cone of Mount Etna, Sicily, I972.
J O H N E. G U E S TSUMMARY
Fumarole temperature increases between March and September x972 suggest a westward migration of magma from below the Bocca Nuova and a possible renewal of activity near the North East Crater.
T E M P E R A T U R E MEASUREMENTS o f fumaroles in the summit crater of Mount Etna were made by Dr. Alison Brown in I969 and I97O (Brown I972 thesis, University of London) from which Guest (i973) concluded that temperature changes in the fumaroles were related to movements of magma high in the volcano prior to the flank eruption that started in April 197 I. If this is so, then fumarole temperatures in the summit region may reflect near surface migrations of magma and might help in predicting new activity (Macdonald I972, p. 416). To test this thesis the author started routine temperature measurements in early i972 and those reported here were made during March and September I972.
Volcanic activity during i972
The geology of the summit cone (Fig. x) and the activity during the two years before the I97i eruption have been described by Guest (I973). The flank eruption that started in April i97i opened at three vents on the southern side of the summit cone, continued by progressive opening of fissures lower down the mountain to the northeast, and by the end of the eruption in June over 7 ° million cubic metres of lava had been erupted (Rittman et al. t 971 ; in Guest & Skelhorn 1973). The erup- tion probably resulted from a surge magma in the central conduit breaking out laterally along fissures to erupt lower down the mountain slopes. During this eruption other activity on the summit cone stopped; but once the flank eruption had ceased magma continued to rise in the central conduit, and in September 197 x the once I ooo m deep Chasm had filled with lava to within i5o m of its lip (Rittman 197 I). Le Guern (i973) noted active vents on the floor of the Chasm late in x97I.
During early i972 many large explosions from the lava filled Chasm were observed. These died down by late March and just after this the author visited the summit region. During the last week of March only about one explosion per day was noted from the Chasm. Bombs oflithic fragments, thrown out by the explosion, were on the snow up to i oo m from the lip of the Chasm and these averaged 5 cm across.
By September 1972 activity was still restricted to the Chasm, but the explosions of lithic debris were no longer observed. The floor of the Chasm was still estimated
312 J. E. Guest
to be about t 50-200 m below the lip. As the crater was filled with dense smoke the
activity on the floor could only be observed during brief intervals when the wind c a u s e d a clearing. T h r e e vents w e r e seen o n t h e floor, e a c h s h o w i n g s t r o n g s t r o m b o l i a n a c t i v i t y t h r o w i n g u p i n c a n d e s c e n t b o m b s as h i g h as a b o u t i oo m f r o m t h e vent. T h e r a t e o f explosion was v a r i a b l e f r o m a l m o s t c o n t i n u o u s explosion from t h e t h r e e vents successively to a b o u t t h r e e explosions p e r m i n u t e . T h e l o u d e s t b a n g s c a m e f r o m t h e v e n t n e a r t h e m i d d l e o f t h e C h a s m a n d m a n y o f these explosions w e r e l a r g e l y o f gas w i t h little i n c a n d e s c e n t m a t e r i a l .
Before t h e I 9 7 I e r u p t i o n it a p p e a r s t h a t t h e v e n t l e a d i n g to t h e C h a s m was blocked (Guest i 9 7 3 ) . D u r i n g t h e l a t e r p a r t o f ~97I e i t h e r this blockage was c l e a r e d b y explosions a l l o w i n g l a v a to well i n t o t h e C h a s m , or a l t e r n a t i v e l y a c o l u m n o f m a g m a was able to p u s h t h e floor o f t h e C h a s m u p as a c y l i n d e r a n d l a v a t h e n b r o k e o u t t h r o u g h fissures on t h e surface o f this raised original floor.
t%..
~ - '
t,,
-.+
.>, \i:ii ;D.
I
f
]
,' / i I : \ . I c i ' - / i I .~ ~, . . i , , - I .-~A_ ,,' ' , 1 ~ . " ~ , / . ' " ] l l S ' ~ , , ' , ' " l . . - J , ~ t " & f :; i -q_~ .~q.. I / ' ' "~ . . . .~ '
""+"~"~ :;~,<"~t.,' , . ' ~ , , . " 1
",..( y.. "-...)t-J~/~ ~ ~ q
Z.-- )./.: ,/
D+,
"+-./ME-
-; '°tl
% \ ::- . .. • . .,.. ., .. ,.- .. I . . . <, ... ~& "~ t~. --.
,'
/ o_L,o;~ .,. ,,,,
/ \ \ "" " . I / v~ni 11971i.. I ~.FIO. I. Geological sketch map of the Summit Cone of Etna. For geological description of the area see Guest ( x 973) and Rittman e t al. ( i 97 x). (Buildings buried by x 971 lavas: (a) Vulcarolo, steam condenser; (b) Volcano Observatory; (c) Funivia Station).
Fxo. 2. The main sites of fumarole fields are pointed to with arrows and numbered as in Table x. The light stippling indicates the area without snow in March x97i. The outer boundary of this area is approximately the 3 °o isotherm at to cm depth as determined by a number of temperature readings across the boundary. The symmetrical distribution of this higher temperature area towards the southeast probably relates to cooling magma in dykes below the 197 x cones.
