Jökull - 01.12.1989, Side 44

Fig. 1. The location of the research area in
southwestern Iceland and the localities mentioned in
the article. The size of the research area is
126x126 km2.
Mynd 1. Staðsetning rannsóknarsvœðisins á Suð-
vesturlandi og staðanöfn sem koma fyrir í greininni.
Stœrð svæðisins er 126x126 km2.
the early eighties. Figure 2 shows the location of the
gravity stations.
A Bouguer correction was applied to the data
assuming an average rock density of 2500 kg/m3. In
reality, the density varies greatly and any single
value for it can do no better than minimize some sta-
tistical parameter of the data. The value chosen
minimizes the roughness of the Bouguer anomaly
surface, as measured by its fractal dimension (Þórar-
insson and Magnusson, in press). The resulting map
is shown in Figure 3. Its main feature is a steep
inland gradient in the gravity; a part of the bowl-
like trend noted by Einarsson (1954) to characterize
the gravity map of Iceland.
To enhance geologically interesting anomalies
and bring out details in the map, the regional trend is
removed by finding a suitable field to subtract from
Mynd 2. Þyngdarmœlistöðvarnar.
the Bouguer gravity data. For this purpose, a succes-
sion of two-dimensional polynomials were fitted to
the data and analysis of variance (ANOVA) applied
to the residual anomaly to test whether a significant
reduction had occurred (e.g. Davis, 1973). Table I
sums up the ANOVA results and shows quite con-
clusively that a second order polynomial should be
used for the regional trend. Figure 4 shows the
second order trend which obviously is a part of the
bowl-like gravity low noted before (Einarsson,
1954). Figure 5 shows the residual Bouguer ano-
maly map after the trend in Figure 4 has been sub-
tracted from the Bouguer anomaly map in Figure 3.
Figure 5 will henceforth be referred to as the gravity
map.
INTERPRETATION OF THE GRAVITY MAP
The largest anomaly on the gravity map in Figure
5 is the gravity low that stretches from the Reykja-
nes peninsula in the southwest to the Langjökull gla-
cier in the northeast. This low corresponds to the
active westem volcanic zone.
42 JÖKULL, No. 39, 1989