Jökull - 01.12.1989, Side 66

chronological methods, Thorarinsson demonstrated
that eruptions in Hekla begin with an explosive
phase producing the acid tephra. The tephra produc-
tion phase is typically short, however, and studies of
postglacial Hekla eruptions have shown that the
tephra distribution is highly directional, being
related to the wind direction at the outset of eruption
(Larsen and Thorarinsson, 1978). Ten tephra sam-
ples from Hekla were analyzed in the present study.
Two samples are from the H5 layer (Sauðafell), a
light coloured fraction (985) and a dark coloured
one (986). The SiOz content in Table II of the light
coloured part is based on Larsen and Thorarinsson
(1978), the value for the dark coloured grains was
estimated from refractive index under the micro-
scope. Four samples from the H4 layer were meas-
ured (988-991, Sigalda), each from a different phase
of the eruption. These samples were analyzed chem-
ically by Gudrún Sverrisdóttir who kindly supplied
the unpublished Si02 data in Table II. The H5 and
H4 layers were described by Larsen and Thorarins-
son (Larsen and Thorarinsson, 1978). Three samples
from different localities were analyzed from the
1104 AD eruption (H,, 1001 from Þóristungur, 1002
from Ófærugil, 1003 from Biskupstungnaafréttur).
The Si02 value is from Thorarinsson (1967). A sin-
gle sample from the 1693 eruption of Hekla was
measured (941, the Si02 value is from Sigvaldason,
1974).
Katla is a subglacial volcano under the Mýrdals-
jökull glacier, South Iceland. The volcano may be
related to the Eldgjá fissure system to the northeast
of Mýrdalsjökull (see discussion in Einarsson and
others, 1980). Tephra from Katla and Eldgjá is rich
in Ti02 and total iron. The chemical composition of
the erupted material from Katla has remained rela-
tively constant from one eruption to another, which
is in marked contrast to the variations observed in
successive Hekla eruptions. Einarsson and others
(1980) used grain size parameters (sorting vs. mean
diameter) of the Katla 1357 tephra to classify it as a
surtseyan type, and also published chemical data.
One sample from the Katla 1625 eruption (940) was
measured in the present study, and assuming that the
eruption took place under the Mýrdalsjökull glacier,
the eruption has probably been a phreatomagmatic
one.
An eruption began on the island of Heimaey on
the 23rd of January 1973. The beginning of the
eruption and its early development and chemistry
was described by Thorarinsson and others (1973).
The Heimaey eruption was of the strombolian-
/hawaiian type (Self and others, 1974; Egilson,
1974). One sample from the Heimaey eruption was
analyzed in the present study (769).
The eruption in Surtsey which began in
November 1963 ofif the south coast of Iceland, was
the first submarine eruption monitored by scientists
in Iceland. It was described by Einarsson (Einarsson,
1965) and Thorarinsson (1966, 1968). Phreato-
magmatic explosions characterized the early stages
of the eruption due to the contact with sea water,
and tephra products resulting from such eruptive
processes were given the term surtseyan by Walker
and Croasdale (1972). The tephra sample analyzed
in the present study (943) was taken from the sea
bottom near Surtsey in February-March 1964, and
the chemical data is from Steinthórsson 1966.
The crater remnant Karl lies just off the tip of
Reykjanes peninsula. Jónsson (1978) has associated
Karl with the Vatnsfell tephra cone, which he con-
siders to have been built up in a submarine eruption.
The crater is built up of layered sideromelane glass
with no signs of alteration. Our sample was col-
lected from the crater remnant on land. Walker and
Croasdale (1972) analyzed tephra from the Karl
eruption and found that it was of the surtseyan type.
No chemical analyses from tHe Karl tephra have
been published, but a tephra layer in a core from
Lake Thingvellir which probably correlates with
Karl has the Si02 content given in Table II for the
Karl tephra (Gudrún Larsen, pers. comm.).
The volcano Askja in North Iceland is situated in
the Dyngjufjöll massif which is largely built up of
subglacial tuffs, pillow lavas and subaerial lava
flows of upper Quaternary age (Sigvaldason, 1979;
Sæmundsson, 1982). The most conspicuous part of
the Dyngjufjöll central volcano is the Askja caldera
and in 1875 a great plinian eruption took place
within this caldera, leading to the formation of a
64 JÖKULL, No. 39, 1989