Geology Reference
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(1992) and Tauxe (2010) but low - stability magnetic
grains, either very small grains close to superparamag-
netic in particle size or large multi-domain grains with
many magnetic domains, are the most susceptible to
acquiring a viscous remanence. Typically, thermal
and/or alternating fi eld demagnetization removes a
VRM, but some VRMs are diffi cult to remove. Storage
in low fi eld space before and during demagnetization
and measurement, more common in modern paleo-
magnetic studies that house measurement and demag-
netization equipment in magnetically shielded rooms
with fi elds of 500 nT or less, minimizes the contribu-
tion of VRMs to the fi nal paleomagnetic result.
A reversals test, in which the anti-podality of
reversed and normal polarity directions is checked
statistically (e.g. McFadden & McElhinny 1990), can
provide a hint of the importance of a VRM in a paleo-
magnetic result. In Tertiary rocks with normal polarity
directions similar to the present-day fi eld, the reversed-
polarity direction is sometimes not as steep as the
normal polarity direction; this suggests unremoved
normal polarity overprinting, typically attributed to
a VRM.
If a rock has been heated in its post-depositional
history, VRM would have been acquired at a higher
rate and the rock would have acquired a thermovis-
cous remanent magnetization (TVRM) overprint. Pul-
laiah et al . (1975) have established a theory for SD
grains that indicates what thermal demagnetization
temperatures are needed to remove a TVRM for the 60
minute heating usually applied in a laboratory thermal
demagnetization. However, Kent (1985) conducted a
beautiful test observing the 10,000 year old VRM in
conglomerate clasts deposited in glacial sediments. The
Fig. 9.4 Flow chart showing the different processes by which sediments are magnetized and then affected
post - depositionally.
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