GeochemicalJournal,Vol.28,pp. 173to 184,1994

C h em ic al c h a r acters of cr ater la k es in th e A z o res a n d :

th e a n o m aly o f L a k e A lb a n o

M ARlNO M ARTlNl,1 L UCIANO GIANNINl,1 FRANCO PRATI,1 FRANCO TASSI,l B RUNO CAPACCIONl2 and PAOLO IOZZELL13 IDepartm ent ofEarth Sciences,U niversity ofFlorence, 50121 Florence,Italy 2lnstitute ofV olcanology and G eochemistry, University ofUrbino, 61029 Urbino,Italy 3Departm ent ofPharm aceutical Sciences, University ofFlorence,50121 Florence,Italy

(Received April23, 1993,･Accepted January 10, 1994)

Investigations have been carried out on craterlakesin areas ofrecent volcanism in the A zores and in Italy, with the aim of detecting possible evidence of residual anom alies associated with past volcanic

activities;data from craterlakes ofCam eroon have been considered for com parison. A m ong the physical- chem ical ch aracters taken into account, the increases of tem perature, am m onium and dissolved carbon dioxide with depth are interp reted as providing inform ation aboutthe contribution of endogene fluidsto the lake w ater budgets. The greater extent of such evidence at Lakes M onoun and N yos (Cam eroon) appears associated withthe disastersthatoccurred there duringthe lastdecade;som e sim ilarities observed atLake Albano (Italy)suggesta potentialinstability also forthis craterlake.

parison. W ith reference to the data collected so INTRODUCT ION far and considering the possibility that the actual

Crater lakes in active volcanic system s have chem ical characters ofcrater lakes are influenced been investigated with reference to change s oc- by residualtherm al anom alies in the hosting vol- curing in w ater chem istry in response to different canic system s, an effort has been m ade to verify stages of activity, and interesting inform ation is w hether and to w hat extent these anom alies can available about R u apehu (Giggenbach, 1974), be revealed by sim ple observations.

K usatsu-Shirane (O ssaka et al., 1980), C hich6n B e this pr ocedure sufficiently correct, m oni-

(C asadevall et al., 1984), Po~s (Casert ano et al., toring a larg e num ber of crater lakes w ould be-

1985), and M aly Sem yachik (Fazllu llin, pers. com e a via ble program even w here high-level com m .). M uch of the atte ntion of scientists and technology is not available and a w ealth o finfor- public officials, how ever, has bee n recently cap- m ation co uld be e asily collected. A ccording to the tured by crater lakes not influenced by active intensity oftherm al and chem ical anom alies, pos- volcanism ; in fact, the events in C am eroon, at sibly related to past volcanic activities, diff erent

Lakes M onoun (Sigurdsson etal.,1987) and N yos levels of probability could be estim ated for po-

(Sigvaldason, 1989), represent the consequences tentially hazardous events. of a rather uncom m on type of geologic hazard. Crater lakes appear thus to deserv e serious atten- V OLCANOLOGICAL SETTINGS tion as potential sources of disasters significantly affecting the populations in the sur ounding areas. Given the general aim of this investigation, System atic investigations of w ater chemistry m ain attention w as paid to the location oflakes have been car ied out at crater lakes in areas of within volcanic system s, and the difference be- recent volcanism in Italy and in the A zores; other tw een craters representing the w hole structure or results from Cam eroon have been used for com- pert aining to larger calderas is not supposed to 173

174 M. M artinietal.

OCor¥vo A g O R E S O FIores Graciosa ¥~ e) ~'~ sanJ~~rg:e o Tercetrcl FoialO ~ ;pico _ san Miguel ~ ~~ )

S.t~a M aria

~ SeteCidades F'ogo

Furnas

S. M I G U E L L_ _ JloKm

Fig. 1. Location of the craterlakesin the Azores.

