Re: Big Craters

["Jerry D. Harris" <LOKICORP@compuserve.com>]

Message text written by "James R. Cunningham"
>From another source (Jerry, I don't have the reference in front of me --
will
fish it out for you -- turn about is fair play, and you are playing fair).

               Crater sizes

Name               Age               Dia. (km)
Chicxulub           64.98 Myr      170
Morokweng      145                    70
Vredefort       2023                   300



This source  estimates Morokweng at 100 km smaller than Chicxulub.   Still
other sources place Chicxulub at =>300.   I think its only fair to note
that
many estimated crater sizes are awfully tenuous.<

        I'll have to check that reference -- thanks for posting it!  I
agree that crater size determination can have a significant margin of
error, particularly when the crater is subsurface (as in Morokweng and
parts of Chixilub).  Unless we can actually drag a tape across the edges of
a visibile crater (as in Meteor Crater in AZ), we have to rely on more
indirect evidence (seismic, gravity anomaly, etc.), and since large craters
can have numerous ringed structures, determining which are actual crater
edges and which are rebound, collapse, etc. structures.  In addition to
Morokweng, I recall that there was some debate about Chixilub's size as
well -- someone had interpreted some seismic data as an outer ring that was
the "real" size of the crater, making it almost twice as large as current
estimates.  As with numerous things in paleontology, it appears that
geophysicists have to wait for a consensus to be reached as well!


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                     Jerry D. Harris
                 Fossil Preparation Lab
          New Mexico Museum of Natural History
                   1801 Mountain Rd NW
               Albuquerque  NM  87104-1375
                 Phone:  (505) 841-2809
                  Fax: ; (505) 841-2808
                LOKICORP@compuserve.com