Jökull - 01.12.2003, Page 32

Thordarson et al.
1783
17841783
1010 10 10 10
10 10 10 10 1020 20 20 20
20 20 20 20 20
May June July August Sept.
Nov. Dec. Jan. Feb.
Oct.
eruption episode
goshrina
explosive phase
sprengivirkni
subplinian
sprengigos
lava surge
eldhlaup
lava prodiction rate
framleiðni
earthquake swarm
jarðskálftar
low
high
high
low
II III IV V VI VII VIII IX
X XI
?
I
?
?
? ?
Oct.
phreatomagmatic
þeytigos
Grímsvötn eruption
gos í Grímsvötnum
Figure 6. Schematic illustration of sequence of events during the Laki-Grímsvötn eruptions. Extent of earth-
quake swarms is indicated by wiggly lines; fluctuations in lava discharge shown by shaded area (not to scale);
eruption clouds denote explosive activity at Laki fissures; eruption clouds with a cone at the base denote ex-
plosive activity at Grímsvötn volcano; arrows indicate onset and termination of Laki eruption. The solid bars
show the extent of each eruption episode, labelled I, II, III etc. Modified from Thordarson and Self (1993). –
Myndræn framsetning á framvindu Skaftárelda.
position of lava flow fronts at various times during
the eruption.
Descriptions of recurring “eldhlaup” (i.e., lava
surges) from the Skaftá River and the Hverfisfljót
River gorges clearly indicate periodic but abrupt in-
crease in lava flow rates. These surges travelled at
velocities in excess of 6 km per day (Thordarson and
Self, 1983) and resulted in rapid advances of the ac-
tive lava fronts that tapered off over several days (e.g.,
Appendix C and Figure 5b). It is noteworthy that the
lava surges occurred throughout the eruption and in
each case they emerged from the gorges 3-5 days after
a major explosive phase on the Laki fissures and the
Grímsvötn volcano (Table 2, Figure 6). This synchro-
nisation in the activity strongly indicates that these
lava surges represent episodic increase in lava produc-
tion at the fissures and hence in the magma discharge.
The contemporary accounts do not give unam-
biguous information about the mechanism by which
these surges travelled from the fissures to the active
flow fronts. However, certain inferences can be made
about the most likely transport mode from flow em-
placement structures and the overall volcanic archi-
tecture of the Laki lava flow. The Laki lava flow con-
sists of numerous lava lobes, which range from me-
ters to kilometres in length and decimetre to >20m in
thickness (Thordarson and Self, 1993; Keszthelyi et
al., 2000). Even so, nowhere on the lowlands in front
of the Skaftá River and Hverfisfljót River gorges does
the flow field feature stacked lava flows as defined
by Self et al., (1993), as would be expected if each
of the lava surge had produced their own surface flow
originating at the vents. Furthermore, tumuli, tumulus
ridges, lava rise pits, and lava rise plateaus are com-
mon surface structures in the Laki lava flow field and
filled or drained lava tubes have been found in various
parts of the lava (Figure 7a–e; Thordarson and Self,
1993; Wood et al., 2001). Studies of the flow top
30 JÖKULL No. 53, 2003