POLIGENETIC VOLCANOES OF ARCHIPELAGO (): SIZE, NATURE, ERUPTIVE STYLES AND RELATED VOLCANIC HAZARD

J.C. Nunes, Z. França, V.H. Forjaz, R. Macedo & E.A. Lima

Departamento de Geociências, Universidade dos Açores, Rua da Mãe de Deus, P.O. Box 1422, 9501-801 Ponta Delgada, Açores, Portugal, [email protected] Observatório Vulcanológico e Geotérmico dos Açores, P.O. Box 12, 9500 Ponta Delgada, Açores, Portugal

10 Azores Archipelago (North Atlantic) is located at the triple Position in Relation includes junction of North American, Eurasian and African plates. to theMAR 8,12 3 silicic 8 poligenetic Flores and Corvo islands are placed roughly 100 km to volcanoes (Santa west of the Mid-Atlantic Ridge and the other islands, Azores Bárbara, Pico located eastwards from the MAR, are stretched along a Alto, 6 Guilherme WNW-ESE general trending area, about 550 km long. Moniz) and

4,65 basaltic That WNW-ESE area corresponds to the westernmost fissural 4,01 volcanism sector of the Azores-Gibraltar Fault and marks, in general 3,52 4 areas (in terms the boundary between the Eurasian and African green and

2,5 yellow). Serra Maximum (My) Ages plates. 2,16 do Cume-Cinco 2 Picos is an inactive 0,71 0,73 mugearite 0,55 type central 0,25

Azores Plateau bathymetry after Miranda et al. 1998 0 volcano with 0 caldera. Pico MAR Faial Corvo Flores S. Jorge S. Terceira Graciosa Formigas

Corvo Maria Sta. São Miguel

Flores Graciosa

Terceira Pico Mountain stratovolcano and shield volcano are Faial S.Jorge the poligenetic edifices at Pico Island, the former 2351

Pico D.J.C. Bank m.a.s.l. “Planalto da Achada” is a 55 km long volcanic ridge of basaltic s.l. nature, with about 190 eruptive centers S.Miguel (mostly scoria cones).

S.Maria

Azores Islands (2333 km2) rise from an anomalously shallow and rugged topographic zone (the so-called “Azores Plateau”) crudely triangular in shape and having depths less than 2 km. This plateau occupies an area of 5,8x106 km2 and marks the transition to the adjacent abyssal plains, with depths exceeding 3.5 km.

Summary Table. Morphometry and other parameters of Azores Archipelago 14 poligenetic volcanoes.

PARAMETERS Povoação Furnas Fogo Sete Cidades D. João Castro Bank Cinco Picos Guilherme Moniz Pico Alto Santa Bárbara Caldeira (GRA) Topo Montanha Caldeira (FAY) Caldeirão Reference Po Fu Fo SC DJC CP GM PA SB CaG To Mo CaF CaC Island S. Miguel S. Miguel S. Miguel S. Miguel Sea Terceira Terceira Terceira Terceira Graciosa Pico Pico Faial Corvo Geographic Coordinates 37º 46' 11'' N 37º 45' 38'' N 37º 45' 47'' N 37º 51' 29'' N 38º 13' 16'' N 38º 41' 45'' N 38º 42' 48'' N 38º 45' 13'' N 38º 44' 23'' N 39º 01' 34'' N 38º 25' 48'' N 38º 28' 04'' N 38º 35' 13'' N 39º 42' 34'' N 25º 14' 41'' W 25º 18' 59'' W 25º 28' 21'' W 25º 47' 06'' W 26º 36' 07'' W 27º 08' 15'' W 27º 12' 28'' W 27º 13' 29'' W 27º 19' 06'' W 27º 58' 26'' W 28º 12' 30'' W 28º 24' 08'' W 28º 42' 54'' W 31º 06' 11'' W Minimum Distance to MAR (km) 438 432 419 390 310 251 244 243 234 172 173 154 121 110 Maximum Elevation (a.s.l.) (m) 1103 804 947 856 -12 545 632 808 1021 402 1022 2351 1043 718 High (a.s.o.f.) (m) 3100 2800 3000 2700 1600 1900 2000 2200 2400 2000 2600 3500 2500 2500 Average Base Diameter (at s.l.) (km) 13,6 14,1 13 14 24 17,8 13,8 16,5 13,5 4,4 10,4 19,3 13,6 4,7 Area (km2) 101,9 169,5 126,5 110 410 54,8 24,2 90,7 101 17,9 18,3 276 132,5 17,2 Volume (km3) 47 66 45 40 237 11 7 15 36 28129485 Top Depression Maximum Diameter (km) 783,45,9- - 4,3 - 2,7 1,6 - - 2,1 2,3 Top Depression Minimum Diameter (km) 5,8 5,6 2,2 4,8 - - 2,3 1,8 1,9 0,8 - - 1,8 1,9 Top Depression Average Diameter (km) 6,4 6,8 2,8 5,3 0,45 7 3,3 - 2,3 1,2 1,3 0,55 2 2,1 Top Depression Maximum Depth (m) 724 603 402 629 28 267 174 258 198 270 100 25 470 305 Top Depression Maximum Age (ka) 820 33,95 15 36 - < 300 23 19,6 25 12,2 << 37 1,31 10,25 - Maximum Known Absolute Age (Ma) 0,32 +/- 0,06 0,093 +/- 0,009 0,280 +/- 0,140 0,210 +/- 0,008 - 3,52 0,41 0,0216 1,24 2,5 0,250 +/- 0,040 0,0017 0,73 +/- 0,07 0,71 +/- 0,49 Maximum Infered Age (Ma) 3,2 0,8 0,29 0,8 - --0,1-- 0,3 0,24 - 1 - 1,5 Nr. Intracaldera Eruptive Centres 423813- 1-572-2212 Nr. Intracaldera Scoria Cones 21 ------112 Nr. Intracaldera Pumice Cones -115 ------Nr. Intracaldera Domes 21552 - 1-57---1- Nr. Intracaldera Maars ------Nr. Intracaldera Tuff Cones/Rings -626 - ----2---- Cross Nr. Flank Eruptive Centres 11 69 77 46 - 11454104710210136 sections of Nr. Flank Scoria Cones 7534333- 16126110207125 Azores Nr. Flank Pumice Cones -14------55378 Islands and Nr. Flank Domes 414298 - 2--1- Nr. Flank Maars -1-2 ------surrounding Nr. Flank Tuff Cones/Rings --12 - -3--4-2-- ocean floor. Total of Eruptive Centers 15 92 85 59 - 214591119 10 212 15 18 Bathymetry Nr. Historical Eruptions none 2 1 3 1 none none none 2 none none 2 1 none after Miranda Last Eruption (Y) 650 000 1630 A.D. 1563 A.D. 1811 A.D. 1720 A.D. - - 1000 1998/2000 A.D. - 5000 - 10 000 1720 A.D. 1958 A.D. 80 000 - 100 000 et al. (1998). Population (e.g. Censos 2001) 5827 14154 22371 13139 - 11788 12754 8535 5741 1292 1099 8835 8003 425 Death / Injuries -19510 0 ---0--20-

