Acta naturalia Islandica - 01.02.1946, Síða 72

66
ORIGIN OF THE BASIC TUFFS OF ICELAND
cases the eruptions solely produce ashes (disregarding possible great
amounts of water) and cause devastating floods of meltwater( ?),
generally known as “jökulhlaup”. The eruptions occur at roughly
regular intervals, especially in the case of Grímsvötn, where the peri-
od is about 10—12 years. It appears to have been generally assumed
that the ice sheet is the direct cause of the peculiarities of these
eruptions, especially that the splitting up of the magma into minute
pieces of glass (sideromelan) is caused by the contact of the magma
with ice or meltwater. And in turn this “sub-glacial” origin of
sideromelan has been thought to illustrate the origin of the Palago-
nite Tuffs. But this is really a very unlikely assumption, and more-
over directly contrary to observation. The effects produced when a
lava flows over swampy grounds or into a lake are pretty well known.
The resulting material is either a heap of scoriae or pillow lavas.
A splitting up of the entire m'ass of lava into minute glass particles
does not appear to be on record. In the case of Grímsvötn we might
understand that a heap of scoriaceous material was piled up around
the vent and a small percentage of the magma split up into ash part-
icles. But to explain by the chilling influence of the ice how the
magma is in fact totally split up into ash does not appear to rest
on rational foundation.
The eruptions of Grímsvötn are, on the other hand, easily under-
stood on the same basis as those of Lassen Peak. They are caused
by the water which the ice-cap or more directly the large crater lake
gives off to the magma chamber. This water leads, in the way men-
tioned above, to an extremely high vapour pressure which after hav-
ing gathered force for a sufficient lapse of time results in an explo-
sive eruption and a splitting up of the entire magma extruded into
minute pieces of glass.
Thus these modern eruptions point to the same mode of formation
of basic glass as the studies of the ancient tuffs. They are not
produced by a chilling agency but result from certain physical pro-
perties of the magma itself, namely low temperature and a large
content of water.
On this basis the tuffs and the accompanying porphyritic fine-
grained basalts may now be classified genetically as proposed below.
1. The temperature of the magma is so high that with the per-
centage of gases it still retains it is still fluid after extrusion.
The result is the ordinary fine-grained porphyritic grey lavas.