Acta naturalia Islandica - 01.07.1964, Síða 8

INTRODUCTION
The Eyjafjöll mountain range (Fig. 1), is an igneous Quatemary formation
in the central south of Iceland. The range is capped with two separate ice sheets,
Eyjafjallajökull to the west. Mýrdalsjökull to the east. It belongs to the Pala-
gonite or „Móberg Formation" of Iceland.
I quote Kjartansson (1959): „The móberg (palagonite tuff and breccia)
consists chiefly of brownish basaltic glass that has been subjected to pala-
gonitization, a process of hydration and alteration. In addition, this rock usu-
ally contains fragments, globules and irregular lumps of more or less crystalline,
vesicular basalt, and it ranges in coarseness from fine tuffs to coarse breccias.
Some of these tuffs are regularly stratified and even the coarse varieties may
show rough stratification. In some places numerous veins, dykes or more volumi-
nous irregularly shaped masses of crystalline basalt are intercalated in the mó-
berg. These basalts are usually tightly jointed into prismatic columns trending
irregularly in all directions or roughly radiating from different centres. A typical
pillow structure is common.
Most students of the Móberg Formation assume that the reason why bas-
altic magma consolidated into móberg and its attendant varieties of basalt
instead of into normal lava flows was that it was extruded under water or
thick ice, and consequently they assume a subglacial origin of these rocks“.
Indeed, the forces which built these mountains are still active: Katla, the
well known subglacial volcano in Mýrdalsjökull, erupts at fairly regular inter-
vals, and Eyjafjallajökull has erupted at least once in history. That eruption
started in December 1821, and lasted until the 26th of June 1823, on which
date a Katla eruption started. The eruption has left a record in a thin light-
coloured ash band of acid composition which is well seen in Thórsmörk. All
Katla eruptions, however, are basic (Thorarinsson: pers. comm., Thorodd-
sen 1925, Loftsson 1880).
Towards the end of the Pleistocene glaciation, marine erosion moulded
the high south cliffs of Eyjafjöll. The broad piedmont plain of sands and gravels
is still being formed, Glacial rivers, draining Mt. Eyjafjöll and the highlands