of the Appalachians, I expect them to match African counterparts
better than other N. American.
Note that I am not really talking about the entire Appalachians; I think parts,
like the Adirondacks (?) came from different events.
–Doug
Doug,
Offered FWIW:
>> Indeed, I have heard that the Himalayas have been measured as still rising,
though I cannot recall where. <<
I've read this too, in more than one place. I think somebody measured the
uplifting in the range of millimeters per year.
>> As the cookie matches the dough mat it was cut from, so do the modern coasts
match because the mid-Atlantic ridge cut them from one supercontinent. The
coasts that collided to form the Appalachians may not have been such a good
fit, and may have needed a lot of jamming together. <<
They did, apparently. The Appalachians contain some of the most complex
folding to be found anywhere on Earth. Clearly _folding_ too, not faulting.
I've seen exposed Appalachian folds, and it's quite an experience to stand in
front of a cliff 100 meters long and 20 or 30 high in which the layers are all
bent into a gigantic, sweeping arch. The energy required to do that boggles
the mind.
Interestingly, the orogeny that created the complex folds was the _third_ major
orogeny involving eastern North America. The first one was caused by small
exotic terranes colliding with and attaching to the proto-North America; the
second resulted from the combination of NA with proto-Europe, proto-Britain,
and a couple more exotic terranes into the Old Red Sandstone continent. The
third was caused by the collision of the Old Red Sandstone continent with
Gondwanaland.
>> I dimly remember something about some seafloor-rock trapped in the
Appalachians. If there are pre-Carbonifereous fossils east of the
Appalachians, I expect them to match African counterparts better than other N.
American. <<
The Appalachians actually contain rocks from all through Earth's history. The
Blue Ridge in the center is uplifted Precambrian rock; to its east and west lie
a lot of Paleozoic beds, though many are heavily deformed and metamorphosed to
the east. Over those are some scattered younger beds, Mesozoic and Cenozoic.
Dinosaur tracks have been found in Connecticut, and of course Dr. Leidy found
_Hadrosaurus foulkii_ in New Jersey. I don't know much about fossil
distributions, or how well the fauna matches up with contemporary North African
faunas. I'm not at all sure, but I have the impression that everything exposed
east of the Piedmont Province dates from after the breakup of Pangaea. There
are older rocks there, but they're very deeply buried below the more recent
sediments.
>> Note that I am not really talking about the entire Appalachians; I think
parts, like the Adirondacks (?) came from different events. <<
I think the Adirondacks are thought to be the result of a circular uplift
unrelated to the Appalachians.
Jon W.
>> I dimly remember something about some seafloor-rock trapped in the
Appalachians. If there are pre-Carbonifereous fossils east of the
Appalachians, I expect them to match African counterparts better than other N.
American. <<
In general, yes. There are purported to be some seafloor rocks exposed along
the edges of the Bronson Hill Volcanics (thought to be remains of an off shore
island arc) in New Hampshire, and the Taconic mountains of New York, VT, MA
appear to be a chunk of Cambrian Seafloor thrust faulted over the
Ordovician-Cambrian rocks of the ("Appalachians" – actually the Green
mountains) and into place west of them. The Green Mtns are somewhat folded,
but unlike the Appalachians to the South, there has been massive and repeated
thrust faulting, so they are more stacked than folded.
The fossil faunas of Eastern Newfoundland, the Maritimes, the Boston Basin and
the basin (whose name eludes me) in Southern Massachusetts and Rhode Island are
European-African as are faunas found in Windows in the Carolina Slate Belt.
(The rocks east of there are Mesozoic/Cenozoic marine and continental deposits
with North American faunas). A good section in SouthEast Newfoundland reveals
European trilobite faunas in the same sequence in which they are found in
Europe whereas Cambrian rocks in Western Newfoundland have North American
faunas.
And yes, the Adirondacks appear to be a local uplift unrelated(?) to the
Appalachian chain.
Hi, Don–
Thanks for the info; I was running out of ammo.
–Doug
Hi, Jon–
In Old Red Sandstone, I am pleased to have a name for proto-N.America +
proto-Europe (Laurentia+Baltica), since I gather they were together for a long
time before joining up with Pangaea.
The Jan. 1995 Scientific American has a cover story proposing that Laurentia
rifted away from proto-S.America ~550 million years ago; the coast that would
eventually become the Appalachians separated from what is now Chile, more or
less (modern Scotland would have been in the Arica embayment between Peru and
Chile, more or less). It is suggested that Laurentia might have bumped S.
America twice more while making and end run around it before being collected
into Pangaea. In the mid-Ordovician, the eastern U.S. presumably hit Peru; in
the late Devonian, they have the southeastern U.S. hit again near Peru or
Ecuador (by this time it is really O.R.S., not just Laurentia).
That last collision, in the late Devonian, would have corresponded to the
Acadian orogeny, they figure. Is this the first orogeny you spoke of? Thus S.
America would replace the "small exotic terranes" you spoke of, or perhaps they
were island arcs caught in the squeeze.
–Doug
Doug,
Disclaimer: I'm referring mainly to a basic geology/paleontology text dated
1989, so new info like your SA article may have changed some of this. But
here's what this ref. says about the Appalachians:
1) In the Ordovician Period, Laurentia and Baltica were moving together, with a
subduction zone in between. Sometime in the mid-Ordovician, that subduction
zone was the site of a meeting between Laurentia and an island arc something
like Japan or the Solomons. When they came together, the island arc sutured to
Laurentia. That triggered the first major orogeny, the Taconic. The Taconic
Mountains formed as the island arc's crust overrode Laurentia's crust. Rock
formations seem to indicate that the Taconics were a high, volcanic type of
mountain range similar to the Andes. Erosion had apparently worn down the
Taconics by Mid-Silurian times, because about then is when the deposition of
eroded sediments to the west stops.
2) A new subduction zone formed east of the old island arc, again drawing
Laurentia and Baltica together. The Acadian orogeny resulted from the
collision of Laurentia and Baltica, and their combination into the ORS
continent. The Acadian orogeny started in the north (Greenland/Scandinavia) in
the very late Silurian, and continued in irregular bursts all through the
Devonian and into the Pennsylvanian. We know it was the collision of Laurentia
and Baltica because what European geologists call the "Caledonian" orogeny is
recognizably the same event as the Acadian, from the same time period, sharing
similar faunas and sedimentary rock formations. A little later, well south of
the Laurentia/Baltica collision region, another volcanic island arc was
apparently sutured to Laurentia; that's now called the Avalon terrane.
