Enmar, on Jun 26 2003, 02:54 PM, said:
Delvo, seas don't just come into existence as you decribe it
...We know of places that are slowly covered by the sea, but as you said yourself they aren't that deep... But I never heard of a place that disappeared into the deep
sea... I can't imagine any mechanism for the land to be flooded by shallow water and then the ground below the water implode miles deep... The other option is, of course, that the sea level was miles higher than it is today, but that
is something we would have known.
Wow! I forgot that the story of the Black Sea wasn't background already provided to all here...
As a map can show you, the Black Sea is close to landlocked, with only a small opening to the Mediterranean Sea at Istanbul. At that point, not only do the two peninsulas of dry land come very close to each other, but the "floor" also comes up pretty close under the water's surface there; the two seas dive deep under the surface away from this point like bowls, which means that there's a "wall" of rock not-quite-separating them.
The idea, widely accepted among geologists but not universally accepted, and not exactly yet proven, is that the wall was once just a bit taller when the sea level was lower. And at that time, the Mediterranen Sea had just about as much water in it as it does now, but what is now the Black Sea was dry land that happened to be below sea level, separated from the Mediterranean Sea by what you could essentially consider a natural dam. Then, a warming trend in the climate melted the glaciers and increased sea level until the Mediterranean water had risen high enough to either break through the top of that rock formation via simple weight, or just spill right over it. After that, a flow was established that would have been tremendously massive and unimaginibly fast, and chewed away much of what was left of that broken natural dam.
Now picture this from the bottom side. There might have been a little freshwater lake there where people would probably have lived, and some rivers flowing into it. Then, whether it happened suddenly one day or by gradual erosion over weeks or months, water started coming down over the tops of the nearby highlands. If it was in the form of a sudden waterfall popping out one day, that waterfall would have been many times the magnitude of any that exist today. If it was slower, it would still have seemed "sudden", like a set of large and fast-flowing rivers appearing very close to each other over the course of a spring and summer. Either way, the lake's water level would have risen very fast, the shoreline backing upstream at a rate of miles per day. And all this new water coming in was salty.
Eventually, the water level equalized and the flow stopped; the Black sea as we now know it had formed. Rivers that would once have fed a smaller lake at the bottom of the basin now enter the Black Sea many miles sooner. We know that most of this water skips right over the top of the sea on its way out to the Mediterranean Sea through that narrow isthmus at Istanbul, because the lower Black Sea is saltwater, which is heavier. This is consistent with the idea of the sea being formed by dumping from the Mediterranean, because if it were primarily from the rivers that flow into it, it wouldn't be so salty, and there's no salt coming in there now from any other source. One effect of this layering is that there's no life down there; the lack of mixing with the lighter upper layers means that any saltwater-inhabiting life-forms that came in there with the water originally used up the last of the oxygen long ago, while any new life forms to get added since then wouldn't survive that far down because either the fresh upper layers or the salty lower ones would kill them. Thus, there's nothing there to decompose the organic parts of any archæological find. They've already found a Roman galley down there which was quite well preserved.
How deep is the black sea anyway?
The fresh and salty layers meet at about 60-70 meters; the deepest points are more than two thousand.
We know of places that used to be at the bottom of the sea (that's how chalk and dolomite are created) and are now bare.
They got moved upward pretty far to where they are now, a geological process that takes far longer than the span of human history and prehistory.
the bottom of the sea is very different from the ground. It's a very thin layer of rocks and It is created under the sea.
You sound like you're describing the difference between the tectonic plates under the oceans and the ones that are parts of continents. But sometimes what's called a "sea" is on a part of a continental plate that just happens to have water in it, just like a pond or lake, only bigger. These are called "epieric seas". I'm not sure if the Mediterranean Sea is one or not, but the Black Sea is. Some of the biggest and/or most well known to many people are Hudson Bay, the Gulf of Mexico, the Red Sea and Gulf of Aden, the English Channel, and a few other "seas" along western Asia and among all of those islands from there to (and including) northern Australia's Gulf of Carpentaria. These seas can come and go with changes in sea level and also with changes in the shapes of the continents; for example, much of North America's southern "Midwest" was once such a sea (basicly a larger Gulf of Mexico), but that was when the Appalchians were taller and the Rockies were shorter...
Edited by Delvo, 27 June 2003 - 01:08 PM.