[BR-Crater] Circle assumption

[Ian Kluft]

On Fri, Mar 30, 2007 at 12:03:28AM -0500, Scot Wilcoxon wrote:
> I realized there's an assumption underlying seeing circles in scenery:
> 
> *Multiple arcs which form a circle are unlikely to happen randomly, so 
> it is likely that a single event formed any discovered circle.

We've chatted on #br-crater but I'll fill in additional info for the
mail list.

Looking at Google Maps/Google Earth can play tricks with your eyes
because of different shading of different satellite imagery, visibility
of artificial structures and inability to differentiate positive and
negative relief due to shadows going in inconsistent directions.
So we need to double-check against topographic maps when we think we
see circular structures.

Arcs can happen fairly randomly.  A river can cut a nice arc.  Hudson Bay
in Canada has a strange circular-shaped side which was once thought to be
a possible impact site but turned out not to be - probably sea ice erosion
processes.  (Though a pair of confirmed impact sites, Clearwater West and
East, are next to it.)

Even if one finds something circular-looking on a map, it doesn't mean
much alone.  It only begins to make more sense as a possible impact site
if you can make a complete circle out of it and, for large structures
over 10 miles in diameter, you can see something to match a central
uplift or inner ring.  Even if it's heavily eroded, if it's on the
surface, there should be something there which looks like it eroded
around something in the middle.

Also, if it has visible lava flows, it's obviously volcanic.  An example
of that is at the McDermitt Caldera on the Oregon/Nevada border around
41.967 N 117.979 W.  It clearly has lava that flowed - you don't need
to look for any other evidence to conclude it's volcanic.

We've talked a bit on the #br-crater IRC chat about "unresearched circular
features", which I've abbreviated UCFs.  We've spotted some on satellite
photos and topographic maps.  There are two others in the Black Rock region
that pass the initial test that have circular features and central uplifts
that topographic maps show erosion has gone around.
   UCF at Soldier Meadows   41.465042 N 119.366280 W   15 miles diameter
   UCF at Cottonwood Creek  41.164069 N 119.568155 W   12 miles diamater

Some additional info...
* The UCF at Soldier Meadows matches an area on the Stuart 1980 paper
  where he noted a different structure to the Soldier Meadows Tuff
  which is more lava-like.  The location inside the circular feature
  is consistent with an impact melt pool.  But it's also exactly where
  the geologists were looking for a volcanic caldera as a source of
  the Soldier Meadows Tuff since 1972.  It's easy to see now with
  computer mapping.
* The UCF at Cottonwood Creek is adjacent to the Lund Petrified Forest
  where trees were buried in ash.  It's easily accessible since Washoe
  County Hwy 34 goes through the middle of it.  But it's also at 6000'
  elevation so it's probably only now losing its snow and remains muddy.
  The Fox Mountain K7UI Amateur Radio repeater is on the south rim of
  the UCF so we should have good communications in the area.

So I'm withholding any conclusion on these places until we go visit
them later in the Spring.  We don't have enough information.  Unlike
McDermitt, there are no visible lava flows around these places.

On-the-ground observations at the UCF at Soldier Meadows will be the 
next big chance for a cross-check of volcanic vs impact origins of
the Soldier Meadows Tuff. 

> When we see a circle we are likely to think the entire circle was 
> probably created due to something which makes circles.  We should 
> examine the reasons for various arcs and not assume the circle was 
> created due to a single reason.

Correct.

Without something else to support an impact origin, a circular area
should be treated with caution and skepticism - it isn't ever going to
be a discovery anyway unless it survives criticism.  You may as well
start with your own criticism and see how well it fares before others
put your idea to that test.  Before I posted the web page about the
Black Rock Crater, I had numerous chances to cross-check photos vs
mining documents vs impact-related texts vs Brad's preliminary satellite
data.  There were sink-or-swim moments when there have been chances to
cross-check infcormation - each time the impact theory has stayed afloat.
It kept coming up either consistent with an impact or leaving it plausible.
After enough rounds of that, I became confident enough to post the page
and ask for attention from geologists.

At Black Rock, the southern portion of the crater has very clear
circular bands on satellite imagery all the way around the circle
south of the Black Rock playa.

> Ian has found pieces of evidence which point toward the existence of an 
> impact event in the area.  Are there clues in the various arcs which can 
> reveal why each arc is visible?  Are we seeing crater-edge fractures, 
> changes in impact alterations, surface extensions of deep impact-caused 
> faults, mineral fertilization of plants along an old crater rim?

Let's try not to get in over our heads.  There are things we can check
and anything else should be left to the pros.