/ N ¥?) O

･ s Y~ e eo LsENA <)

evlco ~~ Ac~ANO 'OMARTIGNANO

R ~ALBANO

eN EMI

T Y R R H E N I A N S E A RNO Napoli

Fig.2. Location of the craterlakesin Italy.

Craterlakesin A zores and Italy 175 affectto a significant extentthe chem ical charac - com m .). M artignano lake pertains to the sam e ters un der consideration. structure. The activity of A lban Hills volcanic

Signi ficant volcani c activity isreported fort he com plex occur ed during the period 0.70-0.036

A zores, crossed in a north-sout h directi on by the m.y. b.p. with hydrom agm atic explosions from axis o f the M id- Atlantic ri dge (Fig. 1); besi des eccentric craters in its final phase; N em i and t he last surtseyan eru pti on of Capelinhos at Faial Albano lakes are filling tw o oft hese craters (D e in 1957-58 (M achado et al., 1962), strom bolian Rit a et a l., 1991). Phlegrean Fields are located activity w ith lava n ow s occurred in 1761 at inside a large caldera possibly produced during a

Terceira, and four explos ive eru ptions arerecorded giant ignim brite eru ption about 0.035 m.y. ago for S. M iguel in 1563, 1638, 1640, and 1811 (R osietal., 1983). The crater hosting Lake A vern o

(B ooth et al., 1983). Fogo lake fills the sum m it is considered as a m onogenetic tuff ring form ed depression of A gua de Pau volcano, about 3 km during the recent subaerial period; it is dated at

about 3.800 y.b.p. The last eru ptive event in the w ide; its present m orphology results from area occur ed in 1538 with the build up of a 140 cratering, volcano-tectonic collapse, and landslides. Sete Cidades lake is located inside a polygenetic m high tuff cone. caldera, 5 km in diam eter, produced during a long series ofplinian events,the lastprobably occur ed SAM PLlNG AND A NALYTICAL PROCEDURES in 1444. Furn as lake occupies the sum mit ofthe eastern m ost active stratovolcano, tru ncated by a W ater sam pling has been carried out along a large polygenetic collapse caldera, about 6 km vertical profile above the deepest point of lake wide. A w ell doctim ented historical eru ption is bottom ; the position of the boat w as constantly reported in 1640; intense therm al activity is at checked by m eans of a portable G P S (Sony IPS- present occuring inside the caldera. 360). Sam ples from depth have been collected in

W ith referenceto Italian lakes (Fig. 2) B olsena closed containers. Tem perature w as m easured in lake is a large volcano-tectonic depression, about situ, pH at the surface, at the sam e tim e of the 13 km in diam ete r,probably produced by a series collection of sam ples for dissolved C 02 determ i- of huge ignim brite forming eru ptions during the nation,in 100 m l glass bottles containing 10 m l period 0.60-0.35 m .y. b.p. (V arekam p, 1980; of a I M N a2C 03 solution. The C 032- excess Gillot et al., 1991). The volcanic activity contin- subsequent to the reaction: H20 + C0 2 + C0 32 ued in the we stern V ulsini area probably up to => 2H C 03- allow s then the calculation of dis- solved C 0 2. O.125 m.y.ago,w here the w ellshaped polygenetic L atera caldera develop ed (N appi et al., 1991; The analytical m ethods used are as follow s. Turbeville, 1 993). A nions and dissolv ed C 0 2 ~> A utom atic titration The activity of V i co volcano occu rred f rom with a Titroprocessor M etrohm 636 equipped with ion selective electrodes.C ations and B oron ~> Ion- 0.40 to 0.09 m .y. b.p.; its sum m i t appears as tru ncated by a polygenetic collapse caldera, about chrom atography with a Dionex D X IOO. N H4 ~> 8 kn wide(Bertagnini and Sbrana,1986),and half Potentiom etric determination. Li ~> A tom ic ab- of the caldera fl oor is occupied by the lake, sorp tion spectrophotom etry . probably established after a giant ignim britic eru ption dated atabout O.15 m.y.(Locardi, 1965). O BSERVATION D ATA AND DISCUSSION Explosive paroxysm al eru ption with subordinate effusive episodes characterized the Sabatini area The analytical results of m ajor com ponents for

during a period of0.60 m .y., andthe developm ent the sam ples collected from crater lakes at the of the volcano-tectonic depression of surface and at approxim ately the m axim um depth lake follow ed the exhaustion of the eru ptive ac- are given in Table 1. tivity about 40,000 years ago (D e Rita, pers. O ur att entio n has been m ainly focussed on an-

