Geology Reference
In-Depth Information
76 km
2
, all of which owe their present
confi guration to a colossal volcanic
eruption that took place about
3600 years ago (estimates range from
1650
B
.
C
. to 1596
B
.
C
.). Indeed, the
eruption was responsible for the pres-
ent islands, the origin of the huge
caldera, and it probably accounted for
or at least contributed to the demise of
the Minoan culture on Crete. Further-
more, some authorities think that the
disappearance of much of the original
island during this eruption was the
basis for Plato's story about Atlantis
(see Chapter 9).
As you approach Santorini from the
sea, the fi rst impression is snow-
covered cliffs in the distance. On closer
inspection, though, the “snow” is actu-
ally closely spaced white buildings that
cover much of the higher parts of the
largest island (Figure 1b). Perhaps the
most impressive features of Santorini
are the near vertical cliffs rising as
much as 350 m from the sea. Actually,
these cliffs are the walls of a caldera that
measures about 6 by 12 km and is as
much as 400 m deep. The caldera-
forming eruption, “known as the
'Minoan eruption,' ejected into the air
30 cubic kilometers of magma in the
form of pumice and volcanic ash. This
material buried the island as much
as 50 m deep] and its civilization.”
Today, approximately 8000 people live
on the islands, and we know from
archaeological evidence that several
tens of thousands of people resided
there before the Minoan eruption
when Santorini was larger. However,
a year or so before the catastrophic
eruption, a devastating earthquake
occurred and many people left the
island then; perhaps there were
signs of an impending eruption by
this time.
The fact that the island's residents
escaped is indicated by the lack of
human and animal skeletons in the
ruins of the civilization, the only
exception being one pig skeleton. In
fact, archaeological excavations, many
still in progress, show that the people
had time to collect their valuables and
tools before evacuating the island.
Their destination, however, remains a
mystery.
1
The two small islands within the
caldera, where volcanic activity
continues, appeared above sea level in
197
B
.
C
. and since then have grown to
their present size. The most recent
activity occurred in 1950 on the larger of
the two islands (Figure 1a).
Santorini volcano began forming
two million years ago, and during the
last 400,000 years it has erupted at least
100 times, each eruption adding new
layers to the island, making it larger.
1
Vougioukalakis, G., Santorini: The Volcano (Institute for the Study and Monitoring of the Santorini Volcano, 1995) p. 7.
plate boundary is also present in Africa as the East African Rift
system, which is well known for its volcanoes (Figure 5.15).
As we noted, these volcanoes at convergent plate bound-
aries consist mostly of lava fl ows and pyroclastic materials
of intermediate to felsic composition. Remember that when
mafi c oceanic crust partially melts, some of the magma gen-
erated is emplaced near plate boundaries as plutons and
some is erupted to build up composite volcanoes. More vis-
cous magmas, usually of felsic composition, are emplaced as
lava domes, thus accounting for the explosive eruptions that
typically occur at convergent plate boundaries.
Good examples of volcanism at convergent plate
boundaries are the explosive eruptions of Mount Pinatubo
and Mayon volcano in the Philippines; both are near a plate
boundary beneath which an oceanic plate is subducted.
Plate Boundaries
Nearly all of the large active volcanoes in both the circum-
Pacifi c and Mediterranean belts are composite volcanoes near
the leading edges of overriding plates at convergent plate
boundaries (Figure 5.15). The overriding plate, with its chain of
volcanoes, may be oceanic as in the case of the Aleutian Islands,
or it may be continental as is, for instance, the South American
plate with its chain of volcanoes along its western edge.
Search WWH ::
Custom Search