Jökull - 01.12.1976, Page 11

64°30'N
22°W 19°W
VOICANIC
ZONES
Fig. 1. An index map showing the position of Hekla within the tectonic framework of southern
and southwestern Iceland. Hekla is located on the junction between the eastern volcanic zone
and the zone of large earthquakes as defined by Tryggvason (1973). The Reykjanes Peninsula is
the direct landward continuation of the submarine Reykjanes Ridge, which is a part of the mid-
Atlantic ridge system.
Mynd 1. Hekla er i vesturjaðri eystra gosbeltis og við austurenda upptakabeltis stórra skjálfta á
Suðurlandi. Reiturinn umhverfis Heklu er sýndur steekkaður á Mynd 2.
ZONE OF LARGE
EARTHQUAKES
o
CENTRAL VOLCANO
4 SEISMOGRAPH STATION
• EPICENTERS JUNE 28,
JULY2 AND AUGUST 2
years, outranged only by the great Laki fissure
in the eastern volcanic zone.
The bulk of Hekla’s products is andesitic.
The silica content at the beginning of an erup-
tion is a linear function of the time elapsed
since the last eruption, and during the course
of the eruption the silica content decreases.
Sigvaldason (1973) presented a model to ex-
plain the observed chemical variadons. His
model assumes that Hekla’s magma is derived
from basalt by partial melting in two stages.
The basalt, erupted to the surface in the west-
ern volcanic zone, subsides as it is carried east-
ward by crustal drift. As the basalt reaches a
suitable depth a silicic liquid is produced by
partial melting and at a deeper level andesitic
liquid is produced by further partial melting.
The temperature anomaly of the eastern vol-
canic zone finally brings the two liquids close
to the surface where they mix to a limited
extent and erupt.
Eruptions of Hekla are accompanied by con-
siderable earthquake activity. Tremors and
earthquakes are generally felt when the erup-
tion begins and fairly strong shocks, exceeding
intensity VI on the Mercalli scale have some-
times been observed during the initial phase
and occasionally during later phases of the
eruption, several weeks or months after the
outbreak. In some eruptions the earthquake
activity near Hekla lias been followed a few
months later by destructive earthquakes in the
seismic zone farther west (Thorarinsson 1967,
Björnsson and Einarsson, 1974).
JÖKULL 26. ÁR 9