176 M. M artinietal,

~:) ~ O,r} ee ･~,-4cYl olr) -Lr~ (~ c~] C~ 0 ~0 00 a$ cs o o oe o (,o~ - c~ e~cf)Lr) C~](~1 - ~ ~rcf)cf) c~] cf) cf)~O tr)(~ -c~] c~] ~r~o- c~]

(~ ~r~ ~ c~o~cr)~ ren~~o ~5 Oe C,O ~(~~¥ O Ot¥ Oecf)en ~l~･-1 =~¥ OLr)(~ c; c; c; c~ c; ~i -1,-4(~].~ rrl c,~ -.-1 ~t ~tcf)cf)(~ C~].-,C~(~~~¥O

C:~cf] ~: ~:cf] o c~ c~ ~t¥o Lr O)eOe c:-)cfO) -Q V-) Cl)O~ (~ Qflcfl,N (~i O~ o - - oe (~ c~ o c~i(~j(Y~ cr; ~ ~o C;(:;(:i(:~ c; C; - (~ ~r ~r c~]cl~ - (~-c~f)

(Lc+) ~ cf) - ~ ~rl/~ oe O O ~O Cr~ Cr~ ~r Otr)(~q¥O ~,r] ~r ~t ¥O O�(~ ~ O (~ -Lr) C~1(~ C~¥O ~,!)Lf) o ¥OLr~Lr)¥O~O eO OOcf) oo (~ Cn O Q~II!)~O ~ (~1C~ - -(~ C~l - -(~](~ - - cf) ~O = ¥C hC :~

¥O O�e~ ~r cfl -Lr) o c~] ~ (:;,-1.-4 -~ ~~'-r)1.Lr~),c-n'.c-~1~,c-~~.c-~.-~,L-r1)'-1~,t-~¥.,-~1,(~-1~,c-f4],~-1o c¥i(~i a~ ~c)r ~:(e~n

~o 0 ~r ~o c~ cF~(~ t¥ cf! ~ c;cf;ef;(~ (~i[c~l c~~lt(e¥]n(cf~) c(~1r cn~(c~]1c(¥~]c(~l ~ro,~~-(~1 c-nc~~r '-1 c~¥c,c･f)~ ~t cr~

c~1.(~.~O r:th. v) ~ QnLr) o(~ ~: ･~ ~O ~c;cll,~~t~tCt~rf)(~~tLcr~) ~rOc(･f~ -~,-1r~r¥ ~ore c~](c~･a~~O~t,~f) ~; e~i¥c; r; r(

