Náttúrufræðingurinn - 1966, Page 39

NÁTTÚRUFRÆÐINGURINN
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SUMMARY
A Comparison of Tablemountains in Iceland
and the Volcanic Island of Surtsey off the South Coast of Iceland.
by Gudmundur Kjartansson
Museum of Natural History, Departmenl of Geology and Geograpliy, Reykjavik.
Terms like Inselberge, tablemountains and several others liave been sug-
gested by foreign explorers for a special type of volcanic mountains in Iceland.
In Icelandic tliey are called stapar (sing. stapi). Tliese are isolated mountains
with steep sides and flat or gently convex tops. Near the base and usually up
to a level above tlie middle of their sides the stapis consist of móberg (i. e.
basaltic hyaloclastic and more or less palagonitisised rocks) and pillow-lava,
but the top with its sharp eclges is made up of lava flows. All these rocks are
of Late Pleistocene age (Fig. 1).
ín the first decades of this century it was debated by geomorphologists, mostly
Germans, whether these volcanoes owed their peculiar shape to erosion or to
tectonic forces, i. e. whether they were Zeugenberge or Horste. The latter view
gained ground as time went on, without being proved, however, in the case
of any single stapi. In 1943, after studying some stapis in South-western Iceland,
the present writer pointed out a third possibility: that the stapis were piled
up by subglacial eruptions. According to this hypothesis the móberg and pillow-
lava structures at the base are the result of rapid chilling in the melt water,
whereas tlie normal flows of lava on the top were extruded subaerially when
the mountain had emerged above the ice surface. The steep walls of the sur-
rounding ice prevented distant spreading of the erupted material and moulded
the mountain almost into its present shape.