SECCION
RIESGOS VOLCANICOS PELIGROS VOLCANICOS
Geroge P. L. Walker*
VOLCANIC HAZARD
Resumen: PART L VOLCANIC HAZARD IN GEÑERAL.
Los peligros volcánicos pueden ser evaluados l. How is volcanic hazard assessed? There are utlizando: 1) Registros históricos, 2) el registro three information sources. geológico, 3) a través del conocimiento del com- portamiento general de los volcanes. Esta. informa- (a) The historical record of eruptions of a vo1ca- ción es mejor presentada en mapas de zonificación no, as a guide to possible future activity. Ge" de peligros volcánicos indicando peligros inmedia- nerally this record covers only the past few tos y potenciales para flujos de lava y flujos piro- hundred years. elásticos, y otros tipos de erupciones volcánicas. (b) The record of eruptions of a vo1cano detemu- Los volcanes de pendientes moderadas presen- ned by stratigraphic mapping/volcanological tan el riesgo más elevado porque sus productos son interpretation of the voIcanic products (each ampliamente dispersados durante sus erupciones. yolcanic eruption leaves a layer which, when Los valles de los ríos pueden actuar como canales deciphered, reveals the characteristics of the para lahares. permitiendo el transporte de los mate- erupÜon l. Generally this record CQversa much riales fragmentarios más allá del área alrededor del more extended period and is eorrespondingly volcán. Las erupciones de ignimbntas son el tipo a more reliable guide to possible future de erupdones volcánicas más peligrosas debido a behaviour than (a). que causan ia devastación de grandes áreas. Los vo1can~s que hacen erupclón raramente ofrecen un (e) Knowledge of the behaviour of volcanoes in problema especial porque por lo general no son ob- general. servadas. To give an example of (a), the hlstOflcal re" En Costa RIca hay muchas zonas de pehgros: cord of Irazu sine e 1700 is one of fairly frequent los volcanes Poás e lrazú han producido numerosos ash-producing exploslve eruptions. The probability depósitos freá ticos; San J osé y el Valle Central es- seems hlgh that eruptions of a similar type and on tán cubiertOs por ignimbritas y coladas de lava; de- a similar scale will take place at a comparable fre- pósitos de nubes ardientes se encuentran alrededor quency in future. To give an example of (b), the del Volcán Arenal. Existen numerosos volcanes jó- venes en Costa Rica. que como el Arenal, pueden reac tivarse. . Hawaü lnstitute of Geophysics, 2525 Conea Road. Honolu. lu, Hawaii, %822.
41 record of the past ca. 40.000 yearS determined by established to monitor the volcano closely, contin- stratígraphic study of pumice deposits from Taupo gency plans be drawn up should a volcanic emer- and Okataina rhyolitic vo1canoes (New Zealand) gency develop, and the people be educated regar- reveals that majar explosive eruptions occur ding the vo1canic hazards they may have to face. It roughly once per 1000 years, the latest being 700 might moreover prove desirable to restrict cons~ and 1800 years ago, and similar infrequent but tructíon ar development within certain hazard large explosive outbreaks may be expected in zones. An example where safety features have future. An example where general knowledge of been incorporated into the design of a construction volcanoes is helpful is Mt. St. Relens. In 1980 a works IS the provision of gatcs on intake tunnels at bulge developed on the northern side of this steep the hydroelectnc power plants near Ruapehu vol- cone and this side of the cone becarne mechani- cano (New Zealand) which can be ciosed when la. cally unstable. The consequent failure and collapse haIS are approaching. Consideration might be gi- of part of the cone had devastating consequences. ven to no-cost/low-cost measures such as the cons. truction of buildings wíth a steep-pitched roof in areas that may be susceptible to pyroclastic falls, 2. How is informatíon on volcanic hazard pre- and the building of simple volqmo shelters (proof sented? The best method is by means 01'volcanic against nuees ardente or pyroclastic falls) as a hazard zone maps which delineate areas liable to possible alternatíve to evacuation in some volcanic specific volcanic hazards, based in part on the past