~ ~~; ~t~oLr! .~~; ~ -O o-e,o-o1Lr-)1!-1Lrfr)･~r~ -o(c~1l ~r- ~tO(C~](O~O cn~c(r~)9 C~¥](¥l- ~tLr) -~,-4cllt-~¥･.~- (c~] ~~ ~O ~:) ~*~ ?~ ~0 ~t 0~0 c,1~O,l~ C7~ o ~ c~lcf) ~r c~]tr)Lr) =C~] ~ ~:) 't*rl .*o *o ~~ ~~Et~ ~rrf] i ee e~ ~c~~O ~r oe~Otr) O ~ tr)cf) oe~O - - ~:'1~ 1: $ 0 ~ cs~ ~~s ee ~t¥ ~ ~ ee oe ee ee ec; r~ r:oc;e~:;Lr; oc; ~: ec; r~ o(; ~~d ~¥i'r) ~ ~ r~ c~ ~!)~ o~- ~l ~) 'J ~ ~~: c~ -~ c,~ cn rf)cll ~t co *~ 0 0 - 0<)~¥ C;,¥Ob~ (･~tr) oo L*r)(~ ~:)C~ Cf)Cn ~:~ ~ ~ '-T'-1 - -~-~ (~q C~g .~(~ = = ,-1 c~1･~ C~C~ Cf~C¥](~ (~ ~~: ~~o~ aH, f1~)~ ,~:o2 ~~ ~:~ l~ '~; eQ +-S/ . q~) eC~ei~ oS~ ~:_ o o o_ o c~ ] c)~ro o o o~,c¥)~ oLr)o o~o o oco~ o coF~r) O Oo O Oo OlO¥r) O~¥l_~ ' C'~ ~ ,O~ ~ ~ ~ ~ - C~ f;i~^ :e~O:~ ¥~l. c:) ~I ~1* ~~) en*O -: ~) *¥~-~~~)~~-~l*~l~~- '~s* :S f~ h: ~) .c!~) i*1~ ~:S ~: ~ ~) ~¥)J ~; ~CeS ~) .i:1' F:;O ce~ ~) ~~._ ~:~ ~~: 7~ Fc:~ .'C~$ ~: o O ~FS ~~~t~~ ~ ~* ~J ~ ~ ~e) ~)O 1:~:P ~F1 *~ ~ ~ O ~ ~-*~~) ;e:~JO ~$ eOe ~ ~!~ Q~ 8 ,C~e ~) q) a) eh QF1 ;~~e'¥~*~ ¥~),-:J~ ~) ~~ ~~ c'D CCI ~: ~:) ~ ~ ~; > ~ z :~ ~,~ ~ ~q ~) a- ~) q) Q) Q)< ~) ~ ~*:~, ~~4$0 ~ ~ ~ce,t ~sｭ: ~! ~c':l ~! ~4 ~~: ~ ~~ ~~ O~t~) O~ rC~:) ~ 't ~l ~ ~l ~] ~ ~l ~ Fa:l$ c~I ~ ~ c~1s ~ ~~ ~ t~l ~;1

Craterlakesin Azores and Italy 177 ions and C 02 distribution, w hich appear to pro- tion of carbon species, observed in m ost of the vide the best inform ation. Figure 3 allows us to sam ples investigated, appear to reeonfr m a per- observe, besides the m inor contribution of sulfur turbed situation w hich does not allow the attain- species, a trend along a C1-H C 0 3 Iine, possibly m ent of a physico-chemical equilibrium . A nom a- resulting from varying extents of chloride or of lous values of C0 2 m put can be derived for N yos, carbon dioxide. The contribution of m arine spray to the chlo- ride concentration can be im portant for the lakes Table 2. Values of the ratio C021H C03 + C03 located near coastlines (Fogo, Sete Cidades, forthesamplesfrom craterlakes (see text) Furn as, A vern o), and C 02 inputs can be infered Sample C021HC03+C03 for the lakes w hose representative points lie near Surface Maximum depth the bicarbonate corner; furt herinform ation, how - ever, can be obtained from the data of Table 2. LakeFurnas 2. 3 0.78 The contribution of C 02 to natural solutions LakeSeteCidades 2.7 l.4 LakeBolsena 0 .2 1 0 .078 gives rise to a series ofreactions,producing H C0 3 0.14 O. 083 LakeVico or C 03 depending on pH conditions; these latter LakeBracciano 0.02 8 0.2 8 species represent the stable form s, unless a con- LakeM artignano 0.27 0.23 LakeAlbano 0,12 0.85 tinuousinput ofC 02takes place. A ccordingly,the LakeNemi 0 ,048 0 .16 values ofthe ratio C021(C 03 + H C 03) can allow O .Il 0. 10 Lake Averno an estim ation ofthe persisting im portance of C 02 LakeNyos 4.2 13.6a contributions. The discrepancies w ith respect to Lake Monoun 0.31 1.8a the theoretical balance betw een pH and distribu- aFrom Kusakabe etal. (]989).

