From: williams@pangea.Stanford.EDU (Tom Williams)
Newsgroups: sci.geo.geology
Subject: Re: rising sea level & continetal drift
Date: 16 Nov 1992 18:28:46 GMT
Organization: Stanford Univ. Earth Sciences
NNTP-Posting-Host: pangea.stanford.edu

In article <1e734kINN5vh@crcnis1.unl.edu> price@helios.unl.edu (Chad  Price) writes:
>jmartin@eniac.seas.upenn.edu (Jason J Martin) writes:
>
>>I have a question about the rising sea level......I'm told that the sea level
>>has risen quite a bit in the past few hundred years.  Could this rise in sea
>>level have to do with the moving continents?  Could the total amout of ocean
>>floor be decreasing?  Something like that would cause a rise in sea level.   
>
>Anything on the scale of a few hundred years is far too fast for plate
>tectonics to have any effect.

Agreed.

>Global sea level changes seem to be almost
>entirely the result of the presence/absence/changing volume of the glaciers,
>and in particular the continental scale glaciers such as Greenland and
>Antarctica.

  Whoa, whoa.  Jason was actually onto something with his original post.
High-frequency sea level changes such as we might need to be concerned 
with on a human time scale are almost entirely due to changing volumes
of landlocked ice.  (Ice on water has little effect since it displaces
it's a volume of water equal to that which it locked up in ice.)

 However,  Jason's intuition is somewhat correct.  Looking at very long time
scales plate tectonic processes can be calculated to have changed the
volume of the ocean basins.  (Walter Pitman, 1978; Bond and Kominz, 1984).
Changing the volume of the ocean basins results in long-term low
frequency sea level changes over periods on the order of 100 million 
years or so.  Such changes are though to be responsible for the high
sea level during the Cretaceous when shallow seas covered much of North
America and other continents.  Global climate was also much warmer then,
so a lack of continental glaciers compounded the tectonic effects.
Increasing ocean basin volume has led to a gradual decline in sea level 
since then.  Various authors estimate this decline as ~200m over the
last ~100 million years.  Glacially-driven sea level excursions are
superimposed on this long term drop.

  Why does this happen??  Well oceanic crust is destroyed at more or less
the same rate that it is created, so it's not really as if a much greater
percentage of the earth's surface is covered by oceanic crust.  The 
effect I'm talking about results from the fact that young, hot oceanic
crust near spreading centers is less dense, and floats higher on the mantle
than older, colder, denser crust.  As a result, when mid-ocean ridges
are spreading rapidly, creating lots of young, buoyant oceanic crust that
rides high on the asthenosphere, the total volume of the ocean basin
is smaller.  By extrapolating past plate movements, calculating the
global average spreading rate and the resulting volume changes,
Pitman (1978) calculated the resulting sea level changes. 
Kominz, (1984) did a rigorous error analysis.

Pitman, W.C., 1978, Relationship between eustacy and stratigraphic sequences
of passive margins: Geological Society of America Bulletin, v. 89, p. 1389-1403.


Kominz, M.A., 1984, Oceanic ridge volumes and sea-level change, an error
analysis, in, J. Schlee, ed., Interregional Unconformities and Hydrocarbon
Accumulation: American Association of Petroleum Geologists Memoir 36, p. 109-127.









kk
--
Tom Williams                       williams@pangea.stanford.edu
Department of Geology				 
Stanford University School of Earth Sciences