Náttúrufræðingurinn - 1948, Blaðsíða 27

NÁTTÚRUl'RÆttlNGURlNN
121
f jöllin við Mývatn vitna um, sé einmitt sú, sem lielzt mætti búast
við á tuglinu, og líking Hverfjalls og tunglfjallanna sé ]iví engan
veginn eins mikil tilviljun og í fyrstu mætti ætla. Ég ætla þó, að
aðeins minnstu tunglgígarnir gætu verið af þessum uppruna, en
það eru gígir með allt að 10 km þvermáli.
Summary
The crater Hverfjall near Mývatn has a rim-diameter of 1 km and
a lieight of 90—150 m above the surrounding country. It is tlius of
a very peculiar form which recalls rather the craters on the Moon
than terrestrial volcanoes.
A study of tliis and another similar crater, Hrossaborg, leads to tlie
conclusion that the feeding magma was very cold ancl highly viscous.
The eruption was exceedingly faint. It consisted in a granulation
of Lhe magma in the topmost part of the funnel, creating an asli-
porridge which was temporarily lií'ted to the surface by steam. But
most of tlie ash ran into tlie funnel again, as tlie viscous magma did
not rise into the empty space, and a ring of ash was left, transformed
ultimately into palagonite tuff. The craters are of late Quaternary
age.
It is suggested that a possible volcanism on the Moon would be
nourished mainly by very cold and highly viscous magmas, and this
might then lead to structures similar to Hverfjall. Considering the
absence of atmosphere and the fact tliat the gravity is l/6 of tliat on
the Earth, grand structures sucli as the smaller ring-mountains might
result.