H C O.3 / 5

e~Y

O Al e Ne Bo e ,3' w

e Ma ev eFu

d' e F:o S Av e C l s 0 4

Fig. 3. Diagram H C03-Cl-S04for the samples from the surface of crater lakes. M o (M onoun), Ny (Nyos), W (Wum), SC (Sete Cidades), Fo (Fogo), Fu (Furnas), Bo (). V (Vico), Ne (), Br (Bracciano), M a (M artignano),Al(Albano),Av (Averno).

178 M. M artinietal.

M onoun, Furn as, Sete Cidades, and Albano; at The ratios am ong the above m entioned com po-

Furn as and Sete Cidades the m ain contribution nents, accordingly, can be related to the prevail- appears to occur at or near the surface, w hile at ing processes operating in the areas, possibly as- N yos, M onoun, and Albano, significantincreases sociated to differentextents oftherm al anom alies. are observed at m axim um depths. A ccording to this interp retation, the data re-

The distributions ofN H4, B, and Liin natural ported in Table 3 and Fig. 4 c an suggest a s eep- w aters pertaining to volcanic system s appeared age of gases throu gh substantially closed system s usefulin investigating the contributions of differ- f or C am eroon and the A zores, w hile therm al ent processes (M artini etal., 1984). A m m onia gas, anom alies ofgreater extent appeart o characterize giving rise to N H4 w hen dissolved in aqueous so- m ost ofthe Italian lakes. The significant differences in am m onium and lutions,can m igrate atrelatively low tem peratures; the migration of boron in natural environm ents is dissolved carbon dioxide obser ved f or sam pl es m ainly associated to vapourphases,w hich require collected at the surface or at m axim um depth, a significant extent of heat contribution; not neg- how ever, seem especially to deserve further con- ligible concentrations oflithium in natural solu- sideration. N atu ral contributions of am m onium to tions alw ays derive from prolonged leaching of the environm ent derive largely from degradation country rocks, enhanced by higher tem peratures. oforganic m aterial,som etim esin association with

Table 3. Distribution of NH 4, Li and B in the samplesfrom crater lakes

Sample Depth(m) NH~ B Li LakeFogo O O.32 0.010 O.002 LakeFurnas O 0.31 0.010 0.003 -lO 0.25 O.OIO 0.003 LakeSeteCidades O 0.29 0.010 0.001 -27 0.38 0.010 0.002 LakeBolsena O 0.17 O.44 0.020 -140 0.10 0.38 O.018 LakeVico O O.18 0.22 0.020 ~}O O.31 0.22 0.020 LakeBracciano O 0.063 0.33 0.01l -162 0.036 O.32 0.014 LakeM artignano O O.14 0.001 0.001 -50 0.16 0.001 0.001 LakeAlbano O 0.014 0.55 0.001 -160 0.31 0,22 0.00l LakeNemi O 0.27 0.29 0.001 -28 0.56 O.29 0.001 Lake Averno O 0.090 1.2 O.38 -39 16 1.2 0.37 Lake Wum O 0.85 O.080 -lOO O.76 n.d. n.d.b LakeNyos O 2.5 O.044 -200 7.9 n,d, n.d.b Lake Monoun O 0.1 n,d. n,d,c -90 12 n.d. n.d, c aFrom Barberietal. (1989). bFrom Kusakabeetal.(1989).

'From Sigurdsson eta/.(1987).