areas. For vokanoes which present potential record of the volcano and in part on general know- hazard it is more appropriate merely to do quiet ledge of volcano behaviour. There is no unique ty- research to establísh the behavioUI pa ttern of the pe of hazard zone map. The simplest type delimit volcano, and to set up some kind of sImple moni- zones of greatest danger, requiring evacuation or torin'g system so as to avoid the posslbilíty of an restricted entry should an eruption be impending, eruption breaking out without warning. or alternatively it shows areas which are judge to be mast liable to be affected for example by pyro- A few general cornments should be made. clastic flows, surges, lava flows, mudt1ows, or pyro- Fírst, it is a principie of volcanology that the more clastic faIls having a thickness greater than a speci- powuerful or violent (and hence more dangerous) a fied threshold. Another type takes into account volcanic event, the less impressive or conspicuous is the frequency or recurrence interval of specific the resulting deposit when viewed on the scale of a events ar (derived frorn that) the probability that rock outcrop (the converse is not necessarily true). such events will happen within a given time periodo It is only when the deposit is mapped and its far- Yet anather type delineates zones which shows the reaching nature recognised that the extent of the pro'oabUity, should
42 lleys, they may present a hazard far outside the Resting on these older volcanoes are very pro- area of the volcano. For example, some prehistoric minent mudflows, and the reality of the mudflow lahars of Mt. Rainier are known to have travelled hazard is shown by the 1964 mudflows that rava- km from the volcano. Note that the distance trave- ged Cartago. I visited the mudflow source area in lled is probably a rather simple function of the the Reventado Valley and saw how extremely uns- lahar volume. table it is. A high-intensity rainfan following a penod of wet weather would readly activate major A fourth cornment is that by far the greastest mudflows perhaps much bigger than the 1963 one. volcanic hazard is that arising frem ignimbrite It seems desirable to attempt to stabilize this mud- eruptioo. Such an event is prabably the only type flow source area, although. it would be very costly which i5 capable effectively of tempararily "knoc- to do so. kíng out" a coun1;¡y the size of Costa Rica, Guate- mala, North Island, New Zealand, Kyusyu, ar New 2. The slopes of Poas and Irazu. Repeated cen- Britam. Apart trom the devastation caused by the tral vent eruptions of Poas have produced many ignimbrite itself, disruption wauld be caused ayer mud~rich phreatic deposits which are up to 0.5 m a great area by the accompanying fall of co.ignirn- thick at 8 km radius, besides strombolian deposits brite ash. and a pumice falI (probably of sub-plinian type) which is 1 m thick at 12 km radius. Irazu has had A final cornment 1S that the most insiduous many strombolian and like eruptions; similar cen- type of volcanic hazard is posed by valcanoes tral vent eruptions may be expected in future. which erupt very rarely, at intervals say of 103 to They pose a hazard to life only very near the erup. 105 years. Such volcanoes are commonly regarded tive vent, but have a senous if temporary effect on as extinct ---an example is Mt. Lamington (New the farming community, exemplified by the 1963- Guinea) which erupted unexpectantly and with 65 ash-falls of Irazu. There is olso a mudflow ha- great violence in 1951 ---and will thus not be zard. Poas and Irazu voIcanoes have a broad shield~ watched. Eruptions which follow a long repose pe- like form, and my irnpression was that lava flows nad tend moreover to be particulary large, power- are a major component of their broad edifices, and fuI or violent events---examples are the plinian are now mostly covered by a thick voIcanic1astic outburst of Somma-Vesuvius in the yeaor 79, and veneer. One lava flow (Formación Cervantes) cave- the ferocious eruption of Taupo in 186 A. D. ring 38 km1 and extending 13 km from vent has which followed nearly 500 years of quiescence. however been mapped on the south side of Irazu and is reatively young.
