Secular Pinoy

I've recently read that the oldest rock found on the ocean floor is about 200 million years old while the oldest continental rock is 3.9 billion years. Why the discrepancy? I know that there is some kind of conveyor belt underwater that recycles the Earth's rocks by swallowing the crust on one place while on another place it pushes up new rocks from the Earth's interior. So if the oceanic rocks get dragged back to the Earth, why does it not eventually swallow the continents, thus completing the rock cycle?

thebeave

New rock for the ocean floor (basalt) is created at cracks in the ocean floor. You've probably seen videos of this on TV wherein red hot lava bursts out from the crack and instantly solidifies. The Mid Atlantic Ridge is probably the most well known of these "cracks" or "ridges". New ocean floor is spreading out from each side of the ridge at a relatively constant rate. Over millions of years (200 mil yrs max) this rock layer migrates, like on a conveyer belt, towards the continental land mass. The basalt ocean floor layer, being denser than the continental mass it runs into, is "subducted" below the continent where it goes deep underground and is re-melted in the earth's interior. This area is called the "subduction zone", and is a pretty geologically active area. You frequently see a lot of volcanic activity and earthquakes above these areas. The Pacific Northwest and Calif coast are examples of this. At least that's how I remember it from college geology class 22 years ago.

Marruk

The Beave

Wow for not taking a geology class for 22 years you really retained a lot.

I think one thing that the Beave forgot to talk about is the difference of not only the density but also of different fault lines. The oceanic to continental are subduction lines just as the beave mentioned, but when two continental plates come together usually there are two different consequenses, although subdution is slightly possible. I don't remember the technical terms due to my geology class was 2 years ago but I can explain it in lay mans terms.

1. The two plates push up against each other. This happens at the himilayas where the asian and indian plates are pushing together and causing the highest mountains in the world and this type of zone will cause the most violent earthquakes. Incedentilly here in the U.S. the rockies are also created from two plates pushing together, so every long while the state of colorado will have a very violent earth quake (long while in a geological sense 1 million plus years).

If you find yourself interested in more I would highly suggest you take a class in geology. It is a simple subject in that most of the work is just observing the world making it very interesting and it is a very fun class.
2. The plates can slide next to each other in different directions. This is very easily seen at the San Andreas Fault and the earth quakes from this kind of movement are usually more frequent but to a lesser degree. One thing that is really cool about the San Andreas Fault is that if you go and visit it you can see a hills and mountains that are split in the middle and it's like the two halves are runnin away from each other.

Secular Pinoy

Ok, thanks for the responses. So the discrepancy is due to the fact that oceanic plates, being denser than continental plates, sink whenever it encounters a continental plate; while continental plates rarely subducts, and just end up either pushing another continental plate to form mountain ranges or just slide past each other. Is that basically it? Darn it sounds so simple, why didn't I think of that? Oh, yeah, I never took geology class.

Surely this is an example of imperfect design! An omnipotent god would want a more efficient means of recycling rocks. The oceans do get to recycle their rocks, but continental rocks just end up old, eroded and baked under the sun. Ergo, this is proof against intelligent design!

thebeave

One other thing that's kinda cool is localized "hot spots" that create islands. The Hawaiian Islands are such an example. In effect, there is an underground "hot spot" that spews hot lava and remains relatively stationary under the ocean floor, while the ocean floor above it moves rather quickly (in geologic time anyway) over it. Currently the Big Island of Hawaii is over the hot spot and is being added on to as we speak. The other islands were, in the not-too-distant past, over the hot spot, and later the spreading ocean floor "conveyed" them away from it. I guess the hot spot sorta turns itself off and on at will, otherwise it seems we would have one long skinny island instead of discrete blobs.

Amen-Moses

Yep continents float, the funny thing is that even in geology class when you get to compare granite with basalt directly it is still a hard one to swallow (especially the granite 'cause it scratches like buggery! ).

Amen-Moses

crocodile deathroll

Just interested

In what ocean floor and where in that ocean floor did they find the 200 million year old rock?

ps418

The oldest oceanic crust, IIRC, is in the northwest Pacific. You can find a handy map illustrating ocean crust ages at the <a href="http://www.ngdc.noaa.gov/mgg/fliers/96mgg04.html" target="_blank">National Geophysical Data Center.</a> There is also some pretty old oceanic crust in the Atlantic, near the continents. Also, older oceanic crust is preserved in the geologic record, in the form of <a href="http://volcano.und.nodak.edu/vwdocs/vw_hyperexchange/ophiolites.html" target="_blank">ophiolites.</a> Ophiolites are often found in former collision zones, marking the line along which former ocean basins closed.

crocodile deathroll

So it looks like the Atlantic Ocean was born as a large lake between Eastern US and Northwest Africa about 200 million years ago

[ December 12, 2002: Message edited by: crocodile deathroll ]</p>

Secular Pinoy

Is there a popular science book for geology? Something comparable to what R. Dawkins or C. Sagan would write? My friend lent me his intro to geological sciences textbook, so I have textbooks covered.