Jökull - 01.12.1974, Page 57

volved because of the exceptional thickness o£
the acid lavas, hyaloclastites and compound
lava flows (especially in Gauss 3). We have also
included for the purpose of calculating the
average rate of growth the lowest part of the
Gauss epoch, of which a section is presented
in Fig. 10.
The Gauss epoch is represented in this area
by an average of 1 flow of 8 m average thick-
ness every 7,500 years or so. This differs signi-
ficantly from earlier estimates (McDougall and
Wensink, 1966; Dagley et al., 1967) of four to
six times slower rates obtained in eastern Ice-
land. The estimates from that area have to be
critically reviewed in tlie light o£ new evidence
regarding the history and age o£ the lava pile
in eastern Iceland (Sæmundsson, 1974). The
new evidence indicates that a hiatus was in-
cluded in the estimate of Dagley et al. and that
the sequence studied by McDougall and Wen-
sink represents an abnormally low production
rate in the early stages of development of the
present day active volcanic zone in northern
Iceland. Piper (1971) reports an average rate
of extrusion of one lava flow per 27,000 years
for SW-Iceland, a value which is far too low,
as recent extensive mapping led by the first
author has shown.
4.2 THE LIFE SPAN OF THE CENTRAL
VOLCANO
The central volcano became active early in
the Kaena event 2.9 m. years ago and the first
acicl phase ceased before the end of that event.
The second acicl phase culminated 200,000—
250,000 years later with numerous rhyolite flows
and the widespread Deildargil ignimbrite. There
are indications that rhyolitic volcanism persisted
in the very core of the volcano during the inter-
vals between two successive phases, but the vo-
luminous intermediate and acid volcanic pro-
ducts were evidently produced during relatively
short periods of paroxysmal activity. Assuming
a similar rate of extrusion for the third acid
phase, as was deduced for the rest of the lava
pile, tliis group with a maximum thickness of
200 m hardly represents more than the first
200,000 years of the reversed Matuyama epoch.
This appears to be maximum value because the
acid phases obviously produced locally an ab-
normally great thickness of rocks during their
Fig. 9. Map of polarity zones within the area
studied. This map is essentially consistent with
Tr. Einarsson’s (1962) paleomagnetic map of
the area south of Hvítá. It shows no similarity
to a paleomagnetic map of Piper (1971) on
which is based the erroneous view of Matuyama
age for the greater bulk of the Húsafell central
volcano.
Mynd 9. Kort,' sem sýnir útbreiðslu öfugt og
rétt segulmagnaðs bergs á rannsóknarsvœðinu.
Kortið er i megindráttum svipað segulkorti
Trausta Einarssonar frá 1962 af sama svæði, en
allmjög frábrugðið korti Englendingsins J. D. A.
Pipers frá 1971.
period of activity. A much lower value is there-
fore more likely. According to this the central
volcano ceased erupting somewhat less than 2.4
m. years ago, its life span being about 0.5 m.
years.
There are clear vestiges of an high tempera-
ture hydrotliermal system in the core of the
volcano following upon an intrusive episode
during the second acid phase. The hydrothermal
system was maintained until the end of the
third acid phase, possibly for as long as 250,000
years. We are unable to tell if there were breaks,
however, within this period.
4.3 THE FREQUENCY AND DURATION
OF GLACÍATIONS
We started our discussion of the Húsafell
section with tlie lowest tillite horizon identified
so far. The main argument for a climatic change
is that red clayey soil or dust interbeds charac-
terizing the lower part of the section and the
Tertiary flood basalts in general gradually gave
way to more coarse grained interbeds of brown-
ish colours, besides layers of tillites, coarse
JÖKULL 24. ÁR 55