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The Tsunami
Generated from the Eruption of the Volcano of Santorin in the
Bronze Age
{1650
B.C. (+/- 50 years)}
George Pararas-Carayannis
(Revised
assessment based on papers first published in Natural Hazards
5:115-123,1992. 1992 Kluwer Academic Publishers. Printed in the
Netherlands, in Sea Frontiers (1972) and in the Grolier Encyclopedia (1974)
Dr. George Pararas-Carayannis / Copyright
© 2004. All Rights Reserved
Abstract
Numerous tsunamis
with short and longer wave periods were generated by complex
mechanisms associated with the paroxysmal Plinian and Ultra-Plinian
phase of the Bronze Age eruption and collapse of the volcano
of Santorin. Based on an approximate reconstruction of volcanic
mass edifice failure geometries, from sector caldera and flank
collapses, and from estimates of volumes and time scales of resulting
kinematic processes - as inferred from geologic evidence and
recent geophysical findings - the near and far-field characteristics
of the tsunamis are evaluated.
A sequence of explosions
involving vertical and lateral blasting episodes, atmospheric
pressure pertrubations, a cone collapse sequence, and mass edifice
flank failures of the Volcano of Santorin - before, during and
after the paroxysmal phase of its Bronze Age eruption in 1650 B.C. (+/- 50 years)
- account for the catastrophic tsunami waves in the Aegean Archipelago
and the Eastern Mediterranean during that period.
Destructive waves
that affected the entire region were generated by volcanic hydromagmatic
explosions and caldera collapses of the volcano of Santorin,
and by volcanically, seismically or gravitationally induced aerial
or submarine landslides and rock falls of the island.
Violent pyroclastic
and hydromagmatic explosions, primarily during the third phase
of the eruption, generated long and short period atmospheric
pressure pertrubations which, in turn, generated a series of
destructive waves by coupling with the sea surface. The massive
caldera collapses and edifice flank flank failures and seismic
events, after
the paroxysmal phase of Santorin's eruption, contributed to the
generation of additional destructive tsunami waves.
The volcano's final
caldera collapse and the massive flank failures resulted from
gravitational instability and may have been triggered by one
or more earthquakes along the NE-SW trending normal fault in
the Aegean Sea where the Santorin volcanic field has developed.
Such extensive loss of land mass and source mechanism can account
for the size and destructiveness of the Bronze Age tsunamis in
the Southern Aegean Sea - with the interesting archaeological
and historical implications about the declination of the Minoan
civilization and Plato's legend of the "lost land of Atlantis"
in the incomplete "Critias" dialog.
Introduction
Numerous large destructive
earthquakes and tsunamis have occurred from antiquity to the
present in the Eastern Mediterranean Sea and particularly in
the Aegean Archipelago. There is historical evidence that such
large destructive earthquakes and tsunamis have ravaged the Aegean
islands and the Greek mainland, resulting in extensive destruction
of the Minoan and early Greek settlements (Pararas-Carayannis, 1973).
Of a total of 613
known historic earthquakes, at least 41 major events generated
documented tsunamis that struck the coasts of Greece. Sixteen
of these resulted in damaging or disastrous tsunamis. Between
1801 and 1958, 482 earthquakes with intensity equal or greater
than VI, and 170 with intensity greater than VIII occurred. Twenty
of these earthquakes resulted in tsunamis, and six of these tsunamis
were particularly damaging or disastrous in the Aegean and the
Eastern Mediterranean Sea (Galanopoulos,
1960).
Thus, the occurrence
of large tsunamis is quite usual for the Eastern Mediterranean
and the Aegean Archipelago. Many destructive tsunamis originated
from a the Hellenic arc region near the island of Santorin. In
AD 365 a destructive tsunami struck the Island of Crete and was
reported as far as Alexandria, where ships were carried inland
and left in the streets of the city. On 26 September 1650, a
destructive earthquake was accompanied by a submarine explosion
from the Colombo Volcano, whose crater lies in the sea on the
northeast of the island of Santorin. There was a devastating
tsunami observed on the island of Ios, north of Santorin, and
waves of up to 16 m were reported. In 1672, the islands of the
Cyclades, and particularly Santorin, were again shaken by an
earthquake. The island of Kos, to the east, was reported to have
been "swallowed up" presumably by the resulting tsunami.
