Jökull - 01.06.2000, Page 57

Geirfinnur Jónsson and Leó Kristjánsson
OTHER ANOMALIES WITHIN THE
MÝRDALSJÖKULL AREA
The positive magnetic field around the caldera can,
at least partly, be explained as overshoot from the
deep magnetic low in the caldera, and it does indica-
te that the nunataks are generally not made of
strongly magnetized material. There is one excepti-
on, however. Northeast of the Katla anomaly there is
a strong positive anomaly (2000 nT by 5 km or so)
which is presumably connected to a linear topograp-
hic depression, a gorge 150-200 m deep and 1.5 km
wide (Björnsson et al., 1994) striking NE towards the
Eldgjá volcanic fissure which was active around AD
930. An anomaly on flight line 10 (Figure 2) is proba-
bly related to the same structure.
CONCLUSIONS
A pronounced negative magnetic anomaly of amplitu-
de about -2000 nT and 12 x 8 km in size is found over
central Mýrdalsjökull. Its location corresponds well to
the position and elongated shape of the active volcanic
caldera of Katla (Björnsson et al., 2000) and with the
position of a magma chamber inferred from modelling
of seismic refraction results by Guðmundsson et al.
(1994). However, the source of the negative magnetic
anomaly is smaller in size than the caldera.
Magnetic anomalies of similar dimensions are
found over several volcanic centers of Quaternary and
older ages in and around Iceland. These are probably
caused by remanence and/or induced magnetization in
caldera-filling material and in intrusions which tend
to occur at caldera rims. However, much additional
geological and geophysical mapping is needed to esta-
blish the source of these anomalies in individual cases.
The present negative anomaly is most likely due to the
relative absence of magnetization in a 7 x 10 km NW-
trending region, within normally polarized basaltic
material. The negative magnetic residuals within the
caldera could be related to a thermal anomaly raising
the temperature of much of the crust to above its Curie
point. This is supported by Guðmundsson et al. (1994)
who conclude that there is a shallow magma chamber
in the crust under the caldera, about 5 km across.
Another possible explanation involves the partial
filling of a subsided region by material of low
remanence (such as palagonite tuffs), but this would
be contrary to the strong positive gravity values
seen in the area (Guðmundsson, 1994a,b). A third
possibility is the presence of advanced geothermal
alteration, as noted for some negative anomalies of
smaller extent, e.g. in Námaskarð (see Kristjánsson
and Watkins, 1977). The overall positive magnetic
fields observed around the caldera rim also appear in
our computed fields because of the vertical boundaries
in the caldera model, but occasional local field peaks
could be caused by strongly magnetized intrusions.
ACKNOWLEDGEMENTS
We thank Úlfar Henningsson of Garðaflug for
his invaluable contribution in carrying out the
1990 aeromagnetic survey, and Finnur Pálsson for
technical assistance and advice. Helgi Björnsson
kindly allowed us to use unpublished data on base-
ment topography under Mýrdalsjökull.
ÁGRIP
SEGULKORT AF MÝRDALSJÖKLI
Í segulsviðinu í rúmlega kílómetra hæð yfir Íslandi er
víða að sjá staðbundin frávik sem tengja má jarðfræði-
legum fyrirbærum, megineldstöðvum, stöpum eða
móbergskollum. Vegna þess hve gisin flugmæligögn
eru hérlendis koma þessi frávik oft ekki fram nema á
einni mælilínu og er erfitt að túlka þau. Tvö slík frá-
vik skera sig þó úr, þau eru svipuð á að líta og tengjast
bæði afar virkum eldstöðvum, Öskju í Dyngjufjöllum
og Kötlu í Mýrdalsjökli. Í þessari grein er fjallað um
segulfrávikið sem tengist Kötlu.
Katla er megineldstöð syðst í eystra gosbeltinu
sem liggur gegnum Ísland, sjálf eldstöðin er undir
jökli, en jökulbotninn er vel þekktur (sjá grein Helga
Björnssonar og fl. í þessu hefti). Það sem einkenn-
ir hann öðru fremur er jarðsig eða askja sem kennd
er við Kötlu og hefur rimi hennar verið teiknaður
inn á myndir 1, 3, 4 og 5. Eystra gosbeltið myndar
um 20 km breitt jákvætt segulfrávik, tengt núverandi
segulskeiði (Brunhes - síðan fyrir 780 þúsund árum)
sem endar í suðri við Eyjafjallajökul - Mýrdalsjökul.
56 JÖKULL No. 49