CompuServe Messages

Hi, Kamron– >> >> The ridges were not so much like a single entity until the transform fault between N.Brazil and W.Africa mostly closed up,<< >> What do you mean on closed up? Remember that jog in the Mid-Atlantic Ridge near the coast of N. Brazil? It was once a far longer jog – after the N. Atlantic opened, S. America started away from Africa, with a _long_ transform fault making the then-long connection between the S. Atlantic ridge and the N.Atlantic ridge. As S. America caught up with N. America, the ridges slid together along the transform fault, so the connection shrank down to the jog we see today, making the two ridges seem near continuous. >> Why would the transform faults still all line up across the Mid OCean Ridge after millions of years of moving in all directions why are they mostly lined up still? The motion has not been "in all directions"; the simple motion away from the ridge at right angles to it has been by far the bulk of all motions. Having seen a mention just today (new Earth mag) that N. America is now moving a bit south of west (instead of its north of west motions during most of the Atlantic's spreading), I am a little readier to accept that some convergence of N. and S. America is part of orthodox theory – but notice it is not like they have stopped all west motion while closing on each other, it is more like two cars running almost parallel while slowly drifting into each other's lanes (bang, ouch!). The major motion is all I am going to see within the flat-map error. My rules of thumb about connecting the originally-juxtaposed continental margins and finding the ridge at the midpoint, with transform faults paralleling the connection, were a gross overall picture, and assumed no spreading or subduction that moved one continent without also moving the ridge and the other continent. Any subduction between the Americas is a very modest violation of that assumption, and the main internal oceans would not be so wide without following the assumption much more than not. This conversation has indeed been interesting; it has forced me to notice weaknesses in my understanding of plate tectonics and fill them. –Doug