The best documented and most recent tsunamigenic earthquake is
the one that occurred on 9 July 1956 near the southwest coast
of the island of Amorgos, killing 53 people, injuring 100, and
destroying hundreds of houses (Galanopoulos,
1957). The waves
were particularly high on the south coast of Amorgos and on the
north coast of the island of Astypalaea. At these two places
the reported heights of the tsunami were 25 and 20 m, respectively
(Galanopoulos, 1960).
Fig. 1. Location map
showing Thera (Santorin) and the Aegean Volcanic chain (dotted
line)
Of all the historical
tsunamis in the region, the best known but least documented has
been the one associated with the explosion-collapse of the volcano
of Santorin during its paroxysmal Bronze Age eruption in
1650 B.C. (+/- 50 years).
There is a great deal of speculation about the effects of this
tsunami on the ancient world, but little is known about its source
mechanism, the time history of events leading to its generation,
and of the maximum wave height distribution in the Eastern Mediterranean.
Most studies have attributed this tsunami to the explosion-collapse
of the volcano of Santorin in forming a large submarine caldera.
Although large tsunamis can be generated by such explosion-caldera
collapse, this mechanism alone cannot account for the large destructive
tsunami waves that occurred in the Aegean and the Eastern Mediterranean
Seas during that period.
Based on recent geological
and geophysical findings, the following is an examination and
analysis of the Bronze Age tsunami waves that were generated
by a sequence of explosions, atmospheric pressure pertrubations,
a cone collapse sequence, and mass edifice flank failures of
the volcano of Santorin - before, during and after the paroxysmal
phase of its eruption
in 1650 B.C. (+/-
50 years). This sequence of catastrophic events, preceeded by
major earthquakes in the 17th B.C. century, can account about the destruction of the
palaces at Knossos and elsewhere on the island of Crete, for
the gradual declination of the Minoan civilization, and for the
legendary "lost land of Atlantis".
Source Mechanism of
Volcanically Generated Tsunamis
 Volcanic
eruptions accompanied by mass edifice flank failures and explosion-collapse
processes in forming submarine calderas, are very efficient tsunami
generators. However, volcanic edifice failure events involve
relatively small volumes of mass and the tsunami or tsunami-like
waves they generate are of short periods and wavelengths. Although
these waves may be catastrophic locally, their heights attenuate
rapidly as they propagate away from the source.
Tsunami and tsunami-like
waves from volcanic sources have complex generation mechanisms.
Destructive water waves may be generated by the explosion and
collapse of a volcanic cone in forming an underwater caldera,
by the coupling of volcanically-generated atmospheric shock waves
with the sea surface, by massive flank failures, by debris avalanches,
and by massive aerial and submarine landslides. To understand
the volcanic tsunami generation mechanism, we must examine submarine
caldera formation processes and all other related concurrent
geotectonic activity that takes place before, during and after
a volcano's paroxysmal eruption or mass edifice collapse.
It is generally accepted
that volcanic calderas of the Krakatoan type are formed by the
engulfment of the unsupported upper volcanic cone into the drained
magmatic chambers below. However, this theory lacks detail and
is somewhat contradictory to evidence. For example, it has bean
observed that the volume of ejected pumice and other pyroclastic
debris is often considerably less than the volume of the caldera
depression. Therefore, the volume discrepancy suggests a possible
mechanism for the explosive removal of the upper volcanic cone,
rather than its total engulfment, or perhaps a combination of
the two processes. This in turn is related to tsunamigenic efficiency.
Fig. 2. Map of the
Thera (Santorin) volcanic field, indicating source areas of volcanic
tsunamis and postulated generating area of major tectonic tsunami.
Also, it is not known
with certainty whether the volcanic collapse phase is sudden
and total, or periodic and partial. The time-history of caldera
collapse and its geometry are very important in understanding
the tsunami source mechanism. Similarly, the triggering mechanism
of the last violent paroxysmal eruptive phase of the volcano
is not clear. It is not known with certainty if this phase is
purely hydromagmatic in origin, the result of extreme gaseous
pressures building below high viscosity magmatic residues or
a combination of the two processes.
