Jökull - 01.01.2012, Page 148

E. Magnússon et al.
Figure 11. a) The bedrock topography of the Öræfa-
jökull caldera. Contours are shown with 20 m interval.
b) Likely paths of jökulhlaups (red arrows) down into
Virkisá river (west) and Kvíá river (east) in case of an
eruption at location shown with star (also shown in a).
Blue lines in a and b indicate the water divides at the
caldera (a) and the southern part of Öræfajökull (b).
– a) Botnlandslag Öræfajökulsöskjunnar. Bláar línur
sýna vatnaskil. b) Líklegar rennslisleiðir frá gosstöð
(rauð stjarna) við koll í botni öskjunnar.
The caldera is rather smooth indicating limited
volcanic activity within it since it was formed. An ex-
ception is a topographic mound situated beneath 400
m of ice at the water divide between the river Virkis-
á, draining west from the caldera and the river Kvíá,
draining east (Figure 11). The mound is likely to be
the result of a previous eruption and should be consid-
ered as a possible location for future eruptions. Such
an event would result in large jökulhlaups both to-
wards east, down Kvíárjökull and west, down Virk-
is- and Falljökull, as may have happened in the past.
Another feature worth pointing out is a topographical
step within the caldera, which we interpret as a sepa-
rate caldera formation. The inner caldera bed is ∼150
m lower than its rim in the northern part of the caldera
(Figure 12). The lowest pass out of the depression
formed by the two calderas, where Kvíárjökull drains
out is ∼200 m above the caldera adjacent floor but at
this location the rims of the two calderas nearly co-
incide. Other passes are higher relative to the inner
caldera floor.
DISCUSSION
The high elevation of the passes out of the depres-
sion formed by the two calderas suggests little erosion
since the collapse of the inner caldera and that the cor-
responding collapse presumably is relatively recent.
A possible candidate for forming the suggested in-
ner caldera may even be the eruption in 1362 AD. Its
dimensions, ∼6 km2 in area and volume of 0.5–1.0
km3, are comparable to the size of the caldera formed
during the Pinatubo eruption in 1991 (Lipman, 2000),
which in many ways was similar to the Öræfajök-
ull eruption in 1362 (Ármann Höskuldsson, personal
communication, 2012). This similarity applies to the
volume of erupted material, but its dry rock equivalent
(DRE) was 4–5 km3 for Pinatubo in 1991 (Lipman,
2000) compared to estimated DRE of ∼2 km3 for the
1362 eruption in Öræfajökull (Thorarinsson, 1958).
The water table within an open ice cauldron
formed during an eruption within the caldera would
not drop below the lowest pass out of it at ∼1570 m
a.s.l. Interaction of magma with water should there-
146 JÖKULL No. 62, 2012