Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 193

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water body that would find a way into the depths of the
fissure and cause explosivity of the eruption. And even after
fragmental rocks had filled up the lakes these rocks would
carry much groundwater that could be drained into new vol-
canic fissures. In the case of Hekla it seems just to be such
water that causes the main explosivity.
These ideas suggest that if a post-glacial eruption has taken
place within a lake, here might have been produced material
corresponding to the Fragmental Rocks. The only relevant
eruption of which I know is that producing the island Sandey
in Thingvallavatn. The observations are most satisfactory.
The material produced, and forming the crater walls, is en-
tirely explosive. Being a mixture of basalt blocks of various
sizes and “sand” it is quite different from the normal post-
glacial craters which are made up of slags and lapilli. The
explosivity in the Sandey eruption was most likely caused
by water penetrating into the volcanic fissure during erup-
tion, more exactly by the steam then developed. While the
coarser material consists of lava blocks, i.e. split-up lava that
now and then must have consolidated in the crater, most of
the finer “sand” consists of sideromelane, not tachylite. This
is a most striking fact and a close link with the Fragmental
Rocks. The origin of the sideromelane here is most likely to
be sought in explosions and splitting of the magma at depth
before a shift in the equilibrium between FeO and Fe203
had taken place.
As a result of these considerations we may provisorily as-
sume that the General layer of Fragmental Rocks is to a large
extent the expression of water entering readily the eruption
process at depth. The role of submergence is uncertain.
Coming now to still younger rocks we have firstly seen
that the table-mountains of the Southwest are composed of
elements of different ages, and have been formed by block-
faulting and uplift. This disproves the theory of a sub-glacial
origin of these structures.
We have further found that the fragmental rocks forming