The Botany of Iceland - 01.12.1914, Síða 42

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THORODDSKN
Iceland, issued from volcanic fissures and crater-rows; these are
not volcanic mountains, but rows of lowT craters established along
the direct line of a fissure, more frequently upon level land. Of
this kind of eruptive vent, 87 are knowTn from Iceland at present.
Each of the craters in such a chain occurs independently and is
built up of scoriæ and lava. They are usually lowr, rarely exceeding
a height of 100—150 metres, while the majority of them are even
considerably lower; they are often very irregularly formed and com-
posed of several rings. Some crater-rows are very long; they often
attain a length of 5—10 krn., and sorne are longer still, as for in-
stance, the Laki crater-row of 1783, which has a length of 30 km.,
and contains about a hundred separate craters of various sizes.
Some crater-rows are so small that they resemble toy-volcanoes.
In some places tlie Iava has wTelled up out of íissures in large
streams without any visible craters. The largest of these lissures is
Eldgjá, north of Mjrdalsjökull; it has a lengtli of 30 km., and has
poured out lava-streams sufficient to cover an area of about 700
square km. In some places “explosive craters” occur, cauldron-
shaped depressions produced by a single volcanic explosion. Tlie
best known of these craters is Viti, on the side of Mount Krafla,
north of Mvvatn. It was formed hy a sudden outburst on May 17,
1724. For a long time afterwards there was a large, boiling slough
at the bottom of the crater, but this is now converled into a greenish-
coloured quiescent lake. The majority of the volcanoes in Iceland
are basalt volcanoes, and have poured out streams of basaltic lava,
and ejected basaltic slag and ashes. Only in the neighbourhood of
Torfajökull liparitic volcanoes occur — of post-Glacial origin, and of
peculiar aspect — which have poured out lavas rich in silica. The
interior of the lava-mass is grev or reddish brown, but the surface
of it is jet-black, as it consists of obsidian, covered here and there
by light, almost w:hite, pumice. The largest liparitic lava-streams
are those called Hrafntinnuhraun and Dómadalshraun. In some
places streams of liparitic blocks in a half-melted condition have
flowed out from craters in the mountain-sides, and several volcanoes
have ejected liparitic pumice, as for instance, the volcano of Askja,
1875. Many volcanoes in Iceland are buried beneath the snow and
the glaciers, and as mentioned above, when they break out into
eruption, large masses of ice are melted and the glaciers burst,
w'hich causes tlie neighbouring level lands to be inundated by
enormous floods of water, with huge fragments of ice tossing on