Náttúrufræðingurinn - 1966, Síða 34

180
NÁTTÚRUFRÆÐINGURINN
beginning in Nov. 1963 as a single eruption, comparable to the “Mývatn Fires”
1725—1729, during which lava broke out from at least four separate fissures.
Tlie fissures that have opened up in the Surtsey area seem to be arranged en
eclielon and so are possibly also older eruption fissures in the Westman Islands
area. This may throw some light on the tectonic forces acting on the rift zone
of Iceland as such fissures are thought to be the result of torsion.
The main types of the Surtsey activity were the (phreatic) explosive and the
purely effusive one. An intermediate stage was characterized by lava fountains.
The explosive activity was also of two types. When the sea flooded the vent
or seaped rapidly through the tephra walls the activity was that of iutermittent
“wet” explosions or “cocks tail” explosions (Fig. 5), typical for submarine erup-
tions, but when the access of the sea was partially or nearly wholly blocked
by a tephra reef or wall the activity was a continuous uprush of lava and
tephra (Pl. I). Fig. 6 shows a typical alternation of these types of explosive
activity. On the whole the Surtsey eruption is a striking example of liow
greatly external conditions may influence the behavior of a volcano. So was
e.g. the change, from explosive to effusive activity on April 4, 1964, entirely
due to the fact that the access of the sea to the vent had become blocked by
a tephra wall built up during some days of calm weather and sea. The com-
position and physical properties of the magma were practically the same imme-
diately before and after this change in the activity. It is also noticeable tliat
the eruption in Surtsey proper started as a fissure eruption and ended as an
eruption l'rom a circular vent. This may be the case with the initial stage of
all shieldvolcanoes. Fig. 7, based on aerial photos, shows the area changes of
the Surtsey from time to time. The results of the area measurements are shown
in Table I and Fig. 9. Fig. 8 shows Surtsey and its lava field Aug. 24, 1965,
but the island was practically the same when the eruption finished there May
17. Its area was then 2.45 km2, whereof lava covered 1.53 km2. The highest
point of the island was then 169 m (173 in 1964). The position of that point
is 63°I8'22"N and 20°36'56"W. The total volume of lava, includiug pseudo-
tephra (hyaloclastite) and pillow lava is, roughly estimated, 250 á 300 mill. m3.
This corresponds to an average volume increase of 7 á 8 m3/sec. during the
131/2 month of effusive activity. The total volume of lava and tephra now
probably exceeds 0.8 km3. Much of the supramarine lava is of the helluhraun
(pahoe-hoe) type (Pl. IV a). Beneath liighwater level one could observe a ten-
dency to the formation of pillow lava (Pl. IV b). Yet no typical pillow lava was
observed, but it may be expected to occur at some depth.
Of a special interest was to watch the glowing lava wlien it came in contact
witli the sea. The crumbling effect of the water on the surface of the lava
tongues was very great indeed and the formation of pseudotephra or hyalo-
clastites, mainly coarsegrained glassy sand, was so rapid that a collar of sand
was piled up in front of the advancing lava flow on which collar one coulcl
walk at low tide without getting wet.
Fig. 10 shows some stages in the development of Surtsey, resulting in a lava
sliield of the Hawaiian-Icelandic type resting on a socle consisting of teplira,