Jazz, I already said those links were too much to ask me to read. YOu need to to do a better job of explaining in your own words.
Once again, did you read my last post in the Links thread? Please answer. Anyway, the links are there for you to follow up on in case you think I am just pulling this out of thin air.
On the positive side. There are lots of good questions here.
So what's causing the rift that the magma is filling?
The rift is caused by the plate moving apart. We do not know why the plates are moving apart. We just know that they ARE moving apart. That is an observation.
WHat's causing the movement of the plates?
No one knows. We cannot observe the mechanics of the mantle. The leading theory is the convection cell theory which says that plates are similar to big conveyer belts of material and move around.
What we do know is that, in the example of the mid atlantic ridge, is that the two plates are moving apart, splitting the crust, creating rifts. That is it. That is the observation. The cause of the movement of the plates is unknown except via theory.
The illustrations do NOT show a rift being filled. They show magma rising up and pushing cooler magma away from the ridge. That's what they SHOW.
I can see where there is some confusion. You are coming from the perspective of wanting it to be that way and thus the animations certainly look like the magma that is coming up is pushing the rocks aside. But think about this critically for a moment. If magma, being a liquid, is rising up to the surface and it has the choice to displace a rock laterally or displacing nothing by moving vertically what do yout think is going to happen? Because it is a liquid it will take the path of least resistance and go up into the empty space above it rather than the filled space to the sides.
If you want to focus your case on a gif animation then first of all realize that it is just a darned gif. Second, think of it in the context of the other information that you know. Magma is prevented from coming to the surface by the crust. When you remove crust the magma will come up. Now look at that animation as if the plates are moving first and THEN the magma fills up into the space. Supplement your knowledge not just based on this one picture but also the text of the sources that explain that picture just how I am explaining it to you here.
There is no need to repeat the distinction from volcanic action. That was never the point and it makes NO difference.
It is very pertinent to the discussion or I would not have said it. One of the primary observations that makes a divergent boundary different from a convergent boundary is the type of volcanism that happens there. IT is one of the observations. We don't want to ignore facts now do we?
I have NO idea what this means. You are not being clear. WHAT "empty spaces?"
A rift is a hole. A hole is an empty space. Before the hole is made there is no way for the magma underneath the crust to come up like it wants to. When a rift is made there is now an empty space. Now the magma can come up and fill that empty space. That is the point I am trying to show you. The magma comes up not because it is pushing on the crust. The magma comes up because the crust moved out of its way. It works the opposite way at a convergent boundary.
What is creating the empty spaces?
Take a cloth and pull on it from two opposite sides. If you pull hard enough you will make it rip at some point in the middle. You have just created an empty space in the cloth. The empty spaces in the crust are created by one plate moving in on direction, another plate moving in the opposite direction. At some point something has to give. What gives is called a rift.
What is a "rift valley" and what does it mean that they are "dropping out?"
Think of a normal valley but instead of a river carving it or something it is created by the land falling at a fault. This should be one of the key points for you because you already know that if it were being compressed that it would be pushing things up like those hills we are trying to describe. This is the opposite of that and is the opposite action that creates it. Pushing makes hills, pulling and rifting makes valleys.
Are you saying that the plates are moving and as they move the previously extruded magma moves with them and that leaves space that is filled by new magma?
Yea. The magma comes up to fill the empty spaces. Then it hardens joining the two plates again. The plates rift more spliting that newly formed rock and more magma comes up to fill the gap.
How odd that the new magma doesn't just pile up on top of the old.
How can it? It won't get past old magma unless the plates cause another rift. Remember, the only reason it is rising is because room was made for it to rise. Once that room is take up again it stops until more room is made by another rift.
How come the rate of movement is precisely right for creating this apparently continuous new sea floor?
It has nothing to do with any rates. The magma wants to come up but the crust stops it. Make a hole in the crust and the magma will come up. You can do this at any pace.
It matters where the movement originates. If not the continental ridge, then where?
Unknown. How the plates move is currently explained by convection cell theory. We cannot see into the mantle AFAIK.
Clearly it doesn't originate in the Pacific trenches. Those are explained as collisions of plates in which one dives under the other -- a movement toward one another.
We cannot say clearly because we do not know. The plate is moving due to some mechanism. The leading theory it is moving due to convection in the mantle meaning that it isn't being pulled or pushed on either "end" of the plate.
It is at the Atlantic ridge that there is this divergent action, and this has to be the driving force that moves the continents.
No it dosen't have to be the driving force. Certainly not just because you say so. IN fact we specifically know it is not the driving force which is the point of this whole discussion.
The best way to simplify this I think is simply that what is happening at a divergent boundary is a weakening of the crust. That is why normal faults occur. That is why the land sinks. That is why the land break apart (rifts). The crust is letting its guard down which is why the magma CAN rise to the surface. It is a passive reaction. These are the things we observe. Period.
This message has been edited by Jazzns, 09-15-2005 05:43 PM
No smoking signs by gas stations. No religion in the public square. The government should keep us from being engulfed in flames on earth, and that is pretty much it. -- Jon Stewart, The Daily Show
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