Iceland / by L. Hawkes [and others].

  • Great Britain. Naval Intelligence Division
Date:
1942
Catalogue details

Licence: Public Domain Mark

Credit: Iceland / by L. Hawkes [and others]. Source: Wellcome Collection.

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    they are best shown in the south-east. Stocks differ from laccoliths in that the lavas adjoining and overlying them have suffered little if any disturbance, and the problem as to how they grew is a much debated one. It seems likely that the block of lavas which these masses replace moved down whilst the rock liquid rose to occupy its place. The best exposed stock is that of Slaufrudal, south-east Iceland. ‘This is composed of light-coloured rock, and its relations to the dark- layered basalts are finely exposed (Fig. 3). The stock is 7-5 km. long and 2 km. broad; the base is not seen; its volume is at least 10 cu. km. The impressive mountain bastions of Austurhorn and Vesturhorn on either side of Lonsfjérdur are carved out of stocks. The rocks of these intrusions are dominantly either light-coloured acid (granite and W.30°N. E.30°S. Fig. 3. Section across the Slaufrudal stock. Horizontal lines—basalts; crosses— granite and granophyre; continuous lines—rocks now seen; broken lines— conjectural, now eroded away. Length of section 3 km. No vertical exaggera- tion. granophyre) or dark-coloured basic (gabbro). ‘The gabbro of Vestur- horn is a handsome coarse-grained rock, and it would provide the finest building stone in Iceland. The appearance of thin slices of the granophyres when seen under the microscope is most striking. The older rocks of Iceland can be matched with those of the same age in Greenland, the Faroes, west Scotland and north Ireland. In these districts there are no younger volcanic rocks; only in Iceland and Jan Mayen are they to be found. This later volcanic activity -continued from Pliocene times down to the present day, but it is not yet known whether there was a period of rest between the older and the younger activity. Before Pliocene times the rocks were disturbed by earth movements and they also suffered prolonged erosion. Had the later volcanic activity not broken out, Iceland to-day would have been a group of islands like the Faroes.
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