Náttúrufræðingurinn - 1995, Page 68

teracts with the sea. The best known example
of this type of activity is the Younger-Stampar
eruption. Accessible outcrops on Reykjanes,
thick deposits of unconsolidated tephra and
tephra layers in soils allow some interpreta-
tions of the eruptive mechanism and eruptive
sequences. With the aid of tephrochronology,
14C dating and written sources the Younger-
Stampar lava is dated back to the early 13th
century AD.
The Younger-Stampar eruption began at the
SW-coast of Reykjanes within an inland-
stretching graben flooded by sea. At this site, a
hydrovolcanic (surtseyan) eruption broke out
forming a small tuff cone approximately 650
m in diameter and 60 m high (estimated) (fig.
11). According to detailed structural and tex-
tural analysis, it is suggested that inflow of sea
water into the conduit gradually decreased as
the eruption proceeded. This is clearly indi-
cated by increasing amount of scoria and
lithics in the upper part of the rim. The fínal
phase is characterised by scoria indicating
Strombolian activity, i.e. only minor water/
magma interactions. Base-surges were the
dominating transport mechanism in the early
stages, indicated by sand-wave bedding, but
later air-fall including ballastically ejected
blocks, gradually took over. The tephra from
the cone is poorly preserved in soils of the sur-
rounding area, suggesting it was mainly car-
ried over the sea.
After a short break, the activity moved 500
m seawards and a new tuff cone was formed,
estimated to have been 1600 m in diameter and
150 m high (fíg. 12). On the SW-coast of
Reykjanes the rims of the two cones overlap
considerably but different structures make
them rather easily discriminated in the field.
No signs of any major hiatus can be identified
in the deposits, suggesting a continuous activ-
ity. With reference to local features, the older
cone is called the Vatnsfell-cone but the
younger, and larger one, the Karl-cone. The
bedding of the Karl-cone is characterised by
planar and massive beds, scattered with some
lithics, and a homogeneity in structure. The
tephra is suggested to have been transported
from source by base surges and air fall.
Vesiculated layers and accretionary lapilli are
prominent features of the Karl-cone. The
tephra layer is traceable inland for few kilome-
tres, covering the whole of Reykjanes and its
immediate vicinity, but as indicated by the
isopachs most of the tephra was transported
seawards (fig. 13).
As the centres of both the cones were totally
eroded away, several different methods were
employed to locate them, including detailed
measurements of thickness and dipping of the
rims and measurements of bedding sags
formed by ballastically ejected lithics. In
many cases, the bedding sags were asymmetri-
cal, stretching away from source, and thus in-
dicating the direction and angle of impact (fig.
9). Detailed measurements of the bedding sags
appeared to be the most effective tool in locat-
ing the centres.
Shortly after the explosive activity ceased,
an eruption of Hawaiian type started on a 4 km
long NE trending físsure on Reykjanes (fíg 1).
This phase of the eruption was characterised
by lava fountaining, building a row of steep-
sided spatter cones and pahoehoe lava fields
fed by lava tubes and some major channels.
The lava covers about 4 km2 and its estimated
volume is 16 million m3. When examining out-
crops at the lava margin, the tephra layer from
the Karl-cone is found immediately below it
(fig. 4 and 5), indicating a minor or no break in
activity between the hydrovolcanic phase and
the lava producing phase. In the coastal area,
lava banked up against the Karl-cone, now
marking its original extent on land.
The Younger-Stampar eruption marks the
beginning of a long-lasting eruption period
(fires), the so-called Reykjanes Fires 1211-
1240 AD. Five lava-flows formed during this
period, including the Younger-Stampar lava-
flow, and according to written sources at least
six eruptions occurred in sea off Reykjanes.
Since the Settlement of Iceland in late 9th cen-
tury AD, two volcanic periods analogous to
the Reykjanes Fires are known to have oc-
curred, in the lOth century and the 12th cen-
tury. At the end of the paper, the volcanic haz-
ards of the Reykjanes peninsula are evaluated
and the importance of further volcanological
research stressed. As this is the most densely
populated area of Iceland, the character of its
volcanic activity and details of the eruption
history must be well documented in order to
ensure proper reactions to future volcanic
events.
PÓSTFANG HÖFUNDAR/AUTHOR'S ADDRESS
Magnús A. Sigurgeirsson
Orkustofnun/National Energy Authority
Grensásvegi 9
IS-108 REYKJAVÍK
230