3. Arenal. The 1968 and 1975 nuées ardent ori- . ginated, as do al! nuees" arden te, on a steep cone PART 2. VOLCANIC HAZARD IN COSTA RICA and may be expected to develop..~:fnfuture' erup- tions. The stratigraphic study of the earhe pro- The following discussion i5. based on impres- ducts of the volcano is weIl in hand and should sions gained during a brief ten-aay visit to Costa .reveal the fuIl capabihty of the volcano. Rica and should not be regarded in any way a defi- nitive account. 4. Guanacaste Province. 1 did not reach the an- desite cones of OroSl, RIncón de la Vieja, and Mira- 1. Sa!l Jos~ and the Central Valley. Extensíve valles, so 1 have no cornments to make On the ha- lava tlCW'í dud igrÜmbrite are seen in many canyon zards they presento 1 did however visit (he extensi- exposures no::..JISan Jose 1 understand that these ve ignimbrite field on the south.wesI slde oI' these ar~ oí" the arder of a mIlhon years old. A pre-ig- cones. A number of ignimbrite eruptions (at least nimbrite phnian pumice fall deposit underlying the 5) have occurred there, and it is cIear that if one ignimbrite thickens and coarsens towards the north such eruption were to occur today it wou!d have and east COilsistent with a SQurce underlying Barva. very serious effects. Stratigraphic mapping of the It i5 possli:Ú tilat Ihese extenslve lava fIows and ig- ignimbrite field and datíng of the individual ignim- nim!:J.rite r~present a long-past period of more vigo- brites is a necessary prelude to assessing hazard rous vc1canism that wiII not recur, but it seerns be- from possible future eruptions. tter not automatlcally to assume that thís is so and to reg3.rd ;mÜlar eruptions in future as possible. S. Other voIcanoes. There are many volcanoes posing a pot.::ntlaI hazard. in Costa Rica which are currently not c1assified as
43 active. The outburst of ATP.nalin 1968 serves as a mant" and "extinct" volcanoes as well as the "ac. reminder of howimportantit is to investigate "dor- tive" ones when assessing volcanic hazards. SECCION
QUIMICA DE GASES BALANCE DEL S Y CL PARA CUERPOS \f AGMA TICOS CALCALKALIN OS POCO PROFUNDOS
WiUiam l. R ose.
S and CI Budgets for Shallow Calc-AIkaHc Magma Bodies.
Resumen ABSTRACT
El presupuesto volátil de un volcán puede ser Particulariy in the case of explosive magmas, d~t~rminado de la siguiente forma: it is almost never possible to dlrectlj' obtam repre- sentative samples of magma tic gases: As a result we ( 1) Midiendo volátiles en inclusiones de vidrio-va- have limited understanding 01'the degasslI1g 01'sha- por en minerales para detenninar el contenido ini- 110wmagma bodies. lf a program of selected geo- cIal de volátiles en el magma; (2) usando eqUIpo de chemical and petrograph.1c observatlons can be ma- sensares remotos y aparatos de muestreo de gases de on volcanic samples, howevtr. it is passibJe {O para determinar la cantidad de volátiles liberados infer much abour the magma inmedlatdy below durante las emisiones de bajo nivel entre las erup- volcanic vents. ,'iones; O) el muestreo y análisis de los materiales eruptados para determinar la concentración de vo- To make an interpretatian of the yolatile látiles retenidos en los materiales eruptados; y (4) budget we need to know at least: 1) The in¡tial midiendo la cantidad de volátiles liberados durante volatBe concentrations of magma when Lt reaches una erupcIón. que es lo más difícil de estimar exac- the shallow reservoir, 2) the amounts of vo!atiles tamente y tiene muchas fuentes de error. released during eruptive actlvity, 3) the amounts released during low~level ernissions betw~en erup. Resultados midales muestran para erupciones t1005. 4) the concentrations of vola tiles remaining r~Cl'~,1tesdel volcán Fuego, Guatemala (1974) yel in the lava and ash samples after eruption and soli- Monte Santa Ekna ( 1980). qUe la gran mayoría de! dlfica tian. Recen t anal)' tical methods al101,1,llS to azufre y doro fueron liberados del magma antc:s y make estlmltes of l by dekrrnmatlOn and intuvre- durante !a enlpción. a pesar de esto una porción tation 01' the chemical composition of glass mdu- signifIcatIva de ambos gases se libera durante los sions trapped within phenocrysts by rapid skdetal períodos J~ emIsión baja entre empclOnes. growth, 3 can be measured by using remote sen- sing equipment and gas sampling. 