Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 94

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retained. In the topmost layer the fragments have in part
lost their angularity. The content of a glassy matrix is very
small and this and the other characteristics tend to show that
we are concerned with scree derived from a higher primary
porphyritic mass. (3) is a 1—2m thick brown tuff contain-
ing scattered worn stones. (4) is on the other hand a primary
stratified breccia consisting of bombs, bomb fragments, and
lava lumps in a brown glassy matrix. The layers dip 20—30°.
Towards the top the dip decreases, the composition changes,
first to a tuff with individual homhs, and finally to nearly
horizontal lavas (5), intercalated with thin tuff layers. All
the material in (4) and (5) is of the same porphyritic type,
no hiatus is found hetween (4) and (5), and both would
seem to belong to one eruption. The difference hetween (4)
and (5) might possibly he explained by assuming a water
level at the top of (4).
The mechanism by which the presence of water favours
the formation of fragmental masses does not seem to have
been clearly expressed. It has sometimes been assumed that
lava masses tend to split or even explode when they enter
water. This is not generally true. Several lava flows in Ice-
land have, after flowing for some distance from the crater,
entered water without anything spectacular happening to
the lava mass. Moreover, it is probable that these lava flows
were in such a state when entering the water that sidero-
melane glass could not possibly be formed. When lava flows
over wet ground, on the other hand, the vapour formed here
is capable of rushing through the lava and cause much frag-
mentation. In the same way imprisoned water at the base
of a subaqueous lava flow may cause much havoc in it.
But it is difficult to see that a breccia mass consisting of
more or less fractured bombs could be due to this process.
The main role of water is in my view the following.
Lava of fissure and shieldvolcano eruptions is produced
very largely in the form of fountains. The lava is thrown
out in the form of lumps of various sizes. The lumps cool