Jökull - 01.12.1989, Blaðsíða 50

Fig. 8. The magnetic map.
Contour lines are 500
nanotesla apart (1 nanotesla
= 1 gamma). Shaded areas
indicate negative values.
Mynd 8. Flugsegulkortið.
Milli jafngildislínanna eru
500 nanótesla. Skyggð
svœði sýna lágan segul-
sviðsstyrk.
form what might be referred to as the Brunhes mag-
netic lineation (Jónsson and others, in press).
A host of smaller anomalies is also present in the
map, some due to geological phenomena and some
due to topography. The fact that they are elongated
NE-SW is for some of them an artifact of the direc-
tional filtering. To properly study those anomalies, a
magnetic map should be gridded on a smaller scale
and the appropriate anomaly separation techniques
applied to each case individually. The filtered map
should be studied with this in mind.
Several anomalies are présent on the Reykjanes
peninsula west of the Hengill area. They reflect the
en echelon distribution of volcanically active fissure
swarms on the Reykjanes peninsula (Jakobsson and
others, 1978; Jónsson and others, in press).
South of Hvalfjörður, an anomaly occurs over the
eroded Stardalur volcanic center (Friðleifsson and
Kristjánsson, 1972; Kristjánsson, 1987), accom-
panied by a localized high in the gravity map (Fig.
5). West of that, an anomaly is seen at the mouth of
Hvalfjörður, presumably a signal from the so-called
Kollafjörður central volcano6. Further north there
are anomalies associated with the Hafnarfjall and
Ferstikla central volcanoes and magnetic lineations
caused by the tilted Pliocene lava pile.
In the northeast quarter of the map are several
notable anomalies. The first occurs over the shield
volcano Ok, the second is over the southwest comer
of Langjökull, the third is over the center of the
6. Both these magnetic anomalies, at the Stardalur and Kolla-
fjörður central volcanoes, actually seem elongated NW-SE on
an unfiltered map. It might also be noted that both anomalies
were observed to correspond to rather large localized highs
(=10 milligals) on the gravity map of Einarsson (1954).
48 JÖKULL, No. 39, 1989