[meteorite-list] Impact Structures - Simple vs Complex?

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Fri Mar 17 13:26:20 2006
Message-ID: <004601c6495a$2882c1a0$182e4842_at_ATARIENGINE>

Hi, Jeff, List,


    The crater categories are:
    1. simple
    2. complex-immature
    3. complex-mature
    4. central peaked crater
    5. peak ring basin (two ring crater)
    6. multi-ring basin

    The factors determining which crater
results from an impact are, in order of
importance:
    1. gravity at the surface
    2. strength of the materials of the surface
    3. total energy of the impact

    On Earth the transition from simple to
complex occurs between a one mile crater and
a three mile crater. On Mars the transition
from simple to complex occurs between a
2-1/2 mile crater and a 6 mile crater. On
the Moon the transition from simple to
complex occurs between a 8 mile crater
and a 20 mile crater.

    When there is a significant impact, at first
there is just a huge blown out hole, called the
transient crater cavity. In the right materials,
on the right body, the crater might just fill back
in, leaving only a circular wrinkle on the surface.
If the center of the impact re-bounds strongly,
there is a central peak. In most craters the
original steep walls slump, shallowing the
crater.

    The Moon's original crust (the "highlands")
was struck with impacts that produced giant
basins, both multi-ringed and flooded. The lunar
crust was probably only 40 to 60 miles thick
at that time. Yet, despite producing basins
1,000 kilometers or more across, no sample
from the Moon has any mantle rock in it, so
it seems that even the biggest impacts don't
dig deeply into the planet. Instead, they heat
and melt vast areas of surface.

    The Earth's impact with the Moon's parent
body, and the subsequent in-fall of debris,
probably re-melted the Earth's crust to a depth
of ten miles or more, perhaps re-melting the
entire crust right down to the mantle. (Just
when it had gotten all solid and settled, too.)

    Yet, there is a NASA pic of a so-called "zap" pit
on a glass spherule from the Apollo 11 soil samples,
a tiny BB of glass that got hit with something even
smaller, which left a little bitty crater. The zap pit
is a ring basin and it's only 30 microns across!
So, the true answer to your question as to how big
a crater has to get to become a complex crater is:

    Well, that all depends...


Sterling K. Webb
-----------------------------------------------------------------

----- Original Message -----
From: "Jeff Kuyken" <info_at_meteorites.com.au>
To: "Meteorite List" <meteorite-list_at_meteoritecentral.com>
Sent: Wednesday, March 15, 2006 6:01 AM
Subject: [meteorite-list] Impact Structures - Simple vs Complex?


> Here's a question for those of you more familiar with impact structures on
> Earth. I believe I saw somewhere that craters fall into 2 main categories?
> simple and Complex with the later having a central uplift, concentric
> rings,
> etc among other things.
>
> My question is: How small can a complex crater be? Is there a definitive
> size restraint or does it completely depend on a multitude of variables
> such
> as the make-up of the impacting body, velocity, impact angle, target rock,
> etc?
>
> Any help is appreciated,
>
> Jeff Kuyken
> Meteorites Australia
> www.meteorites.com.au
>
>
>
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>
Received on Thu 16 Mar 2006 07:31:39 PM PST