Náttúrufræðingurinn - 1997, Blaðsíða 21

Rennsli í Skaftárhlaupum og aur- og efna-
styrkur í hlaupum 1994, 1995 og 1996.
Skýrsla Orkustofnunar, OS-96066/VOD-7.
79 bls.
Wolfe, C. J., I. Th. Bjarnason, J. C. VanDecar &
S. C. Solomon 1997. Seismic structure of the
Iceland mantle plume. Nature 385. 245-247.
■ SUMMARY
An intemational geophysical project named
ICEMELT (the melt that produces Iceland), is us-
ing means of seismology to illuminate the mantle
under Iceland down to approximately 650 km
depth to construct a model of the mantle plume
under Iceland. The plume and its interaction with
the Atlantic Ridge is the main source of the exces-
sive volcanism in this region, and hence the source
of the existence of Iceland.
The plume is imaged by analysing the effect it
has on seismic waves, that are slowed down when
they traverse thc pait of the mantle that is affected
by the heat of the plume. For this task, data were
collected continuously 1993-1996, with broad-
band seismometers, that are sensitive to earth-
quakes anywhere in the world. These measure-
ments have confirmed a remarkably narrow plume
with a diameter between 150-300 km at 100-400
km depth undir Iceland. The plume probably
reaches down to at least 650 km depth, and its
geographic center is under central Iceland and the
western part of Vatnajökull, which is also the Ioca-
tion of the most powerful volcanic systems in Ice-
land. Analysis of surface waves has revealed a large
reduction (approx. 10%) in S-wave velocity at ap-
proximately 50 km depth in the mantle under cen-
tral Iceland and along the volcanic zone north of
Vatnajökull. This velocity reversal probably marks
the upper extent of a partially molten astheno-
sphere, the melting region or oven where the mate-
rials that make lceland are smelted.
Seismic activity associated with volcanoes under
the Vatnajökull glacier was well recorded by
seismometers located on the glacier and other
places (1. and 3. ftgures). Three events of seismic
tremor were recorded originating under the glacier.
In July 1995 and August 1996 tremor was recorded
in association with jökulhlaups from the Skaftá
cauldrons, and in September-October 1996 erup-
tion tremor was recorded from the Gjálp eruption.
Comparison of these three tremor events indicate
that the tremor frorn the Skaftár cauldrons could be
associated with small volcanic eruptions, lasting
approximately two days in July 1995 but only half
a day in August 1996.
Ingi Þ. Bjamason and Bergþóra S. Þorbjamar-
dóttir (1996) have interpreted the sequence of me-
dium size (size approx. 5.0) earthquakes in the
Bárðarbunga volcano as a manifestation of in-
creased pressure in the volcano for at least 22
years, that resulted in the Gjálp eruption in 1996.
The increased pressure is due to flow of magma
front the mantle into the region under the volcano.
It is difficult to predict the continuation of these
events, but an additional medium size earthquake in
Bárðarbunga should be a likely signal for further
eruptions in this sequence.
PÓSTFANG HÖFUNDAR/AuTHORS ADDRESS
Ingi Þ. Bjarnason
Raunvísindastofnun Háskólans
Science Institute
University of Iceland
Hofsvallagata 53 / IS-107 Reykjavík
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