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on Thu, 23 Feb 2006 02:00:38 GMT, George <george@xxxxxxxxxxxxxxxxxxxxx> sez:
` "mirage" <mjohnson37@xxxxxxxxxxxxx> wrote in message
` news:1140638829.112988.245900@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
` >
` > SBC Yahoo wrote:
` >> Generally speaking a oceanic plate is more dense than a continental
` >> plate,
` >> and the densest plate will slide beneath the lighter density plate.
` >> (Subducted, I believe is the geologic term)
` >>
` >
` > That part's clear. What I'm wondering about is the subduction angle.
` > Very old, cold oceanic crust descends into the mantle at a pretty steep
` > angle once it slips under lighter continental crust.
` > In thinking about the spreading center from which the Farallon and
` > Pacific plates
` > emerged, the eastern edge of the Pacific plate is a single unit with
` > the spreading center. In other words, the spreading center and the
` > edge of the oceanic plate are a continuous physical structure (i.e.,
` > there's no single, precise geologic demarcation where the spreading
` > center ends and the oceanic plate begins). When the spreading center
` > is overridden by the continental crust, the spreading center remains
` > bouyant. As the oceanic crust follows the spreading center beneath the
` > continental crust, the oceanic crust is still hot and physically
` > contiguous with the already-subducted spreading center, which isn't
` > descending toward the mantle. In such a case would the oceanic plate
` > remain intact below, and parrallel to, the overlying continental crust?
` > Or would it shear at some point, separating from the spreading center,
` > and descend into the mantle?
` >
` > mirage
` The farallon plate is subducting at a relatively shallow angle compared to
` most plates at subduction zones (the Sumatra zone, for instance). The
` spreading center is relatively close to the coast of the Western coast of
` the continent, so the plate is likely relatively young, and likely more
` bouyant than others.
I'd like to expand this discussion a bit in the same direction. Here
is a Google pic of BC around the north end of Vancouver Island:
http://maps.google.com/?ll=51.69299,-125.36499&spn=4.849696,10.063477&t=h
Now, the main spreading centre for the Pacific is being subducted under
the coast of North America, and all that's left off BC is the western edge
of the Juan de Fuca plate, which disappears under the continent just at
the northern tip of Vancouver Island. If you look at this pic, you will
see a pair of large pinky looking bare spots at top centre (just north of
the yellow line of the highway to Bella Coola), which are a pair of huge
old ash cones, which have odd native names I forget, and are touted as the
tracks of a hot spot drifting eastward, which has spawned more cones not
nearly as obvious as these, and is now supposed to be about a third
farther inland. The thing is, if you estimate where the spreading centre
ought to be under the continent based on the direction it is travelling as
it hits the coast (assuming it wasn't simply subducted away), it looks to
track right across where this hot spot is supposed to be. I don't know
whether the matchup works back in time, but I would expect that a hotspot
and a spreading centre are similar sorts of things, and ought to maintain
their relative position.
So, I would offer this as a candidate for another example of a
spreading centre being linked to a hotspot, but I don't know
what the current thinking is on this among the experts.
--
==========================================================================
vincent@triumf[munge].ca Pete Vincent
Disclaimer: all I know I learned from reading Usenet.
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