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Name: Marvin
Status: Other
Age: N/A
Location: WA 
Country: United States
Date: November 16, 2005


Question:
Some years ago, the National Geographic published a map of the Ocean bottom. That map showed that river beds of most of the world's major rivers ran far out into the ocean, some more than 1000 miles and to depths in excess of 10,000 feet. That proves the rivers were here before the ocean. Where did the world-wide ocean come from?



Replies:
Thanks for your question, Marvin... Firstly, it is important to point out the error in asserting that continental rivers are older than the oceans. From your question, I would guess that you arrived at this conclusion along one of two possible trains of thought (or maybe some combination of both), either: 1) the deep, incised channels that appear on the continental shelves of the world's oceans demonstrate that rivers have been carving into the land surface long before water in the ocean basins rose to appreciable heights, because rivers don't carve channels in land that is covered by several hundred to thousands of feet of water, or; 2) the water contained within the oceans was placed there by the slow accumulation of water delivered to them by rivers over geologic time. As it turns out, both of these trains of thought, though understandable, are off-track.

The map top which you refer is likely the ocean floor map produced by Bruce Heezen and Marie Tharpe in the 1960's. Unfortunately, this map is highly stylized and presents an exaggerated characterization of ocean floor features that leave viewers with a faulty impression of how conspicuous some of these features are. It is true, however, that there do exist many deeply incised channels along the continental shelves of the oceans, but they are not the sole result of flowing river water. Addressing issue #1 stated above, these channels, though they may often appear at or near the spot where major rivers flow into the ocean, are predominantly the result of submarine landslides, called turbidity currents, which act to erode a great deal of loosely consolidated sediment and deposit it further down in the ocean basin. Understand that the continental shelves (the region of relatively shallow water which circumscribe the continents and other large land masses) are comprised of sediments derived from the continents and transported to the oceans by rivers. Because the sediments tend to be loose, even small seismic disturbances are rather effective at causing the sediments to collapse and slide down the gentle slope of the continental shelves. Some of these piles of sediment can be over 20,000 feet thick, and would account for the vertical extent of deeply incised channels. The presence of the continental shelves is, by itself, pretty good evidence that the oceans had to exist prior to any one river which flows into them, for the presence of the ocean allows for the "stacking up" of the continental shelves as rivers deposit more and more sediment along the margins of the ocean basin. For more info on how old the world's ocean basins are, perform an Internet search for "sea-floor spreading" and "mid-ocean ridges". Such geologic phenomena tell us, for example, that the Atlantic basin is approximately 180 million years old, which is older than the majority (if not all) of rivers that drain into it.

Addressing issue #2, although the water on Earth, including in the oceans and on land, is regularly "recycled," and hence, may at one point be delivered to the ocean via rivers, it would be incorrect to say that the continents are the ultimate source of the water now present in the ocean basins. A good deal of debate has ensued over the last century as to the source of the world's water, but the two leading candidates are: A) the interior of Earth, and; B) outer space. As is often with science, however, the answer is a combination of both. Hydrogen and oxygen, the two constituents of pure water, exist in the universe in abundant supply; as such, it would be reasonable to assume that when the planets were formed, water was necessarily (and haphazardly) incorporated into to the planet. In order for that water to accumulate at Earth's surface, however, we must expect that it was delivered there by the action of a process called "outgassing," by which the water migrates from the interior of the planet (e.g.. from the mantle) to the surface where it can exist in its liquid state and accumulate in low-lying areas like ocean basins. As it turns out, volcanic eruptions are very effective at moving water locked within the interior of Earth to its surface. It is reasonable to imagine that a significant amount of water has been release from the mantle during Earth's 4.5 billion year history.

Secondly, it is very likely that water will have been delivered to Earth from outer space by interplanetary debris such as comets (comprised of ice and rocky material) and asteroids (comprised mostly of rocky material but also some small amount of ice). If you were to ask researchers actively pursuing questions concerning Earth's relatively watery condition, I would surmise that the majority would tell you they believed most of Earth's water was derived from cometary impacts, and not from outgassing, though that certainly played a part, too.

Sorry for the lengthy diatribe, but anything less would have been an incomplete answer. Hope this helps!

Scott J. Badham
Ocean Tectonics Research Group
Department of Geology and Geophysics


That river beds extend from their present entries into oceans at their present locations does not "prove" the rivers were there first, only that the rivers have dumped silt from their tributaries into the oceans for many years. In fact, it is the other way around. The rivers have been depositing silt into the oceans for centuries, which just gets overlayed and pushed out from the mouths of the oceans for centuries.

Vince Calder



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