Teide-Pico Viejo (Tenerife, Canary Islands) and Montanha do Pico (Pico, Azores Islands) are stratovolcanoes presently actives, with great scientific interest, ...
Teide-Pico Viejo (Canary Islands) and Montanha do Pico (Azores islands), two potentially dangerous stratovolcanoes in different stage Of Evolution Zilda T. T. M. França1,2, Vicente Araña3,Alfredo Aparicio3, Victor Forjaz1,2 1- Dep. of Geosciences, Azores University, Rua Mãe de Deus, 9501-801, Ponta Delgada, Portugal. 2- Azores Volcanological and Geothermal Observatory 3- Dep. of Volcanology. Consejo Superior de Investigaciones Científicas, José Gutierrez Abascal, 2, 28006, Madrid, Espanha
Introduction
Teide-Pico Viejo (Tenerife, Canary Islands) and Montanha do Pico (Pico, Azores Islands) are stratovolcanoes presently actives, with great scientific interest, because both are placed on the atlantic platform where the volcanic activity (recent and historic) define them as high hazard areas for people and infrastructures. Teide-Pico Viejo complex (T-PV) constitutes a high stratovolcano with 3,718 m and 3,103 m, respectively. The base of the two structures (Caldera de las Cañadas) is 2,000 m altitude and is placed on the cross of NWSE and NE-SW tectonovolcanic lineaments (Araña et al. 1989). Platform and scarps of the caldera collapse are 0.6 My (Abdel Monem et al., 1972) or 0.5My (Ancochea et al., 1989). Last caldera subsidence is 175,000 years old (Mitjavilla y Villa, 1993). Pico Viejo and Teide craters are around 1 Km diameter but a probable 1492 salic eruption (Soler et al, 1984) filled last one (Teide). Between 2,000 m and 2,800 m many adventitious cones are placed on T-PV complex. Some are periferic and other ones, placed on E-W tectonic lineament, are responsible by emission pumice materials (e.g. Montaña Blanca, 2000 years ago). The volcanic materials are an OIB alkaline suite with an important compositional diversity (Araña et al, 1989). It has started with basaltic-basanitic flows followed by trachyts and intermediate rocks. On the present days a 98ºC fumarolic activity is the main emission of this stratovolcano. Known data (Simonsen et al., 2000) explains that magmatic sources are a HIMU model corrected with EMI values. Volcanic stratovolcano Montanha do Pico (MP) rises 2,351 m altitude, has a 16 Km diameter basement and a production of 97 Km3 hawaiian and strombolian materials. A 550-590 m diameter subsidence crater is placed at 2,250 m altitude (with maximum 30 m walls). The crater is filled by a lava lake where a 125 m high driblet cone was formed (named Piquinho). At 2,050 m altitude remains of a first collapse crater (around 800 m diameter) are existing covered by lavas from highest fountain, namely from the driblet cone. A N115ºW fracture is crossing the main crater wall such as the lava cone; it present an alignment of pyroclastic “boccas” and a short clastic lava flow. They are the last volcanic activity on the top of Pico stratovolcano. The present volcanic manifestation are gas emissions on the base and on the top of Piquinho such as on the E-W fracture
existing on the east MP slope, north of the thick and important debris deposit so called Quebrada do Norte. 76º were the highest temperature registered on the top fumaroles.
This stratovolcano is placed on the cross of NNW-SSE, NE-SW and WNW-ESE to W-E tectonovolcanic alignments. Nunes et al. (1998) have proposed an approche age of 240 my to the subaerial part of MP based on the subaerial evolution and volume data knowledges. Several formations of the MP first phase linked with the fossil crater collapse (2,050 m high) have ages around 40,000-5,000 years BP. Second phase looks to follow a period along 5,000-2,000 y BP till the existing actual collapse crater. Piquinho, the lava lake such as the top fissural eruption have an age between 1,500-1,000 BP. The young formation of the island is the main difficulty on obtaining absolute isotopic ages. Charcoal of several formations and
14
C dating associated to
detailed geological maping are the only roots of the proposed ages. The lavic products have a variation from basalts to mugearites and benmoreites including gabbroic and mafic xenoliths; salic samples are not known. The series is alkaline with OIB sequences. França (2000) using Sr, Nd and Pb isotopic data from Pico, Faial, S. Jorge and Terceira islands, all from Azores and geographically neighbouring, explains that the observed differences are reflecting that the mantelic source has, at
different proportions, mixing characteristics between HIMU and DMM reservoirs and a slightest contribution of EM. Contribution toward the characterization of the mantelic plume has been presented by Schaefer et al. (2002) and Widom (2002) based in osmium, helium and oxygen isotopic data. According to Schaefer et al. (2002) the Os radioactive signatures from Faial and Pico basalts, with lower values than the oceanic basalts, suggests the integration in the Azores mantelic plume of a component from a depleted Archean harzburgite mantle. According to these authors the amount of time needed to the Re depleted should be equal to 2.5 Gyr and, therefore, is not necessary to consider, in order to explain the Os isotopic signature, the integration in the plume of an Iberian lithospheric sub continental mantle as proposed by Widom (2002). Chemistry The SiO2-alkalis elements sequences including all the more representative rocks from the two stratovolcanoes make clear the existence of a continuity and an overlapping between them. At MP volcanic materials are essentially basic and they are coincident with the basic sequences of T-PV. At T-PV are not existing gaps on SiO2 contents but at MP a short gap (50-55%) is known. Mugearites and benmoreites, following the general trend, are suggesting that MP evolution is at an initial phase and that differentiation processes inside an incipient magma chamber are enough for an evolution to trachytic compositions. A HIMU magma source is consistent to the two magmas. For the Azores islands Widom and Shirey (1996), based on Os isotopic determinations, reassert the existence of a mantle plume enriched with additional components with EMI-EMII affinities. At Tenerife relationships
Pb / 204Pb –
206
207
Pb / 204Pb (Observi et al.,
1971) also show a mantle source close of EMII, data that is closed of the global chemistry of the canarian
and azorean islands, namely Pico island (França, 2000). Otherwise 87
206
Pb / 204Pb versus
143
Nd /
144
Nd and
86
Sr/ Sr plotting look to stand out the interference of a MORB type component (beyond the former reported
sources). Once that magma sources for the two stratovolcanoes (OIB, HIMU, EMI-EMII) are very similar, differentiation processes involved on the evolution trend is a real probability. Discussion Birth of those stratovolcanoes is not contingent or casual. Otherwise is a general process where several factors represent stages of an uprising and evolution as follows: junction of several structural directions; alkaline sequences of OIB type; predominantly HIMU, DM; EMI-EMII mantle type sources; identical evolution ages (around 200 my); maximum altitudes, since the base, around 2,000 m; successive collapses and calderas/craters generation, etc. At OIB islands the starting process and volcanism development is linked with those stratovolcanoes. Nowadays it is recognized that the oldest Canary islands formations are related with such a type of volcanoes. At Fuertventura (Ancochea