Náttúrufræðingurinn - 1992, Page 34

SUMMARY
The Hekla eruption of 1991 - The
tephra fall
by
Gudrún Larsen
Science Institute, Dunhaga 3
IS-107 REYKJAVÍK
Iceland
Elsa G. Vilmundardóttir
National Energy Authority, Grensávegi 9
IS-108 REYKJAVÍK
Iceland
and
Bardi Thorkelsson
Icelandic Meteorological Office
Bústaðavegi 9
IS-150 REYKJAVÍK
Iceland
The fourth eruption of Hekla in this
century, and its seventeenth eruption in
historical times, began at about 1700 hours
on January 17th 1991. It was preceded by
the shortest period of quiescence known
so far, 9 years and 9 months, as measured
from the activity of April 1981. The first
phases were characterized by mixed activ-
ity that produced a small tephra layer and
severa! lava flows from fissures on the SW
part of the Hekla ridge and on the flanks
of the volcano (fig. 1). The opening phase
of the eruption was of the plinian type,
with an eruption plume rising to 11.5 km
height above sea level in less than ten
minutes. The eruption apparently began
near Axlargígur on the SW shoulder of
the volcano. About an hour later, fissures
became active on the S flank and on the
SW part of the ridge, and then on the E
flank where activity lasted longest in this
eruption.
The eruption cloud was observed on the
recently installed weather radar of the Ice-
landic Meteorological Office, located at
Midnesheidi, some 150 km from Hekla.
Cross sections through the eruption cloud
were taken at elevations of about 2 and up
to at least 12 km a.s.l. The direction of the
cloud at each time, its extent down to a
certain density level, and changes of den-
sity within the cloud were easily observed
during at least the first 24 hours. The sim-
plified drawings on fig. 2 show the erup-
tion cloud in cross section at 2 km a.s.l.
during the first 9 hours, as observed on
the computer screen.
The eruption cloud was carried towards
NNE, across the uninhabited highlands,
to the Mývatnsveit and Öxarfjördur dis-
tricts in NE Iceland. In the vicinity of
Hekla, tephra fall was most intense during
the first 2-3 hours. The tephra fall
reached the NE coast at Melrakkaslétta,
some 330 km away, in about 5 hours and
30 minutes. By midnight on January 17th
tephra fall had reached an area of ca.
21000 km2, forming a thin and narrow te-
phra sector (fig. 3). Minor tephra fall was
observed during the following days but af-
ter the first 12 hours it was very slight.
The H-1991 tephra was deposited on
snow, offering an opportunity for detailed
sampling (fig. 4) and for determination of
mass loading at various distances. In areas
proximal to the mountain, the sampling
was successful but collection of represen-
tative samples was difficult in distal areas,
where deposition of the tephra took place
in high wind. The mass loading along the
axis of thickness ranged from >20 kg/m2
at distances <4 km to 1-2 g/m2 at distances
of 280 km from source (fig. 6 a and b).
The mass loading values were converted
to thickness values (fig. 7 a and b) and
isopach maps were constructed for com-
parison with other tephra layers. The vol-
ume of the tephra deposited during the
first 12 hours is about 22 million m3. This
is about one-third of the tephra volume in
the last two eruptions (table 1). The
H-1991 tephra layer is the smallest Hekla
layer known to have been deposited in
historical times.
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