I had an inquisitive 8th grader ask me, “Why does the ocean floor drop off all of a sudden?” I didn’t know. Why isn’t it more gradual?
I was about to be flip and throw out that it was simply the edge of the tectonic plate on which the continent sits. However, when I doublechecked Britannica, I noticed that geologists do not yet have a complete understanding, either. (So you don’t have to feel bad.)
Beaten to the punch again. Curses!
I dunno, bib, I wasn’t much help. If you’ve got something to add, throw it in.
If I’m remembering my geology classes, continental crust and oceanic crust have different compositions, which is why in a subduction zone (coast of CA) the oceanic crust (heavier) is forced beneath the continental crust, which “rides up” over the oceanic crust. Criminy. Tectonics. This is pathetic. I was a geology major, long, long ago. Got to brush up.
Well, here’s the britannica link, for what it’s worth.
I’m with Alpine. I vaguely remember that there are two main types of plates, composition-wise speaking, light and heavy (these are not the official designations). One of them was mostly basalt, but I can’t remember what the other one’s made of. Granite, maybe?
Glad I could clear that up for you. Yeesh.
RL Mentock should be by here shortly to clear this up.
Yep, any minute now.
::whistling::
Well, until he gets here, continental crust is granitic (lighter) and oceanic crust is basaltic (heavier). The crust “floats” on the upper mantle and, iceberg-like, the lighter density continents rise up higher than the denser oceanic crust. This is called isotasy.
The continental shelf and slope are composed of continental crust. I believe the question originally posed is why the apron of sediment running off the continent has not made a gradual slope.
Oceanic crust (with its covering of continental sediments) is constantly being subducted into oceanic trenches. The oldest oceanic crust is only about 200 million years old while the oldest continental crust is at least 3.9 billion years old. Thus, the apron of sediment that one might think would form a gradual slope from the oceanic abyss up to the shore line is never given the time to develop. It is subducted along with the oceanic crust and a great deal of water and together these are probably the feed stock for the creation of granite.
The last piece of western North America to join the continent, the Salinian Block in California, probably originated thousands of miles away (I have heard that it is similar to some Antarctic rocks). All of the intervening oceanic crust and all of the contintal sediments that had been swept off the earlier “West Coasts” of North America that were in the process of forming the expected slope, were subducted under or squeezed up and welded onto the continent.
mipsman - that’s the way I remember it, too. Thanks for your much more detailed explanation.
But we’ve all forgotten the obvious geologic joke…
Remember that subduction leads to orogeny!
Yeah, but what do they keep on the continental shelf?
Towels. That’s why the land is dry.
Duh. 
I would agree with mipsman, except I believe that lots of oceanic crust is very old. The Atlantic is a very young ocean only 60-80 million years old or so, but the Pacific is very very old, it’s the remenant of Panthallassa, the ocean back when all the continents were joined in Pangea. But I’m not a geologist.
Also, not all oceanic/continental plate meetings are subduction faults. Yes, on the Pacific coast the North American plate is subducting the Pacific Plate. But the Atlantic ocean is spreading and Europe and America drift apart, so there isn’t any subduction. That’s why there aren’t any volcanos and earthquakes on the east coast.
And yes, it’s true that lots of chunks of the west coast used to be “microplates”…little islands floating in the Pacific that the North American plate collected…kind of like if Africa drifted into Madagascar and they stuck together.
If history is any guide, (as Hutton and Lyell said it is), sometime within the next 120 million years, the Mid Atlantic Rift will subside and North America and Europe/Africa will slam together again and all of that Atlantic oceanic crust (and accumulated sediments) will be subducted again. You just can’t find really old oceanic crust. I believe the oldest is out in the western Pacific ~200 million years old.
But you almost need to think of all oceanic crust as a big conveyor belt constantly moving from the mid oceanic ridges to a subduction trench. Any islands, micro-continents or even real continents carried along are destined to eventually choke a subduction trench and/or be welded onto an existing continent. That is why the Pacific is so empty. Everything that was out there is now welded onto western North America.
Probably even your ancestral continent, Lemur866.
It’s true that the Pacific basin is old, in that it didn’t just open recently, but the crust underlying the basin is being subducted at the edges. I believe the oldest remnants that have been found are in the northwestern portion of the basin. The age is ~200-250 million IIRC.
The Atlantic is indeed a younger basin, but the oldest sections are as old as the oldest crust in the pacific. Here’s a map showing the ages of the basins: http://www.ngdc.noaa.gov/mgg/image/crustageposter.jpg
I know I’m going to get trouble for this. (mipsman! instead of trying to spell my name, just use the handy :
: batsignal) The reason is…it is more gradual.
More gradual than one would think, that is. Mapmakers always exaggerate the vertical dimension. They take flak for this all the time, but if they drew it to scale, you wouldn’t be able to see any features. The cross-section diagrams in the textbooks make it look like the continental slope is a cliff.
From the continent plate to the ocean plate, there is a drop off more rapid than the rest of the continental shelf, but the average drop has only about a 4 degree slope. Take a meter stick, lay it on a table, and raise one end 7 centimeters. That’s steep enough, as far as the ocean floor goes, or even your average highway, but it’s not a cliff.
There are steeper cliffs underwater, but the slope of the continental slope is mainly the angle at which the ocean sediments can repose for a long time without sliding. And erosion is not as strong there.
Note: I’m thinking that the oldest ocean floor is only about 180-190 million years old. Even in the Pacific, subduction has obliterated the oldest floor. I looked at those NOAA and Britannica links, and I think that’s what they say too.
180-190 million sounds about right to me. 200 is the number I carry around in my head, and I wasn’t sure if anything older had been found up around Japan. My point was that though the major basins have different histories, the oldest crust is of a similar age in all of them.
Thanks guys. The last one here seems to really nail it. I had implied that it was the sort of cliff that you mentioned. So then he asked me why it would do that.