Jökull - 01.12.1984, Page 70

V.R.M. is mostly in the direction of the present
geomagnetic field, but it has not yet been at-
tempted to separate these two by quantitative
analyses of intensity and direction changes during
demagnetization.
The induced magnetization of a sample in the
earth’s magnetic field F is the vector k-F, where k
is the susceptibility and F is ~ 0.5 Oe. The
Königsberger ratio Q is the intensity of reman-
ence (N.R.M. or T.R.M.) over the magnitude of
k-F. If Qt < 1, it may be difficult to measure the
original polarity using a fluxgate meter in the
earth’s field, but the fluxgate probe should be
directed as nearly as possible at right angles to
the actual field to minimize this interference.
Within a lava flow having a very variable oxida-
tion state, a negative correlation between k-F and
T.R.M. has been observed (Wilson et al. 1968).
However, if the oxidation state within the lava is
high or constant, or between flows, the magni-
tude of both magnetization vectors may be
expected to be proportional to the amount of
magnetite in a sample (Deutsch et al. 1971).
Hence, a positive correlation between k-F and
T.R.M. may be expected from that cause. In
actual samples collected from a large number of
lava flows, the positive and negative correlations
seem to cancel, yielding the graph of Fig. 1.
Note that the proportion of samples having Q
< Vi, where confusing results are liable to be
obtained during field measurements, is at least
10—15%. To circumvent this problem it may
actually be helpful to use an insensitive magneto-
meter or compass. That way, more effort and
patience has to be spent on locating sufficiently
strongly magnetized samples, but these may be
expected to give much more reliable directional
results than the weakly magnetized ones.
To give an idea of the strength of remanence in
a vertically magnetized hand sample, it may be
noted that a sample of 10 cm height and 30 cm2
horizontal cross-section having J = 0.5 A/m will
give a field of 50 nT at 8 cm distance from its
center. Smaller deflections should be regarded as
possibly suspect and influenced by V.R.M. or
induced magnetization.
DISTRIBUTION OF DIRECTIONS
AND INTENSITIES
It is often advantageous to envisage the geo-
magnetic field as being a purely central dipole
field whose dipole moment direction „wobbles“
with respect to the geographic pole (i.e. spin axis)
of the earth. In this way, one may plot positions
of virtual poles (V.G.P.’s) for comparison of
directional data between different areas. A norm-
al-polarity field is generally taken to be a field
direction to which corresponds a south magnetic
pole north of the geographic equator.
The general relation between pole latitude and
field inclination in Iceland is illustrated in Fig. 2.
3.0
*10'3
2.0-■
1.0
vol.
susc.
c.g.s.
Profiles HA-HH
and GD—GH (1980-82)
■ Qt=1/io
45
‘Qt=1/3
,. k
'b
•Q,=1
'Qt=3
80
95 88 75
Remanence (J100), A/m (=10 G)
0.1
—h
0.3
4-
1.0
-)--------h-
3.0
-l-/
•Qt=io
54
10.0
Fig. 1. Relation between mean
magnetic susceptibility and
T.R.M. intensity (Jtoo) in lava
samples from Snaefellsnes and
Myrar, W-Iceland. Bars just
above abscissae show range of
J-values in samples (one per
flow) whose susceptibility values
were averaged for each data
point. Standard-error bars inclu-
ded- Dotted curves indicate
Q-ratios for the primary T.R.M.
in 0.5 Oe field.
1. mynd. Vensl segulhrifstuðuls
og styrks upprunalegrar hitaseg-
ulmögnunar í 437 bergsýnum úr
jafnmörgum hraunum af Snae-
fellsnesi og Mýrum (óbirt gögn).
68 JÖKULL 34. ÁR