[meteorite-list] FOLLOWUP TO Smallest Possible Earth Impact Crater?

From: Charles O'Dale <codale0806_at_meteoritecentral.com>
Date: Thu, 19 Jul 2007 17:23:04 -0400
Message-ID: <00cc01c7ca4a$ff1ab700$a796684a_at_mdguo5m3tdnvnv>

Sterling, thanks as always.

I guess what has to be determined here is whether Merewether is a "cosmic velocity bolide crater", a "terminal velocity bolide pit" or a naturally formed structure whose formation method is unknown. No meteorite material or impact rock types have been found at the site.

An interesting thing about the whole structure is that it "may" be a crater string since there seem to be three impact structures lined up at the site.

Chuck
  ----- Original Message -----
  From: Sterling K. Webb
  To: Charles O'Dale ; meteorite-list at meteoritecentral.com
  Sent: Thursday, July 19, 2007 4:21 PM
  Subject: Re: [meteorite-list] FOLLOWUP TO Smallest Possible Earth Impact Crater?


  Hi, Charles, List,

      I just went and read your excellent article on Merewether:
  http://ottawa.rasc.ca/articles/odale_chuck/earth_craters/merewether/index.html
  so now I know Merewether is 200 meters across. However,
  the key data is that the crater is NOT in a rock surface, but
  a glacial morraine of boulders gobbed up with sand and clay.

      That fact alone would explain the absence of a rim upturn.
  Rims are formed by the explosion "pushing" upward against
  horizontal strata that are significantly rigid and "resist" being
  pushed with a strength beyond the mere weight of the material.
  A conglomerate of boulders and mud is not rigid, hence no
  tilted rim is produced.


  Sterling K. Webb
  ----------------------------------------------------------------------
  ----- Original Message -----
  From: "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>
  To: "Charles O'Dale" <codale0806 at rogers.com>;
  <meteorite-list at meteoritecentral.com>
  Sent: Thursday, July 19, 2007 2:30 PM
  Subject: Re: [meteorite-list] Smallest Possible Earth Impact Crater?


  Hi, Charles, List

      The three most important characteristics an incoming
  body needs if it wants to get to the surface in one piece
  are a) slow entry speed, b) a shallow entry angle, and c)
  an aerodynamic (flattened) shape. Calculations performed
  by John S. Lewis suggest that, with a shallow entry just
  above escape velocity, an an iron of 30 to 100 tons can
  "land" without making a crater.

      HOBA is a perfect example at 60 tons. It sits on a flat
  surface surrounded by red rusty soil that may contain 25
  or more tons of degraded iron shale residue.

      The same calculations for a stone meteorite give an
  upper weigh limit of around 40 tons, however there are no
  stones known that come anywhere near this mass. Stones
  are too fragile; they fragment too easily. (JILIN has the
  record at 1.77 ton.) However, if anyone finds a ten-foot
  diameter stone meteorite, it's fine with me.

      Of course, with a higher incoming speed and a more
  usual angle, that 100 ton iron would make a lovely crater.
  Using the excellently handy LPL Impact Calculator:
  http://www.lpl.arizona.edu/impacteffects/
  we see that (theoretically at least) a 10 cm (4 inch) iron
  ball entering the atmosphere at 15,000 m/sec at a 45 degree
  angle reaches the surface at about 800 mph with a force
  of about 200 pounds of TNT and would make a four-foot
  crater in sedimentary rock; the crater would be about 15
  inches deep with five inches of broken rock in the bottom.
  Oh, and the iron ball would survive intact!

      That is what a good computer model says. Whether this
  actually happens in real life is another matter. The computer
  model only calculates target surfaces of water or sedimentary
  or crystaline rock. Small craters in which the impactor survives
  are technically not impact craters -- they are impact pits. An
  impact pit has the rock or dirt removed by mechanical force,
  not by an explosive event. It takes an explosive event to make
  a technically "true" crater. There are a small number of
  examples of meteorites recovered from impact pits in dirt,
  though.

      What is the size of Merewether? From the look of the picture
  it must 100's of feet across. I took my previous example from
  the impact calculator and re-ran it with larger and larger iron
  balls and got bigger and bigger craters, over 300 feet in diameter,
  until I reached the size where the impactor fragments, after which
  I get crater fields from the fragments. Of course, that's with one
  limited set of parameters; other parameter, other results. The small
  "craters" (less than 20-30 feet) were all shallow impact pits with
  a surviving object. Above a certain size, they were all explosive
  craters, with that characteristically deeper profile.

      Merewether is certainly more than big enough to be an explosive
  crater. This does not say that it is, but if there's an objection that it is
  "too small" to be an explosive crater, that's a mistake.


  Sterling K. Webb
  ---------------------------------------------------------------------------------
  ----- Original Message -----
  From: Charles O'Dale
  To: meteorite-list at meteoritecentral.com
  Sent: Tuesday, July 17, 2007 8:58 AM
  Subject: [meteorite-list] Smallest Possible Earth Impact Crater?


  I am seeking advice on a "small" problem. I am trying to determine what the
  smallest possible impact crater on earth would be. In other words, we have
  to determine the smallest size of a bolide that would impact earth at cosmic
  velosities (>12 km/sec) to create such a structure. Or, the largest size of
  a bolide that would be slowed to terminal velosity by our atmosphere (and
  not creating a "crater").

  The answer may help in adding information to the "enigma" of the Merewether
  structure, could it be an impact related crater?

  http://epod.usra.edu/archive/epodviewer.php3?oid=315776

  Chuck
  http://www.ottawa.rasc.ca/articles/odale_chuck/earth_craters/index.html


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