Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 65

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the decrease of outer pressure makes the extruding magma
explode. Yiolent explosive activity sets in. The pile and its
surroundings will be covered with tephra (pyroclastic mate-
rial, 2 in Fig. 4). If this happens in an ice-sheet the far-
spreading tephra is carried away by the moving ice, hut if
in the sea, the tephra comes to rest on the bottom.
The explosive stage was never reached by the new suh-
marine mounts 88, 67 and 74 (Fig. 3) near Surtsey, and
slightly by 25 (Surtla) by the end of its activity. Only Syrt-
lingur, Jólnir and especially Surtsey itself were explosively
active for months and piled up tephra rings around their
craters high ahove sea level.
Most móberg mountains ceased their activity in the explo-
sive stage, as did Syrtlingur and Jólnir, their last or topmost
product being tephra. But in the case of stapis (tablemoun-
tains) — including Surtsey — the emerging ring of tephra
around the crater became sufficiently watertight to prevent
the inflow of water into the vent. Consequently the eruption
turned effusive. This last or effusive stage in the building up
of a stapi is represented by the lava flows capping this type
of volcanoes or even forming shield volcanoes on top of them.
In Surtsey this change from explosive to effusive activity,
without any removal of the eruptive vent, took place on
April 4, 1964. Lava pouring out the following 13 months
formed a small shield volcano in the southern part of the is-
land. During a pause of 14 months in this kind of activity
the shortlived tephra islands Syrtlingur and Jólnir were form-
Fig. 4. Sections of Surtsey and. Leggjabrjótur (cf. Figs.2 and 3) in
equal scale, vertical exaggeration 2:1. Rock structures below sea
level are suggested. 1, pillow-lava with increasing content of vol-
canic breccia towards the top. 2, tephra, mostly tuff. 3, lavaflows
of shield volcano type, possibly with pillow structure at the base.
4, foreset breccia, probably containing pillows, scattered and in clu-
sters. — Strandlínur, shore-lines, due to ice-marginal lakes.
fönn, névé.