Náttúrufræðingurinn - 1985, Page 54

a few meters more. The rhyolite is here similar
to that in the other exposures, but no basaltic
xenoliths were found. The basaltic dykes are
also missing, although a thin basaltic vein dis-
sects the rhyolite in one place.
The origin of the composite dyke may be
envisaged as follows:
1) Near the top of a magma chamber there is
a rhyolite magma. Although this magma may
from time to time form dykes, they will cool
down rapidly, thereby increasing the viscosity
and thus making it impossible for the magma to
flow a long distance away from the chamber.
2) Into the top of the magma chamber comes
laterally flowing basaltic magma and forms a
dyke. The rhyolite magma then uses this basal-
tic dyke as a channel and is thus able to reach
higher level in the crust than as a purely acid
dyke. Two things make it economic for the
rhyolite magma to flow along the middle part of
the basaltic dyke. First, the tensile strength
across the still molten middle part of the dyke is
essentially zero, so that the rhyolite magma does
not have to overcome any tensile strength. Sec-
ond.the hot basaltic dyke insulates the rhyolite
magma from the cold country rock, decreases
its viscosity, and thus that makes it possible for the
ryolite magma to flow to much greater dist-
ances away from the magma chamber than
would otherwise be possible.
3) Most of the basaltic xenoliths that the
rhyolite contains are derived from the middle
part of the basaltic dyke, near the magma cham-
ber. Next to the chamber the middle part of the
basaltic dyke was molten at the time of intrusion
of the rhyolite magma, and so the roundish
edges of most of the xenoliths is explained. Next
to the chamber the minimum horizontal com-
pressive stress (perpendicular to the dyke) is
higher than at shallower crustal levels, and the
same applies to Young’s modulus. Thus, next to
the magma chamber some stoping is energe-
tically economic, but higher in the crust pushing
the dyke rock apart is more economic for the
rhyolite magma. The angular xenoliths have, how-
ever, been taken up by the magma in the upper
parts of the channel.
4) The rhyolite magma flowed up through the
basaltic dyke and continued flowing for some
distance ahead of the upper end of the basaltic
dyke. However, the viscosity of the rhyolite
magma increased very rapidly after the magma
left the basaltic dyke, and thus the rhyolite dyke
expanded and soon stopped flowing. The fact
that the rhyolite contains few vesicles at the top
indicates that it did not reach the surface.
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