The
Eastern
Rim
of
the
Thera
Caldera
showing
stratified
pyroclastics.
© Alan V. Morgan
In the last issue of WAT ON EARTH I described the first part of the 490 Field Trip taken by students in the Fall of last year. In that issue we covered the terrain from Athens through to Thermopylae via the north shore of the Gulf of Corinth. In this issue I want to describe the latter part of the trip, concluding with the highlight for some, the visit to the caldera at Thera.
After our visit to Thermoplylae we crossed from the mainland to Euboea (Evia) via a small ferry at the north of the island. We had hoped to visit some of the older metalliferous areas of the island where the ancient Greeks had mined silver, and the more modern nickel mines. Unfortunately events conspired against us, and in spite of some rigorous searches, we were never able to discover the entrance to the open cast pits for the modern nickel and bauxite mining operations. We spent the day gazing wistfully at distant hills with the mine roads and passing kilometres of conveyer-belts, which were non-functional, since the mines are temporarily closed. More practically, we also spent a fair amount of time looking at several hillside stability problems along the west coast road of Euboea and the geology around the aesthetically pleasing Calkida Cable Bridge. The cable- stayed bridge spanning from the mainland to the island of Euboea (Evia) opened for traffic in 1993. It is 695 m long, with a main span of 215 m and two side spans of 90 m. The Evian Gulf is a neotectonic graben formed by normal faults trending northwest. The bridge was founded on two horst blocks with an intervening trench. We were now at the mid point of our Greek trip and it was time to turn south to look at the Corinth region.
The Gulf of Corinth was formed some time between upper Miocene and lower Pliocene. During the last 250 years, the average rate of opening of the Gulf is estimated as 6 mm per year from historic data and geodetic survey measurements. The Isthmus of Corinth is a narrow strip of land south of the Gulf, at the centre of a neotectonic graben bounded by active faults. Just as Greece is one of the most seismically active countries in the world, the Gulf of Corinth is one of the most active in Greece.
Historical seismicity reconstructed for the last 2,500 years indicates a pronounced increase in the last 250 years. Signs of this tectonic instability are everywhere, ranging from uplifted marine terraces to modern earthquake damage to various structures and may be particularly well seen in the cut of the Corinth Canal. The southern shore of the Gulf is made up of a series of normally faulted segments of which some at least are seismogenic. In December 373 B.C. a disastrous earthquake destroyed the Town of Bura and the large and prosperous town of Helice was carried to the bottom of the sea. The Helice fault which runs about 2-3 km south of the Gulf of Corinth was reactivated in 1881 with the development of a fresh scarp 2m high and 13 km long. It can be seen on air photographs as a prominent scarp. Seismologists have estimated an average return period of about 100 years for a Magnitude 8 event along this fault. First in 1963 and then in '68, '70, and '82 I have crossed over the canal on the road or rail bridges and gazed down into the waters which link the Aegean and Ionian seas. 1994 was the first time that I had ever travelled through the canal by boat, and we all had a keen sense of anticipation as the harbour pilot launch drew alongside the quay to embark us on a return trip. As we moved along the canal, sandwiched between two other larger vessels, we were reminded of the seismic activity of the region. Many ENE-striking subvertical normal faults occur in the vicinity of the Canal, and are considered active. The north side of the canal showed some "classical" normal faults, many exhibiting up-turned and down-turned ends adjacent to the fault plane.
The Corinth Canal is 6.3 km long, 21m wide, and 79m maximum height, with a water depth of 8m. About 11 million cubic metres of rock were excavated in its construction. The Pliocene marls exposed in the central part of the Canal are overlain by several cycles of near-shore sandstones, conglometerates, silts and clays of the Pleistocene. Recent research has revealed an upward transition from brackish water facies (grey to blue-grey marls) to marine sediments (light yellow marls).
In Roman times, a 4m wide stone-paved roadway called the Diolcos was used to drag small ships across the Isthmus; it had been constructed during the fighting for ancient Corinth. Nero started excavation of the canal in AD 66 using 6,000 Jewish prisoners. He was killed before the work could be completed. We had the opportunity of seeing a small monument to Nero carved into the soft sandstones near the western terminus of the canal. The modern canal was completed in 1893 after excavations by the French, and later, Greek Corinth Canal Companies under the direction of General Turren, an associate of Ferdinand de Lesseps. Small excavations cut into the walls illustrate where construction workers scaled the near vertical walls. It is something of a miracle that the slopes have shown only minor instability problems in spite of their significant length, very steep inclination, the strong earthquakes that have shaken the Corinth area, and the degree of ignorance of geomechanics at the time of construction.
