Ken Madrell (UK)
Most visitors to the Cyclades islands will gravitate to the island of Santorini to see its stunning caldera and the magnificent sunsets from the northern town of Oia. The island is part of the Aegean volcanic arc formed by the subduction of the African plate under the Aegean Sea. About 3,600 years ago, the island suffered a violent volcanic eruption in which much of the rocks were removed, causing the volcano to collapse and produce the caldera.
About 160km northwest of Santorini and also situated on the volcanic arc is the island of Milos. The island is a more peaceful alternative to the bustling crowds of Santorini and the rich volcanic soils are renowned for producing excellent wines and vegetables. Milos is a ‘must-visit’ island for any traveller with an interest in geology visiting this area of the Greek Islands.
There are a number of designated Geo Walks on the Milos (see How to Explore the Island below). These can be up-loaded at https://www.milosminingmuseum.com/en/the-museum/miloterranean-geo-walks/. Readers may also wish to refer to these while reading the text of this article.
The geology of Milos
The oldest rocks are a basement of metamorphic rocks, such as schists, gneiss and quartzites of Mesozoic to Palaeogene ages (250 to 25Ma). The basement rock is overlain by Miocene to early Pliocene (25 to 5Ma) conglomeratic and calcareous rocks.
The main character of the island we see today was formed by volcanic activity, which began during the Pliocene about 3.5 million years ago. The earliest volcanic deposits were pyroclastic sediments consisting of felsic, pumice-rich flows and tuffs. These sediments accumulated under subaqueous marine conditions. The pyroclastic deposits are exposed along much of the coast and have often been eroded to form dramatic rock formations such as the one illustrated in Fig. 3.
During the upper Pliocene (2.7 to 1.6Ma), these early pyroclastic deposits were succeeded by lava domes and lava flows of rhyolite to andesitic composition, accompanied by the eruption of newer pyroclastic deposits. As shown in Fig. 4, the lunar-like landscape at Sarakiniko is an excellent location to view rhyolite pyroclastic deposits. If following Geo Walk 6, you will see many examples of this type of scenery.
Lava domes are distributed widely across the island. Fig. 5 shows an example from the northeast of the island, close to the town of Pollonia.
Extensive hydrothermal activity associated with the volcanism has resulted in the alteration of some of the pyroclastic deposits and tuffs into valuable industrial clays such as kaolinite, bentonite and perlite. The extraction of bentonite and perlite in the north-eastern area of the island demonstrates a delicate balance between the mining industry and the island’s main source of revenue – tourism. If following Geo Walk 5, after leaving Pollonia, you will pass through this area, which contains extensive mining operations, some with viewing platforms, before reaching the famous Papafrangas beach and the archaeological site of the settlement at Fylakopi.
You will then walk across the lava dome shown in Fig. 5, before returning to Pollonia.
Hydrothermal activity is also responsible for the colourful rocks found around the island. One of the best locations to observe these are at Paliochori, in the southeast of the island (Fig. 8). The colouration of the rocks is due mainly to the release of hydrogen sulphide. This continues in the present day and can be recognised by the smell (rotten eggs) and bubbles rising through the shallow waters.
Other mining operations in the history of the island have included the extraction of silver, sulphur, obsidian, millstone, manganese, alunite, gypsum and siamite. Some of these are described in Some examples of what to see along the Geo Walks below.
How to explore the island
Your starting point should be a visit to the Mining Museum of Milos, located in the town of Adamas. The museum is laid out on three levels. On entering the building, the ground floor contains exhibitions related to the history of mining on the island. Display boards are in English and are supported by a range of artefacts, photographs and maps. On the first floor, there is an excellent collection of rocks and minerals, showing the mineral wealth of the island. On the lower ground floor, there are video shows covering the mining history of the island and display boards illustrating seven Geo Walks (see https://www.milosminingmuseum.com/en/the-museum/miloterranean-geo-walks/) on the island of Milos and its nearby neighbour, Kimilos (Walk 5). Time studying the exhibits will enhance your understanding of the Geo Walks. Geo Walk guides can also be purchased from the shop on the ground floor.