Furthermore, certain
other large depressions that resemble Krakatoan calderas - though
associated with regional volcanic activity - result from tectonic
subsidence along fractures controlled by regional fault patterns
which may create step faultings and double pit craters as those
observed on the volcano of Kilauea on the island of Hawaii and
on the Piton De La Fournaise volcano on Reunion island in the
Indian Ocean. Such fault patterns may be localized along ring
fractures and may indeed form circular caldera-like depressions,
but may also be associated or triggered by earthquakes or larger
tectonic displacements along major fracture zones. This may have
been the case for the volcano of Santorin during its Bronze Age
eruptive sequence. Large tectonic displacements caused by a large
earthquake in the area could have triggered the final collapse
of the cone, the formation of the large caldera, and the mass
edifice failures which generated the destructive Bronze Age tsunami
or tsunamis.
Source Mechanism of
the Bronze Age Tsunami(s) in the Aegean Archipelago and the Eastern
Mediterranean Sea
 It
is not known how high the waves of the Bronze Age tsunami were,
but researchers (Marinos
and Melidonis, 1959)
found evidence of inundation on the west side of the island of
Anaphi - wich is 25 km east of the island of Santorin - at heights
ranging from 40 to 50 m above sea level. Other evidence of tsunami
inundation was presumably found as high as 160 and 250 m on the
northeastern side of the same island. At a greater distance away
from Santorin, evidence of the tsunami height was found at 5
m above sea level north of Jaffa-Tel Aviv. This tsunami height
- when corrected for eustatic change in sea level in the Mediterranean
for the last 3500 years - would have been at least 7 m during
the Bronze Age.
Fig. 3. Vent development
and caldera collapse during the four phases of the Minoan eruption
of the Volcano of Santorin (after Heiken and McCoy, 1984).
Could the tsunami
generated by the explosion/collapse of the volcano of Santorin
- or any tsunami for that matter - have been as high as 160 and
250 m on the northeastern side of the island of Anaphi ? Such
tsunami runup appears to be extremely unreasonable for either
tectonically or volcanically generated tsunamis on any open coastline.
In view of recent studies of the Santorin caldera formation ,
and other geophysical measurements and observations, the following
is an evaluation of the tsunami source mechanism from caldera
collapse alone.
The extension and
normal faulting within the Aegean plate are consistent with a
NE-SW trending graben along which the Santorin volcanic field
has developed (Figure 2). Eruptions of Santorin have occurred
from fissures located within this graben. Furthermore, there
is evidence of a much older flooded pre-Minoan caldera present
on the southern half of the volcanic field of Santorin before
the 1490 BC eruption (Heiken
and McCoy,1984).
This caldera was approximately 5- 6 km in diameter. The original
depth of this caldera is not known, but its present average depth
is 280 m. The presence of this preMinoan caldera would reduce
the volume of the Minoan caldera to 19 km3, which is reasonably
close to the estimated volume of magma extruded during the Minoan
eruption (Watkins
et al. (1978), namely
between 13 and 18 km3 (dense rock equivalent). Furthermore, the
vent development and caldera collapse occurred in four phases
(Figure 3) (Heiken
and McCoy,1984).
Thus, a long-time history of caldera collapse is inferred.
Scenario
and Time-History of Santorin's Bronze Age Explosive Eruption
and Mass Edifice Collapse
The following sequence
of events must have taken place: First, a vent was developed
during the initial phase of the eruption. Subsequent eruptive
episodes extended the vent(s) into the flooded caldera. A subsequent
series of large phreatomagmatic episodes must have occurred which
widened extensively the vent(s) and increased the volume of emitted
pyroclastics. It was not until the third phase that the more
massive phreatomagmatic episodes begun and a more massive subsidence
started at the western part of the island . Finally, the eruptive
episodes of the fourth phase, although with a large phreatomagmatic
component, were mostly from subaerial vents in the east. It was
during this phase that the final collapse of the Santorin caldera
occurred and the northern flank was blown by a lateral blast
episode. During this phase, the collapse extended from west to
east and the Santorin caldera enlarged to about 81 x 9 km with
a depth which averaged about 380 m below sea level. Subsequent
eruptions of Nea Kameni in more times have somewhat altered the
Minoan caldera, but not significantly.