During the second world war, the railway bridge across the canal was blown up causing serious stability problems to the neighbouring slopes. A new railway bridge was put into circulation in 1948. During construction of the abutments, joints with aperture up to 10 cm were uncovered in the canal's rock walls, which opened further during earthquakes in 1953 and 1981. Glass telltale monitors and displacement transducers were installed. To minimize the risk of instability, the abutments were reinforced with untensioned grouted dowels.
We stayed on a campground just south of Corinth with a fervent hope that earthquakes, tsunamis and other disasters would stay away for the 48 hours or so that we were in the area. The following day, Sunday, the "rest day", was devoted to classical archaeology, starting with a drive along the coast road to the well-preserved theatre at Epidavros. Here I was able to prove to the students that the acoustics were considerably better than our lecture theatres at Waterloo by providing a few minutes of monologue on what we might see in the Cyclades!
From Epidavros it was west to Nafplion, a small coastal town at the southern edge of the Plains of Argos. Nafplion, the capital of the prefecture, is reputed to be one of the loveliest towns in Greece, with neoclassical houses, picturesque streets, wooden balconies with cascading flowers, Turkish fountains and mosques. Kapodistrias, the first governor of Greece was assassinated in Spyridons Church. If you feel like a climb, (I think I was the only one that attempted it) 857 steps lead up to the Venetian fortress where the view over the Plain of Argos is worth the effort. From here there is a magnificent view to the north toward Tiryns and Mycenae. The students spent most of their time gazing at a sleek, and probably horrendously expensive, jet black yacht moored on the quay facing the fortified islet of Bourtzi which stands in the middle of the bay. After a lunch break we moved on through the dusty and hot plain to Mycenae.
Mycenae was the most powerful influence in Greece up to 1100 B.C. Old and new Mikines both lie just off the main Corinth-Argos road. Mikines was one of the great natural defensive locations in Greece. Whoever held it could survey the plain north to the Dervenakia pass and south as far as Argos. Mikines is ringed by defensive walls and one enters through the massive "Lion Gate", one of the oldest examples of monumental sculpture in Europe. Inside, the excavations reveal six royal tombs, courtiers' houses and sanctuaries. Cut into the nearby hill of Panayitisa, has been found the treasure of Atreus, also known as the beehive tomb of Agamemnon. Its lintel weighs 120 tons. Most of the more exceptional finds have been removed to the national archeological musuem in Athens, including frescos, gold jewellery and the gold mask said to have belonged to Agamemnon. The ruins of Mycenae are really quite impressive. Since many members of the 490 class were geological engineers we left it to their imagination to explain how the cyclopean walls were built of massive blocks of limestone. We spent about three hours wandering through the fortress looking at the the walls which are over 6m thick in many places; going into the rock- cut shaft which descended to the water supply (interestingly now completely dry; - a problem for the hydrogeologists to ponder), and thinking about the mechanical problems of moving stones which weighed up to 50 tonnes each.
All too soon, the sun was setting and we re-embarked for a fairly hair-raising trip to Athens, together with (seemingly) half the population of the city who had ventured out into the countryside for the weekend. Driving in rush hour traffic in Toronto or London is bad enough but memories of a night-time drive into the city centre will stay with me for a long time!
The following day we had the opportunity to see part of the construction of the new Athens subway system. The basin in which Athens is situated has a NE - SW axis and about 22 km long and 11 km wide. Along the centre of the basin rise a series of hills including the 156m high Acropolis. Bedrock consists of the Athens "schists", much of which is covered by Quaternary layers of varying thickness. The Athens "schists" actually include sandstones, slates, marls, phyllites, cherts and masses of brecciated or crystalline limestone. The upper units of the Athens "schists" include marls, platy limestones, sandstones, conglomerates, and breccias. Limestones crown the hills of the town. The ages are Triassic, Jurassic or Cretaceous. The limestone caprock of the Acropolis is of upper Cretaceous age. The shales and sandstones correspond to an upper Cretaceous flysch and enclose peridotites, spillites, and tuffs.