3) In the upper Mississippian, another major orogeny occurred as a result of
the ORS continent's collision with Africa to form Pangaea. This one is called
the Alleghenian orogeny. It involved a lot of uplifting, folding,
metamorphing, thrust faulting, and a number of igneous intrusions. The
Alleghenian mountains were probably a lot like the Himalayas in their
characteristics. The Alleghenian orogeny is also known from Europe and
northern Africa. In Europe it's called the Hercynian orogeny; in Africa, it's
the Mauritanides orogeny.
By the late Mesozoic, even the Alleghenian mountains were gone, leveled by
erosion. The modern Appalachians are the result of slow uplifting during the
Cenozoic, possibly mixed with isostatic adjustment.
Jon W.
Hi, Jon–
The author of the article (Ian W. D. Dalziel) seemed to have the strongest
conviction that Antarctica was connected to Laurentia before the Cambrian; the
various collisions of Laurentia (proto-N.America) with the S. American part of
Gondwana seemed to be worded as more speculative, or perhaps that was more
because he was referencing other people's notions that fit in well.
The article has a series of maps showing Baltica and Laurentia starting to
collide between the mid-Ordovician and mid-Silurian maps, not a bad fit for the
Taconic orogeny. In the next late-Devonian map they are more firmly connected
and the southeastern U.S. is scraping along Peru – this is the event they
associated with the Acadian orogeny, thus I guess Laurentia is getting mashed
from two directions at nearly the same time (it is comforting that you describe
the Acadian as long-lasting). The Canadian/Greenland coast is clearly affected
mainly by Baltica in these events. Perhaps that Avalon terrane was far enough
south to be involved in that presumed Peru contact?
If all of the fold mountains from the pre-Pangaean continental collisions wore
away, we have been barking up the wrong tree in trying to explain the
Appalachians in such terms. Were they completely worn away all up and down the
east coast, or did significant parts survive? Perhaps the modern Appalachians
are still mainly shaped by the folds of those older mountains? This would help
explain the frequency of missing/fallen anticline tops we spoke of by adding a
potential mechanism; as weathering exposed a tough layer at the top, it would
be attacked before the flanks of that tough layer became exposed.
–Doug
<<If all of the fold mountains from the pre-Pangaean continental collisions
wore away, we have been barking up the wrong tree in trying to explain the
Appalachians in such terms. Were they completely worn away all up and down the
east coast, or did significant parts survive? Perhaps the modern Appalachians
are still mainly shaped by the folds of those older mountains?>>
I agree with you, Doug. As deep and complete as the erosion of a faulted and
folded region could ever be, it would still be differential between layers.
Resilient beds, and/or inclined beds, will always produce relief. Nothing in
the Central Appalachians, at least, has been eroded flat. The inverted
topography is a result of eroded folds, and it will continue to invert as
previously buried resilient layers and previously buried soft layers continue
to be alternately exposed.
Until, of course, there is deposition on top of the stubs, as there is already
in GA and AL where the southern range disappears under the Mesozoic. Should
that area ever be uplifted and subsequently eroded, newly re-exposed faults and
folds will once again provide relief.
Hi, Ed–
The picture I am forming in my mind is of a great range of fold mountains (as
big as the Himalayas?) forming during the collisions as/before Pangaea formed,
with correspondingly great roots below them.
Then, sort of like I explained to Kamron, as the tops wore down, the roots
floated up correspondingly, perhaps keeping the general level always high. (I
goofed in treating the roots as equal in depth to the mountains' height,
momentarily forgetting the density difference between air and mountain is far
greater than the difference between continental rock and mantle rock, thus
should have made the roots deeper by a factor of 5 or 10, more like an
iceberg). By now the mountains have apparently worn down below the original
ground level, but differential wear on deeper folds from the collision keeps
them mountainous.
I wonder if they have worn down to the original cratons yet?
I also wonder if I have this all right – the water gaps where rivers cut
straight through these mountains, mentioned in other messages in this thread,
are a bit annoying.
–Doug
<< The picture I am forming in my mind is of a great range of fold mountains
(as big as the Himalayas?) forming during the collisions as/before Pangaea
formed, with correspondingly great roots below them…Then, sort of like I
explained to Kamron, as the tops wore down, the roots floated up
correspondingly>>
I see what you're saying here, and I think you're basically right, but I don't
see your "roots" going down very far. Remember, these are sediments which have
overflowed the edge of the continental craton being pushed back by the plate
collision like you'd push back the cuticle of your fingernail (ouch…!) So
while the margin of the craton could be expected to subside a bit because of
the additional mass of the folded, faulted and crumpled sediments on top of it,
we're really talking about only a few thousand feet of overthrusted sediments
across a relatively narrow band parallel to the continental margin, and that
can't make TOO much difference along a continental margin that has been
accumulating sediments since the end of the Precambrian. I rather think the
bulk of the uplift in any plate colllision, or at least a good portion of it,
is from the upturning of the continental crust as the colliding plate plows
under it. The difference in the density of oceanic crust and continental crust
with sediment on top of it CAN'T be so great that the oceanic crust subducts
without plowing up the continental crust as it goes.
I think you're right though that isostatic adjustment constantly renews the
erosion process and exposes deeper and deeper folded and faulted layers to the
elements, unless something happens that either causes the whole region to
subside or causes sea-level to rise and thereby allow deposition on top of the
eroded stubs.
<< the water gaps where rivers cut straight through these mountains, mentioned
in other messages in this thread, are a bit annoying.>>
Danged annoying <g>. I live at the head of one gap and downstream of another,
cut by two separate streams. We tend to think that a stream which breeches an
anticline consequently pre-dates that uplift. I try to think that it merely
pre-dates the most recent EXPOSURE of that structure. But then we look at WV's
ironically misnomered New River, which flows the wrong way directly across the
Appalachian range. My thought is that it must have risen in a localized chunk
of high country to the east which pre-dated the additional pushing and shoving
which progressed to the west. FWIW, Western PA's Youghiogheny River also runs
the wrong way, and cuts two spectacular gorges through Laurel Hill and Chestnut
Ridge, two afterthought anticlines — still intact, not inverted — along the
western flank of the Allegheny Front. These low ripples apparently occurred
after a larger uplift to the east and the already-established river cut them as
they formed.
