Jökull - 31.12.2001, Blaðsíða 6

Jukka Káyhkö et al.
northwestem part of our study area coincides with the
fifth major fissure swarm in northern Iceland, Þeista-
reykir.
Fissure fracturing may involve vertical movement,
whereby tectonic depression valleys or grabens are
developed. One of the largest grabens in our study
area, Sveinagjá, is 4-17 m deep and up to 2.5 km wide
(Guðmundsson and Báckström, 1991). During the
eruption of Askja in 1875, the northern part of Sveina-
gjá subsided some 3-6 m. Grabens may act as impor-
tant natural channels for lava flows, meltwaters and
even aeolian sand transport.
Lava-flow processes and morphology
During the early postglacial period, large volumes of
lava erupted in the Northern Rift Zone, and magma
output was at least an order of magnitude greater than
at present times (Guðmundsson, 1986; Sigvaldason
et al., 1992). As a consequence, the most spatially
extensive geomorphological features in Odáðahraun
are lava fields. A lava field forms during an effu-
sive eruption or a series of eruptions, and commonly
consists of many adjacent and overlapping lava flows.
At Odáðahraun, lava eruptions may take place at (1)
fissure eruption sites (mainly aa lava fiows); (2) cen-
trally located vents resulting to the formation of shield
volcanoes (mainly pahoehoe lava flows) or, (3) major
central volcanoes of Krafla and Askja (both pahoehoe
and aa) (Rossi, 1997a). Lava flows normally occupy
topographic depressions but may construct conspicu-
ous positive relief features such as shield volcanoes.
They also alter drainage systems and sand transport
routes.
The most common type of eruption in the study
area is a fissure eruption that is generally short in du-
ration (days to weeks; cf. Rossi, 1997b). In the open-
ing phase of the fissure eruption, the supply rate from
the fissure is generally high and a sheet of lava (sheet-
flood pahoehoe) usually forms (e.g. the central part of
the Krafla lava field). After a few days of eruption,
magma outflow concentrates into a single crater vent
and an open-channel lava flow (mainly aa) develops
(see Rossi, 1997b; Wylie et al„ 1999). The roughness
of aa surfaces vary from clinkery (stone-sized surface
rubble; e.g. many flows at Krafla) to blocky (e.g. Búr-
fellshraun) (Kilburn and Lopes, 1991; Rossi, 1997b;
Lammi et al., 2000).
Lava flows from shield volcanoes (e.g. Trölla-
dyngja and Kollóttadyngja) normally exhibit pahoe-
hoe morphology. The most widespread lava flows in
our study area are dense and hummocky pahoehoe
flows that were fed by lava tubes (Swanson, 1973;
Walker, 1991; Wilmoth and Walker, 1993; Rossi
and Guðmundsson, 1996). Shield volcanoes are es-
sentially monogenetic structures, and lava fields at
shield volcanoes presumably form during a continu-
ous string of eruptions lasting from several weeks to
several decades (Rossi, 1996).
Other volcanic structures
In addition to lava flows, the northern rift zone con-
tains numerous other volcanic features. Mild explo-
sions at fissure eruption sites have created structures
such as spatter cones, spatter ramparts, scoria cones
and hornitos. Eruptions with higher explosivity have
formed tuff rings and led to caldera collapses at Krafla
and Askja.
The most significant volcanic features from the
glacial periods are hyaloclastite ridges (e.g. Dyngju-
fjöll ytri) and table mountains (e.g. Herðubreið).
These were formed in subglacial eruptions and con-
sist of sequences of pillow lavas, hyaloclastites and
subaqueous and subaerial sheets of lava (Wemer and
Schmincke, 1999). Postglacial erosional processes
have modified these mountains, and released material
for fluvial and aeolian transport on the lava fields.
Glacial floods (jökulhlaups)
The average eruption frequency under Vatnajökull
during the past century has been approximately one
per decade (Larsen et al., 1998). Vigorous volcanic
activity beneath the ice cap has produced widespread
and recurrent tephra layers as well as catastrophic out-
burst floods, or jökulhlaups (Bjömsson, 1992). Two
major volcanic systems, Bárðarbunga and Grímsvötn,
lie beneath the glacier. They have developed large
subglacial calderas. Kverkfjöll volcano, also regarded
as a potential trigger for jökulhlaups, lies at the north-
ern margin of the ice cap, between the outlet glaciers
Dyngjujökull and Brúarjökull. The Grímsvötn sys-
tem is currently the most active of the three. Geo-
thermal activity gradually melts the overlying glacier
4 JÖKULL No. 51