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See discussions, stats, and author profiles for this publication at:https://www.researchgate.net/publication/222315683 Volcanic geology of Furnas Volcano, SãoMiguel, Azores Article in Journal of Volcanology and Geothermal Research · September 1999 DOI: 10.1016/S0377-0273(99)00064-5 CITATIONS 70 READS 522 8 authors , including: Some of the authors of this publication are also working on these related projects: The eruption of trachytic magmas within a basaltic fissure zone: insights into the shadowzone hypothesis of São Miguel, Azores View projectPetrogenesis of the peralkaline ignimbrites of Terceira, Azores View projectAngus M. DuncanUniversity of Liverpool 90 PUBLICATIONS 1,327 CITATIONS SEE PROFILE Nicolau WallensteinUniversity of the Azores 48 PUBLICATIONS 370 CITATIONS SEE PROFILE All content following this page was uploaded by Jose M. R. Pacheco on 08 May 2015. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the srcinal documentand are linked to publications on ResearchGate, letting you access and read them immediately. Ž . Journal of Volcanology and Geothermal Research 92 1999 1–29www.elsevier.com r locate r jvolgeores Volcanic geology of Furnas Volcano, Sao Miguel, Azores ˜ J.E. Guest a, ) , J.L. Gaspar b , P.D. Cole a,c , G. Queiroz b , A.M. Duncan c ,N. Wallenstein b , T. Ferreira b , J.-M. Pacheco b a Uni Õ ersity College London, Planetary Image Centre, 33–35 Daws Lane, Mill Hill, London, NW7 4SD, UK b Departamento de Geociencias, Uni Õ ersidade dos Ac ¸ ores, Rua Mae de Deus, 9500 Ponta Delgada, Portugal ˆ ˜ c Centre for Volcanic Studies, Department of En Õ ironment, Geography and Geology, Uni Õ ersity of Luton, Park Square, Luton, LU1 3 JU,UK Received 1 November 1998; accepted 20 April 1999 Abstract Furnas is the easternmost of the three active central volcanoes on the island of Sao Miguel in the Azores. Unlike the other ˜ two central volcanoes, Sete Cidades and Fogo, Furnas does not have a well-developed edifice, but consists of a steep-sidedcaldera complex 8 = 5 km across. It is built on the outer flanks of the Povoac ¸ ao r Nordeste lava complex that forms the ˜ eastern end of Sao Miguel. Constructive flanks to the volcano exist on the southern side where they form the coastal cliffs, ˜ and to the west. The caldera margins tend to reflect the regional r local tectonic pattern which has also controlled thedistribution of vents within the caldera and areas of thermal springs. Activity at Furnas has been essentially explosive,erupting materials of trachytic composition. Products associated with the volcano include plinian and sub-plinian pumicedeposits, ignimbrites and surge deposits, phreatomagmatic ashes, block and ash deposits and dome materials. Most of theactivity has occurred from vents within the caldera, or on the caldera margin, although strombolian eruptions with aa flowsof ankaramite and hawaiite have occurred outside the caldera. The eruptive history consists of at least two major calderacollapses, followed by caldera infilling. Based on 14 C dates, it appears that the youngest major collapse occurred about12,000–10,000 years BP. New 14 C dates for a densely welded ignimbrite suggest that a potential caldera-forming eruption Ž . occurred at about 30,000 years BP. Recent eruptions - 5000 years old were mainly characterised by alternating episodesof magmatic and phreatomagmatic activity of plinian and sub-plinian magnitude, forming deposits of interbedded ash andlapilli. An historical eruption is documented in 1630 AD; new evidence suggests that another occurred during the earlyoccupation of the area at about 1440 AD. q 1999 Elsevier Science B.V. All rights reserved. Keywords: geology; Furnas Volcano; Azores; trachytic pyroclastics; calderas; 14 C dates 1. Introduction Furnas, near the eastern end of Sao Miguel island ˜ Ž . Fig. 1 , has been, and probably continues to be, one ) Corresponding author. Department of Geological Sciences,University College London, Gower St., London WC1E 6BT, UK of the most active and dangerous volcanoes in theAzores Archipelago. It is a trachytic centre, and themajority of its activity has involved explosive vol-canism. However, throughout its history, it has ex-hibited almost all known eruptive styles rangingfrom mild effusive activity to caldera-forming explo-sive events. From a hazard perspective, it poses 0377-0273 r 99 r $ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0377-0273 