Hi, Ed–
I chose to speak of "roots" as the simplest way to refer to something getting
pushed into the mantle as mountains were piled atop it and rebounding as the
load was removed; I am not fussy about whether it is some inverted image of the
mountains or a general bowing of the crust over a wider region.
It does make a difference in terms of what is rising and getting eroded –
the wider-region uplift would accelerate erosion of things beyond the original
mountain range's area, and offer less uplifting of the folds from the
collision(s).
–Doug
>>>Until, of course, there is deposition on top of the stubs, as there is
already in GA and AL where the southern range disappears under the Mesozoic.
Should that area ever be uplifted and subsequently eroded, newly re-exposed
faults and folds will once again provide relief.<<<
I'd think that if erosion proceeds to the point of actually flattening out the
countryside and the area is then uplifted again, new drainage patterns will
develop away from the center of the uplift. There may certainly be traces of
the former drainage pattern. What makes me sceptical of the Appalachians as a
reuplifted range is that the major drainage patterns either follow the folds
or, in the case of rivers like the New and Susquehanna seem to predate the
folding entirely. The drainage is what you'd expect to see in a folded mountain
range, not a folded range that has eroded, then re-emerged.
There is also the question of why No Middle or Lower Mesozoic sediments occur
around the ranges. There are, not suprisingly, some sedimentary layers in the
Triassic Jurrasic Rift Valley deposits along the East Coast, but no particular
evidence (that I'm aware of) that the sediments originated in the Appalachians.
I really can't see any hard evidence that the Applachians are older than the
Early Cretaceous.
The whole thing looks kind of like bending the facts — moderate erosion —
maybe 100,000,000 years worth to fit modern theory that requires the mountains
to be three times that old. Don't know why this bothers me. I don't really
much care how old the Appalachians are.
Of course I don't know enough about geology (other than what sediments are
found where) to even qualify as an informed amateur so what do I know?
<<I'd think that if erosion proceeds to the point of actually flattening out
the countryside and the area is then uplifted again, new drainage patterns will
develop away from the center of the uplift. There may certainly be traces of
the former drainage pattern.>>
Yes, certainly. The eroding folds and faults remain the determining factors in
the topography of the central and southern Appalachians. And my point was that
they always will be the determining factors until they themselves are buried
with sediment.
<<I really can't see any hard evidence that the Applachians are older than the
Early Cretaceous.>>
No big argument here either, just a qualification that the several orogenies
we've discussed here which formed the mountain chain apparently proceeded from
north to south over time. Devonian beds are clearly involved in the folding in
northeast PA and southern New York, and these had to have come from an earlier
event northward and eastward. Jon Wolff described the individual events in
excellent order in an earlier message. In another previous post I said I was
pretty sure that the Permian beds which begin in extreme southwest PA just west
of the first discernible Appalachian upfolds lay conformably atop the
Pennsylvanian. If that IS true, it puts the major orogeny along that section of
the chain at least into the early Mesozoic. Sorry I'm not good for much
information on the eastern flank of the range. I figured the Triassic and
Jurassic in the rifts was deposition of sediments washing down off these ranges
as the Atlantic started to open up, but haven't seen enough of that area to be
able to say with any certainty.
While I think that much of the theory about the creation of the Applachians is
probably correct, I also think there are an awful lot of inconsistencies and
things that need to be explained better than they are. It sort of annoys me to
see probable facts that are supported by lots of data stirred up with shakey
hypotheses and the whole thing presented as gosphel truth.
Not tidy sir, not tidy I tell you …
Doug,
Mr. Dalziel may well have the right of it. I don't know. I only know that my
text describes exotic terranes that don't clearly match either Laurentia or
Baltica as being involved in the Taconic orogeny.
>> In the next late-Devonian map they are more firmly connected and the
southeastern U.S. is scraping along Peru – this is the event they associated
with the Acadian orogeny, thus I guess Laurentia is getting <<
This is a bit surprising. My text says that Gondwanaland was assembled from
continents and microcontinents very early in the Paleozoic, or even in the late
Proterozoic, and stayed pretty well in one piece until it broke up in the
mid-Mesozoic. I'd be very interested in learning how the SE US could "scrape
along" Peru, on Gondwanaland's _west_ coast, then swing north around the
northern coast of Gondwanaland in time to suture to Baltica in the late
Devonian or early Pennsylvanian.
>> If all of the fold mountains from the pre-Pangaean continental collisions
wore away, we have been barking up the wrong tree in trying to explain the
Appalachians in such terms. Were they completely worn away all up and down the
east coast, or did significant parts survive? <<
As I understand it, the original Taconic, Acadian, and Alleghenian mountain
ranges are all long gone from erosion. What we see today as the Appalachian
Mountains are the long-buried roots of those mountains, which were recently
uplifted enough for erosion to strip away the overlying Mesozoic sediments and
expose the ancient folds and faults.
Jon W.
Hi, Jon–
>> My text says that Gondwanaland was assembled from continents and
microcontinents very early in the Paleozoic, or even in the late Proterozoic,
and stayed pretty well in one piece until it broke up in the mid-Mesozoic.
In Dalziel's reconstruction in the Sci Am article, the pieces of Gondwana are
separated into three in the earlier supercontinent Rodinia; as Rodinia breaks
up, proto Antarctica+Australia+India rift away from Laurentia (today's W.
coast) and swing around to meet proto-Africa (today's E. African side), which
in turn seems to be pushed into proto-S.America. Gondwana is thus assembled
into its familiar configuration by the Cambrian.
I am a slightly puzzled at the dates; his "latest Neoproterozoic" map (with
Gondwana more or less complete) shows an age of 550 million years, but I had
thought of the Cambrian as beginning 570 million years ago. Is my date info a
bit obsolete here?
