Volcanic geology of Furnas Volcano, São Miguel, Azores

Journal of Volcanology and Geothermal Research 92 Ž1999. 1–29 www.elsevier.comrlocaterjvolgeores

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

UniÕersity College London, Planetary Image Centre, 33–35 Daws Lane, Mill Hill, London, NW7 4SD, UK Departamento de Geociencias, UniÕersidade dos Ac¸ores, Rua Mae ˆ ˜ de Deus, 9500 Ponta Delgada, Portugal Centre for Volcanic Studies, Department of EnÕironment, Geography and Geology, UniÕersity of Luton, Park Square, Luton, LU1 3 JU, UK b

c

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-sided lava complex that forms the caldera complex 8 = 5 km across. It is built on the outer flanks of the Povoac¸aorNordeste ˜ 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 regionalrlocal tectonic pattern which has also controlled the distribution 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 pumice deposits, ignimbrites and surge deposits, phreatomagmatic ashes, block and ash deposits and dome materials. Most of the activity has occurred from vents within the caldera, or on the caldera margin, although strombolian eruptions with aa flows of ankaramite and hawaiite have occurred outside the caldera. The eruptive history consists of at least two major caldera collapses, followed by caldera infilling. Based on 14C dates, it appears that the youngest major collapse occurred about 12,000–10,000 years BP. New 14C dates for a densely welded ignimbrite suggest that a potential caldera-forming eruption occurred at about 30,000 years BP. Recent eruptions Ž- 5000 years old. were mainly characterised by alternating episodes of magmatic and phreatomagmatic activity of plinian and sub-plinian magnitude, forming deposits of interbedded ash and lapilli. An historical eruption is documented in 1630 AD; new evidence suggests that another occurred during the early occupation of the area at about 1440 AD. q 1999 Elsevier Science B.V. All rights reserved. Keywords: geology; Furnas Volcano; Azores; trachytic pyroclastics; calderas;

1. Introduction Furnas, near the eastern end of Sao ˜ Miguel island ŽFig. 1., has been, and probably continues to be, one )

Corresponding author. Department of Geological Sciences, University College London, Gower St., London WC1E 6BT, UK

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C dates

of the most active and dangerous volcanoes in the Azores Archipelago. It is a trachytic centre, and the majority of its activity has involved explosive volcanism. However, throughout its history, it has exhibited almost all known eruptive styles ranging from mild effusive activity to caldera-forming explosive events. From a hazard perspective, it poses

0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 6 4 - 5

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Fig. 1. General map of Sao ˜ Miguel Island showing the main volcanoes and structures Žmodified from Queiroz, 1998.. Inset right shows Atlantic setting; inset left shows the principal structures of the Azores Archipelago area Žmodified from Gaspar, 1996..

considerable problems as the current caldera complex is inhabited, and some towns outside the caldera complex are close enough to suffer fatalities from even a small explosive eruption. A general study of the island of Sao ˜ Miguel, together with the preparation of a 1:50,000 geological map, was carried out by Zbyszewski et al. Ž1958., Zbyszewski and Veiga Ferreira Ž1959., and Zbyszewski Ž1961.. Based on this work, Assunc¸ao ˜ Ž1961. studied the petrography of the rocks of the island. The petrology and geochemistry of the main volcanic complexes, including Furnas, were considered in the works of Fetter Ž1981. and Rodrigues et al. Ž1989.. A preliminary volcanological map of Sao ˜ Miguel was made by Forjaz Ž1976., who also defined the main volcano-stratigraphic units for this island ŽForjaz, 1984.. George Walker and his colleagues, Basil Booth and Ron Croasdale, put the Azores on the world volcanological map. Their detailed studies of the island of Sao ˜ Miguel, the principal island of the archipelago, led to considerable advances in knowledge of the mechanisms of explosive volcanism

ŽWalker and Croasdale, 1971.. In their study of the explosive history of the island during the last 5000 years, Booth et al. Ž1978. established a detailed stratigraphy of tephra associated with Furnas and the other 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. Ž1983.. A 1:50,000 generalised hazard analysis map was published by Forjaz Ž1985.. During the years 1980r1983, Richard Moore of the US Geological Survey made a 1:15,000 scale geological map of the Furnas Volcano ŽMoore, 1983.. This was later incorporated into a 1:50,000 geological map of the whole of Sao ˜ Miguel ŽMoore, 1991a.. Moore Ž1990, 1991b. discussed the geology of Furnas as well as the other two major central volcanoes on the island. Based on radiogenic dating by him and others ŽShotton, 1969; Shotton and Williams, 1971; Adel-Monem et al., 1975; Moore and Rubin, 1991., Moore Ž1990. studied eruption frequencies. Furnas was designated one of the six European Laboratory Volcanoes by the European Science Foundation in 1990.

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2. Atlantic setting The Azores Archipelago, which consists of nine inhabited islands, sits astride the Mid-Atlantic Ridge. The islands are associated with a region of positive gravity and residual depth anomaly which is interpreted as the surface expression of a mantle plume ŽWhite et al., 1976; McKenzie and O’Nions, 1995.. The archipelago is located where the American, Eurasian and African lithospheric plates meet at a triple junction ŽFig. 1.. Within this framework, the North Atlantic in the area of the Azores is characterised by three main tectonic features. These are: Ž1. The Mid-Atlantic Ridge ŽMAR. which crosses the archipelago between the islands of Flores and Faial ŽKurase and Watkins, 1970; Steinmetz et al., 1976.. The ridge trends 108 to the north of latitude 38850X N and between 108 and 208 to the south ŽSearle, 1980.; Ž2. The East Azores Fracture Zone which extends broadly east–west from the MAR to Gibraltar ŽKurase and Watkins, 1970.; and Ž3. the Terceira Rift which extends from the island of Santa Maria northwest to the MAR ŽMachado, 1959.. In addition, the fracture systems of Sao ˜ Jorge and FaialrPico have a general WNW–ESE trend ŽAgostinho, 1932.. For the Azores Platform ŽNeedham and Francheteau, 1974; Lourenc¸eau et al., 1968., the boundary between the American and Eurasian plates is well-established. However, the location and nature of the eastern branch of the Azores triple junction is still controversial, and is the subject of several geodynamic models ŽMadeira and Ribeiro, 1990..

3. Furnas and its geological setting The active centres of Sao ˜ Miguel ŽFig. 1. consist of three major trachytic central volcanoes, linked by rift zones. At the trachytic centres, explosive volcanism has dominated, while in the rift zones, although occasional trachytic eruptions have occurred, activity is characterised by basaltic effusive eruptions accompanied by strombolian cone building. At the western end of the island is Sete Cidades Volcano which has been built up from several phases of activity including the production of a lava shield and numerous explosive eruptions. Queiroz Ž1998.

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identifies at least 17 explosive eruptions that occurred here during the last 5000 years. In addition, there have been three offshore eruptions recorded in the last 400 years. The edifice is truncated by a complex caldera which contains two lakes. Within the caldera are cones and domes representing the present phase of caldera infilling, the most recent of which is dated at about 600 years ago ŽBooth et al., 1978. just before the island was inhabited by Portuguese settlers. The only known historical eruptions on land Žpost about 1440 AD. in this part of the island occurred in 1652 AD ŽWeston, 1994. on the so called ‘waist’ region, the rift between Sete Cidades and Fogo Volcano to the east. The 18 km long waist consists of hundreds of cinder cones, fissure vent systems and associated lava flows. Fogo Volcano Žalso known as Agua de Pau., is largely made up of pyroclastic deposits and is also truncated by a caldera complex. There have been at least four eruptions from this centre during the past 5000 years ŽBooth et al., 1978., the most recent being in 1563 AD ŽWeston, 1994.. The deposits from a major eruption that occurred about 5000 years ago were first recognised by Walker and Croasdale Ž1971. and termed Fogo A. This eruption produced one of the best documented of all plinian deposits; it is also distinctive in the field and serves as a widespread marker horizon in the stratigraphy of Sao ˜ Miguel ŽBooth et al., 1978; Cole et al., 1999-this issue.. Another, mainly basaltic, rift links Fogo to the next trachytic centre, Furnas. This rift is known as the Achada das Furnas complex and is 5 km long; it consists of cinder cones and lavas, and includes a small, young trachytic centre known as Congro ŽFig. 1.. Furnas Volcano is, from a topographic point-ofview, the least impressive of all the volcanoes on Sao ˜ Miguel. It has no well-developed positive edifice: to the west, its products interdigitate with lavas from the Achada das Furnas rift; to the east, it banks up against the deeply eroded Nordeste complex of lavas that once formed a basaltic shield, much of which has been destroyed by marine and fluvial erosion ŽMoore, 1991a,b.. It also cuts into the Povoac¸ao ˜ caldera, which, in turn, cuts the Nordeste complex. This old caldera is about 6 km across and partly open to the sea at the mouth of the main

