By Tony Cross
Greece and its geology are a wonder of nature, with the nation a paradise blessed with high mountains, blue seas and over six thousand islands. But it’s all a big geological accident, the result of millions of years of violent earth movements on a planetary scale.
Geology in Greece: in the beginning…
The story of Greece and its geology begins around 250 million years ago when the continents had all come together into one single land mass that geologists call Pangea.
The area that would one day become Greece lay on the southern shore of what would eventually become Europe and on the northern edge of a great ocean called Tethys. On the southern edge of Tethys lay the continent that would one day become Africa.
The Earth’s crust is not all the same, nor is it a single unit. The crust making up the continents is very thick; 30 km to 40 km (18.6 to 24.85 miles) thick — and thicker still under mountain ranges. The crust under the oceans is quite thin, however, only around 7 km (4.3 miles) thick.
In addition, the crust is not one single unit but is broken up into various sized chunks known as tectonic plates. These plates move relative to one another because they are literally floating on the deformable layer of the upper mantle beneath them, in much the same way that a ship floats on the sea.
In some places these plates are moving together and where oceanic crust is pushed into continental crust the thinner oceanic crust is forced beneath the thicker continental crust and down into the mantle, where it begins to sink and melt. Geologists call this type of plate boundary a subduction zone.
The Greek landscape and geology that we see today is here because of a subduction zone. Without it, Greece would simply not exist.
The compressive phase
Around 150 million years ago the great continent of Pangea started to break up. The African plate began to move northwards and the Tethys Ocean started to shrink. The northwards movement of Africa meant that the oceanic crust beneath Tethys was subducted under the southern edge of the continental crust of Europe.
As the oceanic crust under Tethys slid beneath the continental crust of Europe, all of the rocks that had formed on the ocean floor over many millions of years were scraped off by the leading edge of the European continent. These rock scrapings, which would have been hundreds of metres thick and many kilometres long, were piled up one on top of another on the southern edge of Europe.
This rock pile (geologists call it a nappe) was likely many kilometers thick in the end. It contained all the rocks that would eventually form Greece’s geology, but in a huge pile and all in one place.
The photo shown here is of a large sea cliff near Kavousi on Crete. The rocks on the left are a grey colour with clearly defined horizontal layers. The rocks on the right are a greenish brown colour with a near vertical layering. Clearly this cliff is composed of two very different rock types.
The rocks on the left are limestones and those on the right are phyllites. The compressional forces of the subduction zone forced the phyllites over and on top of the limestones. The junction between the two (known as a thrust fault) lies roughly in the center of the picture, running diagonally up from right to left.
Millions of years of weathering and erosion have ground both sets of rocks down so that to the casual observer today they appear to be a single unit.
The tensional phase
Around 65 million years ago the continent of Africa finally collided with the continent of Europe and closed the Tethys Ocean forever. It would eventually be reborn as the Mediterranean Sea.
When two continental plates come together there is no subduction, since they are both too thick. Instead the continents themselves are deformed and mountains are created. In the west, this collision formed the Alpine mountains, in the east it formed the Balkan mountains.
In these mountain areas the continental collision destroyed the subduction zone but in the area in between, where modern Greece lies, the subduction zone remained active.
Even though Africa could no longer move northwards as fast as it had done, the oceanic plate in the area of Greece was still sinking into the mantle. As it sank, the subduction zone itself rolled back southwards. This rollback of the subduction zone put the nappe pile under enormous tension.
When rocks are placed under tension they break, causing normal faults. One side of the fault moves downwards on a sloping surface to relieve the tension. Normal faults often occur in parallel and in swarms leaving alternating areas of high ground with lower ground in between.
The rollback of the subduction zone caused massive parallel swarms of normal faults in the nappe pile. Because the subduction zone is fixed in the east and also in the west the rollback created an arc, an arc that is ever expanding as the rollback progresses.
The photo above is of a small section of the north wall of the Corinth Canal. The rocks here are nicely layered; we can see yellow, white, red and black layers.
The two diagonal lines in these rocks are normal faults, breaks in the rocks caused by tensional forces due to the rollback of the subduction zone. The rocks to the right of each fault have dropped down relative to the rocks on the left; you can see this clearly in the displacement of the coloured layers of rock.
The total vertical displacement here is only a few meters but in the massive regional faulting that shaped Greece and its geology, the displacements are measured in kilometers.
The modern topography of Greece
Looking at a topographical map of Greece today you can see how a subduction zone, starting roughly in the area of the north Aegean and rolling back southwards in an expanding arc would create the “ripped” and “torn” appearance that Greece has today. You can also see how regional faulting created the alternating series of high mountain ranges and islands, with lower plains or sea in between.
The Pindus Mountains, for example, the backbone of mainland Greece, run southeastward in a gently curving arc. On both sides are lower plains. These mountains, like so many others in Greece, are bounded by massive regional faults.
The expanding arc of the subduction zone caused extensive local faulting too. On Crete for example, all of the mountain ranges are bounded by faults. They stand tall because the ground around them has dropped down due to faulting. Similar local fault-bounded structures can be seen all over Greece.
What about the volcanoes?
There are many volcanoes in Greece — on Santorini, Milos, Nisiros, Methana, Sousaki, and so on. Some are active, like Santorini; most are dormant, like Milos, and one or two are extinct, like Sousaki.
If you look closely, all the Greek volcanoes sit on an arc that parallels the arc of the subduction zone but is north of it by about 100 km.
As the oceanic plate is subducted deep into the mantle it begins to melt. Magma from the melting plate rises to the surface where it erupts, forming volcanoes.
The hot springs of Thermoplyae (of Spartan fame) sit at one end of this volcanic arc; the hot springs of Pamukkale in Turkey sit at the other. In between are all the Greek volcanoes, formed above the spot where deep in the mantle the subducted oceanic crust is melting.
Greece’s geology continues to change
The subduction zone today runs in a great arc, down the western side of the Ionian Islands, around the Peleponnese and south of Crete, then curving up northwards again past Kasos, Karpathos and Rhodes.
The Greece and the geology we see today is not an end point however; it’s simply the way things are right now.
The subduction zone is still active, the oceanic plate is still descending as Africa creeps northward. The subduction zone is still rolling back and the arc is still expanding. That’s why we have so many earthquakes in Greece — we’re still being torn apart by tectonic forces.
We don’t need to worry about this too much though — these geological processes happen on a timescale that is measured in millions of years. Chances are, that beautiful Greek beach in the travel brochure will still be there when you arrive.