>> I'd be very interested in learning how the SE US could "scrape along" Peru,
on Gondwanaland's _west_ coast, then swing north around the northern coast of
Gondwanaland in time to suture to Baltica in the late Devonian or early
Pennsylvanian.
Soon after the assembly of Gondwana, Siberia, Baltica, and Laurentia all detach
from proto-S.America (today's W. coast) and each other to go their separate
ways. Laurentia is rather frisky, circling around Gondwana/S. America with
several brief collisions; Baltica does not have as far to go, but is generally
close to Greenland and hits and sticks halfway through Laurentia's end run.
Indeed, since Laurentia stays near the equator during much of the action, it
might be easier to think of Gondwana as swinging around Laurentia and Baltica,
which never get as far from each other as Gondwana gets from the pair of them.
–Doug
All of what you said seems reasonable to me except the erosion of the
Applachians. The modern Applachian chain is very clearly a folded structure.
The Easternmost fold — Blue Mountain — stretches for many hundreds of
kilometers broken only by the occasional water gap. To further complicate
things, one of those water gaps — the New River gorge — runs backwards with
the New River originating East of the Applachians and draining into the
Mississippi basin.
The other question would be when the modern Taconic mountains were emplaced.
They are mostly Cambrian age seafloor slates with a few carbonates. They
appear to have come from the East. After the ancient taconics of your model
eroded?
Don,
>> All of what you said seems reasonable to me except the erosion of the
Applachians. The modern Applachian chain is very clearly a folded structure.
<<
True, it is. All I know is that three different texts in my library agree that
the modern Appalachians are the roots of the ancient mountain ranges, mildly
uplifted in the Cenozoic by isostatic adjustment and/or epeirogeny. Once
uplifted, they started eroding again, revealing the synclines and anticlines
that were all that was left of the ancient ranges. As soft layers eroded and
harder layers resisted, the landscape formed into huge synclines, anticlines,
and ridges up to hundreds of kilometers long, plunging at one end or the other.
The entire Appalachian chain, from Maine to Georgia, is like that.
Jon W.
>>>True, it is. All I know is that three different texts in my library agree
that the modern Appalachians are the roots of the ancient mountain ranges,
mildly uplifted in the Cenozoic by isostatic adjustment and/or epeirogeny.<<<
Maybe — I must say that I'm more than a little sceptical. Sounds to me like
adjusting the facts to fit the theory rather than developing the theory to fit
the facts. There's a "hole" in the Appalachian chain between Southern New York
and Quebec, and in that hole we see evidence of lots of uplift and erosion —
e.g. 5 or 6 miles of sediment apparently eroded in the Upper Conneticut River
Valley. On both sides we have these tidy folded mountains that are purported
to have eroded only a little — only a few thousand feet of sediment missing
— in 300,000,000 years. I don't have any problem with localized uplifts, but
I have a lot of trouble with the selective erosion that seems to be necessary
to make the model work.
I'd suggest that the facts might point to the Appalachians being a much more
recent structure than we can currently account for them being.
Don,
>> On both sides we have these tidy folded mountains that are purported to have
eroded only a little — only a few thousand feet of sediment missing — in
300,000,000 years. I don't have any problem with localized uplifts, but I have
a lot of trouble with the selective erosion that seems to be necessary to make
the model work. <<
Based on the above, I'm not sure you quite understand. My refs all describe
the sequence of events in eastern NA as follows:
1. Taconic orogeny, mid-Ordovician, creates a mountain range something like
the modern day Andes: lots of igneous rock, lots of volcanoes.
2. Taconic Mountains are worn down by erosion, which process is basically
finished by the early Devonian. At that point, eastern NA is an old landscape,
rolling hills, old meandering rivers, very little relief.
3. Acadian orogeny, late Devonian through early Pennsylvanian, raises a new
range of fold-type mountains somewhat east of where the Taconics were.
4. Acadian Mts. worn down by erosion, but that process may not have been
complete by the time the Old Red Sandstone continent hit Gondwanaland to form
Pangaea.
5. Formation of Pangaea in Late Missippian/Early Permian causes third
(Alleghenian) orogeny, a third range of mountains in eastern NA. Lots of
folding, lots of thrust-faulting, especially in the south. In some places,
coastal rock was thrust-faulted inland for dozens of miles.
6. During the Mesozoic, the relief in eastern NA was again worn down by
erosion, to an old landscape with rolling hills, old rivers, and little relief.
7. During the Cenozoic, exactly when I'm not sure, epeirogenic activity (one
text describes this as 'arching' due to upwelling from the mantle, possibly a
very very early stage of pre-rifting activity) and isostatic lift (after the
ice ages) gently lifted the entire eastern part of the US. This lifted the
remnants of the Taconic, Acadian, and Alleghenian ranges by two or three
thousand feet, enough for serious weathering to start up again. Streams formed
and started downcutting, faster in soft rock than in hard, and so on. Because
all that was left of those ancient ranges after so long was the folded, faulted
roots, that's what was uplifted, and that's what is now eroding.
So they haven't eroded "just a little in 300 million years" — they eroded a
lot, in fact just about all the way, and what we see today is the product of
maybe ten to thirty million years of erosion. Maybe less. Maybe a lot less.
Jon W.
I understand the hypothesis, I think. I'm a little hazy on where the original
Taconics were and whether there are modern traces, But it's quite clear from
the sediment sequence that there was a land mass to the East of New York in the
Upper Ordovician, so I'll take that on faith. I might point out that the
modern Taconics are something of a problem as they appear to have been thrust
faulted in from the East, and that needs to have occured when there was no
intervening mountain range — unless you want to hypothesize thrust faulting a
big block of countryside up one side of the mountains then down the other. If
the modern Taconics were emplaced during the Taconic or Acadian Oregenies then
why didn't they erode along with the monster mountains to their East?
I don't have any problem with the Acadian Oregony. There's lots of evidence I
think.
My problem is with the Applachians as a product of the Alleghenian orogeny.
It's clear that the mountains are post Pennsylvanian and pre-Cretaceous. It's
also clear that they are (in most places) a folded structure and now that we've
decided that continents move (I'm old enough to remember when they didn't) that
a continental collision is a handy mechanism for creating them.