 Based on this scenario and reconstructed
time-history of events, it is highly improbable that Santorin's
caldera collapse alone could have generated the tsunami waves
of the size that have been suggested in the literature as occurring
on the island of Anaphi to the east of Santorin, or the island
of Crete to the south. The caldera collapse process - as that
of the 1883 Krakatau volcano in Indonesia - occurred in phases
and probably lasted many hours or even a day or two (if not more).
Undoubtedly, several smaller tsunamis were generated during the
earlier phases of the eruption from the partial caldera collapse,
the atmospheric pressure waves of the violent phreatomagmatic
explosions, and from flank failures and debris avalanches within
the caldera and on the outer perimeter of the island.
Much greater tsunamis
must have been generated during the third phase when more intense
phreatomagmatic episodes begun and the preexisting caldera to
the south further collapsed. Finally, the fourth phase of the
eruption had episodes that reached Ultra-Plinian intensity. Apparently,
one or more lateral blast episodes and a mass edifice failure
obliterated Santorin's northwest flank and created the opening
which presently separates the island of Thera from Therasia.
Additional blast episodes and flank failures enlarged the southwest
opening of the PreMinoan caldera between what became Therasia
Island and the small island of Aspronisi. During this paroxysmal
fourth phase there was further massive collapses of the PreMinoan
caldera and a total engulfement of any remaining volcanic cone
to the north. The massive collapse created the northern part
of Santorin's caldera. The extenive caldera collapses to the
north and to the south and the mass edifice failures on the northwest
and southwest of the volcano generated destructive tsunami waves.
However, most of the tsunami energy from Santorin's caldera collapses
and these flank failures propagated primarily in a north-northwest
and a southwest direction into the Aegean Sea and cannot account
for the high tsunami inundation that presumably occurred at Anaphi
island and Crete. Therefore, the caldera collapse and failure
of Santorin's flanks along the aforementioned regions was not
the only mechanism of tsunami generation. Other massive flank
failures and debris avalanches must have occurred on the southern
and northeast sides of Santorin which generated even larger tsunamis
that affected Crete to the south and the neighbor islands to
the west. Santorin's coastal geomorphology, existing ring dikes
and submarine topography indicate that such mass edifice failure
occurred. It is also possible that a large earthquake along the
NE-SW trending fault triggered these land mass failures and additionally
contributed to tsunami generation - although not necessarily
at the same time as the eruption. Such flank failure events could
have occurred at different times before or after Santorin's eruption
in 1650 B.C. (+/- 50 years).The geometry of the collapse
would have allowed the generation of high tsunami waves only
from the two openings that were formed in the west and northwest
of the volcano. Most of the tsunami energy from the final caldera
collapse would have propagated in a westerly and northwesterly
direction.
The fact that the
fourth eruptive phase was primarily from subaerial vents - as
indicated from the emission of ignimbrites (Heiken and McCoy,1984) - with only a small phreatomagmatic component,
indicates that the bulk of the Santorin volcano was still up.
Thus, a different source mechanism for the final caldera formation
and tsunami generation must be found. What triggered the final
caldera collapse and the largest of the Bronze Age tsunamis,
could not have been a phreatomagmatic eruption, but some other
geophysical event. Such an event could have been a large earthquake
with an epicenter close to Santorin island along an about NE-SW
trending fault that, we suppose, also generated the 1650, the
1672 and the 1956 tsunamis. Such an earthquake may have been
responsible for triggering the final Santorin volcanic caldera
collapse and can account for the large tsunami that resulted.
Present
State of the Volcano of Santorin
A tsunami source mechanism
is proposed here resulting from the collapse of the volcanic
cone in forming a submarine caldera, following the last violent
eruptive phase, but augmented by additional larger scale crustal
movements. Only such a mechanism can account for the generation
of the extensive catastrophic sea waves documented for the Aegean
Sea, following the eruption and collapse of the volcano of Santorin
in 1490 BC.
There is substantial
evidence that on the island of Crete large earthquakes destroyed
the Minoan palaces of the island, including Knossos, throughout
the life of this Kingdom. The first major destruction of the
Palace of Knossos by earthquakes occurred around 1720 BC. After
the palace was rebuilt and restored to its original splendor,
it was again destroyed by the earthquakes of the fourteenth century
BC (Pararas-Carayannis,
1974). So, evidence
of large destructive earthquakes exists and Sir Arthur Evans,
in his excavations at Knossos, verified that. Specifically, he
found many Minoan houses ruined by huge blocks of rock, displaced
as much as 6 m from their original positions (Pararas-Carayannis, 1973). Only an earthquake of great magnitude could
accomplish this. Such an earthquake could have been associated
only with the seismic zone of the convex side of the Hellenic
arc (Hellenic Trench), which is an established tsunamigenic region
(Papazachos et al.,
1985, 1986).
