Jökull - 01.12.1984, Page 87

Length
On Fig. 10 all lineaments in the southern part
of the area in Figs. 2 and 6 have been drawn from
air photographs. Field observations have shown
that most of these lineaments are dykes.
Nevertheless, a few of the lineaments have been
identified as normal faults, as indicated in Fíg.
10.
N
S
Fig. 7. Stereoplot of the poles to 388 dykes in the
research areas.
Mynd 7. Pólar 388 ganga á rannsóknarsvœðun-
um.
Fig. 8. Thickness distribution of 365 dykes in the
research areas. The average thickness is about
4.3 m.
Mynd 8. Pykktardreifing 365 ganga á rannsóknar-
svœðunum.
Table 2 gives the inferred length and the calcu-
lated length/width ratio of some well-defined
dykes in the research areas. When estimating the
length of a dyke, I followed the rule to count as a
single dyke any lineament that could be traced as
a single line, though in places it might be offset or
otherwise discontinuous. When tracing the dyke
across fjords, the criteria used were lithology,
thickness and direction. The lengths of the dykes,
as given in Table 2, are presumably minimum
lengths. In eastern Iceland where I traced a few
dykes along their length (Gudmundsson 1983a)
some were longer than estimated from air photo-
graphs. Also, because only a few thickness
measurements were made on each dyke in north-
western Iceland, whereas tens of measurements
were made on each of the traced dykes in eastern
Iceland, the length/width ratios given in Table 2
are less accurate than those of the dykes in east-
ern Iceland.
The dykes seen as lineaments on the air photo-
graphs are much fewer than the dykes observed
in the corresponding mountain sections. In gene-
ral, only dykes thicker than 3—4 m are seen on
the air photographs, whereas in well-exposed
sections along cliffs most or all the dykes are
seen.
Form
The dykes vary in shape; some are nearly
straight with parallel edges while others are
irregular and have nonparallel edges. Some
dykes are sinuous and vary little in thickness,
others have an apparent pinch-and-swell struc-
ture. Often a dyke thins when passing through a
thick lava flow. The observations show that the
dykes use subvertical joints in the lava flows as
channels (Gudmundsson 1984). Some thin dykes
when dissecting thick lava flows deviate consider-
ably from the vertical where they follow inclined
joints.
In general, the columns of the dykes are nearly
horizontal, that is, they are perpendicular to the
walls of the dyke-fracture. But in a few dykes the
columns in the middle part are bent upwards.
This is taken to indicate that the magma was
moving upwards in the still mobile middle part of
the dyke after the outer parts had become solidi-
fied and immobile. No lateral bending of columns
was observed.
Most of the dykes are multiple in the sense that
they are composed of many parallel rows of
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