Jökull - 01.12.1984, Blaðsíða 72

ANG. DISTANCE FROM
CENTRAL AXIAL FIELD
Fig. 3. Average intensity of remanence after 0.01 T (100 Oe) demagnetization, plotted as a function of
three angular measures of the field. Same data base as in Kristjansson and McDougall (1982), 2163
lavas with a95 <23.5°. Upper curves: Arithmetic averages, with standard-error bars. Lower curves:
geometric averages. Transformation to pole not applied.
3. mynd. Meðal-segulmögnunarstyrkur hraunlaga á íslandi (Miósen-neðra Pleistósen), teiknaður sem
fall af a) halla sviðsins b) fráviki sviðsins frá meðalsegulsviði jarðar og c) fráviki segulpóls frá snún-
ingspól.
N of lavas J100
Western NW-Iceland 348 3.4
Eastern NW-Iceland 469 3.1
Central N-Iceland 398 3.0
Borgarfjordur 342 3.1
Esja area 306 3.6
Snaefellsnes / Myrar 397 2.8
Of these, Esja is the youngest, but Western
NW-Iceland possibly provides the least altered
samples. As first shown by Wood and Gibson
(1976), remanence intensity of lavas in E-Iceland
decreases with increasing depth of secondary
burial. Other collections in Iceland have been
taken from higher up in the zeolite zones, and in
fact the mapping of these zones is not yet com-
plete in those areas. At this stage we shall hence
only investigate the effects of altitude above sea
level on primary remanence intensity. It is seen
(Fig. 4) that altitude effects are generally slight,
so a uniform magnetization may be assumed in
modelling the magnetic effects of the exposed
lava pile in many areas of Iceland. However, in
some areas, particularly in the vicinity of volcanic
centers, a marked decrease in intensity may be
seen with decreasing altitude, accompanied by an
increase in V.R.M. Therefore, field mapping
should avoid outcrops at less than 200 m altitude
when possible.
If our data from exposed lava core samples are
taken at face value, the most correct intensity
value to use in anomaly interpretation would be
the average projection of observed remanence
vectors on the present or the mean geomagnetic
field direction. The average angle of projection
(from Fig. 3b) is 19°. Using the data of Fig. 3b
and including also results from unreliably magne-
tized lavas, we obtain a mean value for the pro-
jection of J10o amounting to about 3.1 A/m. How-
ever, we must add to this value the estimated
amount of original remanence removed by 100
Oe treatment, which brings the mean up to 3.8 A/
m. Finally, by allowing for the presence of
relatively non-magnetic sediments and clastics in
the pile, we may round this value off to 3.5 A/m.
If landscape effects are to be modelled, the aver-
age induced magnetization in the lava pile (1 A/
m) must also be taken into account.
However, we must realize that the lava sam-
ples collected for paleomagnetic purposes may
not be representative of the lavas in general.
These samples are, for the most part, collected at
a height of 0.5-2.5 m above the flow bottoms,
and scoriaceous zones are avoided altogether.
Given the large and systematic variations of
70 JÖKULL 34. ÁR