Jökull - 01.12.2003, Blaðsíða 31

The 1783–1785 Laki-Grímsvötn eruptions
Two more subplinian explosive phases are indi-
cated by the available data. There are no records of the
first one in the historic accounts, but a subplinian fall
unit derived from fissure 7 is preserved in the proxi-
mal tephra deposit at the kipuka Innri-Eyrar (Figure
3). The Fissure 7 subplinian fall unit rests directly
on the phreatomagmatic fall unit produced by explo-
sive activity on fissure 6 (see below) and thereforewas
formed later, most likely towards the end of August
or at the very beginning of September. The second
phase took place around 7 September as is indicated
by records of tephra fall near Vestmannaeyjar. At the
time the wind was blowing from the northeast (King-
ton, 1988) and the tephra-laden plume would have
been carried directly over the Mýrdalsjökull glacier.
Thus, no or very little tephra would have fallen in
the Fire districts (Figure 5f). This tephra fall unit and
later falls (if they occurred) have not been identified in
the proximal deposit, in part because the three north
easternmost fissures of the Laki vent system are com-
pletely surrounded by lava (Figure 3). Consequently,
the proximal part of the tephra fall that may have been
produced by the explosive activity on these fissures is
either buried beneath the lava or completely removed
by erosion or both.
The explosive activity that commenced on 25 June
and peaked on 27–28 June produced a black erup-
tion plume that “twisted and spun over the moun-
tains forming bolsters” that dispersed “sandy” tephra
over eastern part of the Fire districts (B41–43, Fig-
ure 5c). This description of the plume is consistent
with our knowledge of ash-laden and water-saturated
plumes that are formed by phreatomagmatic erup-
tions (Thordarson, 1991). On the basis of the prox-
imal stratigraphy and timing of events, this tephra fall
is correlated with the phreatomagmatic fall unit that
originated from the tuff cone that is located on the
central part of fissure 4 (TC1 on Figure 3).
Similar plume and tephra-fall was dispersed over
the eastern parts of the Fire Districts on 29 July (B62,
Figure 5e). Phreatomagmatic explosions that pro-
duced the tuff cone on fissure 6 (TC2 on Figure 3) pro-
duced this tephra fall. This explosive phase marks a
significant change in the eruption because it coincides
with a shift in the activity at the Laki vents. From this
time on, the main activity was centred on vents to the
northeast of Mt. Laki.
Four of the reported tephra falls are considered to
be from explosive activity at the Grímsvötn volcano.
The tephra fall reported on 18 July is undoubtedly
from Grímsvötn and this is clearly indicated by Stein-
grímsson in his description of the eruption (B52).
The westerly dispersal and the aireal coverage of the
tephra fall on 21 June, 19 July and 14 September
rule out origin at the Laki fissures and point toward
a source farther east (i.e., B34, B54, B70; Figures 5b
and 5d). In light of other evidence presented above
(see p. 18–19) showing that eruptions at the Gríms-
vötn volcano coincided with the activity at the Laki
fissures, it is reasonable to assume that the source of
these tephra falls was explosive activity at Grímsvötn.
The reports of tephra falls clearly show that the ex-
plosive activity at the Laki fissures was episodic and
not simply confined to the beginning of the eruption
as thought previously (Helland, 1886; Thoroddsen,
1879, 1894, 1925; and Thorarinsson, 1967, 1969).
The records appear to give a realistic representation of
periods of vigorous explosive activity at the vents, be-
cause in general terms their occurrence conformswell
to the established tephra stratigraphy and the dispersal
of individual fall units (Thordarson and Self, 1993).
These conclusions are strengthened by the percepti-
ble synchronisation of earthquakes, explosive activity
and increase in lava production (Figure 6, Table 2).
THE LAVA SURGES AND OTHER FLOW
RELATED PHENOMENA
Steingrímsson’s meticulous descriptions of the Laki
lava as it flowed across the plains in front of the Síða
highlands are truly remarkable. Not only do these de-
scriptions provide accurate information about the ad-
vance of the lava but also are a valuable source for
assessing the flow dynamics. A detailed analysis of
these descriptions and their implication for the char-
acteristic emplacement mechanism of the Laki lava is
beyond the scope of this study and will be presented
elsewhere. However, two aspects of Steingrímsson’s
descriptions are particularly important for evaluating
the progress of the Laki eruption; the occurrences and
timing of lava surges and descriptions revealing the
JÖKULL No. 53, 2003 29