4 can oe estima- Los presupuestos gaseosos pueden ser usados ted by samphng and analysis of materials erupted. para ( l) estimar el tamaño de cuer:Jos magmáticos 2 LSthe hardest to accurately estimate and the superficlales; l2) detectar el arrivc de nu~vo mag- sources of error iD its determinatíon ar¿ numerous. ma: (3) detenn-inar el grado de dt gasificación del Nevertheless because of atmospheric samp!mg of magma '5uperficial: y (4) detennin.., cambias en la temperatura y fugacidad de] oxíge; "o en e! magma Michigan Technological Um'i. *
47 eruphvc douds in recent years and because os stu- composltlOn:) or emisslOns vark" conslderdbly 'Wlrh Jies of fresh pyroclastics collected before and du- time. ring faIlout we have some primitive estimaks. In terpretatlon af the gas budgets af shallaw Results so far have focused on S and C1 bud- magma bodies offers informatian of obvious value gets for the recent eruptions of Fuego. Guatemala in forecasting of activity. 1) The sizes of shJJlow (1974) and Mount St. Helens. USA (1980). The magma bodies may be estimateJ. 2) The arrival of magmas lose the vast majonty of S and most CI be- new magma fram depth may be detected. 3) The fore and dunng eruptlon. There is considerable gas degree of degassing of [he shaIlow magma may be from mtruslve magma whlch participates 1fl explo- mferred. 4) Changlng temperature and oxyg:en fu- SlVI:'eruptions. Mafic magma ma}' have hlgher S gacity wndl1lOnS in the magma body may be tn- contenl. A significam portion of both gases is lost ferred. during low level emlssions between eruptions. The EMISION DE DIOXIDO DE AZUFRE Y OTROS GASES EN EL VOLCAN MASAYA, NICARAGUA
Stoiber, Richard E., *
Sulfur Dioxíde and Other Gas Emission From Masaya Volcano, Nicaragua
El volcán Masaya se encuentra en un estado A group of scientfsts- from Dartmouth Colk- de alta emisión gaseosa, produciendo 1000 a 2000 ge (Stoiber, R. E., S. N. Wilhams. and N. M. Jolm" toneladas de S02 por día. Estos estados ocurren son), Umversity of Virginia (R. A. Parnell). Stan- aproximadamente a intervalos de 25 afios y se ex- ford University (William Winner). and Colorado tienden por un lapso de 5 años. Lluvia ácida con College (B. Huebert) are studying the geology of un PH de 2.5-3.5, se precipita de la pluma de gas the Masaya Caldera, Nicaragua wlth special att~n- del Masaya. Filtros especiales se han colocado en tion to the gas emissíon fram its crater, Santiago. el cráter y en la dirección del viento para colectar We are now in the midst of a penad of high emis- S02, HCI, HF, HBr, S04' Cl y F (ver tablas). Estos SlOn, 1000 to 2000 tons per day 502 and often datos combinados con los datos suministrados por more. These periods have recurred at aproximatcly un espectrógrafo de correlación permiten hacer es- 25 year intervals and last about 5 years. tL'11aciones de la emisión de gases diaria. Un anali- zador de gas Intersca electroquímlco de S02 fue Gas is almost a!ways at ground leve! as It pJS- usado para medir las concentraciones de S02 y HCI ses over a ridge WhlCh is traverse to the prevajlmg en la pluma de gases. Estos datos permiten evaluar wind direction, 15 km downwind frcm ~he Cr.Ikf los peligros ambientales derivados de la degasifica- Concentrations of up to 1 ppm saz are fOllnd hefl:'. ción del volcán Masaya. MeJ.surements of 502 flux passing over the ridge were made in 93 m5tances In January to March 1981. At this time an average rate of 1300 toos was indicated.
There 15 a wide vana tia os in emlSSl0n, greater than W~ hav.; measured at other vokanoes, i.:h quite :jurel)' ret1ects a changing output at the crat::r."" Our collaborator lohnson finds strongly acid rain f.llls tl1ru plume, pH 2.5-3.5. Tht soil reactioos are und~r study. Winner is descnbing the dekte- nous effects of the gas on the coffee plan ts.
Special filters appropriate for the determinJ- tlOn of certam gases have been placed in the gas plu. ,. Departmcm of E.uth Sciences me both at the era ter and downwind at Llano Pal'J.- Dartmoutb Collcges, Honovcr, N. H. 03755. USA. ya. The 3 filters m each se! allow the weight of
49 S02, HCI, HF (and infrequently HBr) which passes gas ratios, aerosol ratios and gas to aerosol ratios' through them to be measured. The filters also ca- have been cakulated (Tabk 1). Hect aerosols: 804 ", CI -, F -. From these data
Table 1
Average ratios of weights of gases and aerosols fram filter samples
Sampie Loation Gases AelOSOIs Gas to aerosol - Santiago Crater SOZ/HO Ha/HF HO/HBr SO4 ~/0 - la - IF- 502/504" HCL/CL HF/F Masaya VolcaDO 3.4 >370 840 1.3 2.3 90.5 29.5 H.5 <.5 1.77 H .68 < .68 Uan? Pacaya, 15 km 6.7 > 10 Not 10 3.4 10.8 downwind foom Santiago detennined
Our present data indicate that ratios from 3 and least for HF. fIlter collections. a11 on Llano Pacaya, a11on the We have calculated the probable S02, HCI same day. are very similar. The majar differences . in these ratios are the contrast between ratios from and HF flux at the era ter using the ratios and the f1lter eollections at the era ter and downwind as downwind correlation speetrometer average value. shown in Table l. The aerosol component increases The ratios allow one to take account of the S02 downwind. This is marked in the gas to aerosol ra- 10ss due to aerosol produetion downwind from the .. tios for S02/S04 . and especially HCl/CI -. The era ter and to include the arooums in aerosols at the crater (Table 2). gas aerosol ratio is greatest for S°.1.' less for HCI
Table 2
Tons per day and per year of selected gases emitted fraro Santiago Crater
}Gr. Rate 01 Emission 5°2 HO HF
Tons/day 1300 400 5 0.5 TonslYeaI 500000 150000 3300 180 '" based on very few data
Concentrations of 502 and HCl in the plum e The data relative to Masaya volcano degassing were measured using an Interscan electrochernlcal are being used in a continuing study of the enviran- SOz gas analyzer at the era ter and downwind. mental impact downwind fraro the Santiago era ter. Because of the difficulty of sampling the most con- The study emphasis the importance of volcanic gas centrated part of the plume, values (Table 3) must plumes as a volcanic hazard under eertain eondi. necessarily be viewed as mínimum only. tions of winds and local topography.
50 Table 3
Concentrations (ppm by weight)
Method LocadoR SO: HO
Filter data Crater 96 30 Calculated) Llano Pacaya 2.8 i) ; . ..-. . . ..-.... -- . . .. -. ~-. 10 lnterscan data Crater » I llano Pacaya 1 Aircraf traverse 0.5 Over Llano Pacaya I 20 km downwind 0.4 ground level SECCION
SISMOLOGlA VOLCANICA ANAL YSIS OF VOLCANIC TRE~fOR FROM PAVLOF, FUEGO, PACAYA, SAN CRISTOBAL ANO MASA YA VOLCANOES,
Steven McNutt*, Análisis de tremor volcánico de los volcanes Pavlof. Fuego, Pacaya, San Cristóbal y Masaya.