Whilst it is relatively easy to walk the Geo Walks numbered 4 (Aggeria), 6 (Sarakiniko) and 7 (Nychia), the others are a long way from the main centres of population, and they are long and tend to be linear. You must remember walking in the heat of the Greek Islands in the summer months can be very challenging. For the other Geo Walks, you are best hiring a four-wheel drive vehicle that can go off-road. For example, Geo Walk 1 (Vani), a must-do trail to the manganese mines, is nearly 25km and over half the distance is along dirt tracks. You must be careful to note the small directional arrows and symbols marking the Geo routes, as they can easily be missed. The same is true for Walk 3 (sulphur mines), another must-do trail. It is 13km in length, mostly along dirt tracks and, if you wish to visit the obsidian mines, this would be another 5km.
Some examples of what to see along the Geo Walks
Geo Walk 1 – Vani
Shortly after leaving Adamas, opposite the power station, evidence of current volcanic activity can be seen along the shoreline by the presence of hot springs. Around the town of Agia Marina, there is extensive evidence of the extraction of kaolin. After leaving the town and heading northwards, evidence of the Upper Pliocene volcanic activity can be seen with the presence of successions of andesitic tuffs and volcanic bombs. Further along the route at Xerokambos, examples of the earlier volcanic activity are seen in the form of pyroclastic deposits dating from the Middle Pliocene (3 to 2.6Ma).
Approaching the Vani mine, the appearance of the landscape changes from the green of the garrigue to the vivid red-coloured rocks surrounding the mine. Searching through the spoil heaps, specimens of rock-bearing manganese ore can easily be found. The manganese mineralisation was caused by hydrothermal activity during the Upper Pliocene, when hot mineral rich fluids passed though cracks in the volcanic tuff, depositing thin veins of the mineral.
Geo Walk 3 – the sulphur mines
On the journey to the sulphur mines, shortly after passing through the town of Zephyria and climbing off the plain, extensive evidence of the extraction of gypsum can be seen along the sides of the valley (Fig. 10).
On entering the ravine, evidence of the extraction of kaolin can be seen from the presence of many galleries and the former sites of larger mines. A diversion from the main route will take you to Demenagaki, the site of a prehistoric mining for obsidian. Fragments of the dark shiny rock are easy to find.
Returning to the main route and nearing the coast, evidence of sulphur mining starts to become evident. Small piles of rubble can be seen and it is worth a brief to stop, as these can yield some very nice sulphur crystals. On reaching the coast, the vast extent of the sulphur mines can be seen (Fig. 11). The area was first exploited by the Greeks and Romans, and extraction continued until 1958. The existing remains are well worth exploring before taking a cooling dip in the sea.
The Milos Mining Museum has a good online site, including information about the Geo Walks: https://www.milosminingmuseum.com/en/the-museum/.
The following papers are recommended for those who want to investigate the geology of the island in greater detail:
- Calvo, J.P., Triantaphyllou, M.V., Regueiro, M. and Stamatakis, M.G., (2012). Alternating diatomaceous and volcaniclastic deposits in Milos Island, Greece. A contribution to the upper Pliocene–lower Pleistocene stratigraphy of the Aegean Sea. Palaeogeography, Palaeoclimatology, Palaeoecology, 321, pp.24-40.
- Fontaine, F.J., Rabinowicz, M. and Boulegue, J., (2003). Hydrothermal processes at Milos Island (Greek Cyclades) and the mechanisms of compaction-induced phreatic eruptions. Earth and Planetary Science Letters, 210(1-2), pp.17-33.
All photographs are from my own personal collection and were taken in May 2019.
My thanks go to Dr Annette McGrath for developing and encouraging my interest in geology through my engagement in The Geology of Yorkshire and northern England, an online Postgraduate Diploma programme delivered through distance learning at the Centre for Lifelong Learning at the University of York. I would also like to thank Annette for suggesting the publication of this article and for her guidance, support and editing the content.
For information on the course, see: https://www.york.ac.uk/study/postgraduate-taught/courses/pgdip-geology-northern/.
About the author
Ken Madrell is a retired educational adviser. He has recently completed the post graduate programme in the Geology of Yorkshire and northern England at York University and is now studying for an MSc in Earth Sciences with the Open University.