Geology of the Greek Islands

Full text

frequency of the environmental dynamics (geological and cli-
matic), both in the past and at present, drastically diminishes
the time available for immigration, extinction, and differen-
tiation to establish conditions of equilibrium. In addition,
they provide an excellent case study for the dynamics of the
interplay between human activities and biodiversity. At the
same time, as a result of being a favorite destination for mil-
lions of tourists each year during the last several decades,
their biota is under serious threat. There is an urgent need
for intensifi cation of conservation efforts so that their value,
both scientifi c and cultural, can be preserved.
SEE ALSO THE FOLLOWING ARTICLES
Endemism / Greek Islands, Geology / Refugia /
Relaxation / Vicariance
FURTHER READING
Blondel, J., and J. Aronson. . Biology and wildlife of the Mediterranean
region. Oxford: Oxford University Press.
Poulakakis, N., A. Parmakelis, P. Lymberakis, M. Mylonas, E. Zouros,
D. S. Reese, S. Glaberman, and A. Caccone. . Ancient DNA
forces reconsideration of evolutionary history of Mediterranean pygmy
elephantids. Biology Letters : –.
Sfenthourakis, S., S. Giokas, and E. Tzanatos. . From sampling sta-
tions to archipelagos: investigating aspects of the assemblage of insular
biota. Global Ecology and Biogeography : –.
Stamou, G. P. . Arthropods of Mediterranean-type ecosystems. Berlin:
Springer-Verlag.
Thompson, J. D. . Plant evolution in the Mediterranean. Oxford:
University of Oxford Press.
GREEK ISLANDS,
GEOLOGY
MICHAEL D. HIGGINS
University of Québec, Chicoutimi, Canada
The geological diversity of the Greek islands refl ects long
and complex interactions between the Eurasian, Medi-
terranean (African), and Anatolian tectonic plates. The
Mediterranean climate and common paucity of soil
have augmented the infl uence of geology on the cultural
development of these islands for the last  years: The
nature of the bedrock and water supply has controlled
agriculture; exploitation of marble and metals have been
important economic activities; volcanic eruptions and
earthquakes have directly infl uenced the lives of the
inhabitants. In turn, study of the geology of the islands
has contributed much to our knowledge of geological
processes elsewhere.
INTRODUCTION TO THE
GEOLOGY OF GREECE
Two hundred million years ago the Tethys Ocean lay
between Eurasia and Africa, opening out to the east.
Since that time, continental fragments have spalled off
Africa and been propelled toward Europe by the creation
of oceanic tectonic plate material to the south and its
consumption in a subduction zone to the north. When
these mini-continents collided with Europe, they made
mountain ranges—for example, Italy’s collision created
the Alps. During these collisions some rocks were forced
deep into the Earth, where the action of temperature and
pressure metamorphosed the rock, changing its mineral-
ogy and appearance. Greece has been the locus of many
such collisions, which have contributed to its complex
geology.
At the present time the Mediterranean tectonic plate is
being subducted beneath the Aegean Sea. Melting of the
plate produces molten rock, which rises to the surface as
the South Aegean volcanic arc (Fig. ). The region north of
the arc is expanding, opening up tectonic valleys, such as
the Gulf of Corinth, and forcing Crete southward. In addi-
tion, the Anatolian plate is pushing eastward, separated
from the Eurasian plate by the North Anatolian fault and
its extension, the North Aegean fault zone. Movements
along these plate boundaries occur during earthquakes, and
FIGURE 1 Plate tectonics of the Aegean region overlaid on a MODIS sat-
ellite image. The Anatolian plate is moving west into the Aegean region.
The Mediterranean (African) plate is subducted along a major thrust
fault and melts at a depth of 100 km to feed the active volcanoes.
100 km
Folding
Normal Fault
Strike-slip Fault
Thrust Fault
Volcano
Mediterranean plate
Eurasian plate
Anatolian
plate
392 GREEK ISLANDS, GEOLOGY
Gillespie08_G.indd 392Gillespie08_G.indd 392 4/9/09 1:26:47 PM4/9/09 1:26:47 PM

Greece is the most seismically active part of Europe. Fault
movements also produce vertical changes in the height of
the land, commonly observed by local changes in sea level.