Craterlakesin Azores and Italy l79

N H 4 Mo NY ~~ w eFO eFu

Ne

~ Bo e Av A e eBr L i *10 B Fig. 4. Diagram NH4~ i~~ for the samplesfrom the surface of crater lakes. Ny (Nyos), W (W um), SC (Sete Cidades), Fo (Fogo), Fu (Furnas), Bo (Bolsena), V (Vico), Br (Bracciano), M a (M artignano), Al (Albano), Ne (Nemi), Av (Averno).

depth allow s to rule out im portant therm al im portant therm al anom a lies; Ieaka ge of am m o- a nom alie s n ea r the lake b ottom s , as observed at nium is in factan effective indicator of geotherm al L a kes N yos and M onoun (K usa kabe and Sano, processes. W ith reference to lake bottom s, the 1992) . A sim ilar hom ogeneity can be derived also degradation can occur inside the lake or fluids fo r pH val ues and dissolved C 02, with the ex- derived from sim ilar processes can be fed from c eption of . outside. C arbon dioxide can also be considered as the In addition to chem ical evidences, the m or- product of decay of organic m atter, but signifi- phology of crater lakes in areas of recent and cant increases in its concentrations w ith depth, quiescent volcanism can provideinform ation about without corresponding increases in am m onium, residual phenom ena associated to past activities. appear better justified by contributions of fluids A detailed investigation by m eans of Video Scan from an extern al source. At A verno, w here only Sounder revealed the shapes ofthe ancient craters am m onium anom alies are detected in the deep w here the Italian lakes are hosted (Figs. 8-10). sam ple, a degradation of organic m aterial occur- B esides m inor particulars, Lakes B olsena, Vico, ring inside the lake appears as a sufficient expla- Bracciano, M artignano, N emi, and A vern o,appear nation; at M onoun, N yos, and Albano, som e ex- as characterized by substantially flat bottom s, tent of fluid contribution from outside seem s to possibly produced by continuous sedim entary re- fitthe chem ical com position better. filling, w hile a funnel shape pertains to Lake Figures 5, 6, and 7 show vertical profiles of Albano. A quiet sedim entation can be thus re- tem perature, pH, and dissolved C 02 for the Ital- garded as a norm al stage for m ost of the lakes ian lakes. The B and Li concentrations in these considered; at Albano, on the contrary, a "clean- lakes appear as related to therm al anom alies, but ing" by circulating fl uids can be inferred. the constant patterns for tem perature values at

180 M. M artinietal.

o 5 10 15 ~0 oc o

Nemi so Vic Averno Martignono hoc,) ~o ~: ~ VERTICAL PROFILE OF Q100 T E M P E R A T U R E

BoIsena leo Albu Bracciano

Fig. 5. Bathym etricprofile oftemperaturesforItalian lakes.

5 5,5 6 6,5 7 75 8 8,5 9 o

eml 50 Averno Vico Martignano 1~) 8Q~ ~ 100

PROVFEIRLTEICOAFl P H

Isena ISO Albano Bracciano

Fig. 6. Bathym etricprofile ofpH-values of ltalian lakes.

Craterlakesin A zores and Italy 181

o 50 100 150 2 0 250 o ppm

Nemi Vic Averno 50 tignano VERTICAl* PRaFll'E oF' C 0 2

~O ~; ,H1, a~il IO�

150 Bolscna Bracciano AIbano

Fig. 7. Bathymetric profiles of dissolved C02forItalian lakes.

BOLSENA LAFLE btAxl~wMOEPTH: t56N, Exaggerated vertical scaIes o -as -50 / o -75 -Ioo _ -125 ~ / -co . d 3KN ~ . '! ~ ~ ~ - ~ BRACCIANO LAKE MAXIMUN DEPTH 165Pt,

-as O N -50 . -?5 - 25 ' / '~ -10,~ ,. .

