Nomination of

SURTSEY – NOMINATION FOR THE UNESCO WORLD HERITAGE LIST
View to the west over the crater Surtungur. (Photo:
Daníel Bergmann 2003).
Tephra
During the explosive submarine phase of the
Surtsey eruption, from November 1963 to April
1964, basalt tephra (volcanic ash) was produced
through the quenching of hot magma by cold sea
water. The resulting tephra formed two crescentshaped
cones which merged, and today cover an
area of 0.34 km 2 . The cones are built up of tephra
layers which to a considerable extent have
consolidated into tuff due to posteruptive
hydrothermal activity. Geologists call craters of
this type tephra or tuff rings, depending on which
material dominates. The eastern tuff cone, named
Surtur or Austurbunki, has a crest-width of 0.43
km, and the western tuff cone, Surtungur or
Vesturbunki, has a crest-width of 0.52 km. On
the scale of other Icelandic tuff or tephra craters,
the Surtsey tuff craters are medium-sized.
Characterised by poor sorting and varying
grain size, Surtsey tephra ranges in its
constituents from silt to boulders (Lorenz 1974),
although it is mainly made up of fine glass shards
less than 2 mm in diameter, along with mineral
phenocrysts and small rock fragments. The tephra
deposited above sea level forms finely-bedded
layers and is poorly sorted, contrasting to the
tephra below sea level, which is more chaotic in
structure and whose grains sometimes demonstrate
considerable size sorting. The porosity of
the tephra layers above sea level must rate as very
high, or 45–50% by volume.
Surtsey has come to be the world type locality
18
for Surtseyan tephra (Walker and Croasdale 1972;
Walker 1973), i.e. tephra produced by hydromagmatic
explosions in shallow water. The island
is also an important site for accretionary lapilli
(Sveinn P. Jakobsson and Moore 1982), whose
individual pellets there reach the exceptional
diameter of 3.5 cm. Also, Surtsey was one of the
first localities in the world where vesiculated tuff
was noted and described, i.e. tuff that contains
independent vesicles between particles, where
steam was trapped while the original tephra was
being deposited (Lorenz 1974).
Lava
The Surtsey eruption evolved from an explosive
phase into an effusive phase at the western crater
(Surtungur) in April 1964, when the island was
large enough to isolate that vent from inflowing
seawater. Altogether, seven Surtsey craters and
crater fissures emitted lava between April 1964
and June 1967 (Fig. 2.6). The first major effusive
phase (1964–1965) produced a lava shield
extending 100 m above sea level and containing,
when finished, a total volume of 0.3 km 3 , while
the second phase (1966–1967) produced a 70-mhigh
lava shield with a volume of 0.1 km 3 . Both
of these shields consist of two structural units: an
inner lava cone and an outer lava apron
(Thorvaldur Thordarson 2000). Together they form
a lava field that slopes gently to the south and
east. Added to this are five small lava flows on the
slopes of Austurbunki. Today the lavas cover an
area of 0.72 km 2 .
When lava flows into water it brecciates and
gradually builds up a submarine delta in front of
the shore. Such a delta is foreset-bedded and
consists of the products of the quenchfragmentation
of molten lava and the
disintegration of subaerial lava flows. At Surtsey
the final layers in the western lava crater have
achieved a total thickness of 230 m, resting on a
130-m-thick brecciated foundation which lies
below sea level.
Due to heavy marine abrasion, high lava cliffs
have formed on Surtsey and now tower a height
of 80 m over the southwest shoreline. These cliffs
provide excellent outcrops for viewing the internal
structure of the subaerial lava shield, which was
constructed by multiple lava flows whose thickness
varies but averages 1–2 m. The morphology
of the lava surface is very interesting, particularly