Náttúrufræðingurinn - 2003, Síða 67

Tímarit Hins íslenska náttúrufræðifélags
Næsta spurning, hversu lengi
móbergskjarni Surtseyjar muni
standast áganginn, er öllu erfiðari.
Hér er einungis hægt að beita óná-
kvæmum samanburði við aðrar út-
eyjar Vestmannaeyja, sbr. 2. mynd.
Aldursgreining á mó frá Garðsenda
í Stórhöfða í Heimaey (Guðmundur
Kjartansson 1967) sýndi að Stór-
höfði myndaðist líklega fyrir tæp-
um 6000 árum (óleiðréttur geisla-
kolsaldur). Samaitburður á gjóskulög-
um í Bjarnarey og Elliðaey við
gjóskulög í Stórhöfða benda til að
þessar þrjár myndanir séu allar af
svipuðum aldri (Sveinn P. Jakobs-
son 1968). Þar sem Bjarnarey og El-
liðaey hafa myndast á sama hátt og
Surtsey og eru eins upp byggðar,
má telja líklegt að móbergskjarni
Surtseyjar muni standa uppi um
þúsundir ára.
SUMMARY
The marine abrasion of Surtsey,
Iceland: areal changes 1967-2002
and future development
During the initial phase of the Surtsey
eruption, from November 1963 to April
1964, basaltic tephra was deposited as
air-fall tephra, creating two large hor-
seshoe-shaped craters. Basalt lava start-
ed to flow from the western crater in
April 1964, continuing until May 1965.
A lava shield was gradually built up
and reached a thickness of 100 m at the
western lava crater. In the spring of
1965 a small island, Syrtlingur, was for-
med by submarine explosive activity
east-northeast of Surtsey; this islar.d
was washed away by wave action in
October 1965. Another small island,
Jólnir, was created southwest of Surts-
ey between December 1965 and August
1966 but disappeared as a result of
wave action in October 1966. Lava er-
upted on Surtsey again between Aug-
ust 1966 and June 1967, this time from a
new fissure inside the eastern tephra
crater, forming an irregular lava shield
towards the southeast. In December
1966 and January 1967 small lava flows
erupted from five different fissures in
the eastern tephra crater.
Heavy storms, rnainly during
7. mynd. Reiknilíkcm (lína) á grundvelli mælinga á heildarflatarmáli eyjarinnar
1967-2002 (rauðir tíglar). Likanið spáir pví að líða muni um 160 ár par til móbergs-
kjarni Surtseyjar verður einn eftir ofansjávar. - Observed area of Surtsey (red di-
amonds) and the estimated model (continuous curve). The model predicts tlmt it ivill
take about 160 years from the end of the eruption until only the palagonite tuff remains
above sea level.
winter, produce high wave activity on
the southwest coast of Iceland. Mar-
ine abrasion has therefore caused
rapid sea-cliff recession on Surtsey.
The loose unconsolidated tephra was
easily eroded, even during the phreatic
phase of the eruption. The Surtsey
basalt lavas are generally thin and
fractured and they have also been
heavily abraded. Hydrothermal acti-
vity within the tephra craters caused
rapid alteration of the tephra, gradu-
ally producing a dense palagonite
tuff. At present some 90% of the vol-
ume of the tephra above sea level has
been transformed into palagonite tuff
which has proved very resistant to
marine abrasion. This is consistent
with observations on the other is-
lands of the Vestmannaeyjar archipel-
ago. Longshore currents have deposit-
ed a sandgravel spit at the north side
of Surtsey. The material is primarily
derived from the western and eastern
lava cliffs, carried by heavy surfs
towards the north.
The cumulative area change of
Surtsey has been measured using 27
aerial photographs taken between 1967
and 2002. Surtsey reached its maximum
size at the end of the eruption in the
spring of 1967, 2.65 km2, but as a result
of marine abrasion the island had
shrunk to 1.40 km2 by 2002. The three
principal geological formations of
Surtsey, tephra, lava and palagonite
tuff, react quite differently to marine
abrasion, and it appears that since 1967
the erosion of the lavas has been the
most signficant factor in the reduction
in the size of the island. The data sug-
gest that the marine abrasion will pro-
ceed relatively rapidly until the core of
palagonite tuff, volcanic necks and lava
resting on palagonite tuff has been
reached, and will then slow considera-
bly. The size of the core is estimated to
be 0.39 km2.
The erosion is extrapolated in
accordance with a simple model
where the shape of Surtsey is approx-
imated to a cone. The rate of erosion
by volume is taken to be proportional
to the length of coastline. This model
is inputted with the observed values
and the extrapolated curve coincides
with the area of the core about 160 ye-
ars after the end of the eruption. Alt-
hough the fit is fairly close, it is not
entirely satisfactory by statistical
criteria. The discrepancies indicate that
the model is more likely to under-
estimate than overestimate the dura-
tion of the lavas.
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