RESUMEN tos magmáticos en un edificio o maCIZOvokánkü que pueden considerarse como llna zona de la baja Tremor volcámco estrechamente relacionado resistencia mecánica cuyos esfu~rzo5 mtcrno..; L'sedn en el tiempo con la actividad eruptiva de cinco es- gobernados por fuerzas de cuerpo (gravedad) en tratovo1canes de los arcos circumpacíficos fue ana- vez de ser por fuerzas tectónicas ..;omo en la pro- lizado y comparado con señales de baja amplitud fundidad. produ~idas por agua fluyendo a través de túneles de descarga de la r~presa Tarbela en Pakistán. Aná- hSIS de frecuencia fueron hechos para cada volcán Vo1canic tremor 15generalJ}' detlned as contl- usando los métodos de la discreta transformada rá- nuous seismic ground oscillations J,>so iatl'd \1,'Jth pida de Fourier y el análisis Burg de máxima entro- volcanic activity (Tokarev, 1981). Tremor ha~ pí::. La represa y los volcanes tienen resonancias been classified in the literaturc in gt'vcrdl \\a~..., b:J- de tubo de órgano. exhibIendo picos angostos e sed upon' 1) auptIve vs non-erupti\t' actl'llty of igualmente espaciados. the volcano: 11)frequency cantent and ""ismlc wa. ve type: UI) time dl.lration (canunuOl.~ or mtcr- Usando la frecuencia de la vibración. una velo- mittent on a time seale 01' minutes lO month<; ¡; :.JI1U cidad de 2 Km/sec para las ondas P en el magma, la IV) source mechanism, including dos.:-d \" IH m e- longItud de onda puede ser determinada. De la 10n- chanisms (jerky crack growth. contll1uous hlock gÜud de onda, s,," puede calcular la longitud del motian, magma chamber oscHlatlOns). and open conducto magmático (tabla 1). que es positivamen- vent mechanisms (exptosions. continuous gas anll te correlacíonable con las alturas de[ volcán. lava ejection. magma flow in ca ndu lts. gJS 05..:11!J- tlOns) (Tokarev, 1981: Kubotera. 197-+: Shimozuru. Altanatlvamente, si los conductos están abier- ¡ 9fj 1. 1971 t This srudy 15concern~J w¡th tr --'mor tos en ambo:> ~xtrt.'mos. en vez de ser solo en la par- which is c1ose1y related in time to surfac.:- t'ruptivL' te superior como en el modelo anterior, ]OS con- activityat five circum-Pacific lsland are \trutovolca- ductos serían más largos por un factor de 2. Si el noes; Pavlof (Alaska), Fuego and PacJya (Guatema- magma contIene burbuJas, entonces la velocida.d de la). San Cristóbal and Masaya (Nicaragua), Bnd las ondas "P" dc.::recería reduciendo ]a longitud del descriptlOns of tremor episodes at th~sL' volcano\.'s conducto calculado por un factor hasta de 5. follow.
Todos los casos estudiados aquí involucran tr¿mores superfk [ales. que put.'den representar tin ... Columbia Universlty fenómeno de resonancia asociada con los conduc- Lamont Doherty GeoL Obs.. Pahsadcs. NY. 10%4, USA.
55 Tremor at Pavlof volcano is observed as short for the fundamental mode of vibra tlon for an Of- bursts of 2-26 minutes time duration and also as gan pipe wlth one open and one closed cnd- continuous tremor of 1 1/2 days duration aecom- panying major strombohan eruptions. At Fuego volcano, tremor episodes often begin with or are A accompanied by large explosions from the surnmit 2 =4 erater: these episodes last fram several hours to ma- ny days duration. Tremor froro Pacaya volcano occurs during strong strombelian eruptions and is where Q '" length in km, A= wavelength m km. Re. of several hours duration. At San Cristóbal volca- sults are shown in Table 1. We -note a rough poslti- no, tremor increased in amplitude gradually over a ve correlatlOn between the length determined by perlOd of severa! days prior to a strong ash and this method and the heigh of the volcano above its steam eruption on March 10, 1976. FinaUy, con- surroundigs. tinuous tremor of low amplitude is observed