Jökull - 01.07.2003, Blaðsíða 24

Leó Kristjánsson
1
2
3
4
5
6 7
8
9 10
11
12
13
14
15
16 17
18
19
20
21
22 23
24
25
26
27
28
29
30
31
32
22° 21°30
64°10
0 10km
Figure 1. Paleomagnetic sampling localities in the “Reykjavík gray lavas”, see Table 1. The extent of these lava
sequences is shown by Jóhannesson and Sæmundsson (1998). – Staðir þar sem sýnum úr Reykjavíkurgrágrýtinu
var safnað.
tions (and length of time interval covered) required for
a trustworthy average direction. The common pres-
ence of geomagnetic excursions will clearly affect the
answer to this question.
Attempts to locate reversed directions in the Late
Quaternary volcanic sequences of Iceland have been
ongoing since the mid-1970’s. The first suggestions
regarding such occurrences were made by Einarsson
(1976) on the basis of measurements in the field at
two localities within the Hengill active central vol-
cano (not far east of Figure 1). His suggestions were
followed up at one of these sites by Peirce and Clark
(1978). The latter authors interpreted their findings
as evidence for two reverse units and one transitional
unit. However, the directions in their outcrops are
scattered, and intensities reach very high values. Sub-
sequent work (L.K., unpublished data, 1982) has in-
dicated that samples collected close together have dif-
ferent characteristic directions after AF demagnetiza-
tion. The apparent reversal could therefore be due
to a lightning strike causing localized heating. The
other outcrop described by Einarsson appeared to the
present author to be a large pillow- like object which
may have rotated after cooling.
The present study describes work on paleomag-
netic directions in late Quaternary lavas in two lava
sequences in South-Western Iceland. One of these is
the interglacial lavas around Reykjavík, and the other
is a series of lavas with transitional magnetic direc-
tions found in the south-central part of the Reykjanes
peninsula (Levi et al., 1990). For the location of var-
ious minor landscape features mentioned in this pa-
per, the reader is referred to the 1:50,000 and 1:25,000
scale maps of the Iceland Geodetic Survey.
22 JÖKULL No. 52, 2003