Jökull - 01.12.1973, Page 52

There is probably another gap both shal-
lower and narrower in the rim of the caldera
at the head of Sólheimajökull, making it pos-
sible for the jökulhiaup, at least partly, to
emerge there, when the eruption breaks out in
the neighbourhood of the gap.
The jökulhlaups probably break up the
glacier where the pass of the caldera is narrow-
est and sometimes actually have found their
way out through there to the surface of the
glacier. Due to this the name Katla and Kötlu-
gjá became attached to the subglacial gap above
Höfðabrekkujökull. The name Katla probably
originally means “one who makes kettles” (i.e.
cauldrons in the snow field). In many of the
Katla eruptions the jökulhlaups appear repeat-
edly, indicating that the gap can become block-
ed partly or wholly during the eruption allow-
ing water to reaccumulate in the emptied space.
In the event that an eruption takes place in
Mýrdalsjökull outside the downthrow, the jökul-
hlaups can emerge almost anywhere.
Fig. 2 reveals that a substantial part of the
volcanic fissures of the Dyngjufjöll area lie
outside the Askja caldera, although the volcanic
activity during the last 100 years mainly took
place within its boundary. By comparing
Dyngjufjöll and Mýrdalsjökull (Figs. 2 and 4),
it can be seen that the neovolcanic zone tra-
verses the former while it seems to terminate in
the latter, e.g. the volcanic fissures, Eldgjá and
some others, run under the glacier from the
northeast, but no active ones extend from it to
the southwest. The volcanic activity is probably
more confined to the caldera rim of Mýrdals-
jökull than at Dyngjufjöll, where the volcanic
rift zone is more active.
In accordance with the opinions in this
paper, a basic eruption occuring in the central
area of Mýrdalsjökull with a jökulhlaup via
Mýrdalssandur is most probable, but a rhyolitic
pumice eruption cannot be excluded. It should
also be considered aí fairly likely that the Katla
eruptions can cause jökulhlaups in Markarfljót
or Jökulsá á Sólheimasandi. In fact, jökulhlaups
can occur from Mýrdalsjökull at any place.
Katla is situated in the vicinity of fertile in-
habitated areas, which have suffered many a
severe blow from this mighty neighbour. There-
fore there is ample reason for a thorough in-
vestigation of its behaviour and imminent
50 JÖKULL 23. ÁR
threats. The ideas explained in the text are
put forward to remind of that it is unwise to
expect Mýrdalssandur to be the sole playground
of the jökulhlaups from Mýrdalsjökull.
REFEREN CES
Björnsson, Flosi. 1970: Lega Kötlugjár. Jökull
20: 49.
Einarsson, Thorleifur. 1962: Askja og Öskjugosið
1961. Náttúrufr. 32, 1: 1-18.
Eythorsson, Jón. 1945: Um Kötlugjá og Mýr-
dalsjökul. Náttúrufr. 15, 4: 145—174.
Jónsson, Ólafur. 1945: Ódáðahraun I—III. 1376
pp. Akureyri.
Loptsson, Markús. 1880: Rit um Jarðelda á ís-
landi. 140 pp. Reykjavík.
Ólafsson, Eggert og Bjarni Pálsson. 1943: Ferða-
bók II: 100-115. Reykjavík.
Pálsson, Sveinn. 1945: Ferdabók. 813 pp. Ed.
by J. Eythorsson.
Rist, Sigurjón. 1967: The Thickness of the Ice
Cover of Mýrdalsjökull, Southern Iceland.
Jökull 17: 237-242.
— 1967: Jökulhlaups from the Ice Cover of
Mýrdalsjökull on June 25, 1955 and Janu-
ary 20, 1956. Jökull 17: 243-248.
Robson, G. R. 1956: The volcanic geology of
Vestur-Skaftafellssýsla. Ph.D.-thesis, 259 pp.
Durham.
Sigurgeirsson, Thorbjörn. 1973: Personal com-
munication.
Sigvaldason, Gudmundur E. 1964: Some geo-
chemical and hydrothermal aspects of the
1961 Askja Eruption. Beitráge zur Miner-
alogie und Petrographie 10: 263—274.
Sveinsson, Gísli. 1919: Kötlugosið 1918 og af-
leiðingar þess. 59 pp. Reykjavík.
Scemundsson, Kristján. 1972: Jarðfræðiglefsur
um Torfajökulssvæðið. Nátt.fr. 42: 81—99.
Thorarinsson, Sigurdur og Sigurjón Rist. 1955:
Rannsókn á Kötlu og Kötluhlaupi sumarið
1955. Bráðabirgðagreinarg. Jökull 5: 43—46.
Thorarinsson, Sigurdur. 1959: Um möguleika á
því að segja fyrir um næsta Kötlugos. Jökull
9: 43-56.
— 1963: Askja on Fire. Reykjavík.
— 1965: Hekla and Katla. Iceland and the
Mid-Ocean Ridges. Ed. by Sv. Björnsson.
Soc. Sci. Isl. 38: 190-199.