Náttúrufræðingurinn - 1952, Page 28

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this gravel layer (Figs. 14, 15). On the planc just S of Jarðbaðshólar stacks of this tuff
(Fig. 11) regarded by Barth as belonging to the plagonite formation, rest on a post-
glacial lava, which tliey also do between tlie crater Rauðaborg and Hverfjall. These
tuff layers increase in thickness and grain-size towards Hverfjall and no doubt tliis tuff
was deposited by the eruption which built up Hverfjall, and is a direct continuation
of the tuff in the crater walls.
Between Hverfjall and Jarðbaðshólar the tuff is superimposed on humus soil in
which the tephra layers H3 and H,( (from Hekla) are found (Fig. lö; profile X on
Figs. 18 and 19). From its position in bog profiles in N. Iceland the age of H3 can
be cstimated to 2500—3000 years. The Hverfjall tuff is probably 100—200 years younger
than H3.
Thus Hverfjall was built up by a single explosivc eruption ab. 500 B. C. A more
exact dating of the eruption will probably Ijc possible by the radiocarbon method.
Further studies in the Mývatn area have revealed that a black tephra layer, which
the author previously attributed to an eruption in the crater row Þrengslaborgir, bc-
longs to Hverfjall. In the soil profiles it is designated as li.
The volume of Hverfjall is ab. 130 mill. m3. The total volume of tephra produced
by the Hverfjall eruption (including the volume of Hverfjall itself) may be estimated
to 300—400 mill. mS corresponding to ab. 200 mill. m3 of dense rock of the same
chemical composition.
The tuff laycrs in the crater walls of Hverfjall show no inward dipping. In the
author’s opinion this is not due to sliding after the eruption had ceased but due to the
explosive activity having during the eruption kept the inner slopes of thc crater wall
steeper than thc natural slope of the ejected material.
From the clistribution of teplira and the structure of the crater walls the author con-
cludes, that the main Hverfjall eruption dicl not last more than 1—2 days. Probably
the eruption was rather similar to the eruption which formed Monte Nuovo (Fig. 24)
in 1538. A hypothetical eruption diagram is sliown on Fig. 23.
In the Mývatn area no lava flows have been emitted between layer h and layer H4,
which is ab. 4500 years old. Thus the Hverfjall eruption occurred after a very long
period of quiescence in the Mývatn area.
The statement that Hverfjall (and also Hrossaborg) is a single-eruption explosive
volcano and not formed at all in the w’ay maintained by Einarsson does not of course
exclude the possibility that some breccias belonging to the palagonite formation may
have been extruded as a mixture of palagonite glass and fragmented solid basalt as
Einarsson maintains. Stratified tuffs of the Hverfjall type are however not uncommon
in the palagonite formation. Such tuffs are e. g. found at Kleifarvatn, on Vestmanna-
eyjar, and in Askja (Fig. 25). These tuffs probably belong to the pleistocene inter-
glacials. But without denying the possibility that breccias may have been produced ac-
cording to Einarsson’s thcory the author wants to stress the fact, that a considerable
part, probably the main part, of the palagonite breccias belongs to the glacical periods
of tlie pleistocene and is thus formed beneath an ice cover. So are e. g. many ridges
fonned along volcano-tectonic lines such as Jarlhettur S of Langjökull, the ridges
around Möðrudalur in NE Iceland and Námafjall E of Mývatn, mainly constructional
and buill up under an ice cover, and every explanation of the structure of the pala-
gonitc breccias must take this fact in account.