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Originally posted by wmscott:
The crust of the earth that makes up the land has a lower density than the ocean crust. This is why the land rises above the level of the ocean floors. The lighter a material is, the higher it floats.
But the Hawaian islands are volcanic and are made of the same stuff as the ocean floor-- ie, lava welling up from the mantle through the crust. Yes? That being the case how does this apply?
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This extra weight causes his side of the seesaw to sink while the other side rises. This happens when at the end of an Ice Age large amounts of water is returned to the oceans, the ocean floors are depressed and adjoining land areas are correspondingly raised.
But if instead of a seesaw, you imagine a piece of plywood balanced at its center and then press down evenly on all sides as water in the ocean would do, then you don't get this seesaw effect.
Or for a better example, build a plywood platform say 100' by 100' and float it on some very dense but still liquid material like tar. Build a mountian of rock about six foot tall in the center. That mountain would depress the platform. Ok. Now flood the platform with water to a height of five feet. The platform depresses a bit more. Now vary the water depth by six inches or so. There isn't going to be much change in the amount of deflection.
[quote][b] For example, the sudden flooding of the Black Sea pushed the former lake bottom down an estimated 200 feet. The weight of Antarctica and Greenland ice sheets has depressed parts of the land beneath them to levels below sealevel.[/quote]
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Not applicable. You have different materials involved. Dirt is more subject to compaction than volcanic rock. And you are talking about a few hundred feet, not the warping of miles thick crust.
But no argument with the general concept of water weight crushing the land beneath it. I just don't understand how a relatively small volume of water can move the oceanic crusts as much as it seems it would have to do.
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The weight of returning water of even only a few feet of increased ocean depth is enormous because the increased pressure is felt across the entire ocean. every square foot on the ocean floor is under that much more pressure, adding up the entire square footage of the world's ocean bottoms adds up to a huge weight.
I think that you are missing the fact that this added weight is distributed over the whole ocean floor. It isn't pushing down at one spot. In my analogy above, it is like applying weight over the entire platform. My platform would have to be a sphere filled with tar to get a better picture and the water would have to surround it and be pulled toward the center. Not easily replicated in the lab. You can't push down at one spot without conpressing the tar or haveing it just out at another spot.
[quote][/b]The effect of this is that even a seemly small increase can cause a shift in the earth's crust. We have the example of the Mississippi river delta in the Gulf of Mexico, the gradual increasing weight of this delta is slowly depressing that part of the Gulf of Mexico. When large dam reservoirs are filled, the weight of the water depresses the earth's crust a bit, bending it and causing earth quakes. Here in the case of the dam reservoir, we see that even a fairly small concentrated weight of water is enough to cause shifting in the earth's crust.[/b][/quote]
But in all cases you are adding mass to a specific area, not the entire ocean. You can depress the surface of a balloon with your finger, but that isn't the same as applying preasure to the whole surface of the balloon.
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Now in the case of ocean islands and the deluge,
First off, my initial comments concerned a specific mechanism used to describe a specific feature of the Hawaian islands. Now on to the flood.
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one of my estimates for the depth of the flood waters is 4,000 ft. If all this water was taken up by deepening ocean basins, this would have resulting in a ocean depth increase in the range of 6,000 ft.
Ok. Then it seems you are postulating an original pre-flood sea floor covered by 2000 feet of water? Is there evidence of beaches or coral formations around that depth?
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Now the bone we have been fighting over in this argument that you have jumped into
Is this not an open forum?
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is that water has only less than a third the density of rock, so how could the lighter water push the heavier islands above the water?
It isn't lighter if the volume is great enough. No problem there.
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I have been invoking the fact that the total pressure felt by the large area of the ocean floor is much greater than the small area supporting the islands.
But the islands sit on top of that crust. Push the crust down the islands go with it. At the base of the island they are tied to the crust via a very substantial footprint. The water would be pushing DOWN on that foot as well as on the surrounding ocean floor.
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Using plastic movement caused by the general depression of the ocean floor, a resulting local uplift in the hot material beneath the islands
Resulting is volcanic eruptions perhaps? This seem more likely, as the channel for escape is already there.
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The islands only have their elevation due to the fact that the hot material beneath them is lighter than the surrounding cooler material. The islands themselves are cold, and sit on top of hot bulge like a rock on a hot air balloon.
Ok. I am sure this is correct to a degree, but don't you think some that elevation is due to volcanos constantly adding material to the top of the islands?
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This is why the islands are often in a deep pocket surrounded by a rise.
I looked up some ocean floor topology maps at
a USGS site and I don't see these pockets. Can you point me to some maps that show these features?
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this causes the bulge to be forced back into a smaller area which raises the island.
Due to the massive footprints of these islands, I can't see this happening. The preasure would be much too broadly distributed.
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Like pushing a plate down into the mud, the mud squeezes out and up around the edges of the plate.
But you are not squeezing a plate into mud, you are squeezing hydraulic fluid inside a hydraulic piston-- albiet this piston has leaks.
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the depression of the ocean plate has similar effects. Areas where the trapped plastic material beneath the plate can ozoo out are uplifted.
Again, the maps I found do not show this. Show me maps and I will retract any effected criticisms.
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Another effect that comes into play is depressing the ocean floor moves it closer to the center of the earth and slightly increases the effect of gravity.
Ok. Small but measurable effect.
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This would also increase the amount of island uplift by magnifying the density difference between the hot material below the island and the surrounding sea floor.
The islands being much more massive would be pulled down more than the relatively thin crust (locally). I don't see how this supoorts what you are postulating.
Take care.
Thanks for the indepth reply.
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[This message has been edited by John, 08-09-2002]
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