Fróðskaparrit - 01.01.1971, Blaðsíða 16

24
Magnetic Anomalies Around The Faeroe Islands
Interpretation of the Anomalies
Because the electrical supply onboard “Ole Rømer” was
rather sparse it was only possible to get echograms along
selected parts of the sailing routes. As the magnetic field are
influenced by the topography of the sea-floor (an extreme
example is shown in Fig. 2) I shall in the following only
discuss the influence of geology on the magnetic recordings
obtained in the western part of the area where the sea-floor
is rather smooth and the topographic “noise” therefore less
dominant.
The main features of the magnetic anomaly pattern can be
recognized by an examination of Pdate 1. It is seen that the
anomalies obtained over the middle series generally are smooth
with low amplitudes and frequencies, this might be expected
as we know from the geology (Rasmussen and Noe-Nygaara
1969) that the middle series are very homogeneous, consisting
of thin flow units, welded together to form compact masses.
As a contrast the anomalies obtained over the lower series
show much higher amplitudes and frequencies. This difference
in magnetic pattern between the two lava series might be ex-
pected as the lower series, consisting of gently dipping thick
flows (10—30 metres) intercalated with tuff-clay sedimente,
are alternately normally and reversely magnetized (Abraham-
sen 1965 and 1967), while the middle and upper series all are
reversely polarized. The dominant positive anomaly found at
the boundary (the geological A-horizon) between the lower and
middle series should then be the effect of the upper normal
polarized part of the lower series cropping out below the
reversely polarized middle series.
The magnetic anomalies found in areas where older rocks
might be present e.g. in the area north of Mykines show that
a magnetic basement must be present just below the sea-floor.
The pattern of the anomalies here look quite like profiles
measured along the gneissic west coast of Sweden (Schrøder
1968); however, inhomogeneously magnetized basalts, inter-
sected by dykes, might give a similar pattern.