Finally, faults provide channels for surface water to descend
deep in the Earth and rise as hot springs.
The most common rock in the region is limestone or
its metamorphosed equivalent, marble (Fig. ). Erosion
of these rocks produces a special landscape with closed
basins, springs, and caves. Early agriculture was enabled
by the perennial springs, and caves were important as shel-
ter and for religious purposes. In some areas subsurface
water evaporates before it reaches the surface, cementing
beach sand to make “beach rock.”
It should be remembered that we live in geologically
unusual times: Just , years ago, much of the North-
ern Hemisphere was covered by ice, and the sea was  m
below its current level. Most of the Greek Islands were
connected to the mainland, and there were vast expanses
of shallow sea, with abundant molluscs. The shells of the
molluscs were worn down to sand that formed dunes,
which were rapidly cemented and transformed into a
porous limestone. This useful building material is locally
called Poros or Panchina and has been much used in the
region for rough construction.
ISLANDS OF THE SARONIC GULF
The Saronic and Corinthian Gulfs are broad, partly fl ooded
valleys produced by almost north–south extension of the
crust. The oldest rocks are hard, gray limestones (–
million years old) that were deposited in shallow seas to the
south. These rocks are well exposed on Salamis, the island
closest to Athens. They are also seen on Aegina, the largest
island of the group, but only in a small area. Volcanic erup-
tions started  million years ago and covered the southern
and eastern parts of the island with lavas and tuffs. After vol-
canism ceased, the northern part of island was submerged,
and marls were deposited. Volcanic activity has continued
recently on Methana, a peninsula  km to the south, and
on the island of Poros, close to the Peloponnese.
EVVOIA
Evvoia (Euboea) is a long island that runs parallel to the
Greek mainland, separated from it by a strait that narrows
to only  m at Khalkis. Here, tidal movements in the
North and South Evvoikos gulfs interact to produce chaotic
currents that reverse  to  times a day. The oldest rocks on
the island are schists and marbles. The marble in the south
of the island was exploited extensively by the Romans, espe-
cially for columns—it is now called Cipollino (Italian for
onion) because layers rich in muscovite and chlorite give the
appearance of an onion (Fig. ). Variegated colored marble
(Fior di Pesca) was also exploited in antiquity near Eretria.
Closure of small ocean basins thrust parts of the ocean
fl oor over the metamorphic rocks. The whole package
was then uplifted and weathered under tropical condi-
tions to produce iron- and aluminum-rich “soils” called
laterites and bauxites. The former were exploited in antiq-
uity as a source of iron and more recently as a nickel ore.
Finally, parts of the island sank down to form swampy
basins. Low-grade coal, lignite, formed here and has been
exploited for power generation.
IONIAN ISLANDS
Kerkira (Corfu) lies close to the mainland and was indeed
connected  years ago when sea level was lower.
FIGURE 2 Simplifi ed geology of the Aegean region and the Greek
Islands.
0100km
Kerkira
Levkas
Kefallinia
Zakinthos
Methana
Salamis
Salamis
Milos
Thera
Crete
Rhodes
Nisyros
Kos
Naxos
Paros
Syros
Syros
Ikaria
Chios
Samos
Aegina
Aegina
Evvoia
Lesvos
Lemnos
Imroz
Samothrace
Thasos
Young sedimentary rocks
Limestone
Shales and sandstones
Marble and schist
Serpentinite
Granite
Volcanic rocks
FIGURE 3 Partly fi nished Roman columns 5 m long, from the Cipollino
marble quarries on southern Evvoia.
GREEK ISLANDS, GEOLOGY 393
Gillespie08_G.indd 393Gillespie08_G.indd 393 4/9/09 1:26:52 PM4/9/09 1:26:52 PM

However, the western coast of the island follows a fault,
and the sea-fl oor drops rapidly to over  m. The old-
est rocks are hard, gray limestones (– million years
old), which crop out in the north and make the highest
hill. Further south, the rocks are younger and softer and
have developed thick, red soils. Paleolithic implements
have been found in this soil, testifying to the long occu-
pation of this fertile island.
The southern Ionian islands (Levkas, Ithaca, Kefallinia,
and Zakinthos) lie close to the western edge of a tectonic
plate, which is why earthquakes are so common (Fig. ).
Over the long term, such activity has produced strong
relief, which is expressed as hills, islands, and lakes. All the
islands are dominated by limestone (– million years
old), which has been cut by many faults during compres-
sion of the region. On Kefallinia this combination has led
to an extensive system of sinkholes, caves, and springs.
Near Argostoli, on the west coast, there is a very unusual
phenomenon: The sea drains into a sinkhole (katavothre)
and reappears, mixed with freshwater, on the other side
of the island. The process is driven by density differences
between seawater and freshwater.
The oldest rocks on Zakinthos (Zante) resemble those
of the islands further north but have been overlain by
younger rocks. These include gypsum that was formed
when the Mediterranean almost completely dried up
 million years ago. There are natural pools of bitumen
(pitch) in the southern part of the island, which formed
by seepage of petroleum and evaporation of the more
volatile components. Bitumen was used extensively in
antiquity for waterproofi ng ships and jars, as well as for
medical purposes. However, there are no signifi cant oil
deposits in this region.
CYCLADES
The Cyclades are part of a band of complex metamorphic
rocks that stretches north to Attica and Evvoia. Marble
and schist dominate, but there are traces of less common
minerals and rocks: The blue/mauve mineral glauco-
phane is widespread and was used as a pigment, jadeite
from Syros is a form of jade that may have been used
in Neolithic times to make axe heads, and corundum
(emery) from Naxos was used to shape and polish marble.
White marble was exploited in antiquity from Naxos and
Paros; the translucent nature of that from the latter was
particularly prized. Granite was intruded into the meta-
morphic rocks and is abundant on Naxos, Mykonos, and
the sacred island of Delos.
Milos and its surrounding islands are dominated by
volcanic rocks but have a foundation similar to their
neighbors. Volcanism started  million years ago with the
eruption of tuffs and lavas and has been expressed most
recently by swarms of phreatic explosions, the latest about
 years ago. In Paleolithic to Neolithic times, the nat-
ural volcanic glass obsidian was exploited for the produc-
tion of blades. Two domes of obsidian were used, both
from the area north of the Bay of Milos. More recently,
volcanic rocks have been exploited to make perlite. Melos
is also a major producer of the clay bentonite, which is
formed by hydrothermal alteration of volcanic rocks.
The most famous and spectacular volcano in the
Aegean is on the island of Thera (Santorini, Fig. ).
The volcano was built on a foundation of marble and
schist, now exposed on the hills around Ancient Thera.
Volcanism started . million years ago in the southern
part of the island, but the main phase only dates from
, years ago. There have been many major erup-
tions, which are exemplifi ed by the Minoan eruption
about  years ago. This started with the rapid erup-
tion of  km of volcanic ash, which buried a Bronze Age
town in the southern part of the island near Akrotiri. The
volcanic summit then collapsed, leading to the forma-
tion of a caldera that now makes up the northern part of
the Bay of Thera. Construction of a new volcano started
shortly afterward with the eruption of lavas in the cen-
ter of the bay. Volcanic activity continues on the Kameni
Islands, which last erupted in . The Colombo Bank
underwater volcano, located only  km northeast of
Thera, erupted in . It will probably make a new vol-
canic island in a few thousand years.
CRETE
Crete is part of the Hellenic Arc, a series of islands and shal-
low water that extends from the Peloponnese to Turkey.
It formed in response to the subduction of the African
FIGURE 4 The cliffs of the Thera caldera at Oia. Gray lavas at the base
of the cliff are covered by red agglomerates. The pale tuff at the top of
the cliff is from the Minoan eruption in 1640 BC.
