Lava tubes and magnetic reversals

[P A Hill & E V Goodall]

Patricia N Kambesis wrote:
> To answer Paul on affects of magnetic reversals - all iron bearing
> materials (lava, sediments, rocks being formed etc.) will take on the
> existing magentic direction of the time (first humanly observed from sea
> floor dredgings which showed "stripes" of sea floor basalt with reversed
> magnetic directions. )

Thanks for the reminder about reversals. That is what came to mind when I 
was thinking about what IS frozen in the rocks.  I am not sure I have been 
in a tube that is more than a few thousand to a few 10s of thousands of 
years old.  It is a rare flow that I have been too that has enough top soil 
development to support bushes let alone trees, but I have heard of much 
older examples even right here in Utah.

But I was not worried about some really old lava wall holding a 180 degree 
out of north field, just the "oldish" wall holding something that is off 
from the current mag. north by more than the precision of the instruments 
thus throwing in more than the usually pertibation into the data.

John H's point about mag fields getting into the rock while still just a 
little plastic certainly suggests an explanation for some wall spots being 
off by a whole lot while most spots only having a slight effect.  Plastic 
flowing lava isn't going to get too twisted too often, but some bits 
occasionally just might get really bent.

Going back to effects of mag. drift, I again go to
http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl
Or as Gary recently pointed out, you can download a program from Garry 
Petrie's site that does something similar.

I see that over the last 100 years in a point close to or in Hawaii
(as an arbitrary example); Lat 19.75 N Long 155.5 W; the change is
currently -3 minutes/year, apparently about a degree and a half per
century.  Of course, I must add the caveat that the change is not linear so 
don't extrapolate the number too far forward or back. Note also that 3 
min/year should be more like 5 degrees in 100 years, not the published 
~1.5, so the non-linear terms show up significantly even in a -- 
geologically speaking -- short period like 100 years.

Just a geologically young flow of a few thousand years might be generally 
off by many degrees or even tens of degress from the current magnetic 
north. This doesn't count those very curious local examples which have been 
mentioned which can be attributed to jumbled bits of lava that are part of 
the walls, or breakdown neither of which is in its original orientation.

Thanks for the great examples folks!

So do those who often survey lava tubes bring their own targets (maybe a 1 
meter stick), just to keep themselves off the walls, rocks and ceilings?

To bring this back around to another thread on this list, that would 
suggest a much larger positioning error than a 1 cm mark on a rock, but 
certainly less than shooting to each others headlamp or any other such 
lower precision technique.

Happy Surveying,
-Paul Hill
NSS 17696