Jökull - 01.12.1984, Blaðsíða 71

Percentages of total number.
< 5% x 15'A x 42 % x37'/2%>
53 '/i%
X
35 '/2%
X
7%
X
4%
Fig. 2. Possible values of V. G. P. latitude (shad-
ed) corresponding to any observed positive
paleomagnetic inclination in Iceland (65° lat.)
Note that high latitudes and inclinations are
much more common than low ones; the mode of
the frequency distribution in inclinations (using
data base of Kristjánsson and McDougall, 1982)
is close to 72°.
2. mynd. Möguleg gildi á breiddargráðu sýndar-
segulpóls, sem svara til tiltekins gildis á halla seg-
ulstefnu frá láréttu á íslandi (skyggt).
Although in most cases relatively steep inclin-
ations of remanence may be expected in Iceland,
inclinations below 30° may occur in about 4% of
all lavas, and occasionally the polarity of the
inclination (if < 43°) may even be opposite to
that of the respective pole Iatitude. In field map-
ping, a slight improvement in the consistency of
results may be expected to occur if the orienta-
tion arrow on this sample in situ is not vertically
upwards but rather in the direction opposite to
the average geomagnetic field (which has D = 0°,
I = 77° down). Secondary tilting of the lava pile
should also be allowed for if possible.
It has not yet been ascertained, by measure-
ments in Iceland or elsewhere, to what extent the
picture of the ancient geomagnetic field as a
purely wobbling dipole field is a realistic one,
particularly during excursions of the V.G.P. to
low latitudes or during reversals. Various models
of the field involving different higher-order con-
figurations have been put forward in the recent
literature, but none has been proved to be either
correct or advantageous to use. Differences in
e.g. calculated mean pole positions resulting
from the use of different field models are only of
the order of a few degrees.
As the V.G.P. moves away from the spin axis
during excursions and reversals, the intensity of
the field observed in Iceland will in general be
diminished. This may be seen by averaging re-
sults of T.R.M. intensity measurements from
large numbers of lava flows. One way of plotting
this decrease is that of Kristjánsson and
McDougall (1982, Figs. 5,6), where each lava
intensity is transformed to its respective V.G.P.
by multiplying it by a factor 2/(1+3 sin2X)1/2. (X =
paleolatitude).
Without such a transformation, the observed
mean remanence intensities may be plotted dire-
ctly as a function of three angular measures of the
geomagnetic field, as shown in Fig. 3. Data are
averaged over 10° intervals.
It is seen that the right-hand curve yields the
most regular dependence of local field intensity
on angle. This gives circumstantial support for
the hypothesis that dipole fields dominate over
higher order fields during major excursions of the
field.
The rapid drop in relative field intensity with
angle, as seen in Fig. 3, is another reason why it is
often difficult to ascertain the primary polarity of
transitionally magnetized lavas by fluxgate mea-
surements in the field.
AVERAGE REMANENCE INTENSITY
A question important to the interpretation of
magnetic anomalies observed over the lava pile is
that of the mean intensity of remanence in lavas
of either polarity.
Here, one first has to enquire whether this
intensity depends to some extent on variables
such as geographical location or secondary altera-
tion.
As to location effects, these seem to be minor.
In various regions of Iceland we find the follow-
ing arithmetic mean intensities after 100 Oe
demagnetization (averaged as scalars, excluding
sites of unreliable remanence directions and light-
ning strikes):
JÖKULL 34. ÁR 69