Excepting Santa Maria and Corvo, the other seven azorean islands have Holocene volcanism, with variable eruptive styles, ranging from plinian and sub-plinian silicic composition eruptions, to hawaiian and surtseyan basaltic volcanism. On the total, there are 9 active poligenetic volcanoes in the Azores (Furnas, Fogo, Sete Cidades, D. João de Castro Bank, Pico Alto, Santa Bárbara, Graciosa caldera, Pico Mountain and Faial caldera) - TABLE. Seven active fissural basaltic areas are spread over these islands and also São Jorge island. Active (basaltic?) submarine ridges are located nearby the islands, some of them with historical volcanism (like “Mar da Prata” ridge, south of São Miguel island, and Graciosa Caldera Volcano (with an elliptical caldera) is the smallest poligenetic volcano Pico Island eastern submarine ridge). of Azores Archipelago (TABLE) and occupies about one third of Graciosa island area.

S. Miguel Island, the largest (747 km2) and most populated (131,609 inhabitants) of the archipelago, includes 4 poligenetic volcanoes with caldera (SC, Azores Poligenetic Volcanoes Fo, Fu, Po – see TABLE) and 2 areas of basaltic volcanism (Picos Volcanic Complex and Nordeste Area), the latter considered extinct. For the purpose of 2500 Faial Caldera Volcano occupies the central part of Faial Island. On its NE part, an old this work, Congro Area (with about 40 monogenetic eruptive centers), was included in Furnas Volcano area. Shield Volcan is almost completely covered by Holocene pumice fall and pyroclstic 2000 flow deposits from the Caldera. 1- caldera rim; 2- Pedro Miguel Graben fault scarps; 3-

vulcano-tectonic lineaments; 4- domes; * scoria cones; surtseian tuff deposits in gray. 1500

High (m) High 1000 FINAL REMARKS

500 (cf. TABLE):

0 1- Fogo Volcano 0 5 10 15 20 25 30 (Fo) has the CORVO For the 14 poligenetic volcanoes of Azores islands Average Base Diameter (km) higher ISLAND lcanoes), several morphological and population living (9 are active vo Azores Poligenetic Volcanoes around it; volcanological parameters were determined, allowing 4 2- Pico Mountain 3,5 CP a quantitative approach on their study. Data is Po (Mo) has 212 3 secondary cones summarized in the TABLE. 2,5 CaG on its flanks; 2 3- a total of Among these parameters are: location (Lat., Long.), 1,5 SB 1350 monoge- 1 CaC distance to the MAR, elevation a.s.l., average base Maximum Infered Age (Ma) Age Infered Maximum CaF SC Fu netic volcanoes 0,5 GM To Fo were identified Mo PA diameter, size of top crater/caldera, and their ages. 0 -200 -100 0 100 200 300 400 500 on active areas; Minimum Distance to MAR (km) 4- the 9 active For these 14 poligenetic volcanoes of Azores islands central Azores Poligenetic Volcanoes volcanoes have it was also evaluated the number of intracaldera and 25000 caused ~200 Fo flank eruptive centres (namely of scoria cones, 20000 victims, being Furnas Volcano pumice cones, domes and coulées, maars and tuff 15000 Fu the most deadly; GM SC CP cones/tuff rings), areas, volumes, number of 10000 5- there is not a Mo CaF PA linear relation Po SB historical eruptions, date of the last eruption, Population (Censos 2001) 5000 between age CaGTo 0 DJC CaC versus distance population living in the area and deaths caused. 0 20 40 60 80 100 120 140 160 180 200 220

Total of Eruptive Centers to the MAR.