Acta naturalia Islandica - 01.02.1946, Blaðsíða 71

ORIGIN OF THE BASIC TUFFS OF ICELAND
65
the steam pressure is such a magma is suddenly lowered during ex-
trusion the magma will become too viscous to crystallize. This glassy
consolidation may be effected before or during extrusion or it may
occur after some flow on the surface of the earth, depending on the
temperature. In this way we may understand that in some cases
streams of glass fragments are extruded and in other cases a vis-
cous magma which comes to rest on a steep slope or builds ex-
ceptionally thick layers, partially or wholly glassy.
To this picture must be added the factor of explosiveness which
will prevail in such cases where the pressure of the magma is releas-
ed very suddenly. Explosive eruptions will then result and the magma
is thrown up into minute pieces of glass. Deep explosions will in
some cases tear away pieces from the wall of the volcanic funnel
and a mixture of glass fragments and blocks, more or less worn off,
may be extruded to produce a mudflow of morainic appearance.
It has been shown by G. W. Morey24) how extreme vapor pres-
sures may be developed in a relatively cold and viscous magma. When
such a magma absorbs water, rapid crystallisation will be inaugurat-
ed on account of the lowering of the viscosity, but thereby the vapour
pressure is enormously increased and devastating explosions may be
the result.
In this way the terrific volcanic blasts of the Peléean type have
been explained. Allen and Day20) explain the explosive activity of
the Lassen Peak as due to a high vapour pressure in a cold and viscous
magma. It is assumed that surface water reached a very viscous
magma mass and was absorbed in it. The resulting decrease in
viscosity would cause a sudden onset of crystallization through which
the vapour pressure was increased. The pressure would continue to
increase until the roof of the magma chamber yielded. Then the
magma would be thrown up as fine-grained ash.
Eruptions of this type would reasonably be expected to have oc-
curred in Iceland, and we pointed out in an earlier chapter that to
them must probably be ascribed the striation which is found in a
few places in connexion with the conglomerates, and some of the
sheets of conglomerate may also reasonably be explained as mud-
flows caused by such volcanic blasts. And even to-day such eruptions
occur in Iceland.
Beneath two of Iceland’s ice-caps are buried very active volcanoes,
Katla in the Mýrdalsjökull, and Grímsvötn in Vatnajökull. In both
Origin of the basic tuffs of IcelancL 5