Introducing Volcano Furnas

[BrickFielder]

Introduction
My intention is to produce a series of articles (depending on interest) which introduces ideas about volcanic and earthquake activity. I want to go slightly off the beaten track to explore oddities, volcano hazards, analysis methods, Wonders and Mankind’s impacts. Keep in mind I am not expert (corrections gratefully received), but hopefully these will at least give a flavour of some different places in the world and provide a few minutes escape from people’s troubles.

Introducing Volcano Furnas

Reason for Interest
Sao Miguel is Another beautiful island with explosive volcanoes with typical volcanic repose times of 300 years which makes activity more likely in the near future. What really catches my eye though is the fact that some villagers live in houses where CO2 levels can get dangerously high currently. 

The Setting
Located off the west coast of Portugal is the Azores, a slice of heaven in the form of a nine island archipelago. Sitting approximately 1,643 km from Lisbon, Sao Miguel is the largest island (40 miles by 10 miles) in the Portuguese Azores archipelago. Sao Miguel is famous for it volcanic landscape peppered with bubbling geysers, colourfull flora and rich marine life, including whales and dolphins. Known as the "green island" or "Hawaii off the Atlantic" São Miguel is the center of government and the Azores. Important industries include agriculture, fishing and tourism. The island grows tea and tobacco and is known for its diary farming. 

Flank volcano collapses and high magnitude sea cliff landslides are known to have occurred in several Azores islands. The major reported landslide-triggered tsunami took place in 1847, when a rotational slide occurred in a sea cliff with more than 350 metres high at the NW coast of Flores Island. The entrance of material into the sea originated a wave that affected both Flores and Corvo islands, being responsible by the death of 10 people and more than 100 injured. In the last 500 years at least 12 tsunamis generated by earthquakes or landslides reached the Azores archipelago. Recent studies revealed that the highest magnitude event was the tsunami generated by the 1755 Lisbon earthquake with had an epicentre in the North Atlantic and had a maximum run-up of about 15 metres. One major earthquake occuring in 1757 killed more than 20 percent of the population of Sao Jorge island and the largest measured event in 1980 on Terceria island had a magnitude of 7.3. Several major landslides have occured on Sao Miguel including the one which destroyed the previous capital of the island Vila Franca do Campo. 

São Miguel has a subtropical climate and similar to other islands in the archipelago, São Miguel is influenced by ocean currents and winds, and, in particular, the cyclonic Gulf Stream. This stream functions as a moderating force in the islands, keeping temperatures hovering between 14 °C and 23 °C throughout the year. The island's location also makes it susceptible to some Atlantic storms, and precipitation tends to be elevated during the winter periods. 

In the east of the island is Furnas volcano and nestled within the crater is the town of Furnas.

It is surrounded by lush green vegetation and home to the famous Terra Nostra Botanical Gardens where you can swim in the mineral rich healing volcanic waters that flow at a constant 35 to 40 degrees Celsius.

The Tectonic environment
The main tectonic features that dominates the Azores region is the spreading Mid-Atlantic Ridge (MAR) which runs North to South to the west of most of the Azores. To the south is the Azores Gibraltar Fracture Zone that constitutes the Eurasian Nubian (Europe and Africa boundary) plate boundary and extends from the MAR to the region of Gibraltar. Most of the Azores lie in the Terceira Rift (TR) fault which is an complex rift fault zone trending NW to SE extending from the MAR to the Gloria Fault.

The concept of an ‘Azores mantle plume’ has been widely debated, and the existence of an Azores hotspot questioned. An alternate explanation is an enriched upper mantle domain with more volatiles than normal or Decompression of a wet mantle caused by rifting tectonics. Another explanation proposed is a block tectonic model for the Azores Rise that has multiple blocks. They move relative to one another in strike – slip motion or rotate clockwise depending which fault systems are active. This swapping back and forth of motions controls buildup of volcanoes and current seismic activity. Interestingly two shallow depth velocity anomalies have been identified with one below Faial, Pico, and Graciosa and the other between Terceira and Sao Miguel so perhaps two hotpots rather than an actual plume.

 

The Volcano
Furnas is the easternmost of the trachytic active central volcanoes of São Miguel and is thought to be the youngest volcanic center on the island. In the last 5000 years Furnas has had 10 moderately explosive eruptions. Two of these have taken place since the island was settled in the mid-fifteenth century. There are at least two calderas, a younger one that is 6-km wide and a larger older 8 km wide. The eastern wall of the 500-m-deep Furnas inner caldera overlaps the western wall of the Nordeste Povoaçao caldera. The southern rim of the volcano is cut by marine erosion to form high steep cliffs. Young pumice cones (less than 5000 years old) are distributed on the floor of the inner caldera.

 
Since the early 1990s geochemical studies carried out in São Miguel Island showed that some villages are placed in anomalous high degassing areas, and indoor measurements performed in various dwellings highlight the risk to the population. These high indoor CO2 concentrations are not only measured in areas classified as high degassing areas, but lethal CO2 concentrations are also registered in buildings located in areas previously defined as low- and medium-risk zones.

Beneath the crater lake degassing plumes have been found in the north of the lake. The highest density of degassing plumes correlates with recent submerged secondary craters formed around the caldera rim. More recent mapping of the lake floor (Lagoa das Furnas) also revealed a volcanic cone in the southern part of the lake.  Analysis of deposits nearby suggest this was formed as part of the 1630 eruption.

 

Past Eruptions
Some time between 1439 and 1443 a Portugese priest entered the valley of Furnas and reported tongues of fire and tremors with vapour rising from the three lakes. It is thought this corresponds to the latest eruption from the Gaspar dome.

Several first hand observations have been recorded of the most recent 1630 eruption. One reports clouds of fire from two of the lakes and an explosion that blew away the elevation between two lakes. Another observation reports on ash which turned day to night with tremors which made the whole island feel like it was in a swale. It seems likely that of the three lakes only the main current one survived the eruption. Analysis suggests the eruption took place from a vent in the southern part of the caldera of Furnas volcano. with seismic activity caused extensive damage destroying almost all buildings within a 10 km radius and probably triggered landslides on the southern coast. Throughout the course of the eruption more than six discrete ash layers were generated by magmatic explosive activity. Isopleth maps suggest column heights of up to 14 km with ash falling up to 550 km away.

Recent Activity
During February of 2018 an earthquake swarm was reported in central Sao Miguel island. Dozens of mostly small quakes at shallow depths between 1-15 km in an area about 6-7 km west of Furnas volcano, were recorded some of which were felt by local inhabitants. It is unclear whether the earthquakes are volcanic in origin or not. If so, they could have been caused by a small magma intrusion at the lower crust and indicate an early sign of possible re-awakening of the volcanic system.

Discussion

Furnas is closely monitored and no eruption is indicated in the near future, yet there are so many different hazards associated with the area that it would make me feel uncomfortable living in an active caldera as people do.

Edited April 21, 2021 by BrickFielder

[BrickFielder]

Cannot add all the pictures at the moment.