Náttúrufræðingurinn - 1975, Side 22

148
NÁTTÚRUFRÆÐINGURINN
Tröllatungu (1. mynd). Reyndist hún hálffull af eins konar brota-
bergi úr kísilrunnum viði og basalti, en ekki var mögulegt að sjá,
hvort kvika hefði raunverulega runnið inn í rörið.
Afsteypum eins og hér er sagt frá, hefur einnig verið lýst frá
Oregon í Bandaríkjunum (Nichols, 1941), Hawaii (Krejci-Graf,
1936), Kamerún í Afríku (Hyde, 1951) og Ástralíu (Walcott, 1900).
SUMMARY
Lava pseudomorphs of tree trunks in Tertiary basalts in lceland
by
Leifur A. Símonarson, Raunvisindastofnun Háskólans, Reykjavik,
Walter L. Friedrich, Geologislt Institut, Aarhus Universitet, Aarhus,
and Páll Imsland, Norrœna Eldfjallastööin, Reykjavik.
The authors describe pseudomorphs in basallic lava of parts of tree trunks
in Tertiary flows in Húsavíkurkleif, Northwest Iceland, and on Ósland in
Hornafjörður, Southeast Iceland. The pseudomorphs consist of dark, rather
fine-grained basalt witli a surface skin of basaltic glass. Here cylindrical tree-
moulds have been filled in with molten lava.
During eruptions on Hawaii lava has been observed to consolidate in an
isolating — but crystalline — crust around trees without destroying thern. Evi-
dently this also happened here, but tlie breaking-up process of the trees appar-
ently took place rather soon. This opinion is based on the fact that the flows
and pseudomorphs seem to be of the same basaltic material. Thin section exa-
mination made on both pseudomorph and flow supports this. The only differ-
ence of importance is that the pseudomorph is more fresh and, witli tlie ex-
ception of the outer skin of glass, not quite as fine-grained as the flow. The
lava has obviously solidified rather slower within tlie tree-mould, except along
the walls, where it solidified more rapidly producing basaltic glass. Therefore
it was not unexpected to find that soine of the pseudomorphs actually liave a
typical pillow structure in the cross section (Fig. 4).