et al., 1991) and at Tenerife (Marti et al., 1994) it is accepted that very high stratovolcanoes are linked to first stages of those islands. Disposition of thick and high layers, now very eroded, are their landmarks. All around Canary islands are identified Miocene and pre-Miocene layers reported to those type of stratovolcanoes. At Tenerife the “Caňadas series”, the more recents (ages > 200 my) of the old stratovolcanoes, are of salic type (phonolites/trachytes) envolving lava flows, pyroclastic flows and pumice fall deposits. These layers are enough for calling our attention to the last stages of the OIB stratovolcanoes. Last actual explosive activity at T-PV was formed by salic materials (2000 AD, 1492 AD, etc) what is a demonstration of the high hazard of those stratovolcanoes for the surroundings people and social/industrial structures. At Pico island the stratovolcano MP is at a beginning evolution stage still as it is shown by last volcanic production (Mistério de Santa Luzia). However it is accepted that several products were not output and that MP can be close of salic phase and the consequent hazard. The present existing fumarolic activities at each one of the stratovolcanoes are data very interesting for future correlations and advise for the actual volcanic hazard for the two volcanoes. As conclusion, the big stratovolcanoes at islands of type OIB represent the main volcanic cycle of each one. Last evolution stages (salic eruptions) are the beginning of subsidence/collapse processes which is equivalent to a new magmatic cycle. Referencias
Abdel-Monem A.,Watkins N., Gast P. (1972) Potassium-argon ages, vulcanic stratigraphy and geomagnetic polarity history of the Canary Islands, Tenerife, La Palma and Hierro. Am.J.Sci. 272, 805-825
Ancochea E., Fuster J. M., Ibarrola E., Coello J., Hernan F., Cantagrel J. M., Jamond C. and Cendrero A. (1989). Cronoestratigrafia de las Series Antiguas de Tenerife. ESF Meeting on Canarian volcanism, Lanzarote. Ancochea E. Cubas C.R., Hernan F. Brandle J.L. (1991) .-Edificios volcánicos en la serie I de Fuerteventura. Rasgos generales del edificiuo central. Geogaceta 9 , 60-62 Araña, V., Barberi, F., Ferrara, G. (1989). El complejo volcánico del Teide-Pico Viejo. En: Los Volcanes y la Caldera del Parque Natural del Teide (Tenerife, Islas Canarias). Ed. V. Araña y J. Coello. Mº Agricultura, Pesca y Alimentación. Icona, Serie Técnica: 101-126. França, Z. (2000). Origem e evolução petrológica e geoquímica do vulcanismo da ilha do Pico. Açores. Tesis Doctoral. Universidad dos Açores. 372 pp. Marti, J., Mitjavila, J., Araña, V. (1994). Stratigraphy, structure, and geochronology of the las Cañadas Caldera (Tenerife, Canary Islands). Geol. Mag. 131, 715-727 . Mitjavilla, J., Villa, I.M. (1993). Temporal evolution of the Diego Hernández Formation (Las Cañadas, Tenerife) and confirmation of the age of the Caldera using the Ar40/Ar39 method. Revista de la Sociedad Geológica de España, 6: 61-65. Nunes, J.C., Cruz, J.V., França, A., Sigualdason, G., Carvalho, M.R., Garvin, J. e Alves, J.L. (1998). Production rates and age of Pico stratovolcano (Azores islands): an estimation from historical eruptions data. Proceedings da “1ª Assembleia Luso-Espanhola de Geodesia e Geofísica”. Fevereiro Almeria, Espanha. Obersby, V.M., Lancelot, J., Gast, P.W. (1971). Isotopic compositions of Lead in Volcanic Rocks from Tenerife, Canary Islands. Journal of Geophysical Research, 76: 3402-3413. Simonsen, S.L., Neumann, E.R., Seimk, K. (2000). Sr-Nd-Pb isotope and trace-element geochemistry evidence for a young HIMU source and assimilation at Tenerife (Canary, Islands). Journal of Volcanology and Geothermal Research,103: 299-312. Schaefer, B.F., Turner, S., Parkinson, I., Rogers, N., Hawkesworth (2002). Nature, 420, 304-307. Soler, V., Carracedo, J.C., Heller, F. (1984). Geomagnetic secular variation in historical lavas from the Canary Islands. Geophysical Journal of the Royal Astronomical Societyu, 78: 313-318 Widom, E., Shirey, S.B. (1996). Os isotope systematics in the Azores: implications for mantle plume sources. Earth and Planetary Scxi. Lett. 142: 451-465. Widom, E. (2002). Ancient mantle in a modern plume. Nature, 420, 281-282.
