Náttúrufræðingurinn - 1969, Side 30

134
N ÁT TÚRU F RÆ ÐINGURINN
hátt og sá, er myndaðist norðan í Surtsey í lok febr. 1964 og þá líkega
myndaðir af eldæðum tengdum eldrás hraungígsins ofan sjávar-
máls. I sept. 1964 gubbaðist hraun upp í nokkra daga úr slíkri eldæð,
er kom upp á yfirborð á miðri hraunsléttunni S af hraungígnum.
Varð af þessu nokkur kúfur þarna á sléttunni, en færðist síðar að
mestu í kaf í hraun, en þó sér hans enn merki.
SUMMARY
The last phases of the Surtsey Eruption
by S. Thorarinsson
In a previous issue of tliis periodical (Vol. 35, 1965. pp. 153—181) the present
writer outlined briefly the history of the Surtsey eruption frorn its visible be-
ginning on Nov. 14, 1963 to the beginning of the year 1966. In the present
paper this history is continued to the end of the eruption on June 5, 1967.
By then 3 years, 6 months and 3 weeks liad passed since tlie eruption first
became visible and the eruption had become the second longest witnessed in
Iceland, surpassed in length only by the „Mývatn Fires" in the 1720’s, which
lasted some months longer.
The third explosive (phreatic) phase of the eruption began on Dec. 26, 1965
(Fig. 1), when an eruption became visible 0,8 km SW of Surtsey. This phase
lasted until Aug. 10, 1967. The island, named Jólnir (Christmas island), which
resulted from this eruption, became visible for the first time on Dec. 28, 1966.
During the winter it fought a hard fight for its existence and was washed
away five times, the last time on April 7. It reappeared for the last time
a week later ancl grew more or less steadily until early July, when it had
reached about 70 m heiglu and an area of about 0.3 km2. In late May cauldrons
began to dcvclop on the norlh side of the crater as the result of subsiding
along concentric semicircular faults and gradually the crater becamc nearly
separated from the rest of the island by a lagoon (Figs. 2 A and B). The lagoon
on Surtsey had been fornied in a similar way in Febr.—March 1964.
The activity in Jólnir (l’l. I a and b. II a) cante to an end on Aug. 10, 1967
and by Sept. 20 nothing was left on the island except a reef which nearly
disappeared at high tide (Fig. 4.).
The average tephra production during the Jólnir phase was about 5m;l/sec.
Figs. 5 A and B show the thickness distribution of Syrtlingur and Jólnir tephra
on Surtsey towards the end of the respective eruption phases.
On Aug. 19 effusive activity began again in Surtsey froni a 220 m long fissure,
running N 10° E, that opened up in the crater Surtur senior (Surtur I, Fig. 3
and Pl. III a and b). This activity went on continuously until June 5, 1967 cover-
ing with lava an area of 1 km2 and increasing the area of Surtsey to 2.8 km2 (Fig.