Jökull - 01.12.1984, Page 96

a 1 a b c d C b a 4
a b a 2 a b c d e d c b a 5
a b c b a 3 a b c d e f i r~ ! i 1 1 e d c b a 6
Fig. 19. Formation of a dyke
which is composed of 12 col-
umnar rows. (1) Intrusion of
the first magma phase (a).
This magma phase cools
enough to form columns
before the next magma phase
(b) splits (a) in two parts
(2) . The process continues
(3) -(5) until the final magma
phase (f) is intruded (6). The
columns of (f) are slightly
bent, thereby indicating the
direction of movement of the
magma. The section can be
either vertical or lateral.
Mynd 19. Myndun gangs sem
samanstendur af 12 stuðlaröð-
um. Stuðlaröð (a) er elst,
stuðlaröð (f) yngst. í hvert
skipti sem kvika treðst inn
klýfur hún nœstu stuðlaröð á
undan í tvennt. Yngsta stuðla-
röðin (f) er sveigð í miðju í
flæðistefnu kvikunnar. Sniðið
getur verið hvort sem er lárétt
eða lóðrétt.
is essentially the same as the tensile strength of
the country rock. Thus there must be some time
limit between successive magma phases of a
single dyke.
Using the conduction cooling models of Jaeger
(1968), it can be shown (Gudmundsson 1984)
that the cooling of a two meter thick magma
phase from the initial temperature of 1200°C
down to 800°C (i.e. when columnar joints start to
form (Jaeger 1961)) takes about 270 days. Thus
the time between successive magma phases, for a
typical dyke, should be of the order of a few
hundred days. This time is comparable with the
time between successive phases of intrusion in
the Krafla volcano in northern Iceland
(Björnsson et al. 1979).
DISCUSSION
The present results of dyke studies in north-
western Iceland are generally similar to those of
eastern Iceland (Gudmundsson 1983a).
Nevertheless, there are some dissimilarities. For
instance, although the average dyke thickness for
northwestern and eastern Iceland is essentially
the same (4.3 m and 4.1 m respectively), the
thickness distribution is different. Thus thin
dykes are relatively more common in northwest-
ern Iceland compared with eastern Iceland.
About 17% of the dykes in northwestern Iceland
are less than 1 m in thickness, but only about
10% of the dykes in eastern Iceland. Also, dyke
strike and dip is more variable in northwestern
Iceland. These differences are associated with the
trend of individual dyke swarms in northwestern
Iceland, which varies, whereas the dyke swarms
of eastern Iceland all have a similar trend. The
dyke swarms of northwestern Iceland are not as
well defined as those of eastern Iceland, a fact
that is at least partly explained by greater degree
of erosion (referring to the original surface of the
lava pile) in eastern Iceland.
94 JÖKULL 34. ÁR