near the active lava Jake of Masaya volcano. We also note two possible sources of error In this type of analysis. It is p.ossible that the funda- Signals resembJing volcanic tremor are also mental mode of vibratíon 18 that for a plpe wlth observed on seismograrns from stations near Tarbe- tWQ open ends; we cannat determine this unambi- la dam and reservoir in Pakistán. Tms signal is ob- guously. If true, our lengths gíven in Table 1 shourd served when water is flowing through the dam's be multiplied by a factor of 2. There is also a large outilow tunnels. Frequency analysis have been uncertainty in the value to assign for the velocity perfomed on the tremor fram each volcano and on of P-waves in magma. Addltion of even a few per- the Tarbela signals using two spectral analysis me- eent of bubbles to the magma drastically lowers thods: Discrete Fast Fourier Transfonn (FFT) and the P-wave velocity, so our value of 2.0 km/sec is Burg maximun entropy (MEM). The Tarbela signaJs probably a maximum. Values as low as 0.3 kmísec are shown to be organ pipe resonanees in the out- have been measured in the field (Aki d a1., 1978). fIow tunnels, with both even and odd hannonics Using this value reduces the lengths given in Table present O. e., i\" 1/4,Q,1/2,Q, 3/4,Q 2, 5/49., ete.). 1 by a factor of 6. The observed spectra change with location around the reservoir and with the size of the outllow ope- All the examples of tremor examined in this ning, which is gate controlled. study appear to be of shallow erigin. ~ e speculate that tremors originatmg at shallow depths may re- We infer that the volcanic tremor signals are present a resonance phenomena associated with also organ pipe resonances, presumably of a mag- magma conduits within the volcanic pile. which ma~f1l1ed conduir. The tremor spectra exhibit na- can be regarded as a zone of relatively low mecha- rrow, evenly~spaced speaks, similar to the Tarbela nical strength whose internal stress field i<;largely daro signals (Figure 1). We measure the frequeney governed by body forces (gravity) rather than tec- of the fundamental mode of vibration, and by assig- tonic forces at depth. ning a velocity of 2.0 km/sec to P~waves in magma (Murase and McBirney, 1973), we are able to deter- T ABLE 1 mine the wavelenght from the relation: Lengths of resonating structures v = XC or i\ = v/r with v: 2.0 km/sec ~ where V velocity in km/sec, A. wavelength in Pavlof 1.6:t 0.3 km km, and f'" frequency in Hz. Based on our results Fuego la:!: O.l km from Tarbela daro, we assume tha! both odd and Pacaya L 1 :!:0.2 km even harmonics are present. We then ealculate the San Cristóbal 0.7 :!: O.I km lengh of the magma conduit based on the relation Masaya 0.5 =: 0.1 km
56 REFERENCES
AKl, K., B. CHOUET, M. FEHLER, G. ZANDT, R. KOYA. NAGI, J, COLP, and R. G. HAY, 1978. Seismic pro- perties of a shallow magma reservoir in Kilauea 1ki by Active and passl'le experiments, J. Geophys. Res.. 83 (B5), 2273.2282.
KUBOTERA, A., 1974. Volcanic tremors at Aso Volcano, in Pbysical Volcanology. edited by Civetta el al., Developmenu in Soljd Earth Geophysics, 6. Elsevier, Amsterdam, 29-47.
MURASE, T., and A. R. McBIRNEY, 1973 Properties of some common igneous rack:>and their melts at high temperatures, GeoI. SocoAmer. BulI., 84,3563.3592.
SHIMOZURU, D., 1961. Volcanic micro-seisms, discussion on the origm, BuII. Vole. SocoJapan, 2, no. S, 154- 162.
SHIMOZURU, D., 1971. A seismological approach to the prediction af 'lolcanic eruptians, in The Surveillance and Prediction of Volcanic Activity. UNESCO Earth Science Monograph No. 8, París, 19-45.
TOKAREV, P. I., 1981. Volcanic tremor, 1981 IAVCEI Symposium on Are Volcanism, Abstracts Volume, Volcanol. Soc. Japan and IAVCEI, 386.
