Náttúrufræðingurinn - 1979, Page 137

Aboul 6 km furtlier west there is a big
crater simply named Eldgigur (= volcanic
crater). It is about 800 m in diameter
and rises 80 m above the surroundings.
Eldgígur is taken to be the easternmost
part of the eruptive fissure Rauðhólar
which accordingly is about 29 km long.
The eruptive fissure is supposed origin-
ally to have continued between Rauðhól-
ar and Eldgígur but a part of the íissure
is now covered by the glacier. Eldgígur
and Rauðhólar are petrographically iden-
tical, but Eldgígur seems to liave been
active somewhat later than the main fis-
sure. The Rauðhólar eruptive lissure is
situated within a narrow graben which
cuts the Miklafell mountain. The vertical
displacement is in places 8—10 m. A tec-
tonic lissure cuts along the craters indicat-
ing sonie tectonic movement after the
eruption. The area ol the Rauðhólar lava
flow is 228 km2 and the volume is calcu-
lated about 6,84 knt3.
Rauðhóll and Bunuhólar at
Hervararstaðir.
Rauðhóll at Hervararstaðir is a specta-
cular crater built up of volcanic cinder,
bombs and blocks but without a lava
flow. It is on the northern end of an
eruptive fissure, the Bunuhólar, at the
upper end of the Holtsdalur valley (dalur
= valley). From there a lava flow went
down the narrow Holtsá canyon but
disappears under sediment at the lower
end of the valley. The extension of this
ílow is not known. Petrographically it is
similar to the above mentioned flows.
The Hálsagigir west of Skálarfjall is an
eruplive fissure with imposing lava and
cinder formations. The lava flow clisap-
pears under the sediments of the Skaltá
river and under the Skaftáreldahraun
(Laki lava) of 1783. Accordingly nothing is
known of the extension ol the lava flow.
The Hálsar lava is similar to tlte L;iki
lava, but somewhat richer in olivine.
The age of the eruptions.
Two light-coloured (acidic) tephra
layers give important information con-
cerning the age of eruptions under dis-
cussion.
The age of the lowermost of tltem,
H-4 (cf Thorarinsson 1971 and Jónsson
1975) is according to C14 age determina-
tion about 3800 C14 years old. The same
tephra layer is about 25 cnt above the
Rauðhólar lava flow at Miklafell and
15—20 cm above the cinder of the Bunu-
hólar craters. Accordingly Eldgígur- Rauð-
hólar and Rauðhóll-Bunuhólar have erup-
ted at approximately the same time.
The second acidic tephra layer (H-3?)
which has been dated 3520 C14 years, as
well as H-4, is underneath the 120 cm
thick pumice from the Hálsar eruption
which accordingly can hardly be more
than about 2000 years. The material
used for these age determinations is stems
of birch iu the tephra itsell'.
It has been stated by Thoroddsen
(1894), Sapper (1908) and several other
authors that the disastrous eruption of
the Laki eruptive lissure in 1783 is not
the first eruption on the fissure in post-
glacial time. Several older craters can be
recogni/.ed along the eruptive fissure of
1783. M hc lavas of these two eruptions
are identical.
ln the Meðalland district an olcl lava
l'low is now mostly covered by sand de-
posits. This tlow, here named Botnar
lava, is superimposed by the Eldgjá (Lantl-
brot) lava, and a sand deposit may be seen
belween the two lava flows at Botnar.
M’his lava llow is first mentioned by Rob-
son (1952). It may be followed from west
of the Botnar larm along tlie Eldvatn
river to Gamlibær east ol' Steinsmýri. The
presents author is of the opinion that
this old lava flow is derived front the
í'irst eruption of the Laki fissure in post-
glacial time, and that it is perhaps 6000—
7000 years old or more. The ages of the
mentioned eruptions are considered to
be appoximately: