Part 2: The Volcanoes

In this part we are touring the volcanoes of São Miguel. I wanted to write a good tour post on these and as told in part one it is meant as an accompany that you read on your phone when you are visiting the island. São Miguel Island consists of six major volcanic zones that we will tour from east with Nordeste as starting point, to west where we finish the tour at Sete Cidades. You can imagine travelling by rental car doing this journey along the island’s length. I will talk about the large scale sights and main attractions that each locale stop provides. There is enough to see here so I cannot cover everything.

The Nordeste volcano

After the hotel check-in I took the rental car and drove east as far as you can get, to the oldest part of the island, Nordeste – ”northeast” in English translation. It is the oldest volcano on the entire island and is heavily eroded by the heavy rains. It is a ruin of what was once a fissure shield volcano that rose high above its surroundings, very likely much taller and broader than it is today. Its peak, Pico da Vara thrusts into gloomy cloud blankets that cover most of the stunning greenery on this island. The warm wet subtropical lowlands contrast with the cooler peak, but the peak is still much too warm for snow. At the coast, Ponta Sorgesso provides a spectacular viewpoint out over the Atlantic and the steep sea cliffs that drop into the deep blue waves.

Nordeste is a heavily eroded pile of thin mildly alkaline lava flows 1300 m thick and dated to the Pliocene era, 4,01 million years old for the lower lava flows. Lava flows higher in the pile are more potassic than the lower stack member of the pile. Some other datings give a much younger age for the lower lava sequence. Nordeste has no obvious volcanic features on the ground. It is heavily eroded and lava flows are only visible in road cuts, as is the case of most of the Island, but in Nordeste you don’t even find cinder cones. Nordeste is basically a pile of fissure-fed thin Aa flows and there are no signs of major trachytic productivity other than minor exposures. In its youth, it may have resembled Marion Island combined with Hekla (minus Heklas evolved magmas). It would have been a fissure eruption shield volcano dotted with past cinder cones, by now it is highly eroded, cut by younger calderas and landslides and river streams drape the scenery. There are arguments for the pliocene datings from other sites, with genetic studies of the islands land snails that diverged around 4 to 3 million years ago that will support pliocene datings on lava rocks.

Nordeste offers little in terms of visible volcanic features and the coast scenery is the main attraction so I drove west again to the Povocao volcano. There is no clear evidence of a past caldera at Nordeste. Nordeste may never erupt again; it seems pretty extinct.

The Povoação volcano

Driving west from Nordeste the lush landscape opens up towards the spectacular Povoação volcanic complex whose enormous caldera has carved out large parts from the Nordeste’s old shield. The feature measures seven kilometers wide and the caldera wall borders with the Furnas and Nordeste complexes to the west and east. The scenery is spectacular and reminded me of a subtropical version of UK farmlands or North New Zealand’s countryside, very pleasant with small towns and groves. Like Nordeste, Povoação is a very old and extremely tired volcano who has not erupted for a very long time. You don’t find any hot springs or geothermal mudpots here unlike the island’s more westerly volcanoes that have lots of geothermal features. It kind of looks like a landslide, but Povoação does have tectonic fault features of a true caldera, so it is not an erosional or landslide feature.

There are four post caldera extrusive features in Povoação that have been mapped by geologists. one trachyte coulee ( thick stubby flow) and three alkaline scoria cones have been mapped, suggesting that the magma system remained alive after the collapse. Povoação’s floor is gentle and without much terrain other than river valleys. The caldera has lots of trachytic pumice in its soil deposits, not from the caldera itself but trachytic tephra airfall from the nearby Furnas volcano to the west which is a much more active caldera for sure. At the northern rim of Povoação there are 100 m thick trachytic pumice deposits that predate the caldera formation with more than one or two layers responsible for the Povoação caldera collapse itself. At the village of Povoação, steep cliffs drop into the ocean. Povoação offers relatively little volcanic features compared to the other São Miguel volcanoes. The eruption that formed the caldera was enormous and laid waste to wast areas a long time ago. Povoação may erupt again sometime in the far future, but seems pretty dead for now.

The dangerous Furnas volcano

Driving towards Povoaçãos western rim the road climbs into the mountains. At the caldera wall of Povoação you enter another magnificent caldera volcano. Furnas is one of the most dangerous volcanoes on the entire island and it is also one of the most beautiful. It is home to lush forests, a magical lake, geothermal springs, geothermal baths, small towns and a botanical garden. For me it was one of the most beautiful sights that I have ever visited. Furnas caldera is little like a lost world, almost like Jurassic Park or something like that. It is an isolated sheltered world deep inside the caldera, its own little society inside this volcano that has an immense peaceful scenery and atmosphere.

Furnas is often cloudy and mists hang over it and inside it. It adds an otherworldly feel where grey clouds hug subtropical forests that grow in the caldera walls and where huge cryptomeria trees giants rise over the mysterious mist layers. Tree ferns, moss, palms and the huge conifers add a ” Jurassic” feel to Furnas, perhaps not unlike Kilauea’s fern-filled summit area, but here the volcanic landscapes are much more developed by man with agricultural fields and small towns. Despite that it feels very primordial. Primordial for sure, especially so during a cloudy day, when everything here becomes extra green and extra mysterious. The rainforested caldera walls where waterfalls stream down, ferns cling to rocks and the crater lake, often having a low cloud ceiling where drooping tree branches hang over emerald green lilly pad-filled waters, were extra surreal for me. It is a place for meditation and thinking and indeed a neogothic church was built long ago at the volcanic lake shore side. The beaches you find in all the island’s caldera lakes are beige due to the trachyte materials. Furnas together with Sete Cidades, while not my geological highpoints in terms of volcanoes, are still among the most beautiful places that I have ever visited. That is because of their private ”lost world” atmosphere and spooky lush greenery and especially so for Furnas geothermal steam. I spent a lot of time there with friends exploring all the geothermal features and we ate the volcano-cooked local foods.

Furnas unlike Povoação and Nordeste does have a real, alive-and-kicking magma system which remains active even if magma supply is slow. Geothermal activity is everywhere, hot water pools, mudpots and mineral-rich springs can be found. In the center of town, Furnas boasts close to 30 bubbling geothermal features some of which are boiling hot, their steam plumes dragging in the mists add to the mysterious otherworldly, slightly unsettling atmosphere that exists here. There are numerous geothermal baths too, all having warm brown iron oxide rich waters. All of these baths are artificially built but use warm volcanic waters from the magma system. Terra Nostra is the largest bath in Furnas: it can house hundreds of guests. I swam in it briefly. I liked the heat but the water quality seemed like a mudpool, even if it was in reality very clean stuff just with rust in it. Geothermal activity in Furnas can be found both in Furnas-village and on the lake shore where there is another set of hot mudpots with mud so fine it seems like paint. The locals boil some of their foods in these hot holes which are then served at fine dinners. The plentiful geothermal steam is a cruel reminder that 1439 persons live inside a timebomb. The last eruption in the year 1630 was as large as St Helens, 1980. 1 km3, and around 200 persons were killed. Previous eruptions here dwarfed the 1630 event. Eruptions in Furnas caldera are of a highly explosive evolved type. The eruptions are plinian, subplinian, vulcanian, phreatomagmatic; lava domes are often formed in the end. The volcano vents constantly produce CO2 gases that collect in deep lying areas. Many homes have gas alarms in case concentrations get too high. A description of Furnas chronology follows below.

Furnas is a nested evolved caldera structure that has collapsed numerous times. It is almost 8 kilometers wide and many 100s of meters deep. There have been major Pleistocene explosive eruptions here, some of these vastly larger than the most recent explosive eruptions. Post caldera eruptions been smaller pyroclastic events (if the 1630 event can even be called ”small”). All of the magmas are sourced from an evolving shallow large trachytic magma body that is compositionally zoned. In many studies, the violent history of Furnas is seen in the abundance of light pumice and grey ash in outcrops. Eruptions outside the caldera are mafic. Furnas, unlike the other volcanoes to east, has well studied history, even if geochronology is hard for all volcanoes in the greenery of São Miguel.

Furnas, like Agua De Pau and Sete Cidades, is an excellent example of the evolution of a mature Azores volcano from a fluid mafic one to evolved caldera volcanism. Furnas began its life on the seafloor as a slowly active alkaline mafic shield in the Pleistocene, 800,000 years ago, later emerging above sea. Furnas is today a sillicic nested caldera structure. The oldest and largest outer caldera likely signaled the change to trachytic caldera volcanism. The Pleistocene was plagued with sillicic caldera forming events at Furnas. The largest each happened after 1000 s or even 10,000 s of years of dormancy. The older caldera was an enormous event (Tambora sized in collapse area but volume is unknown ) The second large inner caldera collapse, four km wide dates back to the Povoação Ignimbrite that happened 30,000 years ago, when the Furnas volcano had its most recent major caldera collapse. The eruption was enormous and massive pyroclastic flows engulfed everything laying down ignimbrites in valleys. The pyroclastic flows also flowed into Povoação caldera giving this Furnas eruption the name of the wrong volcano even. Around 11,000 years ago there was another enormous pyroclastic eruption that collapsed the inner caldera once again, but smaller than the earlier two.

After the last large caldera collapse the volcano stratography becomes a bit better known with tephra zones from many series of trachytic eruptions. In the last 5000 years there have been 10 smaller trachytic eruptions from Furnas that have laid down their pumice airfall in the caldera. The Furnas C eruption was the largest of the group of later eruptions and dates to 1900 years ago. A 2 km wide collapse feature formed where Furnas town now sits; carbonized ”fossil” leaves have been found in ash from this event..

Since humans came to the Island there have been two plinian eruptions in Furnas, in the years 1439 and 1630. Both ended with a lava dome extrusion as the final gas-poor stuff came up. The 1630 event occured when the caldera was nearly as densely populated as today. It was known as the “Year of the Ashtray” it was disastrous with 200 persons were killed in the eruption with ash and destruction spreading over vast areas.

If we take 5000 years divided by 10 eruptions gives an eruption once every 500 years, but there is no timetable for that. Eruptions at Furnas are very likely caused by injection of a more mafic mugearite or hawaiitic magma into Furnas trachytic magma chamber. Furnas will erupt again and in geological times likely soon. Any new eruption poses a lethal threat to all of the caldera residents who seem to have formed a friendly relationship with the time bomb they are living in. In short, Furnas is a mature Azorian central volcano. It has evolved a silicic heart and has a long history of repeated collapse and refill.

 

The Congro Fissural system

Driving west from the Furnas caldera I entered the first monogenetic field on the Island on our tour from east to west. CVFS Congro Fissural Volcanic System is a series of cinder cones between the Furnas and Auga De Pau central volcanoes. The eruptions here are one go and then erupt elsewhere, caused by numerous small magma pockets. Some monogenetic eruptions here are fully trachytic like Lagoa do Congro an indication that there are many melt pockets here, some which are in an advanced state of fractional evolution. Most cones are less evolved magma and the whole landscape echoes Chaîne des Puys but the climate is even milder and huge cryptomeria trees dot many of the cinder cones. The landscape is as beautiful as it is everywhere on the island, with cow pastures and tree filled cinder cones.

One of the most magical sights in the monogenetic Vulcânico Fissural do Congro field are the hidden forest lakes that you can find inside numerous cinder cones and one such lake in a maar crater. Just as in the Furnas and Sete Cidades lakes it is a private enclosed and slightly mysterious atmosphere where subtropical forests drop into green anoxic looking waters. The easiest of these hidden gems is Lagoa do Congro (lagoon) which occupies the base of an explosion maar crater maar. It is 500 metres of diameter, with steep walls emplaced in basalts and trachytes and a maximum elevation difference of approximately 120 metres. This crater, formed about 3800 years ago, is associated with a hydromagmatic eruption. There is even a trail to it and you walk along laurel and cryptomeria rainforest on the way down to its shores. There are three other monogenetic cinder cones that have such magic lakes here, but they have no trails up to them. One is named Lagoa do Arieiro, which is another very peaceful place where misty forests loom over a lonely cone lake. One of these other cones just north of Lagoa do Congro is likely almost never visited by any tourists. Another similar one is found just west of the Congro maar but seems more accessible.

The area seems dead silent with only birds song and wind, but in 2005 and 2011 – 2012 there was intense seismicity and some ground deformation an indication of magma rising and stalling at shallow levels in the crust. The area has been quite active in the Holocene, but it is nowhere near as large as the Pico’s fissural monogenetic system close to the main city, where you find 100 s of cinder cones (well green as always). An interesting note is that CVFS is one of the most seismically active regions on the entire island.

The dangerous Auga De Pau volcano

Driving west from Congros monogenetic cones, I came up into more hilly terrain and a gentle slope was right ahead, a huge cloud cumulus castle hovered right ahead in the highlands. The massif right ahead was the Agua De Pau stratovolcano, the tallest active stratovolcano on the entire island. The volcano is, just like Furnas, dangerous for the local populations which face threats both from pyroclastic flows, tephra falls, and even fast-moving lava flows, due to its bimodal nature, meaning it has erupted both mafic and sillicic magmas before. Since the island was settled, two eruption events have caused problems for villagers.

Agua De Pau is hardly a magnificent stratovolcano like Etna or Fuji, it is lumpy and its summit is strongly eroded by the subtropical rains in these latitudes. Auga De Pau’s lumpy upper parts are a combination of factors of erosional rainfall in pyroclastic deposits and all trachytic lava domes and coulees (short stubby viscous lava flows ) that have been extruded around the nested caldera complexes. The road upwards to the summit snakes through the eroded trachytic domes. The summit crater provides the Lagoa do Fogo lake, one of the 3 caldera lakes on the island and its highest.

Much of the land surrounding the crater lake is occupied by large parcels of median 283 hectares (2,830,000m2) in size, 61 hectares (610,000m2) of forested cover and endemic Macronesian Azorian species. Among that is the everpresent japanese cryptomeria trees that dot the grey trachytic pumice beaches. The caldera lake was formed 15 000 years ago and that was the last really large eruption, but there been numerous trachytic explosive eruptions since that. The last one in 1563 erupted one km3 of trachytic materials.The caldera also contains very eroded pumice cones and remains of lava domes. Agua De Pau, like Furnas and Sete Cidades, is an excellent example of a mature Azorian central volcano that have developed a sillicic central magma system and bimodal flank eruptions.

This volcano has like furnas a strong geothermal activity and hot baths and springs can be found in the jungles on its flanks. Caldeira Velha (thermal baths ) have wonderful jungle trail and hot baths. A geothermal power plant can also be found on the volcano’s north slope. Agua De Pau shows the typical evolution of an Azorian central volcano from basaltic to silicic magmas.

Auga De Pau has a long and studied history. It began to grow 200,000 years ago as a basaltic shield or basaltic stratovolcano that grew out of the ocean, during this time São Miguel was not one island but perhaps a series of islands. The oldest basaltic rocks in this region have an absolute age >250 ka – most of them were formed in a submarine environment. The volcano then grew and grew forming a tall structure that rose above the sea. The oldest materials from this volcano in cores are about 280,000 years old and consist of alkaline basalt trachybasalt lavas. The volcano later developed into a evolved explosive stratovolcano with bimodal flank behavior. The volcano is draped with tephra and pyroclastic deposits and tephra chronology have been done on available outcrops. The volcano has a long history of explosive tephra rich eruptions, that have draped the edifice in deposits. Some events will be very briefly described here.

Since 40 000 years back the volcano’s eruptive history is much better understood with many layers of trachytic pumice materials emplaced from pyroclastic currents that have formed a series of dated formations. The Porto Formoso sequence is a series of pyroclastic deposits dated to 21,000 years back from a series of trachytic events; ignimbrites with fiamme stones can be found in road cuts. Another eruption formation is the Cha Gatas sequence that forms a whole series of pumice flows and ignimbrites some with syenitic xenoliths in that outcrop. (Syenite is the plutonic version of trachyte.) The Coroa eruption is another one which laid down a single trachytic formation dated to 18,000 years ago. In its later phases it formed a pumice cone and dome visible 2.5 km south east of the Ribeirnha village. Obsidian fragments and coarse pumice blocks can be found in abundance in this deposit in exposures. Roida da Praia formation is an impressive formation almost 100 meters thick in roadcuts from Agua De Pau. Rodia is the product from a group of 65 explosive trachyte eruptions. Five of these eruptions were very large. The older member eruptions have a lot of light coloured pumice and lapilli from subplinian activity which is dated to 34,000 years ago. The middle member of eruption groups is dated 14,000 years ago and has darker materials than the older unit. Lapilli and pumice are mixed with syenitic mantle xenoliths like in earlier deposits.

The Ribeira Cha eruption formation is a particularly spectacular one, from a very violent short-lived eruption that has been dated around 12,000 years ago. Other dating methods yields up to 16,000 years ago for the formation. Syenitic xenoliths (plutonic magma pieces) are common in this deposit too. The ignimbrites from this eruption show many deposits from the same eruption repeated pyroclastic flows. Ribeira Cha is generally agreed to be the culprit behind the larger outer summit caldera formation. The Ribeira Cha eruption is overlaid by smaller deposits from the Pisao formation that contains bubble rich lapilli and pumice stones. The Fogo A eruption overlies this and is one of the most studied in the island. The eruption was very violent and laplli pumice rained down over the entire island and the layer is the best age key in stratography because of that. Another younger unit above that is Lombadas formation that spans 3000 years with three eruption events over that time.

The last major eruption from Agua De Pau was on 28 June 1563 and that was witnessed by the locals. It had a very interesting explosive first plinian trachytic phase in the summit. The 1563 eruption was on the northwest interior flank of the summit caldera and took place after days of tremor in the surrounding towns. It likely started with a loud phreatomagmatic detonation. The Fogo 1563 plinian eruption formed a set of more than 70 stratigraphic sections were documented, allowing to establish eruption layer correlations and to understand the internal structure of the deposit. The 1563 eruption layers is characterized by alternating ash and pumice lapilli layers and it was possible to discriminate two deposits defined by their dominant lithofacies and stratigraphic position, which are representative of two eruption phases. Eyewitnesses reported late at night June 28 1563 an intense glow over the summit and thunderous booms that would be the hot base of a plinian column. During the first night, grey lapilli and pumice fell on the eastern and northeastern parts of the Island. By the 29 June changes in wind directions meant that ash began to fell on Ponta Del Gada leaving a 10-centimeter thick layer.

Four days after the trachytic eruption stopped, on 3 July there was a final very interesting effusive flank event caused by fluid basalt emerging from the deeper parts of Aguas plumbing system. Fast-moving Nyiragongo-like mafic lava burst out at Pico Do Sapatiero, an older trachytic dome through the western tectonic fault, and very fluid lavas rushed down the slope towards the villages below. The locals saw a column of steam and a mean glow litting it up, and reading from accounts from diffuse sources. A fast-moving lava stream ran straight trough the Ribeira Seca settlement where it crashed into the ocean waves. The priest Gaspar Frutuoso wrote down the event as did others,in the ”Phoenix of Angra” Fénix Angrense account. The lava was described as very thin and fluid and flowing like a river in flood, a good sign that it was from deep and primitive materials. Some of these lava flows contain both alkaline basalt and pieces of evolved trachytic materials, a clear sign of magma mixing in the central volcanoes, in a magma system with a zoned composition.

Picos Fissural System

Driving west from Augua De Pau’s eroded pyroclastic hills and trachytic domes, the landscape opens up as you heads down its slope towards the next volcano Sete Cidades. On the way to Sete Cidades you are driving through the youngest part of the entire island, the Picos Fissural system. It is named Picos Volcanic Fissural System and it is the largest monogenetic field on São Miguel. Here you visit more than 200 cinder/scoria cones which dot the landscape and are beautifully green. It’s a beautiful green landscape with lush gardens, pastoral fields, hedges and flower gardens and small towns. Some cinder cones are covered with small subtropical rainforests, others are crop fields with intense blue hydrangea hedges making interesting patterns with the green pastures and crop fields.

Picos Volcanic Fissural System is young, the most recent addition to the island. Before its formation Sete Cidades and Agua De Pau volcanoes were not joined. The whole region is young and I saw almost no signs at all of erosional valleys or rivers. Still eruptions are infrequent enough to allow the subtropical greenery to flood everything leaving no fresh barren lava surfaces. Monogenetic cones are everywhere here, some are half crescent shapes while some others are true cinder, scoria cones. The geological youth is also shown by the coasts not having very steep cliffs, with frequent lava flows called Fajãs ”lava shelfs” extent out in the water making it easy for boats to make landfall. These were almost definitely the cause of why Ponta Del Gada was built in this part of the island. Picos Fissural system is young and compared to the rest of the island is very active. Any new monogenetic eruptions could cause mayhem in this densely populated area. Most messy woud be a fissure eruption upslope of Ponta del Gada sending Aa lava flows steaming through the city.

The Picos fissural system has been estimated to have emerged from the ocean around 30,000 years ago with monogenetic eruptions using tectonics faults, forming cinder cones along fault lines in the crust. Eruptions have been more frequent along the middle of this fault system, forming a distinct ”spine” where the cinder cones been heaped up. More than 270 scoria cones have been mapped in PFVS, they are so abundant that the landscape is almost saturated, they also have buried each other under a thicker and thicker pile. At Miradouro da Reserva Florestal de Recreio do Pinhal da Paz its possible to peek out over this cone filled young landscape. Over the past 5,000 years there have been 19 monogenetic eruptions and that gives an average of one cone forming eruption every 260 years in this region. There may seem to be few other volcanic features to see here, but below the surface there are lava caves scattered about, as is often the case with basaltic volcanism and near Ponta De Gada you can visit Gruta do Carvão lava tube system. The largest lava tunnel on the island of São Miguel. The cave is located on the western edge of Ponta Delgada and extends for 1,650 metres, I visited it last in May 2023. It exceeds expectations with well formed fluidal features and staligmites and well preserved surfaces. The last eruption here lasted from 19 to 26 October 1652 and broke out north east of Ponta Del Gada. It was rare for this area, being an evolved eruption with very tall fountains and explosive likely vulcanian detonations building up cinder cones. Two viscous short almost ”pancake” like dome-like lava flows were also emplaced. Today the eruption site is a forested hill. The eruption was seen by locals who described it in writings as a dark cloud of ash and hot black stones being thrown high in the day skies. The night scene was described as a fire with shooting fireballs going up into the night skies.

Sete Cidades volcano

 

The last volcano on our tour is also the most famous and scenic one on the entire island. Sete Cidades at the western point is one of the most spectacular sights on this island and is a major reason tourists visit São Miguel to hike its awe striking caldera landscape. I had the huge luck of being able to visit this collapsed bimodal stratovolcano just a few years ago. Sete Cidades rivals Furnas in terms of scenery and beauty. Both calderas can be called comparable in natural beauty, but Sete Cidades maybe could be called more scenic and its caldera walls are much sharper than Furnas and Aguas due to young age. This is the youngest central volcano on the island so has the sharpest features and sharpest edifice.

This is one of the worlds best examples of an explosive caldera that can be visited and be walked inside, although the green framework shows it is certainly not frequently active. Sete Cidades edifice is visible as a shield-like bulge if seen from Picos or Agua’s summits on a clear day. Visiting the caldera was a spectacular sight for me. It is an area that evokes awe and also a soothing calm. It is a captivating landscape that is perfect for an explorer who wants to take a long hike and return to same starting spot. There are many trails here one around the caldera rim and many paths to stroll in its lush interior. This is its own little world just like Furnas that is kind of enclosed and hidden away from outside by the caldera walls around. I found this green subtropical caldera immensely peaceful where the subtropical forests crowd the caldera walls and interior pumice cones and where their lush greenery hangs over mystical lake shores of Lagoa Verde and Lagoa Azul thats two peaceful (if slightly overfertilized lakes) that can be found in the caldera. When seen from up at the caldera rim the two lakes can under ideal light conditions get two colors, one green and one blue, lagoa verde and azul, and this is one of the most photographed spots in the caldera. Photographers often add extra saturation and sell them to media advert sites, but on closeup inspection lagoa azul appears just as green and mucky as lagoa verde: the cattle pastures are a source of too much fertilizer. The two lakes are connected but a scenic bridge runs over their narrow connection.

There are four lakes in the caldera. The other two can be found in a very large pumice cone complex, lagoa santiago and lagoa rasa. The caldera rim is often windy and breezy yet it is not cold. The subtropics make a great place to run around in it, which you cannot do in warmer places without overheating. When you hike down into the caldera’s interior after having done the caldera ring road, you are greeted with a scenery that exceeds expectations. I myself found it lush and scenic, scenic pastures and small villages mix with subtropical forests and the trachyte beaches provide scenic views over the lakes. It is a peaceful scenery and during cloudy days with mists, it becomes just as mysterious as Furnas to walk these lake shores during a misty day. The lush subtropical forests rise into misty caldera walls and tree branches hang over mysterious green waters. The caldera floor feels humid and hot compared to the cooler windy caldera edge which was noticeable when I did my hike. Observing it from the shore of the lake, the surrounding caldera looked like a giant green football stadium. The caldera has four enormous post intra caldera trachytic pumice cones whose postcaldera growth has filled the eastern and southern part of the caldera. The pumice cones are named Caldeira Seca, Caldeira do Alferes and Caldeira Santiago, with the last having two smaller freshwater lakes. It’s a serene place where you can stroll the beaches and the cones through the numerous hiking paths that are often lined with blue hydrangeas.

Between Caldeira Seca and Caldeira do Alferes pumice cones is the town with same name of the volcano. I spent quite some time walking its streets which were just as peaceful as those in Furnas. Here man has carved himself a peaceful pastoral existence inside a time bomb. Today almost 800 persons call the caldera home. The beautiful neogothic church of São Nicolau can be explored when you walk around the town. In such a green lush place there may seem to be no evidence of any recent volcanic activity, but the lake beaches have grey sand that turn out to be pumice and the hiking trails and sandy outcrops are littered with trachytic pumice stones. Other more clear signs of ongoing deep volcanic activity at Sete Cidades can be seen if you drive to Ponta da Ferraria tidal hot springs at the volcano’s south west coast. Here geothermal hot water wells up in a bay where you can swim in warm tidal waters that are heated by magma. Close to the geothermal pools is Pico Das Camarinhas, a very nice mafic scoria cone whose viscous looking Aa has formed a little lava delta that can be explored. Interaction between hot lava and seawater has also formed a well formed littoral pseudocone that is one of the best preserved examples on Earth. Such features don’t last very long so this one is an exceptional preservation 100 of years after the eruption.

Sete Cidades just like the other central volcanoes on São Miguel began to grow on the seafloor as a basaltic shield volcano, later evolving into a bimodal caldera. It began to grow before the Picos fissural system emerged. The start of construction of the submarine edifice is not known but its thought to be many 100 000 s of years ago, although it is not as old as the other volcanoes on the island. Sete Cidades emerged roughly 210 000 years ago from the sea as a separate island. Since then the complex above water has grown to about 70 km3. Doing some simple maths 70km3 in 210 000 years that gives us an estimated rate of 0.02–0.03 cubic kilometres (0.0048–0.0072 cu mi) per century, so 20 to 30 million cubic meters of erupted materials per 100 years, which is a lot less than than Hawaii that may do 10000 – 15000 million cubic meters per 100 years. Still the supply is certainly enough for one or two eruptions per century. Eruptions here since settlement have happened on submarine flanks, the Sabrina ephemeral island 1811 being a famous example. Sete Cidades have a long history of both trachytic and basaltic eruptions, flanks mostly produce mafic eruptions making mafic scoria cones, while the trachytic caldera produce explosive pyroclastic eruptions whose pyroclastic flows and tephra falls have laid down pale lapilli and pumice over the entire peninsula, seen in cliff exposures at the coast. The caldera has had no historical eruptions since people came to the island.

The volcano’s chronology for the past 36,000 years has become better known. The Risco formation consists of well-preserved pyroclastic flow deposits and can be found as meter-thick deposits around the coast. It consists of many members as units and some have well-developed charcoal deposits in them. It has been dated to around 35,000 years old. Above that is the slightly younger Ajuda formation that has been dated to around 28,000 years old. That is in turn overlaid by the massive Bretanaha Formation that covers the entire volcano and reaches 6 meters thick at the volcano caldera rim. Pyroclastic flows raced all over and carbonized forest remains have been dated to around 28750 years. The lower layer consists of yellow pumice and fine lapilli, the upper layers consist of block and ash flow deposits. Some of the ash flows were so hot that they almost behaved like lava flows after being deposited. That is in turn overlaid by the much younger Lombas Formation which happened 15,750 years ago, so 13 ka after the Bretanaha tephras. It consists of 12 eruption units of mostly pumice fallout. Not all of these are trachytic, some eruptions were mafic.

One of the most significant pyroclastic formation deposits at Sete Cidades is the Santa Barbara formation, that has a three step chronology. Pyroclastic flows raced down the edifice laying down hot deposits and some of these have fossilized carbonized tree trunks in them. The caldera itself has formed through repeated collapses in the late Pleistocence. After the last Ice Age ended the Sete Cidades caldera had assembled its current form after numerous explosive caldera ”re collapses” that laid down all these trachytic deposits. The present caldera developed in three phases associated with massive paroxysmal eruptions which occurred approximately 36,000, 29,000 and 16,000 years before the present. Post caldera activity are associated with more trachytic tephra layers, such as formation of the large pumice cones in it. In the last 5000 years there have been 17 smaller trachytic eruptions inside the caldera laying down pumice and lapilli. Many deposits have hydromagmatic features. In the same time span there have also been 15 radial, rift alkaline basaltic eruptions on Sete Cidades flanks. These eruptions happened as either mafic monogenetic cone forming, evolved dome forming events, forming cinder cones and domes on Sete Cuidades flanks. Many monogenetic flank eruptions where also submarine, some forming tuff cones leaving behind palagonized cone remains offshore.

If we do the maths 15+17 = 32 thats thirty two eruptions in 5,000 years so once every 156 years for Sete Cidades. There is no timetable and the evolved caldera has not been active since 500 A.D when the Seca pumice cone formed. The last eruption that was well documented on Sete Cidades happened in 1811 offshore in the sea, when a mafic flank eruption burst the ocean surface forming a temporary island that grew. The newborn island was given the name Sabrina Island. The first person to land on the island was commander James Tillard, captain of the British warship HMS Sabrina, who hoisted the Union Jack on the erupting island and claimed sovereignty for Great Britain. Tillard’s great discovery was to be short lived, the islet made of loose tephras was soon washed away by the sea.

Sete Cidades is the youngest and historically (in terms of geology) the most active volcano on the island. It is possible or even likely that it will erupt during our lifetime. A very likely eruption scenario is a flank eruption that happens offshore in the sea. Most of these likely only form warm water plumes and discoloured seawater. The larger ones in shallower waters break the waves forming Surtseyan eruptions and short lived tuff islands. Even larger eruptions offshore may form Surtsey-like islands leaving behind palagonized islands and lava towers sticking up from the blue. An ocean eruption would be a good tourist eruption. A flank eruption on land woud be more problematic and would perhaps resemble Canaries eruptions. Sete Cidades is densely cultivated and small towns are everywhere. A flank eruption will destroy roads and homes and fields and the area won’t be usable for agriculture for well over 100 years due to loss of soil.

The flank eruptions can have numerous compositions. Flank eruptions of alkaline basalt will form fluid Hawaiian style eruptions with fast moving lava flows causing material damage. Slightly more evolved lavas (trachybasalt) may form strombolian eruptions and more slow ”basaltic” flows. A flank eruption of trachyte could either form a pumice cone with explosive action destroying entire flank communities, or if gas poor the extrusion of a slow lava dome or blocky flow ”coulee” with only local minor damage. The evolved caldera is most problematic, here the residents face similar hazards as those in Furnas. Any eruption in the caldera will be highly explosive, and even a small hydromagmatic eruption will cause damage to valuble land. A larger intra – caldera pyroclastic eruption puts the entire caldera population at lethal risk.

Summary

São Miguel is an excellent example showing that even sleepy volcanic islands can have a fascinating and varied volcanic geology. The fact is that most volcanic islands are like this, peaceful and green serene spaces where volcanic activity is infrequent and mostly a past memory. São Miguel is a good spot for a geology student to see almost all types of volcanology phemomena preserved as depoists in certain spots. Even a sleepy volcanic island have a lot to offer and see and because of that it has become a popular spot for geological field trip

Jesper, July 2024

Literature sources: https://www.amazon.com/Volcanic-Geology-Archipelago-Geological-Society/dp/1862397317