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Subduction - from the top down

Geoscientist 17.5 May 2007

Last September, the Geol Soc's Volcanic and Magmatic Studies Group (VMSG) and Metamorphic Studies Group (MSG) ran a joint field trip to the islands of Santorini and Syros. Here Dougal Jerram and Kathryn Goodenough report on the trip’s success...

The trip was the brainchild of John Schumacher (University of Bristol) who organised and led the Syros part of the trip; the Santorini section was led by Tim Druitt (Université Blaise-Pascal) with the overall joint trip organised by Kathryn Goodenough (BGS) and Dougal Jerram (University of Durham). There were twenty participants, representing ten different countries. The field trip aimed to bring together people with a range of different specialisms and was undoubtedly successful with lively discussions continuing over dinner, at the bar or around the pool.

Introduction and background

The Cyclades islands of Greece represent one of the best areas in Europe to study the anatomy of a subduction zone. The northern islands have good exposures of the Mesozoic metamorphic basement, which has been metamorphosed to blueschist facies; blueschist assemblages are well preserved on the islands of Syros and Sifnos. This high-pressure, low-temperature metamorphism has been dated at 40-45 Ma, and is related to subduction of the African plate beneath Eurasia. Subduction in the area continues today, and the southernmost islands of the Cyclades form the volcanic islands of the South Aegean Arc. The most active of the volcanic islands is Santorini, a spectacular caldera volcano with new islands in the centre of the caldera representing the active volcanic edifice.

Top of the subduction - Santorini Volcano

The first three days of the trip were on Santorini, based in the village of Akrotiri on the rim of the caldera. A magnificent view across the caldera greeted us as we emerged from the hotel on the first morning, and set off on a path winding down the cliff-face to the shoreline. The oldest volcanic rocks on Santorini are rhyolitic and dacitic tuffs, around 600,000 years old, which are exposed at the base of the Akrotiri cliffs. These are overlain by the spectacular deposits of Santorini's two main eruptive cycles, both of which included a number of major explosive eruptions, culminating in caldera collapse. The deposits of many of these eruptions can be seen in the cliffs, characterised by layers of air-fall pumice overlain by lithic breccias. Many of the eruptive layers can be recognised by distinctive weathering one layer was described as looking like a wormery! Tim explained that a total of 12 major explosive eruptions have been identified, each with deposits several tens of metres thick. However, these were separated by a much larger number of minor eruptions, which probably occurred every 3 to 4,000 years, and each of which covered the island in several centimetres to a few metres of volcanic ash. Another such event would certainly put the cruise ships off visiting this island!

Santorini and man

In the afternoon of the first day, we visited the spectacular exposures of the Minoan eruption in quarries near Thira. The Minoan eruption, which is dated to about 1650 BC, was the culmination of the second eruptive cycle on Santorini. It began with a Plinian phase (3km high eruption column), which lasted for several hours and deposited a few metres of air-fall pumice. This was followed by base surges and pyroclastic flows, which spread out across the island, covering it in around 50 m of volcanic material. The whole eruption lasted for just 1 or 2 days.
For many years, it was thought that this eruption led to the collapse of the Minoan civilisation in Crete, but archaeologists have now disproved this theory. On Santorini itself, a Bronze Age town (close to modern-day Akrotiri) was buried by volcanic material; but remarkably, the inhabitants seem to have already left, taking their valuables with them. The excavations are currently closed for renovation, but we were able to visit the museum in Thira (Fira) and marvel at some of the delicate pottery and decorative furniture that have been unearthed in the area. This interplay between geology and human history really brings home the importance of understanding how volcanoes work.

Neo-volcanics and the Caldera

Our second day in Santorini started with a trip to the lavas of Skaros Head, part of a shield volcano that infilled one of the earlier calderas developed on Santorini. This locality offered superb views of the caldera, and Tim showed us the evidence for 'fossil' caldera walls, developed earlier in the history of the volcano and exhumed during more recent eruptions. We also saw examples of fresh, porphyritic andesitic lavas, as well as the ruins of the ancient capital of Santorini, destroyed by earthquakes in the 19th century. 

In the afternoon, we boarded a boat for the Kameni islands, the currently active part of the Santorini caldera. Nea Kameni, the youngest island, is composed of dacite lava flows that have formed over the last 440 years, the most recent eruption being in 1950. A short, but hot and sweaty walk took us to the summit of the island and the vents formed at the end of the 1941 eruption, where Vikki Martin explained that each of the dacitic lava flows contains a distinct assemblage of magmatic enclaves, which she is studying to understand magma chamber processes beneath Santorini. Our return journey in the boat took us round the base of the cliffs, with fantastic views of the volcanic sequence, including wonderful 3D exposures of dykes cutting through the older units to feed higher eruptions. Most of the group also took the opportunity for a first swim in the Aegean trying not to think too hard about the 400 m of water between us and the base of the caldera! 