We had a fascinating lecture on the problems of subway construction by the chief engineer at Constitution Square (a focal hub of the new system), and then went underground to see the tunnel construction. As the engineers pointed out they were proceeding rather cautiously. They had allowed six months for the archaeological excavations in the area of the new station, that had finished a few days before we arrived, but two years after the work had started! Unexpected materials found at the Constitution and other nearby stations included Mycenaean remains, Classical Greek remains, Roman tombs and other bric a brac scattered by almost 4,000 years of occupation. More importantly (as far as they were concerned), the roof of the tunnel was barely metres below the basement levels of the Grande Bretagne Hotel and the Parliament Buildings!
We all had plenty of opportunity to see TBM's (Tunnel Boring Machines), laser guided sight lines, roof supports and grouting, and dust prevention facilities. When the engineers had their fill we re-emerged into the blinding glare of an Athenian noon with the omnipresent roar of traffic. By late afternoon we had packed up and migrated to Pireaeus ready for the last part of the venture which was the overnight ferry crossing to Thera in the southern Aegean Sea.
Thera is the ancient name of the Cycladic island of Thira. What is loosely called Thera consists of five islands. The largest is Thera (main town is Phira or Fira); The second largest (part of the caldera rim to the west) is Therasia with the town of Manolas. Next in size, and centrally located, is the basaltic parasitic cone of Nea Kameni (new Kameni), and southwest of that, Palea Kameni (Ancient Kameni), an older cone. The smallest island is Aspronisi (White Island) a remnant of the caldera wall in the southwest. Thera is also known as Santorini, probably in error, since the name was given by the Crusaders after Agia Irini (Saint Irene, hence Santorini) from a small chapel apparently located on Therasia. In the middle of the second millennium B.C. an explosive eruption, like that which overwhelmed Pompeii, buried Thera's flourishing Bronze Age settlements beneath metres of pumice. Subsequent collapse of the magma chamber formed the impressive sea-filled caldera and left three small islands in place of the previous large one. The main island (present-day Thera) was resettled before the end of the Bronze Age and was later (9th century B.C.) occupied by Dorian Greeks, who built their city on a limestone height to the east of the modern village of Akrotiri.
Thera's Bronze Age ruins were first explored in the 1860s and 1870s, while pumice was being quarried for use in building the Suez Canal. In 1967 excavation was resumed at the site of the chief Bronze Age town near Akrotiri. Many of the remarkably preserved buildings were adorned with spectacular wall paintings in a Cretan style, and great quantities of Minoan-style pottery have been found. Comparing the artifacts and ruins of Thera with those found at other Bronze Age sites, archaeologists have traditionally dated the destruction of Thera at about 1500 B.C.. Evidence from radiocarbon dating of plant remains on Thera, anomalies in the world dendrochronological (tree ring) record, and volcanic deposits buried in Greenland's ice sheets suggest that 1628 B.C. is a more accurate date. Due to the new evidence archaeologists may have to redate some events relevant to Minoan culture. Considerable controversy also surrounds the speculations that the Greek legend of Atlantis may be linked to the cataclysmic destruction of Thera.
The general geology of the Thera deposits include pre-Minoan carbonate rocks and subsequent lavas, tuffs, pumice ejecta, and ignimbrites. Much of the sequence is clearly visible in the 350 m high cliffs. Some of the main features (described top-down) are as follows:
Recent
lava
and
tuffs
of
the
Kameni
islands
(1860
-
1950).
Older
stratified
lavas
and
pyroclastics
(tuffs,
ignimbrite,
and
pumice)
of
Thera,
Therasi
and
Aspronisi.
The
most
prominent
ignimbrite,
exposed
in
the
steps
up
to
Fira,
is
a
red
and
black
scorched
band
up
to
7
m
thick.
The
oldest
dacite
lavas
outcrop
at
Akrotiri
and
Cape
Fanari
(SW
part
of
Thera).
Outliers
of
Triassic
meta-limestone
at
Mount
Elias
(SE
part
of
Thera).
Because
of
our
arrival
by
ferry
from
Piraeus,
we
had
some
opportunity
to
look
at
the
principal
features
of
the
caldera
from
water
level.
This
cursory
glance
was
reinforced
by
a
boat
trip
around
the
caldera
the
following
day.
The
first
stage
took
us
out
to
Nea
Kameni,
a
basaltic
volcano
which
was
quite
active
68
years
ago.
Arrival
at
Nea
Kameni
was
at
a
blocky
basalt
lava
landing
area.
From
the
landing
region
a
trackway
leads
to
the
summit
of
Nea
Kameni.