Height of the Bronze
Age Tsunami
There is no doubt
that a number of tsunamis were generated from the gradual collapse
of the Santorin volcano over a period of time. Also, there is
no doubt that a much larger tsunami was generated that acted
as the catalyst in the declination of the Minoan civilization
(Pararas-Carayannis,
1973). There is conclusive
evidence that Minoan cities on the north and east coast of the
island of Crete were struck by huge tsunami waves (Marinatos, 1939). These included Amnisos, Malia,
Niron Chani, Psira, Ghoumia, and Zakros. Nothing is definitely
known about the tsunami on other Aegean Islands. However, a rough
estimate of the Santorin tsunami at Anaphi island, the closest
to the origin, can be extrapolated from the 7 m tsunami (corrected
for eustatic change), as documented at Jaffa-Tel Aviv, 900 km
away. This estimate, based upon geometrical dispersion, neglecting
effects of refraction, diffraction, or resonance, results in
a height of 42 m, consistent with the 40-50 m elevation at which
pumice deposits were found by Marinos
and Melidonis (1959).
Such pumice deposits were found at 350 m inland on the west coast
of the island. Also, the west coast of Anaphi island would have
been closer to the tsunami source area and would have experienced
the highest tsunami waves.
The estimate of 42
m at Anaphi appears to be reasonable. The highest possible tsunami
wave at the source' could not have exceeded 50 m The pumice deposits
found on the northeastern side of the island of Anaphi, at 160
and at 250 m could not have been carried by tsunami waves of
any eruption or of any other earthquake. These values are simply
too high. Furthermore, even a 50 m tsunami cannot be generated
by a tsunami source mechanism that involves only the explosion-collapse
of the Santorin volcano, even on the shortest time scale. Only
a mechanism which involved caldera collapse, in combination with
much larger scale tectonic crustal movements along a prevalent
NE-SW trending fault, caused by a large earthquake, can account
for the generation of the largest of the Bronze Age tsunamis.
Conclusions
Based on the above
evaluation the following conclusions can be reached:
(1) The time history
of caldera formation indicated by Heiken and McCoy is too slow
for large tsunami generation. Thus, a different tsunami source
mechanism must have been responsible for the extreme tsunami
observed in the Aegean and Eastern Mediterranean Sea in the Bronze
Age.
(2) Following several
paroxysmal eruptive phases and partial caldera collapses, the
final Santorin caldera collapse was triggered possibly by tectonic
subsidence resulting from a large earthquake along an about NE-SW
trending normal fault that has also formed the graben along which
the Santorin volcanic field has developed.
(3) Several tsunamis
must have occurred with different source mechanisms. The gradual
collapse of the western portion of the caldera must have generated
several smaller tsunamis originating at the northwest and west
opening of the Santorin caldera. The several paroxysmal phases
probably generated other small tsunamis.
(4) The collapse of
the remainder of the volcano into the empty magmatic chambers,
possibly triggered by a large earthquake, generated a larger
tsunami at the northwest and west openings which may have been
destructive in adjacent islands, in Crete and elsewhere.
(5) The much larger
tsunami, the Bronze Age tsunami, was generated by a combination
of normal faulting resulting from a suspected large tectonic
earthquake and possible underwater landslides of unstable volcanic
tuffs on the outer perimeter of the Santorin volcano. This event
may have occurred concurrently as the tsunami generated by the
collapse of the remaining volcano of Santorin, or at a different
time.
References
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1957, The seismic
sea wave of July 9, 1956,
Prakt. Acad. Athenes 32, 90-101.
Galanopoulos, A. G.:
1960, Tsunamis observed
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4), Rome, 371-386.
Galanopoulos, A. G.:
Undated, The Minoan
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pp. 218-232.
Galanopoulos, A. G.,
1981, New Light on
the Legend of Atlantis and the Mycenean Decadence, Athens.
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1971, The eastern
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S., and Ninkovich, D.: 1978, Volume
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