Re: A geology lesson (was Re: pole shift)

Anton Sherwood (
Mon, 21 Jul 1997 19:45:51 -0700 (PDT)

quoth E.Shaun--
: Actually, there may be a *little* bit of truth in what Danny is
: saying. Feeding off of my geology experience, I seem to recall that there
: are two different forms of tectonic movement: convergent and divergent.
: When a divergent tectonic movement takes place, the plates split apart,
: causing magma to escape...a perfect example is Iceland which is always
: splitting and growing igneous rock. Convergent movement is when two plates
: collide or "converge" producing magma in that way. What I presume Danny is
: getting at is that there may be a large convergent movement without a
: head-on collision, causing the pole or continent to move. However,
: though this is conceptually a possibility (isn't everything!?),

I don't get it. Yes, the plates move in different directions,
that's obvious. If every pair of nearby plates were converging
(or diverging), the planet would have to be shrinking (or growing).
I can't visualize what Shaun is suggesting here - that two plates
may approach each other without colliding? Or that some collisions
have a lateral component (that can't be surprising)?

: it has never happened before on this planet (as far as anyone knows)
: and the odds of it happening are tremendously infinitessimal.

Yeah, I'd say it's unlikely for all the plates at once
to come loose from the mantle and race about.

On another hand, remembering Haldane's Rule --
I saw something on teevee that said the surface of Venus is determined
from crater counts to be all about the same age; okay that isn't
disturbing, except that the age appears to be HALF a billion years!
What caused that catastrophe, and is it likely to happen again?

: P.S.: Just to clarify something, Hawaii is a bit of an anomaly. It was
: --and is being-- formed by weaknesses in the seafloor which were
: punctured under pressure. These punctures sent up spits of magma in
: the form of pillow basalts which, in turn, created the Hawaiian
: islands. The punctures still prove to be very active in Hawaii
: judging by the high amount of volcanic is probable that
: within the next million years there will be many more Hawaiian
: islands...and more opportunities to trap future tourists! ;~)

Blaming Hawaii on weak spots seems to put the cart before the horse.
The conventional story, as I understand it, has a "hot spot" of rising
magma (cause unknown) which, as the Pacific Plate moves over it, pushes
volcanoes through. Presumably every plate has weak points where magma
is more likely to get through than the surrounding area; that's why
Hawaii is a sequence of cones and not a single long ridge.

Anton Sherwood *\\* +1 415 267 0685 *\\*