Jökull - 01.12.1994, Page 46

samples are from units formed in eruptions that are
well documented by historical accounts.
The Reykjanes volcanic system
Reykjanes peninsula is a tectonically and volcani-
cally active landward continuation of the Reykjanes
Ridge which forms a segment of the Mid-Atlantic
Ridge. The term Reykjanes is used to designate a 5
km long tip of the Reykjanes Peninsula (Fig. 2).
Reykjanes lies within the WVZ and is built up of lava
flows and tephra units accumulated during the Brun-
hes geomagnetic chron. The peninsula is largely cov-
ered by postglacial lava flows, but numerous tuff
ridges and table mountains reflect subglacial and/or
submarine volcanic activity during glacials or at rela-
tively high sea level (Jónsson, 1978).
Within Reykjanes peninsula the volcanism is con-
fined to five volcanic systems or rift swarms that
Fig. 3 The Vatnsfell (RG2) and Karl (RG3) tuff cones off
Reykjanes. H3 and H2 refer to the lava flows from the Eldri
Stampar and Yngri Stampar crater rows.
3. mynd. Gígarnir Vatnsfell (RG2) og Karl (RG3) undan
Reykjanesi. Hraunin frá Eldri Stömpum og Yngri Stömpum
eru merkt H3 og H2.
trend SW-NE. Each system is demarcated by a sepa-
rate rift swarm, anomalously high thermal gradient,
and volcanic fissures. The erupted material is of
basaltic composition and consists of picrite, olivine-
tholeiite, and tholeiite, belonging to the tholeiitic
suite. The tholeiites are characteristic products of fis-
sure eruptions while the picrites and olivine-tholeiites
are generally derived from shield volcanoes (Jakobs-
son et ai, 1978). The westemmost one of the volcanic
systems is the Reykjanes system, which mns NE from
the tip of the peninsula.
The Reykjanes volcanic system forms an SW-NE
trending 25 by 10 km long zone. The southemmost 9
km are submarine (Jakobsson et al., 1978). Many of
the eruptive fissures extend across the shoreline, lead-
ing to effusive volcanism on land and explosive erup-
tions in the sea off Reykjanes. Tephra studies on
Reykjanes indicate that there have been at least 10
submarine emptions off Reykjanes since the deglacia-
tion of the area. The eruptions have deposited tephra
units that crop out in soil sections and crater remains at
the SW shore of Reykjanes. The tephra layers and tuff
cones are listed in Table 2, which also shows their age.
Two adjacent craters, the Vatnsfell and Karl tuff
cones, are partly preserved on the SW coast of
Reykjanes. The only tephra layer on Reykjanes that
can be traced to a distinct crater is the R7 layer, which
correlates with the Karl tuff cone. Apparently, other
eruptive centres lie further offshore. Samples from the
Vatnsfell and Karl tuff cones were analyzed by Walk-
er and Croasdale and used to define surtseyan tephra
(Walker and Croasdale, 1972). In the present study,
tephra samples were taken from the units R-3, R-7, R-
9, and the Vatnsfell-Karl cones.
The origin of the tephra layer R-3 lies offshore,
most probably less than 1 km from the present coast-
line. The axis of the maximum thickness of R-3 has a
northeasterly trend. R-3 is the second most extensive
tephra layer from the Reykjanes system. The lower
part of the tephra layer is generally well preserved, es-
pecially close to the origin, but the upper part is gen-
erally redeposited by wind or otherwise deformed. A
very distinctive, brownish unit which is present in the
lower part of R-3 (“the brown layer”) makes it easily
detectable. The brown layer can be traced up to 5 km
44
JÖKULL, No. 44