Jökull - 01.07.2003, Page 32

Leó Kristjánsson
cause the lavas on Reykjanes overlie poorly consoli-
dated tuffs which are being eroded at a rapid rate. For
these reasons, one should also not consider the mean
directions from sample groups A and B+C as being
significantly different.
The excursion samples collected since 1990 carry
similar low remanence intensities as those sampled
previously on the Reykjanes peninsula: most have
values of around 0.3 A/m or less after 10 mT demag-
netization.
New results - Fagradalsfjall and other sites
Generally, only “ordinary” normal-polarity directions
have been seen in the following hills in the area
of Figure 4 and its vicinity: Slaga, Fiskidalsfjall,
Húsafell, Lyngfell, Borgarfjall, Festarfjall, Sandfell,
Stórihrútur, Kistufell (so called in 1:25,000 map),
Keilir and Driffell. Some sampling has also been done
farther east in Vesturháls, Sveifluháls (near the road),
and the top of Langahlíð (east of Kleifarvatn lake).
A few samples were collected farther west in Háley-
jabunga and Sýrfell: all were of normal polarity.
Among results which may be of interest with re-
spect to continued research into the geological history
of the area, are the following:
1. 61 normally magnetized samples have been col-
lected from approx. 46 lava sites in Fagradalsfjall
outcrops. Of these samples, 17 collected at 13 sites
in the unnamed hill at the SE corner of Fagradalsfjall
(278 m in 1:25,000 map) all have somewhat “right-
handed” declinations (average direction about D =
43Æ, I = +68Æ, a.s.d. = 7Æ). Two hills to the NNE from
there (291 m and 264 m) appear to contain few if any
crystalline outcrops. Similar right-handed directions
are e.g. found consistently in Festarfjall, Lyngfell and
Keilir, but this is most likely to be due to coincidence
rather than these sites all being contemporaneous.
Other sampled lava sites in Fagradalsfjall are in
its southern and southwestern parts, as well as sites
reaching from the unnamed gully on its northern
slopes, through the summit Langhóll to the north-
ern part of Geldingadalur. All of these sites exhibit
southerly declinations. The average direction (N =
33 sites with one or two samples each) is at D =
167Æ, I = + 64Æ, a.s.d. = 5.5Æ, 95 = 1.7Æ. Due to
its southerly declination, this direction corresponds
to a virtual geomagnetic pole at 21ÆN, 347ÆE (Fig-
ure 2b). Remanence intensities after 10 mT AF treat-
ment average about 2 A/m. Many of the outcrops are
feldsparphyric. The NW-part of Fagradalsfjall has not
been visited but it appears to contain few if any lava
outcrops. VGPs having as low a latitude as 21Æ are
quite rare in Iceland, and this direction has only been
seen in a few isolated samples at other localities in the
area, namely NW of Sandfell; Stórihrútur; lava just
west of the road southeast of Fiskidalsfjall; and Borg-
arfjall.
2. Samples from 11 sites in Borgarfjall have strongly
right-handed average directions (D>60Æ). Their aver-
age is D = 85Æ, I = +76.5Æ, a.s.d. = 4.5Æ. A few less
right-handed directions also have been noted in Borg-
arfjall, as well as two samples with southerly declina-
tions (cf. above), and two with westerly declinations
on its eastern slope.
3. Several normally magnetized units were sampled
on the upper slopes and top plateau of the Svartseng-
isfell hill (around 63.88ÆN, 337.59ÆE), as well as east
of the road at the Selháls col south of Svartsengisfell.
All the 10 samples from these have westerly declina-
tions, averaging about D = 329Æ, I = +76Æ, a.s.d. =
6Æ. Such declinations are only seen sporadically else-
where in the region covered here.
CONCLUSIONS AND DISCUSSION
Previous results (Kristjánsson et al., 1980) regarding
the relatively low between-flow scatter of paleomag-
netic directions in the Reykjavík gray basalts have
been generally confirmed. It should be kept in mind
that this scatter is in part due to local magnetic anoma-
lies, measurement errors, etc. In the last two centuries
the declination of the geomagnetic field in Iceland has
changed by about 24Æ (see Kristjánsson, 1993), corre-
sponding to 5–6Æ of arc. The current rate of change
of the geomagnetic inclination is relatively small but
changes of 4Æ or more per century are known, e.g.
in Britain 1800–1900 A.D. (Tarling, 1989). As the
angles between remanence directions in successive
“flow units” at many of the sites listed in Tables 1a,b
are only a few to several degrees, it seems likely that
the time between units at most of these sites is of the
30 JÖKULL No. 52, 2003