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drainage system. Only a few outcrops of pyroclastic materials have been associated with the Povoac¸ao ˜ caldera centre ŽMoore, 1990.. Although the caldera is presently open to the sea, and thus could be the result of sector collapse, the opening is narrow and it is more likely that faulting and marine erosion have cut back into the seaward side, breaching the caldera walls. The main features of Furnas are the caldera complex and the southern rim of the volcano cut by high sea cliffs which provide the best exposures ŽFig. 2.. The deposits from this volcano were draped over the Nordeste edifice and have been heavily dissected, and on the seaward side deposition occurred at sea;

constructional forms were cut back with ease by marine erosion. Most eruptions at Furnas have been from vents within the caldera or on the caldera margin, although strombolian eruptions with aa flows of less evolved composition have occurred outside the caldera margin on the flanks ŽMoore, 1991a,b.. Furnas acquired its eroded form first, because it is largely made up of unconsolidated pyroclastic rocks and second, because it grew near the shore of the rugged Nordeste lava pile allowing both marine and fluvial erosion. In addition, most pyroclastic material transported to the south enters the sea, and even pyroclastic flows travelling overland to the east and northeast 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. Roman numerals indicate positions of stratigraphic sections in Fig. 8.

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Fig. 3. Ža. Sketch of the Furnas Caldera looking northeast from approximately 2 km west of Lagoa das Furnas. Žb. Photograph facing northeast from the Pico do Ferro overlook Židentified in Ža. and Fig. 2. showing the outer earlier caldera wall cut into lavas of the Nordeste complex ŽA., top of early caldera infill made up of pyroclastics, lacustrine deposits and lavas ŽB.; and the inner caldera scarp ŽC..

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system and diverted out to sea. The result is that for many eruptions only the proximal deposits are available for study. Moore Ž1990, 1991a,b. identifies only one caldera-forming event at Furnas. He relates this to an outflow deposit that consists of a welded ignimbrite and associated non-welded tephra, of fall and flow origin. This he dates indirectly at about 12,000 BP. We, on the other hand, recognise three welded tuffs of different ages and infer that the outflow deposits mapped by Moore Ž1991a. represent the products of at least two different eruptions separated in age by about 20,000 years. During the last 5000 years, Furnas is known to have erupted explosively at least 10 times ŽBooth et al., 1978.. The 1630 AD eruption has been previously thought of as the only historical activity, but our work now recognises that another eruption occurred during the early settlement of Povoac¸ao ˜ in the mid-15th century ŽQueiroz et al., 1995..

4. Geomorphology of Furnas 4.1. The Calderas Based on the geomorphology of the volcano ŽFig. 2., the 8 = 5 km diameter caldera complex of Furnas consists of at least two dominant structures, together with other minor ones. The earliest recognisable caldera rim forms the steep outer scarp on the northeast and northern sides of the caldera. This cuts into a sequence of lavas that are part of the Povoac¸aorNordeste volcanic complex. The edge of ˜ this caldera is also seen on the southeast side where it is breached by the Ribeira Quente valley. Again, the caldera wall cuts a sequence of Nordeste lavas overlain by Furnas materials. The rim is indistinct on the southern side where it has been mantled by young materials and may have been partly faulted out by a series of normal faults forming a graben. Collapse of this early caldera was followed by a period of caldera-filling eruptions. The top of the caldera fill is preserved as a distinct bench on the northern and northwestern floor of the caldera, where it has a thickness in excess of 200 m and abuts against the Nordeste lavas exposed in the caldera wall. This material is also exposed in the valley of

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the Ribeira Quente where it has been deeply dissected. A second inner caldera about 6 = 3.5 km across cuts into the older caldera wall to the east exposing Nordeste lavas, and forms a distinct scarp to the northwest, exposing the old caldera fill; but, like the earlier caldera, it is covered by younger materials to the south. Formation of this caldera was followed by another phase of infilling and at least two other, smaller, eruption-related collapses. One, the 1.5 km diameter Furnas Lake crater, cuts a pre-Fogo A tuff cone to the northeast; the other, centred on Furnas village, has a well developed scarp on its western side ŽFig. 2. and is probably related to the Furnas C eruptive episode ŽBooth et al., 1978.. 4.2. Cones, pumice and tuff rings, domes Eruptions forming domes, cones or both have occurred within the caldera complex and on the volcano flanks. The two best preserved pumice and tuff rings within the caldera complex are those associated with historical eruptions: that of 1630 AD on the southern side of the main caldera, and Gaspar, the site of two eruptions, the more recent one in the first half of the 15th century. A number of other explosive centres can be recognised but these are all thickly mantled by younger products. The 1630 AD and Gaspar craters contain domes about 700 m in diameter and about 100 m high. These formed after the main explosive phases of each eruption ŽCole et al., 1995.. Domes also occur without a surrounding pumice rampart and the best preserved ones form a chain on top of the early caldera fill on the northwest side of the old caldera ŽFigs. 2 and 3.; the largest of these is Pico do Ferro which has a diameter of nearly 500 m and is about 60 m high. The easternmost dome in the chain has been largely removed by the collapse of the wall of the second of the main calderas. The dome at Bodes, on the rim of the older caldera, also forms an upstanding feature and is cut by the scarp of the second major caldera wall as well as by WNW trending faults. Outside the caldera complex, low hills on the eastern flank have been interpreted as domes by Moore Ž1990, 1991a.. Pumice cones also occur, notably Pico do Canario that straddles the inner caldera

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wall to the east ŽFig. 2.. To the north there are at least 10 cinder cones with associated lavas, all of which are more than 5000 years old.

5. Tectonic structures The tectonic structure of the volcano expresses itself at different levels. There are major faults and complex fault zones with throws of tens to hundreds of metres, and large numbers of faults showing movements of less than a metre. We determined the distribution and orientation of faults and dykes ŽFig. 4. associated with Furnas by direct observation of dislocations in coastal outcrops and in the caldera walls, as well as inferred faults based on geomorphological features such as linear valleys and scarps

ŽGaspar et al., 1995.. Basaltic dykes are exposed in the coastal sections and form offshore reefs. One of the most important fracture systems identified at Furnas crosses the volcanic massif with a WNW–ESE trend and shows a clear normal dip-slip component. Some vents appear to be aligned with this system. In addition, the orientation of some valleys suggests the existence of E–W faults, parallel with the main axis of the island, a trend that has been interpreted elsewhere as representative of old deep oceanic crust fractures ŽGaspar, 1996; Ferreira et al., 1998; Queiroz, 1998.. Another important fracture system consists of conjugate faults with N–S and ŽN.NE– ŽS.SW trends. This system is well-represented on the south coast from Amoras to Ribeira Quente. An additional set of extensional fractures trend NW–SE and are parallel

Fig. 4. Sketch map of Furnas Volcano showing major faults, dykes and the distribution of fumarole fields.

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to the so-called Terceira Rift regional fault system ŽFig. 1.. The following conclusions emerge from the pattern of structures shown in Fig. 4. First, the convergence of the different structural systems may be responsible for the localisation of a trachytic centre at Furnas, a situation similar to that at the two other major trachytic centres on the island ŽQueiroz, 1998., and also found on other Azorean islands ŽGaspar, 1996.; second, the outlines of the calderas tend to mimic the main fault trends, suggesting that the walls of the two major caldera collapses are strongly controlled by structural weaknesses, especially the NE–SW system; and third, the WNW–ESE system

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of fractures played a major role in the transfer of magma to the surface.

6. Petrology The volcanic products of Furnas Volcano form a mildly alkaline suite ŽFig. 5. ranging from basanite through alkali olivine basalt, potassic trachybasalt, basaltic trachyandesite Žshoshonite., trachandesite Žlatite. to trachyte Žnomenclature after Le Maitre et al., 1989.. Of the three central volcanoes on Sao ˜ Miguel, the Furnas suite is slightly more potassic; the more evolved products of Fogo show a decrease

Fig. 5. Total Alkalis vs. Silica plot ŽTAS. with classification after Le Maitre et al. Ž1989. showing the chemical variation of the eruptive products of Furnas Volcano. The analyses presented here represent the stratigraphic units of this paper and were collected systematically during the project. ŽAnalytical data from unpublished report, University of the Azores..

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in K 2 O and have a peralkaline tendency, whereas the Sete Cidades trend is more sodic ŽMoore, 1991b; Queiroz, 1998.. These mildly alkaline suites are typical of many oceanic volcanic islands associated with spreading centres ŽWilson, 1989.. At Furnas, materials erupted within the caldera complex are trachytic in composition, consisting of latites and trachytes. Vents erupting basic magmas are restricted to the flanks of the volcano. It appears that basaltic magmas have been prevented from erupting within the caldera complex during the history of Furnas Volcano. A basanite lava that occurs at the base of the exposed caldera fill at Salto dos Ingleses may have either flowed into the caldera from a vent outside ŽMoore, 1991b. or represent the underlying basement of Nordeste Volcano lavas. The products of Furnas form a compositionally continuous sequence from basalt through to trachyte, though intermediate members are present in only subordinate amounts ŽFig. 5.. The volcanics of Sao ˜ Miguel are broadly bimodal in composition, either basaltic or trachytic, with a scarcity of intermediate products; both Booth et al. Ž1978. and Moore Ž1991b. estimated the relative volumes of material of different composition and substantiated the presence of this Daly Gap. Self and Gunn Ž1976. identified a similar scarcity of intermediate products in the erupted material of Terceira, one of the central group islands of the Azores. This Daly Gap could represent either a genuine scarcity of magmas of intermediate composition or some physical control that inhibits the eruption of magmas of intermediate composition. Self and Gunn Ž1976. suggest that an efficient differentiation system may generate salic residues leaving little in the way of intermediate products. Storey et al. Ž1989., in a study of Fogo Volcano, argue that the presence of a compositionally zoned magma reservoir with a cap of trachytic liquid will prevent the eruption of less evolved melts. It is generally accepted by most workers ŽRodrigues et al., 1989; Storey et al., 1989; Moore, 1991a,b; Widom et al., 1992. that the observed compositional range of the erupted magmas on Sao ˜ Miguel has been largely generated by fractional crystallisation from a basic parent. In terms of a plumbing model, it is proposed that basaltic magma ascends from the mantle along the axis of Sao ˜ Miguel island. Where this axis is cut by

NWrSE grabens this has allowed higher rate of throughput of magma and enabled significant storage within crustal reservoirs. With time, these crustal reservoirs differentiated into zoned trachytic bodies which have given rise to periodic explosive trachytic eruptions. Eruption of basaltic magma is restricted to zones outside these long-standing trachytic reservoirs.

7. Stratigraphy The products of Furnas, being mainly of pyroclastic origin, are deeply dissected, both within the caldera complex and on the flanks. Many deep valleys have been carved along fault zones. Superficially, however, much of Furnas appears to be a bland terrain in which the lush vegetation disguises the valleys and the steep caldera walls that have slopes sometimes more than 508. Marine erosion, together with fluvial activity, has also produced a steep and highly incised coastal region. Despite the deep dissection, establishing the stratigraphy is no easy task because of thick vegetation. The inner caldera walls provide a few discontinuous outcrops formed by landslides and two waterfall sections. Within the caldera there are many road, track and stream cuts that expose some of the younger deposits. The best exposures are in the sea cliffs that cut the southern flank of the volcano in a generally proximal zone. We derive most of our information for the stratigraphy before 5000 years ago from this section. The cliffs show a relatively well-defined sequence of deposits. Fig. 6, which represents a section of the sea cliff from just east of Ponta Garc¸a to just west of Povoac¸ao, ˜ is constructed from detailed studies of outcrops accessible on narrow cliff paths, from seaborne visits to beach sections, and from a set of overlapping photographs taken from a boat along the whole coastline. The key stratigraphic horizon in the coastal section is the Povoac¸ao ˜ Ignimbrite, which can be traced from the Povoac¸ao ˜ area on the flanks of the Furnas Volcano to the western end of the cliff section ŽFigs. 6 and 8.. Below the Furnas sequence is a pile of basaltic lavas belonging to the Nordeste Volcano. They occur

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Fig. 6. Schematic section of the south coast of Furnas showing the main stratigraphical units.

as fault-bounded blocks forming part of a N–Strending graben and horst system. Moore Ž1991a. gives a KrAr date for a tristanite lava that overlies mafic Nordeste flows just west of Ribeira Quente at 93,000 " 9000 BP. implying that activity at Furnas had started at that time. We divide the Furnas stratigraphy into three Groups. The top of the Lower Group is defined by the Povoac¸ao ˜ Ignimbrite Formation, and the Middle and Upper Groups are separated by the Fogo A deposit ŽFig. 7.. Although the division between the Lower and Middle Group corresponds to a major eruption thought to be related to the first major caldera-forming event, the division between the Middle and Upper Groups is based on a widespread deposit from the adjacent Fogo Volcano. It would have been more convenient to base this boundary on the deposits resulting from the second major collapse, but these are not sufficiently well-exposed to form a stratigraphic marker. Type localities for the main geologic units are given in Table 1.

A further problem in correlation lies in radiocarbon dating which has produced anomalous results on Sao ˜ Miguel. Dates for materials deposited during the past 4000 years are consistent with the stratigraphy, but with older dates anomalies occur. It is known that radiocarbon dates on samples from areas where there is considerable CO 2 outgassing are not always reliable ŽSulerzhitzky, 1970; Saupe et al., 1980. and give ages that are too old. At Furnas, high concentrations of magmatic CO 2 occur giving old radiocarbon ages to samples of modern vegetation ŽPasquierCardin et al., 1999-this issue.. There is also the high probability in areas with high rainfall of contamination from modern organic products giving ages that are too young ŽHarkness et al., 1994.. We have now discovered discrepancies at Furnas between samples collected from the same volcanic unit at the same site and between ages of units where there are observed stratigraphic relations. Our attention was drawn to a potential problem by analyses from two carbon samples from the same

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Fig. 7. Summary of the stratigraphy of Furnas Volcano reported in this paper.

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Table 1 Locations of type localities for the stratigraphic units identified in Fig. 6 Upper Furnas Group

Furnas A to 1630 AD — for locations of key sections, see Booth et al. Ž1978., Cole et al. Ž1995, 1999-this issue.

Middle Furnas Group Cancelinha Formation Several good exposures occur on the track northeast of Ponta Garc¸a Salto dos Ingleses Formation Part of sequence exposed below waterfall at GR 466,815 Ponta Garc¸a Ignimbrite Formation Good exposures of non-welded massive flow deposits and stratified on the steep slopes east of Grota do Mouco GR 453,767 Mouco Formation Good exposures occur on the northern side of track leading from Ponta Garc¸a to Amoras beach west of GR 454,772 Gado Formation Exposures on the northern side of tight bend in road GR 549,798. Also many exposures east of Povoac¸ao ˜ Lower Furnas Group

Povoac¸ao ˜ Ignimbrite Upper Amoras Lower Amoras Quente block-and-ash flows Cavaleiro Formation

Exposures on beach in Povoac¸ao ˜ at GR 542,792 On Amoras beach west of Ribeira das Amoras. GR 456,766 On Amoras beach east of Ribeira das Amoras. East of GR 457,766 On coastal track east of Ribeira Quente. East of GR 486,773 Track leading down steep coastal cliffs GR 527,792

unit, the Tufo Formation described below, collected at the same site. These samples yielded dates of 34,000 BP and 5000 BP ŽSamples FS 109 and FS 325 in Table 2.. To investigate the problem, we

collected a further three samples from the same site and horizon. One sample, FS 401, was collected from the same charcoal material as sample FS 109 ŽTable 2.. By courtesy of Patrick Allard, these sam-

Table 2 New 14 C dates for Furnas deposits. All were determined by Beta Analytic except for those marked with an asterisk, which were determined by Pasquier-Cardin courtesy of Patrick Allard. Note that FS 109, FS 325, FS 400 and FS 402 are all from samples collected at the same site and in the same unit Cr14 C

Sample number

Age Žyears BP.

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FS 332 FS 35a FS 35b FS 30 FS 31 FS 325 FS 319

250 " 60 490 " 50 560 " 60 1870 " 120 2310 " 160 5020 " 60 17240" 70

y26.8 y24.6 y26.7 y28.5 y24.7 y26.2 y23.5

FS 326 FS 109U FS 402U FS 400U FS 109 FS 142

22,880 " 120 27,420 " 540 27,510 " 560 27,570 " 410 33,950 " 480 34,980 " 840

y25.3 y23.8 y24.35 y23.52 y24.14 y26.6

FS 143

35,980 " 410

y24.5

FS 38

) 30,000

y24.4

FS 121

) 44,400

y24.9

Stratigraphic unit

Grid reference

Charcoal from lowermost 1630 lapilli Charcoal from central part of Furnas H lapilli Charcoal from central part of Furnas H lapilli Carbonaceous layer from base of Furnas C tephra Wood fragments from within Furnas B tephra Charcoal from basal 1 m of Tufo Formation Charcoal from massive units of the Ponta Garc¸a Formation Ignimbrite Charcoal from non-welded base of welded ignimbrite Charcoal from basal 1 m of Tufo Formation Charcoal from basal 1 m of Tufo Formation Charcoal from basal 1 m of Tufo Formation Charcoal from basal 1 m of Tufo Formation Charcoal fragments soil below Povoac¸ao ˜ Ignimbrite Formation Charcoal fragments soil below Povoac¸ao ˜ Ignimbrite Formation Charcoal fragments soil below Povoac¸a˜ Ignimbrite Formation Charcoal from within valley filling ignimbrite in Albufeira Formation

478,770 497,811 497,811 477,828 477,828 464,767 453,767 456,766 464,767 464,767 464,767 464,767 547,791 546,792 456,766 502,777

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ples were analyzed using a technique that reduces the effect of cosmic ray contamination; this gave a consistent age of about 27,500 BP ŽTable 2.. From these observations, we would argue for great caution in accepting a single radiocarbon date for organic materials on Furnas, especially for date older than a few thousand years. The range of dates for the Fogo A deposits ŽMoore and Rubin, 1991. may indicate that the problem is widespread, and suggests that single determinations of radiocarbon ages can be misleading. 7.1. The Lower Furnas Group Below the Povoac¸ao ˜ Ignimbrite, along the marine cliff section, are a number of different units representing eruptions of substantial magnitude. The oldest rocks at the western end of the Furnas section are tephra that interdigitate with lavas of the Achada das Furnas rift; but the lavas thin out rapidly to the east, and at Amoras, the sequence is entirely of Furnas materials consisting essentially of pyroclastic materials with a few small valley-filling lavas ŽFigs. 6 and 7.. This section between the Ribeira das Amoras and Ribeira do Tufo valleys is depicted on the US Geological Survey map ŽMoore, 1991a. as lavas, but virtually all the cliff section consists of pyroclastic rocks. 7.1.1. Amoras Formation The base of the Furnas sequence is not seen at Amoras ŽSite I, Fig. 8.. Here, non-welded ignimbrites and associated deposits crop out. The sequence is cut by a major fault which has been utilised by the Ribeira da Amoras valley. There has clearly been a downthrow of some 20 m to the west juxtaposing two pyroclastic sequences that are divided by an erosional unconformity. Thus, the Amoras Formation is divided into an Upper and a Lower Formation. The Lower Amoras Formation is about 40 m thick at the type locality at Amoras beach. The oldest units are non-welded ignimbrites which occur in flow units 0.5–5 m thick. These ignimbrites are overlain by at least four units

all separated by paleosols. A few of these units are composed of stratified deposits of pumice lapilli and ash typical of those found on Furnas volcano. The Upper Amoras Formation is up to 35 m thick and separated from the Lower Amoras formation by a well-developed unconformity. The formation is well-exposed on the west side of the Ribeira da Amoras and is composed of pumice lapilli beds interbedded with debris flow deposits. At least six individual pumice lapilli layers each more than 1 m in thickness occur. The debris flow deposits are up to 3 m in thickness; there are both massive and stratified hypoconcentrated debris flow varieties which have eroded the underlying pumice lapilli. At the eastern end of the cliff section, where Furnas materials are seen above Nordeste lavas ŽSite V, Fig. 8., there is a sequence of similar non-welded ignimbrites. Above these pyroclastics is a thick silicic lava ŽGarajau Lava, Sites IV and V, Fig. 8. that forms the base of the cliff for about 1 km until at Ribeira Quente village it dips below sea level. 7.1.2. Albufeira Formation Above the Garajau lava is a thick sequence of pumice lapilli fall, surge and debris flow materials apparently formed from more than one major Plinian eruption. This unit is well-exposed in the cliffs on either side of the Ribeira Quente village and has a thickness in excess of 100 m ŽFig. 6 and Site IV, Fig. 8.. Moore Ž1991a,b. dates rocks in this sequence as being ) 33,000 BP and we have a new date of ) 44,000 BP ŽTable 2. for a valley-filling ignimbrite in the upper part of the unit at Ponta Garagau. They were thus probably formed between this date and at least 93,000 BP. We name this sequence of tephra the Albufeira Formation as it is well-exposed in valley sections just west of the point with that name. The Albufeira Formation is overlain, as seen in the cliff path west of Ribeira Quente, by a thick sequence made up almost entirely of block-and-ash flow material of silicic composition ŽQuente block and ash flow in Figs. 7 and 8.. We infer from this deposit that a major dome formed within a few kilometres of the present coast. Spindle-form bombs

Fig. 8. Selected representative stratigraphic sections on the southern coastal cliff of Furnas Volcano Žlocations are indicated in Figs. 2 and 6..

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in the deposit indicate contemporary explosive activity from the dome as some of the flows were forming. This unit has been dated at 22,000 BP by Moore and Rubin Ž1991., but, since it underlies the Povoac ˜ ¸ao Ignimbrite, it is older than that date ŽFigs. 7 and 8.. Between the block-and-ash flow sequence in the cliff section and the deposits of the next major eruption that formed the Povoac ˜ ¸ao Ignimbrite materials, there are alternating ash and lapilli beds up to 8 m thick including a paleosol and a pyroclastic flow deposit. 7.1.3. CaÕaleiro Formation This rock unit, that overlies the Nordeste lavas just west of Povoac¸oao ˜ ŽSite VI, Fig. 8., consists of more than 18 distinct beds separated by palaeosols. All, except one surge deposit, are fallout beds of ash and pumice. There is no characteristic unit that allows correlation with other sequences, but the position below the Povoac¸ao ˜ Ignimbrite Formation means that these materials are older than 30,000 BP. 7.1.4. The PoÕoac¸ao ˜ Ignimbrite Formation The Povoac¸ao ˜ Ignimbrite Formation ŽSchmincke and Weibel, 1972; Booth et al., 1978. is a distinctive pyroclastic flow deposit, with associated material, that covers much of the floor of the Povoac¸ao ˜ Caldera. It is exposed in valley sides and the coastal cliffs. It has a densely welded zone that achieves thicknesses of up to 60 m ŽMoore, 1990, 1991b; Duncan et al., 1999-this issue. where it fills deeper palaeovalleys. Distribution of the deposit indicates that it originated from Furnas. It occurs in the major valleys leading down from the eastern side of the Furnas Caldera complex rim to Povoac¸ao. ˜ It appears that the major exit for the pyroclastic flows was close to the site of the Bodes dome ŽFigs. 2 and 4., where a deep valley occupying a complex graben, following the ESE structural trend, existed before the eruption. The valley is filled with this ignimbrite which is well-exposed in the caldera wall on the road from Furnas to Povoac¸ao. ˜ Here, on the northern side of the palaeovalley, there is a welded ignimbrite that thickens into the valley overlain by about 25 m of altered tephra

belonging to the same eruption. Although Moore Ž1991a,b. argues that the ignimbrite here is younger than the Bodes dome, the presence of welded ignimbrite in the caldera wall topographically below the Bodes outcrop strongly suggests that the ignimbrite is older than the dome. If this is the case, then the palaeovalley may have had a width on the order of 1 km, and the axis of the valley is roughly below the dome. Both the dome and the Povoac¸ao ˜ Ignimbrite are cut by the wall of the younger caldera indicating that they predate the younger collapse. The Povoac¸ao ˜ Ignimbrite Formation is made up of a number of different lithologies including lapillifall beds, thick surge deposits, massive non-welded ignimbrite units as well as the distinctive densely welded ignimbrite horizons ŽDuncan et al., 1999-this issue.. The welded zones thicken in the valleys. On the valley sides there are some places where more than one welded zone is separated by non-welded material, but these units generally merge into a single welded unit in the axes of all but the smallest valleys. This suggests that the ignimbrite consists of a number of hot pyroclastic flow deposits which became a single cooling unit. Radiocarbon dates from soils immediately underlying the Povoac¸ao ˜ Ignimbrite Formation ŽTable 2. range from 30,000 to 35,000 BP indicating that it is younger than these dates. To the east of Amoras, the Povoac¸ao ˜ Ignimbrite is overlain by the Tufo Formation Ždescribed later. which has a well-established radiocarbon date of 27,000 BP. The age of the Povoac¸ao ˜ Ignimbrite Formation eruption must therefore lie between these dates at about 30,000 BP. Inconsistent with this age is the date provided from charcoal within ashes at the base of a welded ignimbrite exposed on the path from Ponta Garc¸a to the Amoras beach. This gave an age of 22,880 BP implying that, either this is a different ignimbrite, or there are problems with the date as described earlier. However, a date from the soil directly below ŽTable 2. is consistent with this ignimbrite having the same age as that of the Povoac¸ao ˜ Ignimbrite Formation. A younger age for the Povoacao ˜ Ignimbrite of about 12,000 BP is inferred by Moore Ž1990, 1991a,b.. This is based on one radiocarbon date of

Fig. 9. Selected representative stratigraphic sections of the Middle Furnas Group. Inset shows the locations. For lithological key, see Fig. 8.

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about 11,230 BP for a soil overlying a welded ignimbrite exposed near the top of what we interpret as the old caldera fill below the Pico do Ferro dome ŽFig. 9.. As shown earlier, the Povoacao ˜ Ignimbrite is older than the material filling the old caldera and the date of Moore Ž1990, 1991a,b. relates to a younger welded tuff. The eruption that formed the Povoac¸ao ˜ Ignimbrite was obviously a major event in the history of Furnas, and probably the largest eruption on the volcano. It appears to have begun with a Plinian eruptive phase giving rise to lapilli fall followed by a phase of phreatomagmatic activity generating pyroclastic surges that travelled south of the caldera. Given the magnitude of the eruption, it is most probable that it relates to the formation of the old caldera. Moore Ž1991b. identifies the best outcrop of what he refers to as the outflow deposit in a quarry in the lower part of the Ribeira Quente valley on the west side. Exposed here is a welded ignimbrite 40 m thick, with three lithic-rich layers about 0.5 to 1 m thick with a lensoid form some 100 m long. The outcrop is surrounded by pyroclastics and does not occur on the opposite side of the valley. As this is a fault controlled valley, the ignimbrite may be faulted out on that side. It is underlain by more than 50 m of weathered pyroclastics of the Lower Furnas Group as exposed in the waterfall on the west side of the tunnel on the Ribeira Quente road, and is overlain by pyroclastics of the Salto dos Ingleses Formation. We identify this outcrop of welded tuff as the Povoac¸ao ˜ ignimbrite within the old caldera. It is at a lower level than the point where it exited the caldera north of the Bodes dome, and we consider that is was lowered by synrpost-eruption collapse of the caldera. 7.2. Middle Furnas Group Materials of this group are exposed in a number of localities on the southern and southeastern flanks of the volcano and also as a fill in the old caldera ŽFig. 9.. It is not possible to correlate between individual units from one major outcrop to another. 7.2.1. Tufo Formation On the southern flank of the volcano, the Povoac¸ao ˜ Ignimbrite Formation is overlain by a sequence of bedded lapilli and ashes, together with at least two valley filling-lava flows, followed by a 6-m-thick

sequence of bedded ash. The deposits of the next major eruption at Furnas after the Povoac¸ao ˜ Ignimbrite eruption lie above this. We name the deposits from this eruption the Tufo Formation ŽFigs. 6–8.. The type locality is in the sea cliff above Ribeira do Tufo; good examples of the welded tuff within this unit occur as fallen blocks on the beach below. The Tufo Formation crops out intermittently along a 2-km section of the southern coastline between Ribeira das Amoras and Ponta da Lobeira. It is more than 45 m thick at the type locality at Ribeira do Tufo in the cliffs east of Amoras. The lower 50 cm is a moderately sorted pumice lapilli bed which grades up into a densely welded tuff 2–4 m thick. It is made up of flattened blebs of lava, and owing to the absence of a fine-grained matrix and the mantling nature of this unit, we interpret this as a welded fallout deposit. Above the welded horizon, the unit is composed of angular, relatively dense pumice lapilli with scarce matrix also indicating a fallout origin. These fallout deposits occur as massive units 2–5 m thick separated by more thinly bedded discontinuous layers which are probably pyroclastic surge deposits. Massive ignimbrite about 10 m thick occurs locally near the base of the sequence in Ribeira do Tufo. The lower non-welded material is rich in charcoal. As discussed earlier, there were considerable problems with radiocarbon dating of this material, but a consistent date of about 27,000 BP ŽTable 2. is now determined. 7.2.2. Mouco Formation This sequence lies directly on top of the Povoac¸ao ˜ Ignimbrite Formation in the Ponta Garc¸a region ŽSite I, Fig. 6.. The Mouco Formation is 30 m thick and overlain by the Ponta Garc¸a Ignimbrites that have been dated at about 17,000 BP ŽTable 2.. The formation is composed of three separate basaltic scoria lapilli layers, each up to 4 m thick, a lava flow and two separate units of alternating lapilli and ash. The basaltic lapilli layers and lava flow were probably derived from local centres outside the Furnas caldera complex whereas the alternating ash and lapilli layers are more likely to have been derived from within the Furnas caldera complex. A sequence, up to 10 m thick, of discontinuous hypoconcentrated debris flow deposits caps the Mouco formation.

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7.2.3. Ponta Garc¸a Ignimbrite Formation These deposits are exposed at two localities in the Grota do Mouca valley at the eastern limit of Ponta Garc¸a ŽSite VIII, Fig. 9.. The ignimbrite is composed of a number of 1–2.5-m-thick massive flow units which show coarse-tail grading of pumice and lithic fragments. Coarse, pumice-rich pyroclastic surge deposits are interbedded between some of these flow units. The total thickness of the sequence is ) 15 m. Radiocarbon dating of charcoal fragments within this unit yield a date of about 17,000 BP ŽTable 2.. 7.2.4. Cancelinha Formation The deposits of this Formation have been identified only to the southwest of the caldera in the Ponta Garc¸a area, and occur below the Fogo A deposit and above the Ponta Garc¸a Ignimbrite Formation ŽSites VII and VIII, Fig. 9. indicating an age between 17,000 and 5000 BP. It is composed of at least nine tephra fall units. Although it has not been possible to correlate these deposits with those in other sectors around the volcano, some of these units are likely to be broadly contemporaneous with the upper part of the Gado Formation Žsee below.. Some of these deposits are formed by beds of pumice lapilli with a few, diagnostic, thin ash layers that allow correlation between exposures, whereas others are composed of alternating layers of ash and pumice lapilli Žsee fig. 9 in Cole et al., 1999-this issue.. As these deposits thicken and become coarser towards the Furnas caldera complex, they are considered to have been erupted from Furnas. 7.2.5. The Gado Formation The Gado Formation is composed of a sequence of tephra fall deposits which crop out within the Povoac¸ao caldera ŽSites IX and X, Fig. 9.. The unit is named after outcrops in the Feira de Gado area. This sequence occurs below the Fogo A deposit Ž; 5000 BP. and above the Povoac¸ao ˜ Ignimbrite Ž; 30,000 BP.. At least 14 distinct tephra fall deposits have been identified separated by fossil soils. The majority of the units are lapilli fall deposits, particularly within the lower part of the sequence. One distinctive feature is that the lapilli in the lower part of the sequence, immediately above the Povoac¸ao ˜ Ignimbrite, contain crystal-rich pumice

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whereas the later lapilli are crystal poor. Toward the upper part of the sequence, characteristic deposits of numerous alternating lapilli and ash layers occur. It is considered that these deposits were derived by explosive activity at Furnas Volcano and although it is possible to correlate these units around the Povoac¸ao ˜ area, it has not been possible to relate them directly to Furnas materials elsewhere. Exposures on the eastern caldera rim region are composed of deposits up to 24 m of lapilli thick fall units, although thicknesses are more typically ; 10 m ŽSite V, Fig. 8.. These materials may be the proximal facies of Gado Formation. 7.2.6. Salto dos Ingleses Formation Within the outer, older caldera is a sequence of rocks that are cut by the inner caldera wall and, as described earlier, form a bench round the northern walls of the older caldera; we interpret these materials as filling that caldera. Some of the best exposures are in the wall north of Lagoa das Furnas but these are mostly inaccessible. Moore Ž1991b. documents a section of this material in a valley just south of Pico do Ferro, at Salto dos Ingleses, after which we name the unit; however, he considered this to represent the northern outer flanks of the volcano, not recognising that it was within the main outer caldera. The Salto dos Ingleses Formation at the type locality as described by Moore Ž1991b. overlies a basanitoid lava of unknown age. Above this are three beds of lacustrine materials interbedded with pumice fall deposits together with pillow lavas ŽSite XI, Fig. 9.. Radiocarbon dates from the lake deposits range from 23,000 to 27,000 BP ŽMoore and Rubin, 1991., but the dates are not consistent with the observed stratigraphy of Moore Ž1991b.. Although the dates are thus individually inconclusive and emphasise the problems of radiocarbon dates on Furnas mentioned earlier, it is likely that these lake deposits formed between 20,000 and 30,000 BP. The sequence above this is obscured for some 30 m, followed by pumice beds and a latite lava flow. This is overlain by a welded tuff. Moore Ž1991b. interprets this as an ignimbrite and correlates it with his outflow deposit. We consider this correlation is unlikely because there are fallen boulders of welded tuff at the base of the waterfall, above the exposed welded ignimbrite, indicating that there is a younger welded ignimbrite

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higher in the sequence. A welded tuff also crops out at a higher level in the caldera wall just below the Pico do Ferro. Moore Ž1991a,b. dates this at 12,000 BP based on a soil layer between it and the overlying dome. This tuff is probably the upper one in the waterfall section. Within the intra-caldera sequence elsewhere there are sheets of massive, poorly sorted pyroclastic materials. These are exposed in the road between Furnas and Ribeira Quente where most of the outcrops are strongly weathered and it is often difficult to distinguish between primary pyroclastic flow and debris flow deposits. The materials of the Salto dos Ingleses Formation span the ages of deposits exposed in the marine cliff section but a detailed correlation is not possible with the present exposure. However, the Ponta Garc¸a Ignimbrite represents a substantial eruption at about 17,000 BP and could be equivalent to the lower welded ignimbrite at the Salto site. 7.2.7. Inner caldera collapse Collapse of the inner caldera brought to an end this phase of caldera infilling. It is not clear which of the eruptive deposits is associated with this event. One candidate is the welded ignimbrite immediately below the Pico do Ferro domes. If this is the case, then it presents a difficulty as the south easternmost of the Pico do Ferro domes, which lies above the soil layer above welded ignimbrite, is sliced in half by the caldera wall, implying that collapse occurred a significant time after the emplacement of the ignimbrite. Thus, either this ignimbrite is not related to the caldera and may be a small local welded deposit; or it is related and the caldera at this point had more than one collapse episode. Whichever is the case, the last collapse must have taken place at about 12,000 BP or somewhat more recently. 7.2.8. Pico do Canario ´ Formation The Pico do Canario ´ forms a distinct cone that straddles the younger caldera scarp. Its deposits are only exposed in cuts on the road between Salto do Cavalo and Povoac¸ao ˜ ŽFig. 2.. The deposits from this centre are overlain by Fogo A materials and are thus older than 5000 years. This cone, which had several eruptive phases, was probably one of the earliest after the collapse of the younger caldera. It is

abnormal in being a trachytic centre ‘outside’ the caldera. However, it lies at the intersection between the old caldera bounding faults and those of the younger caldera, a situation that may have facilitated movement from the central trachytic reservoir to a position outside the original caldera. It clearly had a well-established link with the reservoir as it erupted more than once. A quarry cut in the east side of Pico do Canario ´ cone shows at least 4.5 m of coarse, proximal lapilli beds with differing amounts of lithic material. We observe at least four soil horizons suggesting that the cone formed during several eruptions. Within sequences from a single eruption, beds of pumice are, in some cases, divided by thin layers of coarse, dark, gritty ash which we interpret as lithic materials formed during vent clearing activity between phases in the eruption. Moore Ž1991a,b. suggests a date of 6600 BP for Pico do Canario ´ based on radiocarbon dating of lacustrine materials on the nearby caldera floor which he interprets as being of similar age. 7.3. Upper Furnas Group The base of the Upper Furnas Group is defined by the top of the Fogo A deposit, and the last eruption to contribute to this sequence was that of 1630 AD. The date of the Fogo A eruption is generally given as 5000 BP ŽBooth et al., 1978., although the range of radiocarbon dates obtained is from 4480 " 160 to 5380 " 210 BP ŽMoore and Rubin, 1991., yet again indicating the lack of consistency in radiocarbon dating. Although the Fogo A deposit came from another volcano, it is one of the best known and identifiable deposits in the area. In addition, the sequence of deposits that follow have been the focus of a number of studies of various aspects of pyroclastic rocks and the hazard posed by explosive activity. We therefore consider this to be a well-defined sequence that is important enough to have Group status. The events from Furnas that followed the Fogo A eruption were first described by Booth et al. Ž1978., and are discussed further by Cole et al. Ž1999-this issue.. Although each of the deposits from individual eruptions in this Group is well-defined and characterised, there is a general uniformity in the type of deposit. The dominant lithologies are fine ashes and

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lapilli beds, normally alternating with one another ŽCole et al., 1999-this issue.. The implication is that each eruption in the caldera at that time involved both magmatic and hydromagmatic activity; this is to be expected given that relatively small eruptions were the result of magma rising through an active hydrothermal system and interacting with intracaldera lakes. Fig. 10 gives a schematic column through the sequence which is described in more detail by Cole et al. Ž1999-this issue..

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This work recognises at least three explosive eruptions at Furnas since the beginning of the 15th century ŽQueiroz et al., 1995.. The products of an explosive eruption ŽFurnas H, Fig. 10. have only a thin weathered upper surface. This suggests that only a short period separated the two eruptions, confirmed by two radiometric dates of carbonised wood ŽTable 2. within the eruption products of Furnas H; these dates suggest that the eruption occurred between 1410 and 1440 AD. Based on historical records

Fig. 10. Schematic section of the Upper Furnas Group representing the last 5000 years of activity from Furnas Volcano, and including deposits from the adjacent Fogo Volcano. For lithological key, see Fig. 8. This figure is not to scale.

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ŽQueiroz et al., 1995., the eruption is unlikely to have been as late as 1440 AD.

8. Historical eruptions

1440s ŽQueiroz et al., 1995.. This eruption almost certainly corresponds to the last eruption of Gaspar Crater ŽFurnas I., during which there was an early phase of explosive activity covering the ground with white ash, followed by the emplacement of a dome as observed by the priest.

8.1. The first historical narratiÕe The details of the discovery and settlement of the Azores are confused ŽAdmiralty, 1945.. Santa Maria is reported as the first island to be settled and this took place in the autumn of 1431 under the leadership of Gonc¸alo Velho Cabral. The island of Sao ˜ Miguel was apparently observed from a high point on Santa Maria and Cabral was sent to search for it. His first landing was at a place on the south coast which was later to become the site of the village of Povoac¸ao ˜ ŽFig. 2.. Leaving some animals on the island, he then returned to Portugal where he collected a group of colonists who returned with him, arriving back on the island sometime between 1439 and 1443. They landed at the same site where they built shelters. But from the beginning they were in great fear because of earth tremors, as well as loud noises, lightning and ‘tongues of fire’ coming from a valley to the west of their encampment ŽDias, 1936.. To discover what was happening, a priest climbed into the valley to investigate. This was no easy task as the terrain was rugged and heavily vegetated, but a track was cut to take him to a point from which he could overlook what we now call the Furnas caldera. He saw vapour rising from a depression that was without vegetation and was completely covered with white material. The vapour sometimes glowed red; we infer he was observing the glow from fresh lava of the dome reflecting in the fumes. Later, the priest went back with others and climbed down into the valley. They discovered three lakes, one of which probably corresponds roughly to the present Lagoa das Furnas. The two smaller lakes were located in the area where the eruption of 1630 AD occurred. The priest also described three distinct fumarolic regions which correspond to those on the north side of Lagoa das Furnas, those now within the Furnas village, and those near the present road to Ribeira Quente ŽFructuoso, 1583; Dias, 1936.. These first-hand observations indicate that an eruption started some time in the late 1430s or early

9. The 1630 AD eruption The deposits and the sequence of events of the 1630 AD eruption of Furnas have been discussed in detail by Cole et al. Ž1995. The eruption involved both explosive and effusive phases, and took place from a site near the southern margin of the caldera ŽFigs. 2 and 3.. At distal localities, the tephra appears as sequences of alternating pumice lapilli and ash beds; deposits of the eruption have been found up to 8 km west of the vent. More than six discrete pumice lapilli layers have been identified within the pyroclastic sequence and are considered to have been generated by magmatic explosive activity. Dispersal directions for the lapilli layers, initially to the southwest and finally northeast of the vent, indicate a change in wind direction during the 3-day explosive phase of the eruption. Ashes are interbedded with the lapilli layers and represent the deposits formed by phreatomagmatic phases that punctuated the magmatic activity. Some ash layers show lateral thickness variations, as well as cross-bedding and sand-wave structures suggesting deposition from low-concentration, turbulent flows Žsurges.. These pyroclastic surges were probably responsible for the 80 people reported burned to death 4 km SW of the vent ŽCole et al., 1995.. High particle-concentration, non-turbulent pyroclastic flows were channelled down steep valleys to the southern coast contemporaneous with the low-concentration surges. Effusive activity followed the explosive activity building a trachytic lava dome within the tuff ring complex formed during the earlier explosive phase ŽFig. 2.. Historic records suggest that dome building occurred over a period of at least 2 months. Our measurements yield a volume of 67 = 10 6 m3 DRE for the explosive products, and ; 20 = 10 6 m3 for the lava dome ŽCole et al., 1995.. Based on contemporary records it is possible to put together the eruption history ŽCole et al., 1995..

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The following are the salient points: Ž1. At the time the eruption started, a group of friars were living in the caldera; and farmers from Ponta Garc¸a cultivated land at Furnas. Ž2. For some days before the eruption, earthquakes were felt; on September 2, 1630 the inhabitants of Sao ˜ Miguel were subjected to numerous almost continuous earthquakes ŽDias, 1936; Homen, 1980.. Landslides occurred, notably a slide from the sea cliffs above the present village of Ribeira Quente, which did not exist then ŽJeronimo, 1989.. ´ Ž3. In the early morning of September 3, the explosive phase of the eruption started and was observed by the friars who escaped to the north thus saving their lives; some 30 people from Ponta Garc¸a ready to start picking fruit and vegetables were killed as a result of the first explosions ŽDias, 1936; Homen, 1980.; and in Ponta Garc¸a 80 to 115 people died from pyroclastic surges and building collapses. Pumice covered the sea near the shore. Ž4. By September 4, the whole island was covered by an ash cloud ŽHomen, 1980.. A total of 80 people left alive in Ponta Garc¸a travelled west to Vila Franca do Campo ŽAnonymous, 1880.. Ž5. On the 5th of September the sky was dark ŽManoel da Purificac¸ao, ˜ 1980. and people tried to clear ash from roofs of buildings. The next day, people of Ponta Delgada had been 3 days with only bread and water and the city was covered with several centimetres of ash ŽHomen, 1980.. Ž6. The explosive phase of the eruption lasted for 3 days, but during that time, the whole island was covered with ash. Ž7. Dome growth lasted until early November. Ž8. At most 195 people were killed ŽCorrea, ˆ 1924; Dias, 1936.. In Ponta Garc¸a, some 100 houses were destroyed; in Povoac¸ao, ˜ one out of 250 houses survived the earthquakes ŽJeronimo, 1989.. ´ Ž9. The population of Vila Franca do Campo returned after about 7 days. The Furnas caldera itself was uninhabited until about 1640 ŽCorrea, ˆ 1924. with no new permanent settlement until 1665 ŽDias, 1936.. 10. Volcanic history of Furnas Volcano Furnas is apparently the youngest of the Central Trachytic volcanoes on Sao ˜ Miguel. It began life as a

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subaerial volcano at least 93,000 years ago based on a date for a lava at the base of the observed volcanic pile ŽMoore and Rubin, 1991.. Prior to the birth of Furnas, the lava shield of Nordeste, which must have been massive, had become deeply eroded. The Povoac¸ao ˜ caldera formed next, on the flanks of the Nordeste shield. Moore Ž1990. has dated rocks at the base of both Sete Cidades and Fogo indicating that they had subaerial lavas some 200,000 years ago. It appears therefore that when Furnas started to build up there existed not only the Nordeste block, but also subaerial volcanoes on the axis of the present island of Sao ˜ Miguel. Their size and extent at that time is unknown, and clearly, they continued to grow as Furnas evolved. Products of the earliest eruptions from Furnas are probably not seen in the exposed materials, but products of early activity were both effusive and explosive, erupting materials of trachytic composition. However, unlike the other trachytic centres on Sao ˜ Miguel, Furnas had no early basaltic shield-forming phase above sea level. In consequence, there was no resistant basement to protect the developing construct from marine erosion. The early activity involved plinian eruptions with heavy pumice fall, non-welded ignimbrites, surges and lahars together with the formation of at least one large dome with associated block and ash flows. Phreatomagmatic activity was not so common in the early phases of the volcano’s history ŽCole et al., 1999-this issue. as it became after the Povoac¸ao ˜ Ignimbrite Formation eruption. A major eruption involving the production of a thick welded ignimbrite ŽPovoac¸ao ˜ Ignimbrite. occurred about 30,000 years ago. Plinian activity ŽDuncan et al., 1999-this issue. was associated with a series of pyroclastic flows that filled valleys on the southern and eastern flanks of the volcano. As this is the largest magnitude and most violent eruption recorded at Furnas, and is also older than the fill of the early caldera, we relate it to the first major collapse. For about the next 20,000 years, eruptions took place in the early caldera, filling it with pyroclastic rocks resulting from mainly fall and surge activity. Lakes formed within the caldera, and lavas entering the lakes developed pillow-like forms. On the flanks,

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pyroclastics were also emplaced including nonwelded ignimbrites, surges and fall deposits, as well as small valley-filling lavas. Extensive coastal erosion continued, removing most of the deposits formed on the southern flank of the volcano. A second caldera collapse occurred about 12,000–10,000 years ago. The deposits associated with this eruption are difficult to identify in the presently exposed sequences. They may have involved a welded ignimbrite exposed in the caldera walls and dated at about 12,000 BP, but the evidence is ambiguous. Since that time, the second caldera has continued to fill with the products mainly of subplinianrplinian eruptions most of which alternated between magmatic and hydromagmatic activity. Dome formation also took place either as the end product of an eruption or as an effusive eruption with little or no explosive activity. By that time, the caldera area had become a wet environment with a high water table and presumably a developed hydrothermal system.

11. The hydrothermal system Fumaroles and hot springs occur in a number of places on Furnas reflecting the hydrothermal system below the volcano ŽFerreira, 1994.. These have been active throughout historical times and their distribution appears to be controlled by the regional fault pattern ŽFig. 4.. In addition, CO 2 is currently being evolved over wide areas in the Caldera complex. Baubron et al. Ž1995. have mapped the concentrations of CO 2 being discharged in the Furnas village area showing that in some parts of the village there are lethal quantities of gas that kill small animals and insects. Further details of the geochemistry and nature of the system are given by Baxter et al. Ž1999-this issue., Cruz et al. Ž1999-this issue., Ferreira and Oskarsson Ž1999-this issue. and Oskarsson et al. Ž1999-this issue..

12. Present status of the volcano One of the purposes of the present study of Furnas was to establish the hazard that this volcano presents. Given the active hydrothermal system and

the prodigious venting of CO 2 , Furnas is considered to be an active volcano that could erupt again. Based on the frequency of eruptions during the 2000-year period before AD 1000, activity might be expected soon ŽGuest et al., 1994.; but using the frequency of the later, more active period, an eruption is long overdue ŽMoore, 1990.. It is also possible that the volcano is in a state of long repose similar to that which occurred between about 1000 and 3000 BC ŽGuest et al., 1994.. Discussion of volcanic hazard is expanded, based on this study, in the work of Cole et al. Ž1999-this issue. and Chester et al. Ž1999.. The statistics of the likely timing of a future eruption based on the distribution with time of previous eruptions during the last 5000 years are presented by Jones et al. Ž1999-this issue.. The Furnas area is also subject to seismic activity independent of volcanic manifestations. In 1998, there was a seismic crisis to the south of Povocac¸ao ˜ with 900 earthquakes, of which 40 were felt ŽFerreira et al., 1998.. Since then to the time of writing, there have been, within the Furnas Caldera complex, occasional seismic swarms each of a few dozen events. Based on the historical account of the 1630 eruption, strong seismic activity is expected in the hours before an eruption, and this would certainly cause disruption in the form of landslides that could isolate communities in the area. Because the volcano is mainly built of unconsolidated pyroclastic rocks, and has been deeply eroded giving steep slopes, it is subject to landslides without volcanic or seismic activity, especially following or during heavy rainfall: long periods of rain and severe rain storms are not unusual in this part of the island. During late October 1997, a nocturnal catastrophic storm initiated hundreds of slides that killed 29 inhabitants of Ribeira Quente, destroyed houses, and cut roads, power and communications on the volcano. Although the scale of this event is rare, individual slope failures are an ongoing hazard. The Lagoa das Furnas also presents a hazard as its surface is above the Furnas village, from which it is separated by a narrow embankment. Should this barrier be breached by slope failure caused by an earthquake, high water levels from enhanced rainfall or landslides from the caldera wall into the lake, then serious flooding could be expected in Furnas village and the Ribeira Quente valley.

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13. Conclusions Furnas Volcano is essentially trachytic in composition, and most of the activity has been explosive accompanied in some cases by dome formation. Basaltic cinder cones and lavas have formed outside the caldera. Because Furnas has built up on the flanks of an existing island, most of the erupted material either entered the sea, or was veneered over ancient topography. Much of this latter material has since been removed by fluvial activity or by marine erosion which has provided good coastal exposures of relatively proximal material. Stratigraphic correlation is not easy on Furnas, first, because of poor exposure, and second, because except for deposits formed during the last few thousand years, most of the remaining materials from individual eruptions are proximal and are thus not traceable over large areas because they were deposited at sea, or have been lost to marine erosion. Important exceptions are the Fogo A deposit, which comes from an eruption on another volcano, and the Povoac¸ao ˜ Ignimbrite from the Furnas Centre. Major tectonic trends have controlled vent distribution, the bounds of caldera collapse, parts of the coastal cliffs and the current surface expression of the hydrothermal system. The caldera complex is the main topographic expression of the volcano, and was the result of two large collapses together with some smaller ones. The first collapse probably occurred somewhat less than 30,000 years ago. Substantial infilling of the first major caldera before the second collapse occurred which exposed the earlier fill material in its walls. This occurred about 12,000 to 10,000 years ago, and since then the caldera has again been in a phase of infilling by the products of eruptions that have taken place at an average rate of one every few hundred years. Several major plinian and welded ignimbrite-forming eruptions have taken place in the history of Furnas, at least two of which we suggest were related to the observed caldera structures. However, the majority of eruptions have been relatively small, involving sub-Plinian and phreatomagmatic explosive activity. Eruptions of more mafic magmas have occurred around the periphery of the volcano, but all activity within the caldera has been silicic. Mafic volcanism is subordinate to silicic activity.

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Furnas must be considered to be an active volcano that could erupt again. Based on what we understand of the last historical eruption in 1630 AD, it is clear that a relatively small trachytic eruption in Furnas could present a serious problem affecting most forms of communication, agriculture and living standards over nearly the whole island of Sao ˜ Miguel. Acknowledgements Field work for this project was funded by a European Commission Environment Programme Grant ŽDG XII., that supported fieldwork as well as PDC as a Post Doctoral Research Assistant at University College London. The work was also supported by the University of the Azores. We especially thank Rui Coutinho ŽUniversity of the Azores., for his valuable help in so many ways, including safely transporting us by boat to the coastal sections of Furnas. Discussions with other workers on the EC Furnas Laboratory Volcano Project were of value, as were meetings with members of the Civil Protection of the Azores and the Mayor of Povoac¸ao. ˜ We particularly thank Steven Self, and Wendell Duffield for comments on an earlier manuscript. References Admiralty, 1945. Spain and Portugal, The Atlantic Islands, ŽBR. 502 Vol. 5., Naval Intelligence Division, London. Adel-Monem, A.A., Fernandez, L.A., Boone, G.M., 1975. K–Ar ages from the eastern Azores group ŽSanta Maria, Sao ˜ Miguel and the Formigas islands. Lithos 8, 247–254. Agostinho, J., 1932. Vulcanismo dos Ac¸ores. Vista geral. A Terra 4, 32–36. Anonymous, 1880. In Castro EVPC. Anno de 1630. Erupc¸ao ˜ no Valle das Furnas. Archivo dos Ac¸ores, 2, pp. 527–547. Assunc¸ao, da ilha de S. Miguel ˜ C.F.T., 1961. Estudo petrografico ´ ŽAc¸ores.. Comunicac¸oes de Portugal 45, ˜ Servic¸os Geologicos ´ 81–117. Baxter, P.J., Baubron, J.-C., Coutinho, R., 1999. Health hazards and disaster potential of ground gas emissions at Furnas volcano, Sao Miguel, Azores. J. Volcanol. Geotherm. Res. 92, 95–106. Booth, B., Walker, G.P.L., Croasdale, R., 1978. A quantitative study of five thousand years of volcanism on Sao ˜ Miguel, Azores. Philos. Trans. R. Soc. London, Ser. A. 228, 271–319. Booth, B., Croasdale, R., Walker, G.P.L., 1983. Volcanic hazard on Sao ˜ Miguel, Azores. In: Tazieff, H., Sabroux, J.-C. ŽEds.., Forecasting Volcanic Events. Elsevier, Amsterdam, pp. 99– 109.

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