Jökull - 01.12.1979, Síða 32

Surtsey, the new island off the south coast. This
shield rests on a socle of hyaloclastite making
Surtsey a similar structure to the table mountains
or tuyas.
The most common type of basalt volcanoes in
Iceland are the crater rows. The craters are ag-
glutinate or scoria cones, but occasionally phreatic
craters may develop on a lava fissure if it cuts
through a lake or waterlogged sediments.
Extrusions of acid lava in historical time have
taken place in the Landmannalaugar-Hrafntinnu-
hraun area. One of these eruptions extruded the
lava flow Hrafntinnuhraun and formed an em-
bryonal tholoid dome.
A special and common type of volcanic activity
in Iceland are the eruptions of ice capped volca-
noes. To these belong two of the most active volca-
noes in historical time, Katla and Grímsvötn, and
the two highest active ones, Öræfajökull and Eyja-
fjallajökull (Fig. 2). As far as is known, the erup-
tions of these volcanoes in historical time have been
purely explosive, probably phreato-magmatic, as
both Grímsvötn and Katla have produced only
basalt. Each eruption of the subglacial volcanoes is
accompanied by a water flood, termed “jökul-
hlaup” (glacier burst) in Icelandic. The jökul-
hlaups from Katla, Öræfajökull and Eyjafjallajök-
ull are all due mainly to melting of ice by the
eruptions themselves. The jökulhlaups from the
Grimsvötn caldera in central Vatnajökull are
caused by storage of water in the caldera, mainly
due to continuous subglacial heat flow through
solfatara activity. Since 1938 jökulhlaups have
occured, on average, every 5th year, reaching an
euphemeral maximum of ca. 10.000 m3/s after a
week or so from their beginning and the total
amount of water released is 3 to 3.5 km3. None of
these floods have been accompanied by a volcanic
eruption. Before 1938 the hlaups often occured
with an interval of about 10 years, the total amount
of water released by those hlaups was 6 to 7 km3,
the max. run-off up to 40.000 m3/s, and they were
often accompanied by a volcanic eruption.
The jökulhlaups from Katla, occurring usually
twice a century, usually last less than a day, but
their max. run-off probably exceeds 100.000 m3/s.
In Iceland the questions what is a single, in-
dividual volcano? what is an active volcano? and
what is a single, individual eruption? are difficult
ones, not least in the light of the volcanic activity in
recent years. Some characteristic features of the
Icelandic volcanoes fit badly in any scheme based
Fig. 2. Subglacial volcanoes and solfataras that
have caused jökulhlaups in historical time.
on classic, viz. Mediterranean volcanic activity.
Studies of the activity in Krafla since 1975 have
revealed that some of Iceland’s central-volcanoes
are connected with fissure swarms, tens of
kilometers in length, along which magma, rising
underneath the volcano, can escape. It is now
regarded as almost certain that the big explosive
rhyolite-producing eruption in Askja on March
28/29 1875 and the basalt producing fissure erup-
tion in Sveinagjá, 70 km farther north, in
February-August of the same year were fed by the
magma reservoir beneath the Askja caldera — as
maintained already in 1881 by W. G. Lock. Should
these two eruption sites then be regarded as one
volcano and the eruptions a single one? The vol-
canic activity collectively called the Surtsey erup-
tion lasted from early November 1963 to early June
1967. It built up three islands, two of which disap-
peared, and one submarine ridge on four separate
fissures arranged en echelon. Are they all together
one volcano? In the following the term “volcanic
system” is used covering both a central volcano and
a transecting fissure swarm.
In the “Catalogue of active volcanoes” are listed
as active all volcanoes which have eruptions known
through written sources. The idea behind this is
that these volcanoes can be expected to erupt
again. In Iceland 18 volcanic systems have erupted
30 JÖKULL 29. ÁR