394 GREEK ISLANDS, GEOLOGY
Gillespie08_G.indd 394Gillespie08_G.indd 394 4/29/09 12:58:10 PM4/29/09 12:58:10 PM

plate beneath the Aegean. The plate boundary is imme-
diately to the south, which accounts for the frequency
of earthquakes. The lowest rocks exposed on the island
are limestones (– million years old), which have
been partly recrystallized. During crustal compression
almost horizontal faulting has emplaced limestones and
other rocks of similar age on top. About  million years
ago, subduction started to the south and, in response, the
Aegean sea to the north expanded. Crete was faulted into
many blocks, which moved independently. Some blocks
became the mountains, whereas others dropped down,
leaving troughs that became fi lled with sedimentary rocks.
These large, and commonly rapid, movements continue
to this day: The extreme relief of the Samaria Gorge in
western Crete was produced by erosion in response to
rapid uplift during the last few thousand years. More
recently the harbor at Phalasarna was uplifted by  m,
possibly during a single earthquake in the fi fth century.
ISLANDS OF THE NORTHERN AEGEAN
Thasos is almost completely made up of schist, gneiss,
and marble and is an extension of the Rhodope meta-
morphic massif on the mainland  km to the north. The
western part of the island has many small metallic mineral
deposits. The oldest mines were for red ochre, hydrated
iron oxide, which was exploited in Paleolithic times for
cult purposes. Indeed, these underground mines were
some of the largest in Europe at that time. From the ninth
century BC, the same ore was used to make iron metal.
There were also signifi cant silver and gold mines, some of
which were reopened in the nineteenth century for anti-
mony and zinc. Thasos was also well known in antiquity
for white marble.
Samothrace lies to the north of the North Aegean
trough, an important plate boundary fault. The island
itself is a horst, a block of rock uplifted along faults to the
north and south. The oldest rocks are parts of the ocean
fl oor, formed about  million years ago. Volcanism
 million years ago was followed  million years later
by more volcanism and the emplacement of granite that
now makes up some of the highest parts of the island.
Lemnos and Imroz (a Turkish island) lie on the south
side of the North Aegean trough. The sea around here is
shallow, and indeed both islands were connected to the
mainland , years ago. The oldest rocks on Lemnos
are sandstones and marls that were shed from a rising
mountain range about  million years ago. Similar rocks
occur in the Meteora region of central Greece. Much of
Lemnos and Imroz are covered by volcanic rocks that were
erupted about  million years ago. Similar rocks also occur
on Lesvos and the Turkish mainland. Although Lemnos is
associated with the blacksmith god Hephaestus, there is no
evidence of recent volcanic activity. One of the chief prod-
ucts of Lemnos from antiquity onward was Lemnian earth,
a medicament. The nature of the earth is not entirely clear:
It may have been ochre deposited from springs or a mixture
of clay and alum.
EASTERN SPORADES
Lesvos (Mytilene) is a large island close to the Turkish
coast. The eastern part of the island is composed of
metamorphic rocks—schist and marble. Further west,
there is a wide band of serpentinite, part of a section of
ocean fl oor that was thrust up during continental colli-
sion. Such rocks do not produce good soils but have been
exploited for magnesite. The western part of the island
is covered by volcanic rocks, lavas, and tuffs, which are
mostly – million years old. They are part of a much
larger volcanic province that extends about  km to the
east. Fossil pine and sequoia trees have been preserved in
volcanic ashes in the western part of the island.
Chios also lies close to the mainland, but its geology is
quite different from that of Lesvos except for minor vol-
canic activity. The oldest rocks are sandstones and shale
shed from a mountain range earlier than  million years
ago. Later, these rocks were overlaid by the limestones that
dominate the center of the island. In antiquity the island
produced a marble called Marmo Chium or Portasanta,
which is salmon pink with red and white inclusions. The
rock is a metamorphosed limestone breccia.
Samos is dominated by marble and schist, which are an
extension of a metamorphic massif to the east. There are
two basins where the younger rocks have been deposited.