57 A
B
e
D)( lLJ I ~ o :::) -~ ~ :E 1'~M~'-- « ~,
G
H
o 2 3 4 5 hz
Figure i
58 SECCION VOLCAN ARENAL PETROLOGY OF ARENAL VOLCANO
M. Se. Eduardo Malavasst R. '"
ABSTRACf basaltic-andesites, to one pyroxene hornblende an- desite, to hornblende dacites. Texture ranges from Petrochemestry of Arenal yolcano is similar to hyalopilitic to trachytic. volcanic rocks of other circum-Pacific Continental margins, characterized by the presence of orogenic Arenal volcano lavas are two pyroxene basal- andesite and related rocks. The MgO, CaO, TiO:z tic-andesites and andesites, gray to dark gr('y in co- and total Fe conrents decrease, and aIkalis increase lor and hyalopilitic to pilotaxitic tex turco Histonc in these rocks with increasing Si02 (see figure 1). Arenal lavas (1968-present) are two pyroxene basaltie-andesites. porphyritlc hyalopil ilie texture: Arenal volcano magmas show typical cal-alka- with exception of the first lava flow erupted in ¡ic trends in the AFM diagram with neither enrich. 1968 (Sáenz, 1977) which had homblende pheno- :11ent non empovenshment in the total Fe relative crysts. to MgO and total alkalies (see tigure 2). Composi- !ians of rocks from Arenal volcano plat in the calc- Chemica! trend of lavas erupkd bl'twe;;::nd alkalic field in the total Fe versus total Fe/MgO, 1968-1973 (Melson and Sáenz, 1973) rdlo::cts the which differentiate between rhe calc-alkalic and extrusion 01' Iess differentiated lava w¡t~, time Sew tholeitic series of island ares and actlve continental chemical analyses of lavas erupted bet\~ l:t'n 1974- margins (see figure 3)- 1978 do not support a simple evolution toward less differentiated lavas with tune smcC' they nave mi- Bilsed O!1 chemical analyses most vo1canic nor chemical variations, but generally are very SI- rocks from Arenal VoIcano are bas;11tic-andesites or milar lnd fairly basic for calcalkaline volcanic andesltes. The exceptions are these daeitic !apilli rocks (see figure 4). layers. a basa!t and several gabbroic accidental blocks. :: PETROLOGIA DEL VOLCAN ARENAL Mmeralogy and mineral proportlOns of Arenal La petroquímica de las rocas del volcán Are- tephra layers show the most contrasting composi- nal es similar a la de las rocas volcánicas de otras tion variatlOns. Juvenile ejects from these layers márgenes continentales de la región clrcumpacífica, range t'rom two pyroxene basalts, to two pyroxene caracterizados por la presencia de andesita orogé- niea y rocas relacionadas. En las rocas del volcán Arenal el contenido de MgO, CaO. TiO:! y Fe total * Program... de Investigación Vu1canológica, Escuela de Ciencias decrece y el contenido de alcalis crece wn el incre- Ge.ográf"I;Cas. UniverSidad Nacional Heredia, Costa R.1ca. mento en silice, ver figura l.
61 Los magmas del volcán Arenal muestran típi- hornblenda. Su textura grada desde hJaloplJítica cas tendencias calcoalcalínas en el diagrama AFM hasra rraquítica. sin incremento o decremento de Fe total relativo al MgO y a1calis totales, ver figura 2. Las rocas del Las lavas del volcán Arenal son andesltas ba- volcán Arenal se ubican en el campo calco-alcalino sálticas y andesitas de dos piroxenos. color gns os- en el gráfico Fe total contra Fe total/MgO, que per- curo a gns, y textura pllotaxítica a hialopilítlca. mite diferenciar entre las series volcánicas calco-al- Las lavas históricas (1968-presente) son andesitas calinas y toleiticas de los arcos de islas y márgenes basáltlcas con dos piroxenos y textura porf¡rítica continentales activas, ver figura 3. hIalopilítica; a excepción de la primera lava erup- rada en 1968 (Sáenz, 1977) que contenía fenocris- La petroquímica sugiere que la mayoría de las tales de hornblenda. rocas del volcán Arenal S~)flandesitas basálticas o andesitas. Las excepciones conocidas.son tres eyec- Las tendencias petroquímicas de lavas erupta- tas juveniles daciticas de las capas de tefra, un ba- das entre 1968-1973 (Melson y Sáenz, 1973) sugIe- salto y muchos bloques accidentales gabroides. ren la extrusión de lavas menos diferencIadas con el tiempo. Nuevos análisis químicos de las lavas erup- La mineralogía y proporciones minerales de tadas entre 1974-1978 no respaldan una simple las capas de tefra evidencian variaciones químicas evolución hacia lavas menos diferenciadas, ya que contrastantes en composición. Las eyectas juveni- ellas tienen sólo variaciones menores pero general- les de estas capas gradan de basalto con dos piroxe- mente son muy similares y básicas para volcanes nos, a andesitas basálticas con dos piroxenos, a an- calcoalcalinos, ver figura 4. desita con homblenda y un piroxeno, a dacitas con
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