In article , Robert Grumbine wrote:
I am not a geologist....however....at the margins, where subduction is
occuring...would there not be shearing and scraping of sediments? Would
there not be lower portions of CRUST being subducted as the mantle that
it's ATTACHED to descends into the planet?

Some portion of the crust/sediment gets scraped off, and some gets
carried down in the subduction zone. Part of the subducted material
is readily melted at the higher pressure and temperature and comes
back up through volcanoes.

For the scraped material, the term to look up is 'accretionary wedge'.
iirc SE Asia has a good one.

Worth looking up on "ophiolites" too, Ken. That's the inverse
effect of bits of oceanic crust and upper mantle getting scraped off and
mixed in with the material of the accretionary wedge, then pushed up onto
the continent. The classical ophiolites are in Cyprus and Oman (good
places for a holiday), but I have heard of nice ones in the Californian
melange too, and I'm sure there are other examples closer to your home.
There's a small ophiolite complex at Ballantrae, for example.

--
Aidan Karley, FGS
Aberdeen, Scotland
Written at Wed, 21 Jun 2006 08:14 +0100, but posted later.