Náttúrufræðingurinn - 1952, Blaðsíða 27

HVERFJALL
169
SUMMARY
HVERFJALL
by Sigurður Þórarinsson
Previous investigations: In niany textbooks on volcanology tlte volcano Hverfjall in
the Mývatn area, Northern Iceland, is described as the best example of the type of
volcano which in English is called tuff ring, in German Ringwall-vulkan. K. Schneider
called volcanoes of this type Homate.
Among the geologists who have studied Hverfjall are F. Johnstrup, Þ. Thoroddsen,
K. Grossmann, W. v. Knebel, K. Schneider, A. Rittmann and T. B. F. Barth (cf. the
List of References). Reck regarded Hverfjall as „Erhebungskrater" but the other
geologists mentioned above regarded it as an explosion crater, but were of different
opinions about the material which builds up the volcano and about the origin of its
inner cone. Only Thoroddsen seems to have studied to sorne extent, although not in detail,
the inner structure of the crater walls.
In 1948 the Icelandic physicist T. Einarsson published a paper on Hverfjall and a
similar volcano, Hrossaborg. He came to the conclusion that the magma feeding these
volcanoes was very cold and highly viscous. The eruptions were exceedingly faint, con-
sisting in granulation of the magma in the topmost part of the vent, creating an ash
porridge, which was temporarily lifted to the surface by steam. The craterwalls were
built up by this ash porridge and the ash was ultimately transformed into palagonite
tuff. This interpretation is on line with Einarsson’s interpretation of the origin of the
basic tuffs in the palagonite formation. Einarsson maintains that Hverfjall is covered
by ground moraine and thus is older than the last glaciation.
The following is mainly based on tlie author’s investigations, which started in the
summer of 1948:
The absolute height of Hverfjall is 452 m, its max. relative height ab. 150 m. Max.
depth of the crater is ab. 140 m. Max. diam. of the crater is 1040 m, minim. diam.
1000 m. The angle of the inner slopes of the crater walls is 25°, angle of outer slopes
20—26°. The crater rim is ab. 15 m broad. The crater walls are covered by a layer of
sand, gravel and blocks (Fig. 5). Bcneath this layer is stratified tuff (Figs. 6 and 9),
dipping regularly outwards. Up to 40 m above the bottom of the crater the dip is less
than 25°, from 40—70 m it is 25—30° and from 70 m to the crater rim it is 30°.
The tuff consists mostly of greyish black pumice (common grain-size 2—10 mm) in-
cluding sporadic pumiceous scoria with felspar phenocrysts. The felspar is plagioclase
(An. ab. 70%). The glass is basaltic glass (ND > 1,60, g 1,61) and shows no signs of
palagonitisation. The thickness of the tuff layers varies between 0.5 and 10.0 cm. The
inner cone is built up of the same material. For the chemical composition of tlie
pumice see Table I.
It is quite obvious that this very regularly stratified tuff is the result of an explo-
sive eruption and cannot at all have flown as a thick porridge. The gravel layer which
covers Hverfjall (Fig. 5) is not groundmoraine but weathered out of the underlaying
tuff (cf. Fig. 6). Contrary to Einarsson’s statement the blocks in the gravel layer which
covers the hills NE of Hverfjall diminish in size and get more sparse with increasing
distance from the volcano (Figs. 5 and 14). Layers of stratified tuff protrude through