Fig. 8. Reconstruction of the shapes of Bolsena and Bracciano lakes,

182 M. M artinietal.

MARTIGNANO LAKE MAXIMUM DEPTH 54M.

e N

o -10 - 20 NEMI LAKE M/AXIMUM DEPTH 3aM ¥ -so ExaggerQted vertical scales d･ l -J- 'I' 1" ~~~40l~p JI- d- d- 'P d - .,r / J-eo- AVERb90 LAK E MAXiMtiM DEPTH 39 M.

o

Fig. 9. Reconstruction ofthe shapes of M artignano, Nemi,and Averno lakes,

ALBA NO LAKE MAXIMUM DEPTH ~77M

o

-25 / -50 e N Exaggerated vertical scales -75 '~,¥ / /, 1~- o / l~~10･- ~. / -10 -1 2 / , -20 -30 -l / -40 -175

O IKM o g, VIC-O 1 0 IJ~KE MAXiMUMD~PTH 45 M Fig. 10. Reconstruction ofthe shap-e 2s0 ofAlbano and Vico lakes. 1't.. sooM. 3 0 'I:~:_ _ :~･ .:.,~' - ' b :~ , b - -

Craterlakesin A zores andItaly 183

C ONCLUSIONS ing sam plesin the A zores. This w ork was fm ancially supported by Gruppo N azionale di Vulcanologia and A ccording to the data collected during thisin- C.N.R. Centro di Studio per la M inerogenesi e la vestigation,in com parison with observations car- Geochimica A pplicata. ried o ut at C am eroonian craterlakes,t he variatio ns

with depth of tem perature, pH, and chem ical R EFERENCES com p osi tion prov ide usefulin form ation aboutthe

persistence of anom alies possibly connected to past Barberi,F., Chelini, W ., M arinelli, G.and M artini, M.

activities ofthe volcanic system s. A m ong chem i- (1989) The gas cloud of Lake N yos (Cam eroon,

cal com ponents, carbon dioxide and am m on ium 1986): results of the Italian technical mission. J. Volca nol. G eoth.Res. 39, 125-134 . seem to reflectth e contribution ofendogene fluids Bertagnini,A.and Sbrana,A.(1986)11vulcano diVieo; to a greater extent. stratigrafia del corn plesso vulcanico e sequenze Fr om these data, besides Lakes M onoun and eru ttive d e lle form az ioni piroclas tiche. M e m. Soc.

N yos for w hich convincing evidence is available, Geol. It.35,699 -713. Bo oth, B., Cro asdale, R. and W alker, G. P. L.(1983) som e kind ofthe above m entioned anom alies can V olcanic H azard on San M iguel ,Az or es.Fore ca sting be infered for Lake Albano. The reconstru ction Volcanic Events(Taz ieff,H.and Sabroux,J.C.,eds.), of the shapes, caried out for the Italian lakes, Elsevier,A m sterdam . appears again to differentiate A lbano with respect C asade vall, T. J., De la C ru z-Reyna, S., Rose, W .I., to the other system s. These differences cannot al- Bagley, S .,Finnegan, D.L.and Z ol ler,W . H. (1984)

low a sufficient evidence of residual phe nom e n a Cra ter lake and post-eruption hydr otherm alactivity, ElChic h6n vo lcano,M exi co.J. Volc anol . Geo th.R es. associated to past volcanic activity; inform ation 23,169-191. from historical records, how ever, seem s to pro- Casertano,L., Borgia, A., Cigolini, C., M orale, L. D., vide further hints about. M ontero, W ., Gomez,M .and Fernandez,J.F.(1985) Titus Livius, w hose "A b U rbe C ondita Libri Invest iga ciones geo fis icas y cara cte risti ca s

CX LII" represent the m ost im portant source of geoquimicas delasa guas hidr otermales:V olcan Poas,

