#Erosion in Ocean Basin
16 messages in this thread
The shallow ocean areas that are actually up on the continental shelf areas
have evidence of massive water flows causing erosion.
I have read that these were caused by heavy silt and sand rivers flowing and
carrying additional materials.
These areas like in the Labrador Sea basin between greenland and the N.
America. These when viewed on a good map show a flow all the way around
Newfoundland past Nova Scotia.
The same features exist all over the planet. I believe these could only be
formed by water carving away material which could only happen if the ocean
water was missing from the planet.
Anybody else wonder about these features?
Kamron
Kamron,
How deep are the features you're talking about? Five meters? Ten? Twenty?
Fifty? By some esitmates, worldwide ocean level may have been as much as a
hundred meters lower than it is now during the Pleistocene ice ages.
Are any of these features believed to be the result of turbidity currents?
Jon W.
Jon
The features in the Labrador Sea basin start as continuations of rivers in
some areas. THese tributaries plunge off the shelf to depths in the basin of
around 2500 to 3,000 meters where they join up and as a single channel that
travels more than 2,000 miles traveling in the gently sloping basin around New
Foundland ending in the deep south of Nova Scotia. It has a deep wide channel
all the way.
Some of this basin has only a very shallow slope.
These features appear all around the margins of the shelf. Where they connect
up to a large drainage basin they continue on through the sedimentary deposits
out to the deeper basins.
There has been some discussion on them , I can't recall where just now but the
stated cause was soil and sand flows cutting them. I don't see how they could
continue to flow unless the water was missing from the ocean basin.
Kamron
Kamron,
Hmm. "Soil and sand flows" sounds a lot like somebody's attempt to explain
turbidity currents, which are basically the underwater equivalent of avalanches
(on a steep slope) or slumps and mudflows (on a shallow slope). Once they get
rolling they can build up a LOT of momentum, and can travel for many kilometers
across the seafloor.
Another thought: could these ocean-floor features be the beds of deepwater
currents? Many shallow currents like the Gulf Stream, Labrador, Kuroshio have
counterparts deep below, near the seafloor.
Jon W.
Jon
The flows are large and river like but not the size of currents like the gulf
stream.
and
They are located mainly around the perimeters of the continents where land
basins drain into shallow basins above the shelf and continue over the shelf
through the sediments at the continental bases.
They look like erosion canyons in the sediments. Continuations of the land
erosion marks in many cases.
Kamron
Kamron,
>>They are located mainly around the perimeters of the continents where land
basins drain into shallow basins above the shelf and continue over the shelf
through the sediments at the continental bases.
They look like erosion canyons in the sediments. Continuations of the land
erosion marks in many cases.
What you are describing are the canyons that eroded into the continental
shelves during the most recent ice age. Sea level was as much as 300 feet
lower then. This allowed the rivers to cut into the esposed surface and erode
the canyons. River volume was much higher because of runoff from the glaciers.
Jim
Yes thats it but they continue on down some go on down to 3,000 meters like
there was no water down at that level for at least a short period of time.
Kamron
>>Yes thats it but they continue on down some go on down to 3,000 meters like
there was no water down at that level for at least a short period of time.
However, if they existed prior to the opening of the Atlantic Ocean in the
Mesozoic, the rivers would have continued headward erosion and never been
submerged. Also, erosion can take place in a sub-aqueous environment. River
currents can extend long distances into the ocean. There are freshwater
currents associated with the Amazon that extend at least 50 miles past the
mouth of the river. Similar currents are associated with the Mississippi, and
Columbia, and the is anecdotal evidence for the same situation before the
Colorado was dammed to nothingness. Sub-aqueous erosion is not as rapid as
sub-aerial, but it does take place. Cosidering that the continental shelves
have been in roughly their present configuration for 65 million years, there is
bound to be headwatd erosion in any canyon.
Jim
Jim
>> However, if they existed prior to the opening of the Atlantic Ocean in the
Mesozoic, the rivers would have continued headward erosion and never been
submerged. Also, erosion can take place in a sub-aqueous environment. River
currents can extend long distances into the ocean.
There are freshwater currents associated with the Amazon that extend at least
50 miles past the mouth of the river. Similar currents are associated with the
Mississippi, and Columbia, and the is anecdotal evidence for the same situation
before the Colorado was dammed to nothingness. <<
The currents would seam to float however if they were laden with soil and
particles they may be heavier than the salt water.
On some of the larger sedimentary deposits like the Indus and Ganges Cones the
canyons have diverted many times like you would expect in a delta formation.
They still cut channels down to 4,500 5,000 meters.
This seams like unlikely behavior. The soil must be falling out of the river
to make the deposits yet it still cuts canyons.
Kamron
Those undersea canyons are well known. I'm not sure that they have ever been
adequately explained, but a lot of ink has been spent trying. The upper parts
of some of them are clearly erosional from the lower sealevels at the time of
the recent glaciations. The lower parts … try an oceanography text.
Don
Hi Don
Thats one way all the ocean frozen at the poles and in the air.
The basins could have been exposed down a lot further.
Would have wiped out most or all of the ocean life though.
