Náttúrufræðingurinn - 1955, Síða 46

152
NÁTTORUFRÆÐINGURINN
öskulaga. Verður það verkefni ekki rakið hér, rúmsins vegna, en þess
vildi ég þó geta, að ég tel norðurgjána hafa gosið aðeins einu sinni.
öskulagið úr þvi gosi, og þar með Eldgjárhraunið austan Kúðafljóts,
er yngra en þykka vikurlagið hið næsta undir hrauninu vestan Hólms-
ár við brúna, en það vikurlag mun jafngamalt hrauni þvi er á því
liggur, og það hraun er, að ég held, hið sama og þekur norðurhluta
Álftavers, og munu þessi hraun runnin nokkru fyrir landnámsöld, en
síðar hefur hraun runnið niður um Mýrdalssand niður hjá Hrút-
hálsum, og er vel til, að það hraun hafi runnið yfir einhverja byggð
og sé þar að leita skýringarinnar á sögnum Landnámu um jarðeld.
Með mælingum á segulstefnu hraunanna má e. t. v. fá úr þessu
skorið.
SUMMARY:
The Geology of Icelund III. Eldgjá,
by Sigurdur Tliorarinsson.
The famous Eldgjé in the highland N of Mýrdalsjökull was „discovered" hy Th.
Thoroddsen on July 22nd 1893, and described by him. Since then many geologists
have visited Eldgjá, among them K. Sapper (in 1906) who mapped the entire
gjá. During the summers 1949—51 the English volcanologist G. R. Robson studied
Eldgjá and its lavaflows, but his work is not yet published.
In conjunction with some photos taken in Aug. 1955 the present author de-
scribes bríefly the northemmost part of Eldgjá, which is separated from its central
part NE of Móraudavatnshnúkar. The description and the map Fig. 6 are based
on author’s observations and aerial photos taken by Á. Bödvarsson in 1954. The
length of this part of Eldgjá is 8,2 km, its. max. width ab. 600 m and its max.
depth ab. 140 m. The inner slopes of the gjá consist mainly of palagonite breccias
and tillite-like breccias. The topmost part of the gjá slopes forms vertical walls
10—15 m high, consisting mainly of scoriæ, lava lumps („Schweiss-schlacken")
and tephratic lava, viz. lava formed by the running together of ejected molten
lava lumps. This lava covers a small zone on both sides of the gjá (cf. the map).
A thick layer of tephra extends towards SE and can be followed to the Sída
district. The author’s opinion is that the described part of Eldgjá has erupted only
once and that the eruption may be divided into three main phases: 1: a mainly
explosive phase forming the tephra layer and some of the tephratic lava. 2: a very
productive effusive phase forming the lava river that has flowed along the valley
of N. Ófæra to Skaftá, and — joining a lava river from the central part of Eldgjá
at S. Ófæra — formed the extensive lavaflows of Medalland and Landbrot. 3: a mixed
phase, extruding small amount of lava and forming craters along the bottom of
the gjá.
The shape of the present gjá, with the spectacular garland contours of its rim
(cf. Figs. 4 and 6) seems to be partly the result of explosive activity as maintained
by Sapper, partly a result of corrosion by lava and slipping of rock as maintained