Replying to LO29671 --
Dear Dan,
I am very pleased to read that you realise which unique part of the world
you live:
> I've learned that our south-central Alaskan region is a jumble of
> colliding terranes, and the volcanoes I've seen erupting out front are
> caused by subduction of the Pacific plate under North America beginning
> 160 miles to my left, running under my feet, getting hot, then
> occasionally melting/exploding through basalt (laid during the Jurassic
> period, 210-145 mya) 40 miles ahead of me and to my right, all moving at
> about 2 cm per year as I recall. Clearly, to me, "It" has had way more
> time than Homo sapiens have had, or are likely to have.
The word 'terrane' is a specialist term. It refers to slivers of continent
that are removed from their original place to a new position due to plate
tectonics (continental drift). Alaska is a refuse dump of several slices
of western N American continent. Parts of California are under way Dan, so
maybe some palm treees will arrive soon :-)
According to my knowledge, the subduction (sliding of oceanic crust
underneath continental crust, causing serious earthquakes and explosive
volcanism) near Alaska is as fast as near10 cm per year; that means that
in a human life time over 5 metres of oceanic crust disappeared. The
Atlantic ocean between N America and Europe becomes each year 12 cm
broader; no subduction occurs in the N Atlantic (only near the Caribean
Island arc), thus no oceanic crust disappears. In my family, I created
allways a lot of laughs when I enthousiastically told about the incredible
velocities of moving plates. When my family heard that it was several
centimetres per year, they sttart laughing at me; for a geologist these
figures are 'unnatural' - we are accustomed to thinbk in millions of
years. Although way more time than Homo Sapiens have had, it is not that
slow.
Dan, please realise that what happens near Alaska, has its effects in
South Africa, Iraq, or Birma. Since our globe doesn't change so much in
shape, and since permanently new oceanic crust is created and old crust
subducting and all these weakness zones are connected and the movements of
all the tectonic plates are influencing each other, plate tectonics will
have its effects all over the earth. The collision of terranes in Alaska
had its effects elsewhere too. Because after the collision movement will
stop, or changes, and of course this effects all other movements of plates
all over the world.
I hope that an earthquake like the one that happened in Anchorage in the
sixties of the former century and which was one of the heaviest ever (I
thought over 8 on Richter's scale) will not happen too soon. The entropy
produced during an earthquake is too much to balance and manage.
Leo Minnigh
--leo minnigh <minnigh@dds.nl>
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