The eastern basin is well known for fossils, deposited  to
 million years ago around small lakes when Samos was
connected to the mainland. The remains include lions,
mastodons, rhinos, gazelles, and Samotherium, an ancestral
giraffe unique to Samos. In antiquity the island was known
for its engineering works, including a -m-long tunnel
cut through a hill to transport spring water to the city.
DODECANESE ISLANDS
Patmos is a small island built on marble but now largely
covered by volcanic rocks – million years old. These are
well exposed in a sacred rock shelter where St. John wrote
the Book of Revelation. Marble and schist continue south
to Kos but have been largely covered by limestone on the
islands of Lipsos and Kalymnos.
The highlands of Kos are made of marble and schist
but also contain the earliest volcanic rocks erupted about
GREEK ISLANDS, GEOLOGY 395
Gillespie08_G.indd 395Gillespie08_G.indd 395 4/9/09 1:26:57 PM4/9/09 1:26:57 PM

 million years ago. Volcanism restarted  million years ago
with the eruption of two volcanic domes (short, thick fl ows)
in the west. Major eruptions , and , years ago
produced tuffs that covered most of the island and also
created calderas, which underlie the sea between Kos and
Nisyros. Cold springs at the Asklepieion deposited terraces
of travertine, which probably initially attracted attention to
the site. Later on, the travertine was quarried to construct
the temple and ancient “health center.”
Nisyros is the easternmost volcano of the active South
Aegean volcanic arc. Volcanic activity started about
, years ago and has continued until recent times.
The island is now a single simple volcano with a large crater
partly occupied by young volcanic domes. The last volca-
nic activity was a series of phreatic explosions in –
(Fig. ). Deep wells have drilled for exploitation of geo-
thermal power, but this resource is yet to be exploited.
Rhodes lies just northwest of a major tectonic plate
boundary, which accounts for the frequency of earth-
FIGURE 5 Recent phreatic explosion craters on Nisyros. These have
partly destroyed young volcanic domes visible to the left.
quakes. One of the most notorious occurred in  BC,
when it toppled the Colossus of Rhodes, a -m high
statue of the sun god Helios, one of the seven wonders
of the ancient world. The early geological history of
Rhodes resembles that of Crete and much of the Pelo-
ponnese: Cherty limestones were deposited – mil-
lion years ago in shallow water to the south. Later on,
overall compression of the crust raised mountains that
were eroded. Finally, basins developed and were fi lled
with marls, which make fertile soils. The oldest lime-
stones are resistant to erosion and form the highest point
on the island.
SEE ALSO THE FOLLOWING ARTICLES
Earthquakes / Eruptions / Greek Islands, Biology / Mediterranean
Region
FURTHER READING
Fassoulas, C. G. . Field guide to the geology of Crete. Natural History
Museum of Crete.
Friedrich, W. L. . Fire in the sea: the Santorini volcano: natural history
and the legend of Atlantis. Cambridge: Cambridge University Press.
Higgins, M. D., and R. Higgins. . A Geological companion to Greece
and the Aegean. Ithaca, NY: Cornell University Press.
Institute of Geology and Mineral Exploration (IGME), Athens, Greece.
www.igme.gr
Jacobshagen, V. . Geologie von Griechenland. Beitraäge zur regionalen
Geologie der Erde, Bd. . Berlin: Gebruder Borntraeger.
Pe-Piper, G., and D. J. W. Piper. . The igneous rocks of Greece: the
anatomy of an orogen. Berlin: Gebruder Borntraeger.
Perissoratis, C., and N. Conispoliatis. . The impacts of sea-level
changes during latest Pleistocene and Holocene times on the morphol-
ogy of the Ionian and Aegean Seas (SE Alpine Europe). Marine Geology
: –.
GREENLAND
SEE ARCTIC REGION
396 GREEK ISLANDS, GEOLOGY
Gillespie08_G.indd 396Gillespie08_G.indd 396 4/9/09 1:26:57 PM4/9/09 1:26:57 PM