99 00064-5 ( ) J.E. Guest et al. r Journal of Volcanology and Geothermal Research 92 1999 1–29 2 Ž . Fig. 1. General map of Sao Miguel Island showing the main volcanoes and structures modified from Queiroz, 1998 . Inset right shows ˜ Ž . Atlantic setting; inset left shows the principal structures of the Azores Archipelago area modified from Gaspar, 1996 . considerable problems as the current caldera com-plex is inhabited, and some towns outside the calderacomplex are close enough to suffer fatalities fromeven a small explosive eruption.A general study of the island of Sao Miguel, ˜ together with the preparation of a 1:50,000 geologi- Ž . cal map, was carried out by Zbyszewski et al. 1958 , Ž . Zbyszewski and Veiga Ferreira 1959 , and Ž . Zbyszewski 1961 . Based on this work, Assunc ¸ ao ˜ Ž . 1961 studied the petrography of the rocks of theisland. The petrology and geochemistry of the mainvolcanic complexes, including Furnas, were consid- Ž . ered in the works of Fetter 1981 and Rodrigues et Ž . al. 1989 . A preliminary volcanological map of Sao ˜ Ž . Miguel was made by Forjaz 1976 , who also de-fined the main volcano-stratigraphic units for this Ž . island Forjaz, 1984 .George Walker and his colleagues, Basil Boothand Ron Croasdale, put the Azores on the worldvolcanological map. Their detailed studies of theisland of Sao Miguel, the principal island of the ˜ archipelago, led to considerable advances in knowl-edge of the mechanisms of explosive volcanism Ž . Walker and Croasdale, 1971 . In their study of theexplosive history of the island during the last 5000 Ž . years, Booth et al. 1978 established a detailedstratigraphy of tephra associated with Furnas and theother volcanoes on the island. Based on this work and other field studies, a hazard analysis for Sao ˜ Miguel, including Furnas, was made by Booth et al. Ž . 1983 . A 1:50,000 generalised hazard analysis map Ž . was published by Forjaz 1985 .During the years 1980 r 1983, Richard Moore of the US Geological Survey made a 1:15,000 scale Ž . geological map of the Furnas Volcano Moore, 1983 .This was later incorporated into a 1:50,000 geologi- Ž . cal map of the whole of Sao Miguel Moore, 1991a . ˜ Ž . Moore 1990, 1991b discussed the geology of Fur-nas as well as the other two major central volcanoeson the island. Based on radiogenic dating by him and Ž others Shotton, 1969; Shotton and Williams, 1971; . Adel-Monem et al., 1975; Moore and Rubin, 1991 , Ž . Moore 1990 studied eruption frequencies. Furnaswas designated one of the six European LaboratoryVolcanoes by the European Science Foundation in1990. ( ) J.E. Guest et al. r Journal of Volcanology and Geothermal Research 92 1999 1–29 3 2. Atlantic setting The Azores Archipelago, which consists of nineinhabited islands, sits astride the Mid-Atlantic Ridge.The islands are associated with a region of positivegravity and residual depth anomaly which is inter-preted as the surface expression of a mantle plume Ž . White et al., 1976; McKenzie and O’Nions, 1995 .The archipelago is located where the American,Eurasian and African lithospheric plates meet at a Ž . triple junction Fig. 1 . Within this framework, theNorth Atlantic in the area of the Azores is charac- Ž . terised by three main tectonic features. These are: 1 Ž . The Mid-Atlantic Ridge MAR which crosses thearchipelago between the islands of Flores and Faial Ž . Kurase and Watkins, 1970; Steinmetz et al., 1976 .The ridge trends 10 8 to the north of latitude 38 8 50 X N Ž . and between 10 8 and 20 8 to the south Searle, 1980 ; Ž . 2 The East Azores Fracture Zone which extendsbroadly east–west from the MAR to Gibraltar Ž . Ž . Kurase and Watkins, 1970 ; and 3 the TerceiraRift which extends from the island of Santa Maria Ž . northwest to the MAR Machado, 1959 . In addition,the fracture systems of Sao Jorge and Faial r Pico ˜ Ž . have a general WNW–ESE trend Agostinho, 1932 . Ž For the Azores Platform Needham and . Francheteau, 1974; Lourenc ¸ eau et al., 1968 , theboundary between the American and Eurasian platesis well-established. However, the location and natureof the eastern branch of the Azores triple junction isstill controversial, and is the subject of several geo- Ž . dynamic models Madeira and Ribeiro, 1990 . 3. Furnas and its geological setting Ž . The active centres of Sao Miguel Fig. 1 consist ˜ of three major trachytic central volcanoes, linked byrift zones. At the trachytic centres, explosive volcan-ism has dominated, while in the rift zones, althoughoccasional trachytic eruptions have occurred, activityis characterised by basaltic effusive eruptions accom-panied by strombolian cone building.At the western end of the island is Sete CidadesVolcano which has been built up from several phasesof activity including the production of a lava shield Ž . and numerous explosive eruptions. Queiroz 1998identifies at least 17 explosive eruptions that oc-curred here during the last 5000 years. In addition,there have been three offshore eruptions recorded inthe last 400 years. The edifice is truncated by acomplex caldera which contains two lakes. Withinthe caldera are cones and domes representing thepresent phase of caldera infilling, the most recent of Ž which is dated at about 600 years ago Booth et al., . 1978 just before the island was inhabited by Por-tuguese settlers. The only known historical eruptions Ž . on land post about 1440 AD in this part of the Ž . island occurred in 1652 AD Weston, 1994 on theso called ‘waist’ region, the rift between Sete Cidadesand Fogo Volcano to the east. The 18 km long waistconsists of hundreds of cinder cones, fissure ventsystems and associated lava flows. Ž . Fogo Volcano also known as Agua de Pau , islargely made up of pyroclastic deposits and is alsotruncated by a caldera complex. There have been atleast four eruptions from this centre during the past Ž . 5000 years Booth et al., 1978 , the most recent Ž . being in 1563 AD Weston, 1994 . The depositsfrom a major eruption that occurred about 5000years ago were first recognised by Walker and Ž . Croasdale 1971 and termed Fogo A. This eruptionproduced one of the best documented of all pliniandeposits; it is also distinctive in the field and servesas a widespread marker horizon in the stratigraphy of Ž Sao Miguel Booth et al., 1978; Cole et al., 1999-this ˜ . issue .Another, mainly basaltic, rift links Fogo to thenext trachytic centre, Furnas. This rift is known asthe Achada das Furnas complex and is 5 km long; itconsists of cinder cones and lavas, and includes a Ž small, young trachytic centre known as Congro Fig. . 1 .Furnas Volcano is, from a topographic point-of-view, the least impressive of all the volcanoes onSao Miguel. It has no well-developed positive edi- ˜ fice: to the west, its products interdigitate with lavasfrom the Achada das Furnas rift; to the east, it banksup against the deeply eroded Nordeste complex of lavas that once formed a basaltic shield, much of which has been destroyed by marine and fluvial Ž . erosion Moore, 1991a,b . It also cuts into thePovoac ¸ ao caldera, which, in turn, cuts the Nordeste ˜ complex. This old caldera is about 6 km across andpartly open to the sea at the mouth of the main ( ) J.E. Guest et al. r Journal of Volcanology and Geothermal Research 92 1999 1–29 4 drainage system. Only a few outcrops of pyroclasticmaterials have been associated with the Povoac ¸ ao ˜ Ž . caldera centre Moore, 1990 . Although the calderais presently open to the sea, and thus could be theresult of sector collapse, the opening is narrow and itis more likely that faulting and marine erosion havecut back into the seaward side, breaching the calderawalls.The main features of Furnas are the caldera com-plex and the southern rim of the volcano cut by high Ž . sea cliffs which provide the best exposures Fig. 2 .The deposits from this volcano were draped over theNordeste edifice and have been heavily dissected,and on the seaward side deposition occurred at sea;constructional forms were cut back with ease bymarine erosion. Most eruptions at Furnas have beenfrom vents within the caldera or on the calderamargin, although strombolian eruptions with aa flowsof less evolved composition have occurred outside Ž . the caldera margin on the flanks Moore, 1991a,b .Furnas acquired its eroded form first, because it islargely made up of unconsolidated pyroclastic rocksand second, because it grew near the shore of therugged Nordeste lava pile allowing both marine andfluvial erosion. In addition, most pyroclastic materialtransported to the south enters the sea, and evenpyroclastic flows travelling overland to the east andnortheast are captured by the Povoac ¸ ao drainage ˜ Fig. 2. Map showing the main volcanological features of the Furnas Volcano and surrounding areas, together with principal villages. Romannumerals indicate positions of stratigraphic sections in Fig. 8.