The difficulty seems to be that, the only available continental collision
probably occured in the late Paleozoic and there apparently isn't enough rock
missing from the Applachian sedimentary sequence for the Applachians to have
been in place all those years. So we have decided that the mountains we see
today for something like 3000 km are the result of the mountains having eroded
flat then being pushed up again in post Cretaceous time.
All I'm saying is that Deus-ex-machina re-emergance theory would look a lot
better if there were a mechanism identified to cause it.
Hi, Don and Jon–
I remember reading about those water gaps. I cannot find the article in
question at the moment, but it offered several theories about how they could
have been formed. One has the river establish its course on flattish land,
then uplift and differential weathering of folded layers creates new mountains,
with the river cutting whatever gets exposed in its path. There were other
theories that allow the river valleys to form while the mountains are present.
But none of the theories seemed entirely satisfactory – the article left me
with the feeling the origin of the water gaps has not been established with
confidence.
The Delaware River's cut through the mountains figured prominently as an
example.
–Doug
>> I see continental collisions as resembling car collisions. A light impact
just crumples the bumper (shoves the sediments and older-rifting detritus on
from the continental margins up on top). A heavier impact crumples the whole
front end of the car (including the continental craton). << I love it. I agree
but its a relative thing.
I used a car analogy in my simulation also…great minds think alike gg
Get cars and line them up with spaces between them in neutral with the break
off.
now push the last one slowly forward at one inch a minute
That is how I view the relative forces in the current Plate Tectonic process.
Relatively of course.
I view the Great Crustal Ripoff event of course as backing up the last car
enough to get it up to fifty miles an hour and ram the next car…thats what
india did to Asia. Relatively of course.
>> Granite is no more ultimately rigid than basalt or any other rock. Rigidity
is relative, as we have discussed before, and looks different in geological
time frames when there is enough time to push solids along cleavage planes
(like slowly bt surely pushing those magnet-studded boards one space at a
time). In human time frames, there is time only to push them hard so they
shatter or melt (if you are in too much of a rush, you can only kick the boards
apart or rip the magnets loose), so granite appears utterly rigid. It is more
rigid than the deep mantle, but not absolutely rigid – if it were, there would
be no granitic gneiss. <<
I like this description very much but I can't melt the granite over time yet.
Because its like the cars..if it could slide it would slide easily once it got
moving. The initial motion would take some action. The premise that it just
stayed moving from the original molten condition is unlikely. So it more than
>> We were talking about _continents_ colliding; how did the ocean floor get
involved?<<
Its on the back side of both coliding continents both subducting and filling
in as the motion occurs.
>> There may be a seafloor-like underlayer to the colliding continents, which
along with lithosphere may still be subducting out from under the collision
point to keep pulling the continents together even after first impact;<<
The forces at play to move the continental mass is much more than the Basin
Mass.
>> I gather this has been proposed in the Himalayas case. The ocean floor
that was following the colliding continents may start subducting at the outer
edges of both continents, but only after some of the impact has been
absorbed.<<
If the impact of the massive continent can be absorbed how could the puny
Basin not get absorbed? Are you talking about the basin plate just south of
India. I see no evidence that is subducting.
>>Transmitting pressures to the other side is what the collision-crumpling is
all about. Slowness might allow the pressures to be transmitted back away from
the collision zone, so the crumpling might be allowed elsewhere. Indeed,
I expect subduction and compression at the far sides of the continents in some
cases.<<
May be if it was hitting something or vibrating apart you could effect the
other side.
This is a biggie to me…
How can you expect the granite structure to reform in time getting thicker
like an earth worm when you let it go…shifting its atomic bonds and at the
same time the crust has to fold to do the same thing?
>>But rapidity would not allow time for smooth deformation of solid rock – it
would all have to be brittle breakage or liquefaction.<<
Only if we are trying to discount all parts of current theories…some bending
of layers obviously has occurred.
>> Signs of faults and melting appear to be much too modest where the
Himalayas and other fold mountains are concerned.<<
Haven't seen um…time to go though. Are there organized tours where we get
eyes on discussion of geological sites? Bus tours of the Ozarks. Or Travel home
tours with radio discussions. Go to 44 to discuss the Devils tower coming up
on mile marker 346.
>>There is less inertia, but still a lot. And as noted above there are likely
forces still being applied while the collision is underway. Indeed, I have
heard that the Himalayas have been measured as still rising, though I cannot
recall where.<< I don't have a big problem with rising areas, I think I see
some shrinking in the pacific plate..if that is occuring then we need a bulging
mountain area or ten of em.
>>Your way has _too much_ inertia given the mind-staggering masses involved;
again, fault blocks rather than folds would be expected, if not a total
meltdown – which we do not see.<<
Again we have a blending of slow and fast processes. Busted and strewn block
sections and folded crust( haven't seen much but believe it has happened)
>> The coasts that collided to form the Appalachians may not have been such a
good fit, and may have needed a lot of jamming together. I am not familiar with
this case. I dimly remember something about some seafloor-rock trapped in the
Appalachians. If there are pre-Carbonifereous fossils east of the
Appalachians, I expect them to match African counterparts better than other N.
American.
I don't have a good grasp of why this seams so.. Note that I am not really
talking about the entire Appalachians; I think parts, like the Adirondacks (?)
came from different events. <<
I have never been through the Eastern US mountains so I never thought of this
possibility. I don't see how the prominence and clarity of continental shelves
could be joined and then separated and still be so cleverly disguised. I see
you are a bit surprised by this also.
Kamron
Hi, Kamron–
>> Get cars and line them up with spaces between them in neutral with the
break off. now push the last one slowly forward at one inch a minute
If we are moving at one inch a minute, it might take mountains on wheels to
produce the big dents. Note that 5 mph is no big deal for colliding cars, but
is quite significant to colliding battleships. The more the mass, the slower
the collision needed to crumple things up. When you are talking continental
masses, you do not need much speed to get a _lot_ of inertia. If you add some
ongoing pulls of similar magnitude, that can only add to the crumpling.
>> I like this description very much but I can't melt the granite over time
yet. Because its like the cars..if it could slide it would slide easily once it
got moving.
