Jökull - 01.01.2013, Side 18

Þ. Björnsdóttir and P. Einarsson
complex where the discharge of magma is highest. A
central volcano may have one or more calderas and
a high-temperature geothermal area. They produce a
significant amount of silicic volcanic rocks in addi-
tion to basalt (Jakobsson, 1979b; Walker, 1993). Fis-
sure swarms are regarded as the surface expression of
dyke swarms. They consist of normal faults, tensile
fractures and volcanic fissures that extend from the
central volcano. The fissure swarms are usually about
10 km wide and their length varies from 30 to over
100 km. The fissure swarms within each branch of
the rift zones are typically arranged en echelon. The
arrangement may be dextral or sinistral depending on
the direction of maximum principal tensile stress that
is parallel to the direction of plate movement (Sæ-
mundsson, 1978). Of about 30 active volcanic sys-
tems in Iceland, 2/3 have a fissure swarm and 2/3
have a central volcano (Jóhannesson and Sæmunds-
son, 1998). The active life of most volcanic systems
is between 0.1 and 10 Ma (Walker, 1993).
The volcanic system of Tungnafellsjökull is lo-
cated in the Central Iceland Volcanic Zone (Einars-
son, 2008), Figure 1. The Tungnafellsjökull system
consists of two or three central volcanoes, the Tungna-
fellsjökull central volcano, the Vonarskarð central vol-
cano and the Hágöngur central volcano, and a fissure
swarm (Friðleifsson and Jóhannesson, 2005). The
Tungnafellsjökull and Vonarskarð central volcanos
are here considered to be one central volcano having
two calderas, one under Tungnafellsjökull glacier and
the other SE of the glacier, in Vonarskarð. The Há-
göngur central volcano is located about 15 km SW of
Tungnafellsjökull. These central volcanoes are char-
acterized by a significant contribution of silicic prod-
ucts signifying their maturity.
Volcanic activity in the Tungnafellsjökull volcanic
system has been low in the Holocene. Only two small
lavas can be associated with the system in this time,
the Dvergar lava and the Tunguhraun lava. Previous
research on the fissure swarm of Tungnafellsjökull has
been limited. A map of the fissure swarm first ap-
peared on a structural map of the neovolcanic zones
in Iceland, in an article by Sæmundsson (1978) which
was later modified by Einarsson, and Sæmundsson
(1987).
In light of the modest activity of the Tungnafells-
jökull system it came as a surprise that an InSAR
study of the Gjálp eruption of 1996 by Pagli et al.
(2007) revealed fault movements of the order of a
few cm in two places within the Tungnafellsjökull fis-
sure swarm. The 1996 Gjálp subglacial eruption be-
neath the Vatnajökull ice cap was the largest in Ice-
land in terms of volume since the Surtsey eruption in
1963–1967 (Guðmundsson et al., 1997; Einarsson et
al., 1997). But it occurred at a distance of more than
35 km from Tungnafellsjökull and within a different
volcanic system. This discovery spurred a field study
of the area to see if surface ruptures could be found
that corresponded to the displacements measured by
InSAR. A map of the fissure swarm was prepared
from aerial photographs and satellite images. Critical
places were then visited during two field campaigns in
2009 and 2010. In this paper we report the findings of
these studies, the most remarkable of which was the
evidence that fractures had opened in several places in
recent years, as recently as the spring of 2010. This
study was a part of an MS-project at the University of
Iceland (Björnsdóttir, 2012).
REGIONAL SETTING
The Tungnafellsjökull area is located at elevation of
800–1500 m and has very little vegetation, water is
sparse. The area is characterized by large areas of
sand and sandy ridges. The landscape was formed by
glacial erosion during the Pleistocene and is covered
by ground moraines to a large extent. There are no
settlements in this area and the few man-made struc-
tures that do exist are simple huts used by travelers
during the summertime. The route to this area is only
passable by 4wd vehicles and then only during the
summer. The obstacles found en route include glacier
rivers, which have to be forded.
Past research on the Tungnafellsjökull volcanic
system has been limited and its classification within
the volcanic zones in Iceland is ambiguous. Friðleifs-
son and Jóhannesson (2005) classified the Tungna-
fellsjökull volcanic system within the Northern Vol-
canic Zone, whereas Jakobsson, Jónasson and Sig-
urðsson (2008) considered it unclear whether it lies
within the Northern Volcanic Zone or the Eastern Vol-
18 JÖKULL No. 63, 2013