The
pathway
is
across
aa
lava
and
cinders
most
of
the
way
and
the
"crater"
is
a
pretty
miserable
excuse
for
a
volcanic
vent
(of
course
I
wouldn't
have
been
expressing
such
sentiments
in
1926).
Minor
eruptions
added
more
basalts
in
1939,
1940,
1941
and
1950.
Today
it
has
some
feeble
exhalations
of
hot
gases
(some
hot
enough
to
scald).
The
yellowish
encrustations
around
some
of
the
steam
vents
may
be
sulphurous,
but
my
feeling
when
I
examined
these
in
1983
and
1994
was
that
they
were
mainly
algal
encrustations.
The
one
advantage
of
getting
to
the
summit
is
that
there
is
a
good
view
across
the
caldera
and
it
is
downhill
back
to
the
boat!
The
class
posed
for
a
photo
at
the
summit.
The boat then went around the south side of Nea Kameni and between Palea and Nea Kameni. We had the opportunity to swim above hot vents coming up from the sea floor. From here we travelled north about 5 km to Oia, and then went on a further 6 km southeast back to Fira. The return journey gave us a good opportunity to see the spectacular stockwork of sills and dykes which make up parts of the caldera walls.
Staying in a small guest house at Perissa on the south coast of Thera allowed us to appreciate several things. Firstly, that groundwater is in short supply and mostly somewhat saline. A water table only develops in the eastern part of Thera and there the groundwater is of medium to bad quality because of sea water intrusion. The only water-bearing rocks of any consequence are the marbles of the Elias Mountain, which contain a karst network below sea level. The yield from the karstic groundwater is only about a few tens of cubic metres per day, and the water quality is only just acceptable, again because of saline intrusion. Secondly, we had the opportunity to travel into Thera for the evening sunset view across the caldera, and finally we also had the chance (because of the infrequent bus service to re-aquaint ourselves with travel on moped type motorcycles (of various vintages and capacities). Some of these ranged from fairly sedate types to souped-up dirt bikes. I took the attitude that my skin regeneration isn't fast enough to warrant one of the latter, and settled for one that I thought I could manage!
Our remaining days were spent looking at a variety of features on Thera and (for some of the class) off-Thera, on the neighboring islands of the Cyclades. One last highlight was looking at the partially exhumed settlement at Akrotiri on the south coast of Thera.
The first archeological excavations on Thera which relate to the Minoan settlements were as an indirect result of the cutting of the Suez Canal by Ferdinand de Lesseps who needed the pozzuolana for the underwater cements for the sides of the canal. The first traces of the early settlements were in Alaphouzos' quarry on Therasia about 1864, and the first formal excavations took place in 1866. They were assisted by the French geologist F. Fouque in 1867. Fouque stayed on Thera during the 1866-1870 eruption and published his observations in "Santorin et ses eruptions" (1870). In 1870 trial excavations were carried out in the general area of the modern excavations at Akrotiri, and these were resumed again in 1895-1900 slightly east of the modern excavations.
Professor Spyridion Marinatos started excavations in Akrotiri in 1967 to prove his theories of the demise of the Minoan civilization, and uncovered abundant well-preserved architecture, pottery and murals. By October 1, 1974, when he died, about 10,000 square metres of excavation had been roofed and some 17 buildings located.
Akrotiri was an urban centre with 2 and 3 storey buildings, mostly residential and all decorated with wall paintings. Typically the ground floors and basements housed mills and grinders, workshops and store rooms, a kitchen and bath tub. To keep them cool, the rooms had a few small windows. Impressive staircases of stone or timber led to the upper stories. The walls were prepared for painting by smoothing with clay and broken straw and coating with plaster which was polished with pebbles. The paintings show men fishing and women gathering saffron. The men wore a loin cloth and the women, long flowing skirts, short sleeved bodices and necklaces, bracelets and earrings of gold, rock crystal and gemstones. Unfortunately all of the murals have been removed from the island and are now in special rooms at the National Museum in Athens.
The buildings show serious earthquake damage and seem to have been under restoration when the eruption occurred. Rapid evacuation of Thera prior to the eruption is supported by the complete absence of bodies under the ruins as well as the lack of objects made of precious metals. The fleeing Minoans took their valuables with them.
Thera was an appropriate place to end our Greek field trip. There are very few places in the world where one can see how a civilisation which dominated a region for hundreds of years came to an end with a single shattering geological event, and it served to remind the students (and the instructors) that despite everything, humans are not yet omnipotent.
Alan V. Morgan