These areas at the bottom of the shelf could have dropped down below the sea
level after the canyons were cut. This could happen in some of the catastrophic
events I like to imagine caused the current configurations on earth. The
problem though brings up the ocean depth problem.
That is if the crust was over the whole earth in a more or less even thickness
shell and the balance of the missing continents were torn off by some event.
The current quantity of ocean on the earth would have flooded the whole earth
deep below the ocean surface.
This brings up a necessary ocean transfer probably from Mars At the end of the
cretaceous. So the saga continues.
The canyons sure look like they were done by forces that could only play out
unsubmerged.
Kamron
Hi, Kamron–
The ocean basins would have to be empty for a long time to let those canyons
be cut into the continental shelf. If the ocean basins were empty only a short
time and something rapidly dumped the entire oceans-full of water onto the
continents, it would just flow off all edges, not bothering to follow
river-courses.
That the deep cuts in the shelf have a rough one-to-one correspondence with
major modern rivers suggests that they were cut by flows not hugely greater
than modern river flows. A few times modern flow rates, say as glaciers melt,
is one thing, but the flows cannot be multiple orders of magnitude greater. For
near-modern flow levels to cut such canyons surely would need considerable
time.
–Doug
Doug
>> The ocean basins would have to be empty for a long time to let those
canyons be cut into the continental shelf. If the ocean basins were empty only
a short time and something rapidly dumped the entire oceans-full of water onto
the continents, it would just flow off all edges, not bothering to follow
river-courses.<<
I agree…but the basins are there..
>>That the deep cuts in the shelf have a rough one-to-one correspondence with
major modern rivers suggests that they were cut by flows not hugely greater
than modern river flows. A few times modern flow rates, say as glaciers melt,
is one thing, but the flows cannot be multiple orders of magnitude greater. For
near-modern flow levels to cut such canyons surely would need considerable
time. <<
I agree
What about a period of say 10 inches per hour of rain for a month or two.
Increasing the water flow a few thousand times.
The erosion level increases dramatically as the flow increases doesn't it?
Just another catastrophic scenario.
Kamron
>>That the deep cuts in the shelf have a rough one-to-one correspondence with
major modern rivers suggests that they were cut by flows not hugely greater
than modern river flows.<<
What Doug says here is valid. The theory is, that over time these stable
geosynclinal deposits graudually subside under the weight of the accumulating
sediments. So in the areas where we find these extensive submerged drainage
features, we generally have stable, subsiding continental margins. Also,
turbidity currents and slumps of recently deposited sediments can extend these
canyon features farther out to sea than we would normally expect.
<< The erosion level increases dramatically as the flow increases doesn't it?>>
This is similar to a question I always had about the inverted topography of the
Ridge & Valley province of the Appalachians. The evidence in many cases is
that the axes of the anticlines have eroded into valleys, and the axes of the
synclines have been preserved as parallel ridges. We would think, "How could
the highest points be eroded below the level of the surrounding topography?"
But then on reflection, we'd realize that the whole of this topography had been
uplifted, and that erosion is fastest at the highest and narrowest and most
exposed points. Over time, the trend of the whole erosional process is to
flatten, and so the most visible differences in topography must be a result of
the different rates of erosion of the variously tilted and folded strata. In
an old, stable area, this is very much the case. Only in a new, active area
are the catastrophic features still prevalent.
Regards, ed
Hello Ed
>> This is similar to a question I always had about the inverted topography of
the Ridge & Valley province of the Appalachians. The evidence in many cases is
that the axes of the anticlines have eroded into valleys, and the axes of the
synclines have been preserved as parallel ridges. We would think, "How could
the highest points be eroded below the level of the surrounding topography?"
But then on reflection, we'd realize that the whole of this topography had been
uplifted, and that erosion is fastest at the highest and narrowest and most
exposed points. <<
This is a question I asked about earlier in another thread. I have not seen
mountains with true anticlines and synclines. It would surprise me to see what
you describe with the top of the fold worn down blow the level of the sides.
Why would the tops of folds wear faster than the synclines?
When I have looked close these were broken sections that split apart and both
fell away from the center…If it happened like this row after row then that
would be unlikely.
>>Over time, the trend of the whole erosional process is to flatten,<<
Where is flattening going on it looks stable to becoming less flat.
>> and so the most visible differences in topography must be a result of the
different rates of erosion of the variously tilted and folded strata. In an
old, stable area, this is very much the case. Only in a new, active area are
the catastrophic features still prevalent.<<
It all looks catastrophic to me but I have seen so very little of the world.
Where is this place you mention, Ridge & Valley province of the Appalachians?
Kamron
PS I was looking at the massive sedimentary build ups at the larger rivers all
seam to have these canyons in them and they run down into the
3,000 to 4,000 meters sometimes to 5,000 meters.
Hi, Ed–
When I see an anticline that has eroded to a pair of ridges, I speculate that
either (a) the ridges represent the most erosion-resistant layers (thus the
center is more eroded because wimpier layers were exposed there), or (b) that
the folding introduced some weakness at the cusp of the fold (cracking where it
was bent most sharply), making it more susceptible to erosion. Anyone care to
comment on which is more common (is (b) even valid?) or fill us in on (c) none
of the above?
–Doug