Jökull - 01.12.1994, Síða 47

Fig. 4 SEM images of tephra particles from the Reykjanes
volcanic system tephra layers. Sample numbers refer to
Table 1. Length of scale bar 0.1 mm.
4. mynd. Rafeindasmásjármyndir af gjóskukornum úr
gjóskulögum á Reykjanesi. Númerin vísa til töflu 1.
Lengdarkvarði 0,1 mm.
Fig. 5 SEM images of tephra particles from the Reykjanes
volcanic system tuff cones Vatnsfell and Karl, and from a
subaerial crater belonging to Yngri Stampar. Sample num-
bers refer to Table 1. Length of scale bar 0.1 mm.
Rafeindasmásjármyndir af gjóskukornum lír gígunum Vatns-
felli og Karli á Reykjanesi, ogfrá gíg á landi í Yngri Stömp-
um. Númerin vísa til töflu 1. Lengdarkvarði 0,1 mm.
from the origin, after which it becomes indistinct. The
tephra of the brown layer is more vesicular and more
coarse grained than other units of R-3 (Fig. 4). Three
samples were analyzed from R-3.
The tephra layer R-7 originates from the Karl tuff
cone (Fig. 3). Most of the tephra was transported
southwards into the sea, and the preservation on land
is therefore restricted. The Karl crater is a typical tuff
cone built up of alternating base surge deposits and air
fall tephra. Pisolites (accretionary lapilli) and firm,
highly vesiculated layers are common in the crater
rim, indicating that the tephra was saturated with
steam when deposited (Fig. 4). The structure of the
crater is characterized on one hand by thinly to thick-
ly (3 - 30 cm) bedded, massive tephra, and on the
other by very thinly to thinly (1-3 cm) planar bedded
tephra. Clearly, air-fall and surges have been active si-
multaneously during the accumulation of the cone,
particularly during the formation of the massive units.
Both the Karl cone and the tephra layer R-7 were
sampled for analysis in the present study.
The Vatnsfell tuff cone which is largely over-
lapped by the Karl cone, has a different structure. The
Vatnsfell crater rim may be divided into a lower, fine
grained, glassy, very thinly bedded part with ripples,
and an upper, coarser and upwards coarsening part
characterized by scoria and rock fragments. The up-
permost unit is nearly pure scoria. The structure of the
Vatnsfell cone indicates gradually decreasing access
of sea-water to the eruptive vent, and finally, com-
plete isolation and strombolian activity. Both major
units of the Vatnsfell cone were sampled in the pre-
sent study. The Vatnsfell and Karl tuff cones were
both formed during the opening phase of the Younger
Stampar fissure, which gradually extended into the
sea. The Younger Stampar lava flow (H2, Fig. 3) was
produced on land by the same eruption, shortly after
the cones were formed.
The R-9 tephra layer originates from an eruptive
site 2-3 km off the Reykjanes coast. The crater has not
been preserved. The distribution axis trends NE and
the unit is found on the whole Reykjanes peninsula
and in adjacent districts (Fig. 1). Possibly, the tephra
layer was formed in an eruption that took place in the
year 1226 AD, an event that is well documented in
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