Náttúrufræðingurinn - 1995, Blaðsíða 85

Reverend Jón Steingrímsson also tells of ruins
of houses and fences in the area which now is
covered by the lava flow from the Laki eruption
cf 1783. Although this information is frag-
mentary, the author favors that the sources men-
tioned refer to the same event, the damage of Skál
and destruction of Tólfahringur.
Only a small part of the volcanic structure
itself is now visible, most of it being covered
by the lava flow from the Laki eruption. The
center of the eruption seems to be a relatively
short fissure, direction northeast-southwest.
Evidently the activity started with heavy fall
of juvenile clasts, pumice and ash and pene-
contemporaneously a pyroclastic flow or rock
and ash flow. The entire erupted material is
characterized by round forrn, particles 1 mm
or less in diameter up to boulders. The occur-
rence of accretionary lava balls is striking. The
uppermost 0.5-1 m of the pyroclastic flow is
loosely consolidated and breaks up in vertical co-
lumnar form indicating high temperature at the
moment of consolidation. Here and there on the
flat surface of the pyroclastic flow there are fun-
nel shaped holes, 8-12 m across at the top and 3-
4 m deep. They are considered to be degassing
pipes of the hot flow. At the bottom of each hole
there is as a rule one big accretionary boulder
formed of a 5-8 cm thick solid lava twisted
around fragmentary lava evidently from the same
eruption.
The destruction of Tólfahringur should, ac-
cording to Reverend Jón, have occurred about
the year 1112 A.D. Remains of vegetation col-
lected from underneath the erupted material
were subjected to l4C age determination. This
material, partly carbonized, was collected at
three different places in the same area and
equally many determinations were made. All
the deteminations gave approximately ihe
sarne 14C age, or about 300 years. Well known
geological facts, however, indicate that this is
entirely out of the question and ntuch to low.
So far, no explanation for this has been found.
The next step was to turn to tephrochronology.
In this area an approximately 1 mm thick layer
is added to the surface of the soil every year.
By measuring several places the distance be-
tween two tephra layers with known age and so
down to the layer from the eruption in ques-
tion, it turned out that the eruption must have
occurred close to the year mentioned by Rev-
erend Jón Steingrímsson, i.e. in 1112.
PÓSTFANG HÖFUNDAR/AUHOR'S ADDRESS
Jón Jónsson
Smáraflöt 42
210 Garðabær
Iceland
193