Clark,
Lightning strikes are usually recognized by paleomagnetists by the
abnormally high intensities of remanence in isolated samples. It is
sometimes subjective because lightning is not a uniform process. I found a
good discussion in a book I have on paleomagnetism under the section on
"sample collection in the field":
McElhinny, 1973, Paleomagnetism and plate tectonics: Cambridge University
Press, Cambridge, 358 p.
He gives two references to studies on lightning strikes (that I haven't
read):
Cox, A., 1961, Anomalous remanent magnetization of basalt: U. S.
Geological Survey Bulletin 1083-E, p. 131-160.
Graham, K. W. T., 1961, The remagnetizaton of a surface outcrop by
lightning currents: Geophysical Journal Royal Astron. Soc., v. 6, p.
85-102.
For discussions of remanent magnetization in relation to petrologic
properties of highly magnetic rocks, I recommend looking up papers by
David A. Clark in primarily Australian publications. Two that give
overviews and good references are:
Clark, D. A., 1983, Comments on magnetic petrophysics: Bull. Aust. Soc.
Explor. Geophys., v. 14, p. 49-62.
Clark, D. A., 1997, Magnetic petrophysics and magnetic petrology: aids to
geological interpretation of magnetic surveys: AGSO Journal of
Australian Geology and Geophysics, v. 17, p. 83-103.
There's also the paper by Grant:
Grant, F. S., 1985, Aeromagnetics, geology, and ore environments: I,
Magnetite in igneous, sedimentary and metamorphic rocks--An overview:
Geoexploration, v. 23, p. 303-333.
Hope this helps.
Tien Grauch
--------------------------------------------------
Dr. V. J. S. (Tien) Grauch
U. S. Geological Survey
Box 25046, Federal Center, MS 964
Denver, CO 80225-0046 USA
phone: 303-236-1393 fax: 303-236-1425
email: [EMAIL PROTECTED]
"Clark Jorgensen"
<[EMAIL PROTECTED] To: <[EMAIL PROTECTED]>
m> cc:
Sent by: Subject: [SEGMIN]: remanent
magnetism
owner-segmin@lists.
geosoft.com
07/23/2002 03:39 PM
Please respond to
segmin
I recently finished doing two mag grids with a lot of remanent magnetic
responses. Both of these grids were located on mountain peaks with highly
magnetic Precambrian rocks. There wasn't any real pattern to the remanent
responses and they were of various sizes (ranging from 5 to 50 meter
diameters). The overall magnetic field varied from about 30,000 to 70,000
gammas with the background at about 56,000.
At first, I thought the remanence was probably induced by lightning
strikes.
After finishing the survey, I compared the local grid responses and
locations to a high resolution helicopter magnetic survey that was flown
earlier over the same area. Both of the grids are located near a major
contact and there did seem to be an overall motley appearance (lots of
small
bull's eye highs and lows) in the helicopter magnetic data near the contact
regardless of terrain elevation. Therefore I concluded the remanent
responses in the ground mag were probably caused by hydrothermal
alternation
from metamorphic activity near the contact rather than by lightning
strikes.
This project forced me to realize how little I know about lightning strikes
and remanent magnetism. How strong are the responses (both in changes to
the magnetic field and aerial extent) likely to be? I am curious to know
if
there is anyone on this list who has had experience with this or knows of
any published works regarding lightning strikes and magnetism.
Sincerely,
Clark Jorgensen
Big Sky Geophysics
P.O. Box 353
Bozeman, MT 59771
USA
(406) 587-6330 phone
(406) 587-6394 fax
[EMAIL PROTECTED]
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