On the third day, we visited outcrops of the Peristeria volcano and lithic breccias of the Cape Riva eruption around Oia, before winding up the Santorini part of the trip with lunch overlooking the caldera in the charming town of Oia. Then we boarded the ferry for Syros, taking with us gifts from the hotel in the shape of several bottles of Santorini wine, which greatly enlivened the journey.

Syros - deep below the volcanoes in the subduction factory

After the busy, touristy villages of Santorini, our base in the sleepy village of Finikas on Syros was surprisingly peaceful. On the first morning, we drove to north Syros, where traffic jams are caused by sheep and donkeys rather than other cars. We walked down the road from the village of Campos, stopping to investigate outcrops of marble and garnet- and glaucophane-bearing mafic schist, whilst John Schumacher and John Dixon tried to urge us along with the promise that they could show us much better outcrops if we would just get moving!

Once we left the road, we arrived at the first of many 'knockers' that we were to see over the next few days. Here large boulders of metabasic rocks represent clasts in a mélange, with a serpentinite matrix that is rarely exposed. The metabasic rocks include both coarse- and fine-grained varieties, and are interpreted as a dismembered ophiolitic sequence. Metamorphism at blueschist facies has produced very aesthetically-pleasing mineralogies of garnet + glaucophane + omphacite (in more iron-rich rocks) or glaucophane + omphacite + zoisite + phengite in the more primitive rocks. 

The mélange units are interlayered with mafic schists and marbles, the marbles containing very clear examples of needle-like aragonite pseudomorphs. We meandered our way between outcrops of the two units, working our way through prickly bushes (aptly named 'spiny bastards' by our leader). Not all the boulders in the mélange had mafic protoliths; we also saw some that are thought to be metamorphosed plagiogranites, and now contain abundant jadeite.
As the day continued, the rocks became ever more spectacular, with the appearance of lawsonite pseudomorphs with distinctive rhomb-shaped cross-sections, and garnets as big as a one-euro coin! What's more, John Schumacher had another trick up his sleeve; our day ended at the beautiful and peaceful Lia beach, where we had time to swim in the bay before our boat arrived to take us back to Finikas.

An extra-ordinary P-T path

The rocks of northern Syros represent the finest examples of blueschists any geologist is likely to see. These spectacular rocks are formed in subduction zones, where cold ocean crust and sediments are dragged down to great depths relatively quickly, following the characteristic blueschist P-T path of high pressures but moderate temperatures. This tectonic setting also allows the rocks to be brought back to the surface quickly with little retrogressive metamorphism, preserving fantastic metamorphic textures.

Metamorphic gems - a blueschist wonderland!

Our second day on Syros began with our local outcrops, in Finikas bay. These blueschists again contain lawsonite pseudomorphs in a glaucophane- and omphacite-bearing matrix. Then it was back to North Syros for yet more spectacular examples of aragonite pseudomorphs, garnets and lawsonite pseudomorphs. John had saved the best for last; towards the end of the afternoon we first visited some outcrops that show superb examples of magma hybridisation and mingling but that have been metamorphosed, and now contain abundant garnets and lawsonite pseudomorphs. One member of the party described these as 'igneous petrology but with jewellery'. 

Our last outcrop of the day was the much-hyped 'Lawsonite Point' which proved to be truly stunning. Bright green, chrome-rich lawsonites are surrounded by a genuinely blue glaucophane-schist matrix. Sadly, some irresponsible visitors have hammered some of the best outcrops, damaging a geological site that certainly deserves to be protected. 

We dragged ourselves away to return to Grammata beach, where our boat was already waiting…. with a floating bar, stocked with bottles of ouzo. This was definitely not what most of us know as fieldwork!

Our trip to Syros ended with a day of 'urban geology', working along the shore beyond the main town of Ermoupoli, where we saw yet more amazing boulders and outcrops of both blueschists and eclogites, before having lunch in a traditional taverna. After a week of sunny weather, that night the heavens opened as a huge thunderstorm boomed overhead. It was a fitting end to the trip!


Our thanks go particularly to Tim Druitt and John Schumacher, for leading the trip and tirelessly answering questions. We were made extremely welcome by the hotels Kalimera (Akrotiri - Santorini) and Olympia (Finikas - Syros), who helped with the smooth running of the trip. This article was compiled by Dougal Jerram ([email protected]) and Kathryn Goodenough ([email protected]).

Further reading/links

Druitt et al. Santorini Volcano. Geol. Soc. Lond. Memoir 19.
Okrusch, M. & Bröcker, M. (1990). Eclogites associated with high- grade blueschists in the Cyclades archipelago, Greece: a review. European Journal of Mineralogy 2, 451-478.
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