Rit (Vísindafélag Íslendinga) - 01.06.1967, Blaðsíða 69

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canic activity was observed in Surtsey during the measure-
ments, but in Jólnir phreatic explosions were going on. On
4 near shore stations all around the islands a distinct mini-
mum of beam transmittance was obtained at 50 m depth.
This minimum seems not to be in connection with the tem-
perature distrihution, and it was not observed on the other
10 stations farther out. It was most probably caused by some
particulate matter.
S. Björnsson: What would prof. Einarsson think is the
cause of explosions at such great depths?
Trausti Einarsson: Intemal vapour pressure in a basaltic
magma is so low that bubbles do not form and expand at a
greater depth than about 100 m in the magma. But the main
explosivity is no doubt due to external vapour, formed by
contact of magma and water, e.g. in Surtsey. Such explosive
activity should be possible in pretty deep water as I said be-
fore.
Th. Sigurgeirsson: Lava entering the sea may develop a
crust which insulates it from the water. If the gas pressure
inside this crust is not too high it may be durable and prevent
intimate contact between flowing lava and water. G. Kjart-
ansson expressed the opinion that solidified lumps of lava
would roll down a slope of 26°—30°. To me it seems very
unlikely that such rough lumps would roll down a slope of
30°. Besides the slope is in many cases less than 20°. This
shows that the lava moves mainly as a liquid also when it
is under water.
K. Saemundsson: I want to point out, that there are many
other basaltic tuffrings in Iceland comparable to the Surtsey
ash cone regarding their material. Some of these are on dry
land, such as the tuffrings near Mývatn (Hverfjall, Lúdent,
Hrossaborg). They are, however, situated on a very perme-
able ground, i.e. young lavas, and it seems evident that the
mflux of water causing phreatic explosions took place under-
ground through the permeable lavas. This has generally been
agreed upon in the case of tuffrings elsewhere in the world
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