Capelinhos, 29 September
1957
The
island of Faial in the Azores is noted for its luxuriant vegetation, but recent
eruptions on its western peninsula have created a new, harsh, mineral world that
juts out defiantly into the Atlantic Ocean. The latest eruption
began in September 1957 in the Atlantic Ocean off the Capelinhos lighthouse on
the Costa da Nau, the westernmost bastion of the peninsula. It
occurred near the Ilheus dos Capelinhos, islets which were themselves remnants
of a former volcano almost obliterated by marine erosion. The growth of
Capelinhos volcano was thus only regaining ground previously lost to the
Atlantic waves. Capelinhos first experienced typical Surtseyan activity: a
trailer to the birth of Surtsey in 1963. In a way, Capelinhos has been upstaged
by Surtsey and its birthright has been stolen, because it had a prior claim to
designating this type of eruption.
Capelinhos
provides a superb example of the interplay between volcanic eruptions and
shallow water, not only in the changing nature of the eruption itself but also
the interaction between the erupted materials and marine erosion. The typically
Surtseyan initial eruptions of Capelinhos expelled weak tuffs that were quickly
eroded; the subsequent Strombolian eruptions produced more consolidated
fragments composed largely of spatter, and especially an armour-plating of
lava-flows, that retarded marine erosion.
The
eruption of Capelihos was the longest since the Azores were settled in the 15th
century, and lasted from 27 September 1957 until 24 October 1958. It
had four distinct episodes. In the first two episodes, the eruptions were
predominantly Surtseyan in character. In the third episode, Surtseyan activity
was gradually superseded by increasingly dominant Strombolian activity. In the
final episode, Strombolian activity occurred practically exclusively. Fumarole
activity continued with decreasing temperatures and frequency until 1979.
Capelinhos
first indicated its impending arrival with more than 200 weak earthquakes
between 16 and 27 September 1957 that increased in incidence as their epicentres
migrated to the westernmost point of Faial. At 08.00 on 27 September the sea
began to boil in shallow water about 80m deep, almost 1 km west of Capelinhos
lighthouse and about 400m west of the Ilheus dos Capelinhos. Gas and steam were
emitted and the sea was discoloured during the next two days. The eruption began
in earnest on 29 September with vigorous explosions of black basaltic ash in
pointed jets, as if they had been fired from a gun. They were shaped like
cypress fronds or cockerels' tails, which subsequently became recognized as the
hallmark of Surtseyan activity. The jets rose 1 km into the air and were
surrounded by billowing white clouds of steam that commonly rose 4km high. The
eruptions gradually focused on the western end of a fissure about 250m long. The
explosive spasms each lasted about half an hour and happened at two-hourly
intervals. By 10 October 1957 a ruff cone, 100m high and 800m wide, had been
built, almost enclosing a broad crater that remained open to the sea on the
west, so that water had no difficulty in entering the vent. This activity ceased
on 29 October 1957 and the weak, unconsolidated tuffs began to slump down the
tuff cone, like a child's sandcastle swamped by the oncoming tide. By 1 November
1957, the ruff cone had been obliterated above sea level and redistributed in a
broad shoal off western Faial.
The
second episode began on 7 November 1957 after a week of total calm. It
replicated the eruptive style of its predecessor, except that the vent was 500m
farther east. A new tuff cone grew around a wide crater open to the sea, and
wind-blown ash built an isthmus joining the new volcano to Faial on 12 November
1957. This ruff cone was 600m wide by mid-December, but it also began to slump
into the sea as activity declined. It seemed as if the second ruff cone of
Capelinhos would suffer the same fate as the first, until a red glow on its
eastern flanks at 22.30 on 16 December 1957 revealed a change in eruptive style
with the first lava emissions.
The
third episode saw the advent of Strombolian eruptions. Lava-fountains and a
lava-flow signified that sea water had failed to penetrate the vent for the
first time, because of the copious accumulations around it. The third episode
was a period of mixed activity. Surtseyan explosions, whenever water succeeded
in entering the vent, were both more numerous and more voluminous. From time to
time, notably on 29 December 1957, sectors of the ruff cone, composed largely of
wet fragments, collapsed into the sea. Lava effusions were relatively
infrequent, taking place especially on 31 January, 1 and 6 February, 6 March,
and 20 and 23 April 1958, but the AA flows that formed an apron around the ruff
cone were vital for the survival of Capelinhos because they retarded marine
erosion and helped keep the sea from the vent. Throughout the winter, however,
the Surtseyan explosions in-creased in violence, and by the end of March the
ruff cone had reached 150m high and l km across, with a crater 450m wide,
covering the Ilheus dos Capelinhos in the process. At this time, ash and bombs
were often being fired 1800m into the air and they whistled down over the
western promontory of Faial. The area became uninhabitable, especially when ash,
mixed with sodium chloride evaporated from the ocean, destroyed crops and buried
houses, and eventually formed a thick noxious blanket from which only the badly
damaged Capelinhos lighthouse still emerged.
The
fourth episode was inaugurated by a phase, lasting from 12 to 14 May 1958,
during which more than 450 tremors were recorded. They were not centred on
Capelinhos, but on the crest of the Caldera of Faial and at Praia do Norte on
its lower northwestern flanks. Capelinhos thus did not cause these tremors. The
western peninsula was warped up and down by more than 1.5m, fumarole activity
was resumed in the caldera, and the earthquakes badly damaged Praia do Norte.
The effect on Capelinhos was radical. The Surtseyan eruptions ceased almost
completely, because, except on rare occasions, the sea could no longer enter the
vent. A large cone, composed predominantly of welded basaltic spatter, grew up
from huge lava-fountains within the ruff cone. In addition, lava-flows issued
from small external fissures, as well as from the central craters, accumulated
in the atrium between the spatter cone and the ruff cone, and occasionally broke
out to form an apron around the volcano. This effusive activity
ended abruptly on 24 October 1958. By then the core of Capelinhos was a
steep-sided spatter cone, 160m high, composed of wine-red basalt encircling a
vertically walled, jagged crater. Around it spread the ruff cone, 150m high and
1 km across, with gender inner and outer slopes rarely exceeding 20. It was
composed of innumerable layers of fine tuffs generally resembling buff sand and
its surface was strewn with remarkable basalt bombs the size and shape of
tortoises. These bombs were not quite solid on landing, so that their bases
flattened on impact, whereupon molten lava burst from the solid carapace to form
the head and legs of the tortoise. The seaboard flanks of the ruff cone were
almost completely girded by lava-flows, and some 2.4 km2 had been added to Faial.
Since
the eruption, the life of Capelinhos has been dominated by its destruction by
marine erosion. In spite of the efforts of its final effusive phase, Capelinhos
did not emit enough lava to protect itself adequately and it has been eroded
extremely rapidly. After 20 years it had already lost more than half its volume
and only 1 km2 now remains, and more than half the ruff cone and almost half the
spatter cone within it have been removed. The jagged crater of the spatter cone
now lies on the brink of vertical marine cliffs 150m high. In the south, the sea
has swept the ruff cone away to re-expose the chief islet of the Ilheus dos
Capelinhos again in the cliffs. On the southern slopes of the ruff cone, the
wind winnows the buff ash from the basaltic tortoise-shaped bombs and sets each
one in relief. Even the site of the vent used during the first eruptive episode
lies once again beneath the Atlantic Ocean. It is doubtful if much of Capelinhos
will remain in a hundred years, unless another major eruption occurs to save it.
However, if the trends of historic time are continued, it is by no means certain
that this eruption will come in time.
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