Only your theory requires the granite to melt. Plate tectonics assumes solid
flow, like the magnets popping one step down the line – just because it has
moved, that does not help it make the next move. Only continued pressure, from
continued pull and/or inertia (on a continental scale) can keep the solid
flowing.
>> >> There may be a seafloor-like underlayer to the colliding continents,
which along with lithosphere may still be subducting out from under the
collision point to keep pulling the continents together even after first
impact;
>> The forces at play to move the continental mass is much more than the Basin
Mass.
This underlayer is not the seafloor from oceans surrounding colliding
continents – I am talking about _directly_ under the collision, where the
lithosphere is also converging. Since the lithosphere is comparable in mass to
the continents, and since it is denser and closer to the "escape hatch", it
would be no surprise if the lithosphere from one of the plates took a dive even
if the colliding continents on top do not.
>> If the impact of the massive continent can be absorbed how could the puny
Basin not get absorbed? Are you talking about the basin plate just south of
India. I see no evidence that is subducting.
I am saying the seafloor south of India might have started subducting (but I do
not see that it has), and may yet start subducting (it is still early in that
particular collision, as I said) near the coast of India. Continents in
collision are too light to "want" to subduct, but let a following seafloor be
confronted with the choice of crumpling and subducting, it is likelier to
choose the latter. Once ocean crust is old enough to be cooler and denser than
the mantle, it is just looking for any excuse to subduct. One caveat – if the
pull that brought the continents together has ceased during the collision,
there is less incentive for trailing seafloor to dive.
>> May be if it was hitting something or vibrating apart you could effect the
other side.
You were just expressing surprise at continents crumpling in slow plate
collisions, implying they should be ultimately rigid. They are not, as I have
explained, but there is some rigidity that _might_ allow the pressure to be
released at some remote place from the collision site in some cases. This does
not seem to be the case where the Himalayas are concerned.
>> How can you expect the granite structure to reform in time getting thicker
like an earth worm when you let it go…shifting its atomic bonds and at the
same time the crust has to fold to do the same thing?
I am not sure I follow. Perhaps you are imagining an earthworm being stretched
and then allowed to snap back to a thicker configuration – but this does not
apply to collisions. Imagine instead the worm being thrown against a wall
headfirst – it may thicken if the impact is direct enough. I assume by "crust"
you meant only the rock atop the granite craton.
If the undercarriage of a car crumples, why shouldn't the body also crumple? If
the bumper crumples, the rest may or may not crumple – it all depends on the
force and angle of collision, the point(s) of impact, and the construction of
the car.
>> Only if we are trying to discount all parts of current theories…some
bending of layers obviously has occurred
Whaddaya mean "we"? You are the one asserting any collisions took place within
days, and trying to claim solid rock cannot bend/fold/crumple without melting
or faulting even on the long time scales of plate tectonics. In your short
time scale, your own assertions become correct – I very much doubt solid rock
can fold so much within a week without very obvious melting and/or faulting.
>> I think I see some shrinking in the pacific plate..if that is occuring then
we need a bulging mountain area or ten of em.
Seafloor might crumple if compressed, but as noted above subduction is more to
be expected. If it crumpled, I do not expect the evidence to last long.
Continental mountains can force down "roots" of light rock that will buoy the
mountains up until the mountains erode. Seafloor material is already too heavy
(except the youngest seafloor) to "float", and I expect it would just sink
again soon after being uplifted, if indeed it does not all fold downwards
instead of upwards. Any projection deeper into the lithosphere may be subject
to being "dissolved" by that lithosphere (since it closely resembles the
lithosphere in composition, or seemingly more likely, to starting a subduction.
>> >> If there are pre-Carbonifereous fossils east of the Appalachians, I
expect them to match African counterparts better than other N. American.
>> I don't have a good grasp of why this seams so..
You had just asked for evidence that the east coast of the U.S. was once part
of Africa. The collision of proto-N.America with proto-Africa which raised
much of the Appalachians was about 300 million years ago, thus any older
fossils east of the Appalachians should have been formed while that area was
part of proto-Africa, and separated by an ocean from proto-N. America. Later
fossils would not help in this matter, since both Africa and N. America were
connected to the region after the collision.
>> I don't see how the prominence and clarity of continental shelves could be
joined and then separated and still be so cleverly disguised. I see you are a
bit surprised by this also.
The old continental craton edges would have met underneath what is now the
Appalachians; burying the best evidence under mountains seems a good enough
disguise for me. The only thing that was surprising me in this matter was a
possible hint that the eastern seaboard did _not_ come from Africa, which
proved not to be an intended implication.
Don's mention of seafloor chunks caught up in the Green Mts. is probably about
as much as one gets in the way of direct evidence for an earlier coastline
thereabouts. He listed some fossil-bearing formations that seem to bear out my
"prediction", provided they are old enough; it seems there is better fossil
evidence for some coastal exchanges between N. America and Europe. It seems we
traded Scotland for coast from east Newfoundland south to maybe RI; they tried
to run off with Greenland also, but got caught.
>> Are there organized tours where we get eyes on discussion of geological
sites?
My college alumni assoc. very occasionally offers me such a chance in southern
Calif., but I have never been in a position to accept. Now that I am moving
back (likely by April), we'll see… Rockhound clubs occasionally offer such
field trips, though they are more often interested only in collecting sites.
The trouble is finding such things when you are traveling. That is surely part
of why there is much interest in a database of clubs. Anyone out there know
some interesting field trips with a geologist for a tour guide?
–Doug
Doug
>>This underlayer is not the seafloor from oceans surrounding colliding
continents – I am talking about _directly_ under the collision, where the
lithosphere is also converging. Since the lithosphere is comparable in mass to
the continents, and since it is denser and closer to the "escape hatch", it
would be no surprise if the lithosphere from one of the plates took a dive even
if the colliding continents on top do not.<<
I see the entire continental crust as about 5 miles thick on top of a base
that is more like 10 or 12 miles thick. I don't see this subducting with the
relative forces that may exist. But even if I simulate it it doesn't want to
keep going down after it softens up.