Rom an history during the first eight centuries,re- Costa Rica. Geofi s.Intern. 24,315-322. D e Ri ta , D .,Funiciello ,R.and R osa,C.(1991) V olca- ports the following: "M any w onders happened ... nic activity and drainage netw ork evolution ofthe one in particular caught p eople's atten t ion; the area (Rom e,Italy). Acta Vulcanologica rising level ofthe w aters ofL ake A lbano, not be- 2, 18 5 l98. cause ofrainfall or of any other natural reason ... Giggenbach, W . F. (1974) The chemistry of Crater the w ater flooded out ofth e lake...". These events Lake, M t. Ruapehu (N ew Zealand) during and after occur ed in the year 364 of R om an calendar, cor- the 1971 a ctive period.N. Z. J. Sci. 17, 33~ 5. Gillot, P. Y., N appi, G., Santi, P. a nd Renzulli, A. responding to 390 b.C.,cannot be interp reted in a (199l) Space-tim e evolution ofthe VulsiniV olcanic sim ple w ay; som einstability, how ever, appearsto Com plexes, CentralItaly. EU G VI, Strasbourg,Earth have chara cterized Lake Alb ano at that ti m e and A bs.,3- 1,445. an emission of fluids at depth m ay have been re- Kusakabe,M .,Ohsumi,T.and Aram aki, S.(1989) The sponsible. lake N yos disaster: chem ical and isotopic evidence in w ater s a nd disso lve d gases f ro m th ree O n the basis oftheinform ation aboutphysico- Carn eroonian craterlakes,N yos, M onoun,and W um. chem icalcharacters ofcraterlakes,it appearsthus J. Volcanol. Geoth. Res. 39, 167-185. possible to distinguish areas for w hich residual Kusaka be,M . a nd Sano,Y.(1992) The origin ofgases influences of the volcanic system s can be hy- in Lake Nyos, Ca m e roon. N atural Haz ards in W e st pothesized; som e even m inor degree of hazard and CentralAfrica (Freeth,S.J., Ofoegbu,C.O.and because of potentialinstabilities can be estim ated O nuoha, K. M ., eds.),Vieweg, W iesbaden. Locard i, E. (19 65) Tipi di ignim b riti di m agmi accordingly. m ed i te rranei. Lei gnim br iti del v ul cano di V ico. Atti

So c. Tosc. Sci , N at. 72,5 5-173. Acknowledgm ents- W e thank Prof. V. H. Forjaz of M achado, F., Parsons, W . H., Richards, A. F. and Universidade dos A zores forthe assistance in collect- M u lford,J. W . (1 962) Capeli nhos e ru ption of Fay al

184 M. M artinietal.

Volcano, A zores, 1957-58. J. Geophys. Res. 67, Rosi, M ., Sbrana, A. and Principe, C. (1983) The 3519-3529. Phlegrean Fields:stru ctural evolution, volcanic his- M artini, M., Cellini Legitim o, P., Piccardi, G. and tory and m echanism. J. Volcanol. Geoth. Res. 17, Giannini, L.(1984) Low tem perature m anifestations 273~288. in volcanic areas.Rend. Soc.It. M in.Petr.39 401- Sigurdsson, H.,D evine,J. D.,Tchoua, F. M ., Presser, 405. T. S., Pringle, M . K. W . and Evans, W . C. (1987) Nappi, G., Renzulli, A. and Santi,P. (1991) Evidence Origin ofthe lethal gas burst from lake M onoun, of increm ental growth in the Vulsinean calderes Cam eroon.J. Volcanol. Geoth.Res. 31, 1-16. (CentralItaly).J. Volcanol. Geotherm. Res. 47, 13- Sigvaldason, G.(1989) Thelak e Nyoseventand natural 31. C02degassing.J. Volcanol. Geoth.Res.39,109-116. Ossaka, J., O zaw a, T., N om ura, T., Ossaka, T., Turbeville,B.N.(1993) Petrology and petrogenesisof Hirabayashi,J.,Takaesu, A. and H ayashi, T.(1980) the Lateracaldera,CentralItaly.J.Petrology 34,77- Variation ofchemicalcom positionsin volcanic gases 123. and w aters at K usatsu-Shirane volcano. Bull. V arekam p, J. C.(1980) The geology ofthe V ulsinian Volcanol. 43,207-216. area, ,Italy.Bull. Volcanol. 43, 487-503.