Our first stop was at a big boulder (5'Hx10'Wx8'D or so) with a number of concentric circles pecked into the north face of the rock. While we were standing around looking at them and scratching our heads as to possible "meaning" or "purpose" for the designs, one of the guys in the group brought out a compass to check for magnetism. Lo and behold, when you got within about 2 feet of the rock in front of a petroglyph the needle turned 180 degrees to point directly at the rock! This was pretty interesting, there was a fairly strong magnet in the direction of the rock. Then he put the compass near the rock, but to one side of a petroglyph. At first the needle pointed north normally, but as he got closer the needle swung toward the center of the petroglyph. As he moved across the petroglyph the needle kept pointing directly toward the center of the circle, even as it passed by the center to the other side! Now this was really interesting. Obviously the petroglyph was centered on the magnetic field within the rock. We tried a couple of other petroglyphs on the same rock, with similar results.
At that point my BS meter gave up, and it turned into a what in the world is going on here question instead. It seemed to me that one of three things happened. (1) The artist knew that the magnetic field existed, and drew the design to identify its location, (2) The creation of the petroglyph somehow caused the magnetic anomoly, or (3) something else created the anomoly and the artist could determine where it was located by some means such as simple observation.
It seems unlikely that the anomoly existed and the artist had some way to detect the magnetic field. I suppose they ancient Americans might have been able to detect magnetic fields, but I have never heard anything about that. I would need additional evidence before I could accept that theory.
The favorite theory by the members of the group was that lightning had struck the rock, someone had seen the strike, and they put the concentric circles around the spot(s). (Option 3) This seems reasonable to me, except it wasn't immediately obvious how the lightning could produce a magnetic in the rock. The first though of course was that all of the electrical current interacted with the iron in the rock, which then became magnetised. My guess is that this is not possible because of the fact that the magnetic field around an electrical current goes around the direction of current flow, and not in the direction of the flow. For the magnetic to be oriented as it is, the geometry is wrong. However, the lightning could have locally heated the rock to a tempterature above the Currie point of the ferromagnetic materials in the rock (around 500C-700C). At this point the magnetic domains in the rock would be free to line up with the field of the earth. Upon cooling, the new orientation would be "frozen" into the rock - hence a localized magnetic anomoly at the location of a lightning strike. In addition, a strike would likely burn off the moss and lichens on the rock, leaving a visible mark that could be identified by the artist. This sounds pretty good and it doesn't require knowledge of, or detection of the magnetic field.
Another interesting possibility is that hammering on the rock caused the magnetic anomoly. If a ferromagnetic material in lined up with an external magnetic field (such as the earth's field) and struck sharply in the direction of the field, enough energy can be imparted to allow the magnetic domains to swing into alighnment with the external field. Since the petroglyphs are on the north face of the rock, the artist had to hit the rock many times in alignment with the earth's field (it must have taken thousands of well placed and energetic strikes to create the designs). This could also account for the presence of the field and would not require knowledge of the fields.
I have no idea at this time about really happened, but I tend to like the lightning heating the rock theory. However, either the lightning scenario or the hitting the rock many times scenario seem plausable to me. What I don't know is what kind of field strength could be created by these processes. I was surprised to find that the field was strong enough to swing the compass needle 180 degrees from a point more than 2 feet in from of the rock.
It would be interesting to find out which effect is really the right one. If it is heating of the rock, then we have a small bit of evidence about what the purpose of the figure was (to mark the location of a lightning strike). If it was caused by hitting the rock, then we still have no idea what the circles might mean.
I would be interesting in hearing from anyone who actually knows something about this. I know that there are lots of folks who have opinions like I do, but those don't help much without some more science behind it).
No comments:
Post a Comment