>>I am saying the seafloor south of India might have started subducting (but I
do not see that it has), and may yet start subducting (it is still early in
that particular collision, as I said) near the coast of India. Continents in
collision are too light to "want" to subduct, but let a following seafloor be
confronted with the choice of crumpling and subducting, it is likelier to
choose the latter.<< I would think it would crumble like ice in front of an
ice breaker.
>> Once ocean crust is old enough to be cooler and denser than the mantle, it
is just looking for any excuse to subduct.<< If it floats as a cold solid it
would surely float as it reheated.
>> One caveat – if the pull that brought the continents together has ceased
during the collision, there is less incentive for trailing seafloor to dive.<<
I agree there is no incentive.
>>You were just expressing surprise at continents crumplng in slow plate
collisions, implying they should be ultimately rigid. They are not, as I have
explained, but there is some rigidity that _might_ allow the pressure to be
released at some remote place from the collision site in some cases. This does
not seem to be the case where the Himalayas are concerned.<<
True …I am not surprised that the folding can occur…it apparently has…my
surprise is that the ocean plate on the opposite side would buckle before the
ten times stronger continent would unless the speeds take advantage of the
inertia.
>>If the undercarriage of a car crumples, why shouldn't the body also crumple?
If the bumper crumples, the rest may or may not crumple – it all depends on the
force and angle of collision, the point(s) of impact, and the construction of
the car.<< I would…my point or what I tried to question was if the craton can
adjust its bonds and flow why doesn't the upper crust do the same instead of
folding?
>>Whaddaya mean "we"?<<
The agreements of which there have been many is the we.
>> You are the one asserting any collisions took place within days, and trying
to claim solid rock cannot bend/fold/crumple without melting or faulting even
on the long time scales of plate tectonics.<<
I have not claimed that rock cannot bend and fold without melting. It
definitely has done this.
>> In your short time scale, your own assertions become correct – I very much
doubt solid rock can fold so much within a week without very obvious melting
and/or faulting.<< well it is very faulted, crumbled and broken so that
happened.
>>Seafloor might crumple if compressed, but as noted above subduction is more
to be expected. If it crumpled, I do not expect the evidence to last long.
Continental mountains can force down "roots" of light rock that will buoy the
mountains up until the mountains erode.<<
This process of floating would create a constant height..as it eroded the area
would rise upward and so maintain its height.
>> Seafloor material is already too heavy (except the youngest seafloor) to
"float", and I expect it would just sink again soon after being uplifted, if
indeed it does not all fold downwards instead of upwards. Any projection
deeper into the lithosphre may be subject to being "dissolved" by that
lithosphere (since it closely resembles the lithosphere in composition, or
seemingly more likely, to starting a subduction.<< If it is heavier than the
material then why doesn't it just sink downward?
>>The old continental craton edges would have met underneath what is now the
Appalachians; burying the best evidence under mountains seems a good enough
disguise for me. The only thing that was surprising me in this matter was a
possible hint that the eastern seaboard did _not_ come from Africa, which
proved not to be an intended implication.<< If the east and west sides of the
App. Mts.
were different sections of crust it would go 15 miles deep or more..I would
expect to see remnants of the crack…and how did they get welded together so
well?
>>Don's mention of seafloor chunks caught up in the Green Mts. is probably
about as much as one gets in the way of direct evidence for an earlier
coastline thereabouts.<< thats a start
>>He listed some fossil-bearing formations that seem to bear out my
"prediction", provided they are old enough; it seems there is better fossil
evidence for some coastal exchanges between N. America and Europe. It seems we
traded Scotland for coast from east Newfoundland south to maybe RI; they tried
to run off with Greenland also, but got caught.<<
I say they were all together also…. with Greenland in between. Greenland
looks like it drifted north while Iceland drifted south. The gap at the top of
N. America could be closed as well as the whole Atlantic gap if these two
motions occurred.
Kamron
Kamron,
>> If the east and west sides of the App. Mts. were different sections of crust
it would go 15 miles deep or more..I would expect to see remnants of the
crack…and how did they get welded together so well? <<
They are, and it does. The reason we don't see any direct evidence of the old
subduction zones below the Appalachians is that in the Alleghenian orogeny,
rock from the east and southeast was thrust-faulted for dozens of kilometers
inland, over the old subduction zones. What wasn't buried that way has been
buried since, by renewed sedimentary deposition. The "welding" was not
particularly good; it was actually very clumsy and ugly, which is why it was an
orogenic event. I'm not sure, but I think the faults in the Appalachians and
along the East Coast date from the Acadian and Alleghenian orogenies, and the
fact that some of those faults are still active after two hundred million years
indicates how rough the suturing was.
Jon W.
Hello Jon
Are there also granitic intrusions in the midst of the thrust faulting?
How about the periodic crustal layers on each side of the Orogeny where it
levels out…are they very different?
Kamron,
>> Are there also granitic intrusions in the midst of the thrust faulting? <<
I don't know. My text doesn't make any references to such, but it's only a
general text. Doesn't mean they aren't there.
>> How about the periodic crustal layers on each side of the Orogeny where it
levels out…are they very different? <<
Not sure what you mean here. Could you clarify, please?
Jon W.
Jon
What I was wondering is how do the thicknesses of the Paleozoic and Mesozoic
layers similar on each side of the Appalachian Mtns and the West African layers
compare?
Hi, Kamron–
>>I see the entire continental crust as about 5 miles thick on top of a base
that is more like 10 or 12 miles thick.
At collision zones like the Himalayas, the crust must be at its thickest, which
is more like 70 km, according to my notes. Below that is lithosphere to 100 km
deep.
>> But even if I simulate it it doesn't want to keep going down after it
softens up.
The lithosphere is colder than the mantle below it because it is more able to
lose its heat to the surface. This makes it denser, and given any opportunity,
it will sink into the mantle. As it sinks into ever hotter mantle, the heat
will be conducted into the sinking plate only slowly; this is rock we are
talking about. It warms up slowly, even as it is sinking into ever-hotter
mantle, so it continues to be colder and denser than its surroundings,
potentially all the way to the core.
A counteracting factor: it also is a bit late in changing to denser packing
structures. But this is evidently not enough to counteract the coldness
factor.
>> If it floats as a cold solid it would surely float as it reheated.
