From: joe@montebello.soest.hawaii.edu (Joe Dellinger)
Subject: Re: Missisippi rift valley?
Reply-To: joe@montebello.soest.hawaii.edu
Organization: School of Ocean and Earth Science Technology
Distribution: na
Date: Wed, 15 Apr 1992 05:57:39 GMT

In article <1992Apr14.184218.10532@nsisrv.gsfc.nasa.gov>, dsc@gemini.tmc.edu (Doug S. Caprette Bldg. 28 W191 x3892) writes:
|> Are you sure about this?   VLBI between Westford Mass. and Ft. Davis in Texas
|> has indicated a decrease in baseline length (about 1-2 mm/yr) averaged over 
|> the last 10 years or so.

	Well, I'm just quoting what I heard at various talks; I suppose I
could be remembering with a sign error...(?) I do remember they were surprised
by the magnitude of the relative motion they found.

|> >	And of course don't forget the state I'm in now (Hawaii), home of
|> >the largest landslides in the world.

	Several people asked me for more info about this via e-mail...
I'm mostly quoting from a talk I attended a few weeks ago by the head of the
Hawaii Volcano Observatory. (I don't do research on this subject myself.)

	Starting a few years ago the US Gov't decided they wanted to know
what they "own" on the seafloor, so they launched a project to do side-scan
sonar imaging of the entire "U.S. Exclusive Economic Zone". (I.E. Everything
within 200 miles of a US coast not owned by someone else.)

	They were surprised by what they saw around Hawaii: what looked like
huge landslides. For example, Tuscaloosa seamount East of O`ahu appears to be
simply a piece of O`ahu's Ko`olau mountains that went sliding off into the
deep. Samples from the seamount seem to confirm this; chemically it does appear
to be a piece of the Ko`olaus. There is also other evidence, such as fossil
coral reefs at various depths around the islands that date from past sea level
stands. (Caused by the interaction between global sea-level change and the
islands own up-and-down motions.) The fossil coral reefs are missing in
places... where they appear to have been mowed down by the chunks of island
sliding past.

	Once they started thinking in terms of giant landslides, some other
mysterious deposits became less mysterious: for example bits of coral
strewn at random orientations 1000 feet above sea level on Lana`i. Now there
is some arguing still going on about exactly how high a tsunami caused that,
when the tsunami happened, how high Lana`i was riding when it happened, etc,
but you get the idea.

	Evidence for how fast the landslides happen comes from the topography
of the sea floor. East of the Hawaiian chain is a depression, apparently caused
by the weight of the islands pushing down on the sea floor. Some of the
landslides go down into the Hawaiian deep _and up the other side_, and on from
there. The mass had to have a lot of momentum to do that. The best guess is
that the landslides happen on a time scale of hours, at most.

	This is all very new research; much remote sampling remains to be done
to pin down where the various landslides started from, exactly when they
happened, how they overlap each other, etc. But there seems to be little doubt
that humongous landslides are an important feature of the way these high
volcanic islands develop, and that if one happened today it would be "very bad"
for most everyone in this state. (Not to mention all the jets left hanging over
mid-Pacific with no place to land!)

	After the landslides are understood, of course, the next project
will be figuring out how to tell _beforehand_ when something big's about
to give way. Chances are the next one isn't for tens of thousands of years,
so we've probably got plenty of time. Something to think about the next
time you're in Hawaii and look up at the beautiful corrugated green cliffs
fronting the sea...

     /\    /\    /\/\/\/\/\/\/\.-.-.-.-.......___________
    /  \  /  \  /Hawaii Institute of Geophysics, Honolulu\/\/\.-.-....__
___/    \/    \/Joe Dellinger, Internet: joe@montebello.soest.hawaii.edu\/\.-.__