Jökull - 01.12.1984, Blaðsíða 75

Inhomogeneous
magnetization
Measurement of sample ing, however, may prevent one from obtaining
in different positions, samples of optimum size and shape from a lava.
averaeed
ANISOTROPY EFFECTS
Interference from
induced magnetization
in earth’s field
Viscous or isothermal
remanence present
Weak magnetization
Large uncertainty in
direction and pole
latitude, especially if
inclination is low
Spinning of sample, or
field nulling by
Helmholtz coils
a.f. demagnetization
until primary direction
is isolated
Variable amplification
of signal
Accuracy of each
measurement a few
degrees. Pole pos.
calculated from 3-5
samples/flow.
To give some crude numbers on the relative
certainty of correct polarity determination
(excluding errors from geological sources, such as
remagnetization), one may estimate that a care-
ful fluxgate determination of lava polarity is
likely to be correct about 80% of the time, but
laboratory measurements will yield an unambi-
guous polarity up to 95% of the time. For this, it
may be necessary to omit occasional samples,
e.g. those suspected of being for some reason
incorrectly oriented.
The reasons for ambiguous results in 5% of
Icelandic lava flows have not yet been fully
investigated, but this will be dealt with below. It
is possible that in some cases secondary remanent
magnetizations have higher coercivities (stability)
than the primary remanence. Resampling flows
at a different site and/or at a different height in
the flow has been found to resolve directional
ambiguities in perhaps every other case, but
sometimes the resampling only brings added con-
fusion.
Very fine-grained lavas, thin lavas, and colum-
nar basalts may be more prone than others to
instability and to acquiring secondary magnetiza-
tion, especially if altered. Felspar-porphyritic
lavas generally seem rather stable. In this context
it should also be mentioned that weathering of
lavas affects only the glassy phase in the rocks but
is not harmful to the opaque minerals. Weather-
It has been noted (e.g. Kristjansson and
McDougall 1982) that paleomagnetic directions
from Icelandic lavas, while apparently very reli-
able and internally consistent, are more scattered
than would be expected by comparison with data
from other formations of similar age.
It has therefore been suggested that possibly
grain orientation effects during flow and cooling
of these lavas may cause magnetic anisotropy,
tending to align remanence directions towards
the horizontal plane.
Anisotropy effects in some basalt lava flows
and dikes have been investigated by Ellwood
(1978), using a low-field torsion balance. He
finds that there is very little evidence for systema-
tic anisotropy effects in Icelandic lavas, and sug-
gests that various randomizing effects during the
flow dominate over horizontal flow- or stress-
induced anisotropy.
The present author has introduced a strong
anhysteretic magnetization into some 15 cylindri-
cal basalt samples. The A.R.M. was applied at
right angles to the drilling direction of these,
which was on average at 33° below the horizontal.
In this experiment the anisotropic component
appeared to be generally at or below 3%, which
was also the level of systematic errors in the
instrument used.
Anisotropy does therefore not seem to be a
serious factor in determining the remanence
direction of basalt lavas. However, its effect may
possibly be larger in acid and andesitic lavas
where flow structures are visually evident. This
has not yet been investigated in Iceland.
At the moment the main reason for the large
between-lava scatter of paleomagnetic directions
in Iceland appears to lie in properties of the
geomagnetic field, caused by conditions in the
Earth’s core.
DIFFERENCES BETWEEN MAGNE-
TICALLY STABLE AND UNSTABLE
SAMPLES
Among the magnetic differences observed
between magnetically stable samples from Ice-
land on one hand and unstable samples on the
other, is the fact that median destructive fields
for the original T.R.M. of stable samples are
JÖKULL 34. ÁR 73