Older, colder seafloor is no longer floating. The only reason it does not sink
is because it is a bit too rigid and monolithic (the same reason the dense
steel of a battleship does not sink until a crack is made in the hull that lets
water in) without a bit of an excuse.
>> I agree there is no incentive.
I said "less" incentive – you are much to casual about ignoring such tremendous
momentum and the "eagerness" of old seafloor to sink.
>> my surprise is that the ocean plate on the opposite side would buckle before
the ten times stronger continent would unless the speeds take advantage of the
inertia.
If there is a balsa-wood frame on the back of a car in a head-on collision, the
balsa frame might collapse even if the rear bumper does not. The frame
collapse will take some of the pressure off the front bumper, but not enough to
save it.
I liken seafloor to balsa because it is thinner, and because it is so prone to
dive (thus it will not hold fast against compressive forces).
>> if the craton can adjust its bonds and flow why doesn't the upper crust do
the same instead of folding?
In a collision, where would either craton or upper crust flow to, if not into
folds? They might fault, but that is determined by the instantaneous magnitude
of the force and the brittleness of the rock.
>> I have not claimed that rock cannot bend and fold without melting. It
definitely has done this.
You have most definitely needed convincing that seafloor basalt and granite can
_flow_ as _solids_. You kept insisting that there should be melting and/or
faulting. It would seem my magnets convinced you, but that is only valid for
the slow-motion case. Moving continents across the globe within days would be
quite different, and would require melting and/or faulting to do any
shape-shifting.
>> well it is very faulted, crumbled and broken so that happened.
I do not believe we see enough faulting and melting by several orders of
magnitude, to explain any mountains built within days.
>> This process of floating would create a constant height..as it eroded the
area would rise upward and so maintain its height.
Imagine an idealized mountain with symmetric root all of the same material. Let
the top mile of a mountain be eroded; I expect its root to rise half a mile and
thus the mountain top to wind up half a mile lower than before. Repeat until
the mountain and root are both gone, and the land will wind up flat.
Imagine two empty barges, with decks at the same level. Load up one and it
sinks a bit, but the top of the load is higher than the empty barge (it is the
mountain). Unload it again (wear down the mountain), and lo, the two decks are
once again at the same level.
>> If it is heavier than the material then why doesn't it just sink downward?
See above about the battleship.
>> If the east and west sides of the App. Mts. were different sections of crust
it would go 15 miles deep or more..
??
>> I would expect to see remnants of the crack…
If the slop on the edge of the colliding continents got squeezed up on top of
the meeting place, piled miles deep, why would you expect to see the old plate
boundaries?
>> …and how did they get welded together so well?
How do colliding cars get stuck together? If rock is flowing sort of like slow
liquid, might it not glue things together? That seam might be quite weak for
all we know; what has tried to tear it apart? Note my quote about New Zealand
in another thread, that implies such old seams may get undone if stressed.
>> Greenland looks like it drifted north while Iceland drifted south.
Greenland drifted more west from N. America when the Baffin Bay rift started up
briefly. But a hotspot arose and seemingly redirected the rifting to between
Greenland and Norway.
Iceland was not there in Pangaea; it spilled out of that hotspot at the ridge
after that part of the Atlantic opened up. I doubt you will find one dino
fossil in Iceland, though I expect they will be findable in all the adjacent
parts of the continents.
–Doug
Hi, Doug and Kamron!
>> The lithosphere is colder than the mantle below it because it is more able
to lose its heat to the surface. This makes it denser, and given any
opportunity, it will sink …<<
I haven't been keeping really close tabs on this discussion, but it seems to me
that one factor that's missing is the fact that both the oceanic crust (mostly
basalt) and the mantle (mostly ultramafics) are both mineralogically denser
than the continental crust (mostly granite and quartz/feldspar rich rocks).
Basalt contains lots of pyroxenes and olivine, as well as lesser amounts of
magnetite and amphiboles–these are all heavy minerals. Ultramafic (mantle)
rocks are even heavier, being mostly olivine and pyroxene with other heavy
stuff like garnet. Granite and quartz are relatively lighter. Thus you have a
good reason for oceanic crust to subduct UNDER continents, quite independent of
temperature factors.
>> (Kamron): Greenland looks like it drifted north while Iceland drifted south.
<<
>> (Doug): Iceland was not there in Pangaea; it spilled out of that hotspot at
the ridge after that part of the Atlantic opened up <<
I agree with Doug…the only drifting Iceland has been doing is the east and
west coasts drifting away from the middle of the island.
Check out these old Scientific American articles on the subject:
May 1972: Plate Tectonics, by John F. Dewey
Mar 1975: The Earth's Mantle by Peter J. Wyllie
Aug 1976: Hot Spots on the Earth's Surface, by K.C. Burke and J.Tuzo Wilson
Nov 1976: Convection Currents in the Earth's Mantle, by D.P. McKenzie and F.
Richter
Apr 1978: Kimberlite Pipes by Keith G. Cox
–Howard
Hi, Howard–
The mineralogical density difference is indeed a reason I had not mentioned
why if either subducts in a collision, it will be seafloor rather than
continent. Kamron and I have discussed it before, thus I sort of assumed it,
but it is best to spell it out when it has been a while. Temperature dictates
that seafloor would "prefer" to subduct rather than crumple, and composition
dictates that continents "prefer" the opposite.
–Doug
Doug
>>He listed some fossil-bearing formations that seem to bear out my
"prediction", provided they are old enough; it seems there is better fossil
evidence for some coastal exchanges between N. America and Europe. It seems we
traded Scotland for coast from east Newfoundland south to maybe RI; they tried
to run off with Greenland also, but got caught.<<
To me there is no mistaking the connection of N. America and Africa.
Even on a flat map that accurately shows the basins it works out.
Cut out the NW bulge of africa including into the Med.
Place the S. Pillar of Hercules a couple hundred miles south of the apex of
the Larenthian Cone.
Keep the coast a couple hundred miles out for wear and tear.
Near exact match.
Tilt Spain back up towards England.
Move Iceland into the pocket at top of Scandinavia.
Move the tip of Greenland south to the top of Newfoundland then North America
arcs into place closing the N. Atlantic into one piece.
Kamron