On Sun, 21 May 2000, meredith lamb wrote:
> Noted a interesting article on the web:
> http://www.discovery.com/news/briefs/20000518/geology_deeprock.html

The original Science paper was in the May 19 issue (Collerson et al.) at
www.sciencemag.org.

> >From my point of view....I can't really understand how they
> could have gotten to the surface from the between the upper
> and lower mantle intact, and, from the Pacific basin itself?
> Meredith Lamb

This is great stuff!  For many years now people have been trying to defend the
idea that mantle plumes, which may be responsible for many "hot spots" such as
the Hawaiian islands, come from the lower mantle.  Some people have even
proposed the base of these plumes to be at or right above the core-mantle
boundary.  The evidence for this specific case is very solid, with crystals of
MgSiO3 type perovskite contained in them.  Perovskite is thought to be the
dominant crystalline MgSiO3 phase in the lower mantle, making it probably the
single most abundant material in the Earth.  The other metal oxides
present suggest a depth even deeper than 670 km which is where the famous
discontinuity is present presumably due to the transformation of
ringwoodite, clino pyroxene, majorite garnet and a few other minerals into 
perovskite and magnesiowustite.  Only problem is that it is terribly difficult
to get it up to the surface, hence its rare appearance on the surface of the
Earth.

The challenge is exactly as Meredith stated: how did it get up?  Perhaps there
are bursts of intense activity at some hot spots and the plumes eject material
faster than usual.  Everything rises as a deforming plastic mass until it
reaches the crust and there melts form and rise to the surface as a lava.  If
this is the case, as this paper suggests, then there are probably no intense
activities currently occurring at volcanoes because this stuff would likely be
found.

John Hernlund
E-mail: hernlund@.......
WWW: http://www.public.asu.edu/~hernlund/

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