Nevertheless it is clear that during i972 up to September there was a column of magma standing in the central conduit below the Chasm to within 2oo m of the summit.
Measurements
Systematic measurement of fumarole temperatures has only been carried out on a few active volcanoes. At some volcanoes the temperature of fumarole gases increases before eruptions, while at others there appears to be no warning of im- pending eruptions from changes in fumarole temperatures. Rainfall can also affect the fumarole temperatures as Rose and Stoiber (i969) showed for Izalco volcano in E1 Salvador.
Measurements made on Etna suggest that temperature changes may relate directly to movements of magma within the volcanic pile and no marked correla. tions with ground water conditions have been noted. Thus at certain locations on the summit cone temperatures have changed markedly but at many other places the temperatures have remained constant throughout changing seasons and therefore were not affected by variations in the amount of melt water from snow. Fumaroles on Etna described here consist largely of H 2 0 and air. In the higher temperature fumaroles (above I oo°C) faint whiffs of SOs are sometimes detected. Some of the fumaroles have small incrustations of sublimates surrounding them and the locations of now dormant fumaroles are marked by these incrustations. Compositions of these have been given by Brown (1972 unpublished thesis, University of London). The apparent amount offumarolic activity on the mountain depends on weather conditions for when condensation is increased, the steam is more visible.
TAB L F. X : Maximum temperatures (°C) for fumarole fields marked in Fig. 2
F u m a r o l e sites M a r c h September x nd 84 Central Crater 2 nd 75 3 85 85 4 74 74 - 8 o 5 65 84 6 76 75 7 nd 79 8 nd 78 9 4 ° 5 ° N E Crater IO n d x35 II nd xI5 I2 52 x35
x3 55 57 April I971 vents.
x 4 > 28o 8o
x 5 62 69
x 6 > 28o n d
314 J. E. Guest
The principal fumarole fields (Fig. 2) tend to lie around the edge of the Central Crater and are presumably related to ring fractures bounding the crater edge and thus to the central conduit at depth. The largest fumarole field in the Central Crater
is site 3 where fumaroles rise from small fissures and holes in ash deposits along the inner rim of the crater. The highest temperatures since 197o are in the middle of the field (marked by arrow in Fig. 2.). Temperatures here rose after the collapse of the Bocca Nuova and before the I97I eruption (Guest I973); directly after the eruption 85 ° was recorded by Brown (in Guest t 973) in the middle of the field and during I972 the temperature has remained at 85 °. Site 4 is a new fumarole field that started after the collapse of the Bocca Nuova in early I97 o. Temperatures here and also at site 6 have also generally remained the same at about 74 °. But a marked increase of I9 ° was noted at site 5 which is on the edge of the Central Crater, being part of an old, denuded, pyroclastic cone.
Presumably temperatures in the Central Crater region are directly related to magmatic conditions in the central conduit. The rise in temperature at site 3 before the t971 eruption and during the uprise of lava in the Chasm can be related to these two phenomena. The only marked change in temperature in the Central Crater is that of site 5. The collapse of the Bocca Nuova took place largely where fumarole temperatures had been rising and the marked increase in temperature at site 5 may indicate corrosion of the rocks below this site by gases associated with the Bocca Nuova, and may suggest that future collapse of this crater could occur in an easterly direction to include the area of site 5. The Bocca Nuova has been increasing its size slowly by collapse since it was first formed as a major collapse crater in 197o: continued collapse on the western side has been sufficient to carry the lip of the crater to within a few metres of the edge of the Chasm, and it seems likely that in a short time the Chasm and the Bocca Nuova will coalesce. This continuing evidence of collapse suggests that future collapses may occur, possibly towards site 5. However, the original collapse seems to be related to a SE-NW fissure formed in i956 (Guest I973). The present gas emissions came from two vents on the line of this fissure within the Bocca (marked in Fig. I). Temperatures here (site 6) have not risen.
Fumarole temperatures in the z97z craters on the southern flank of Etna have remained fairly similar, although an increase of 7 ° was noted at one fumarole on the fissure of the Vulcarolo vent. However systematic measurements along the fissure show that temperatures have remained the same or declined within the cone. During March there were high ( > 2 o o °) temperature (SO~) fumaroles (eg. sites 14 and t6) on the edge of these craters. These were in lava and spatter surrounding the upper part of the cones. By September the temperature of site 14 dropped to 8o °, and SO~ was no longer detectable. This area of vents is now slowly cooling.
One of the last pre-i97i eruptions of lava from the North East Crater was a massive flow from vents in the col between the Chasm and the North East Crater (Le Guern I973). The thicker parts of the flow (sites io and I I) were as high as 135 ° at i o cm depth, but were less than 2oo ° at several metres below the surface (paper thrown into these fissures did not ignite).
occurred at the lava vent of site 12. This vent, which was the last to e r u p t on the N o r t h East C r a t e r before April 197 i, consists of a steep conical m o u n d of lava t h a t caps the g r e a t fan o f l a v a s t h a t extend northeastwards from the N o r t h East Crater. E x t e n d i n g from this lava vent to the southwest are fissures which in M a r c h had relatively low t e m p e r a t u r e s , ( m a x i m u m a b o u t 52°) excluding one now d o r m a n t area t h a t gave 74 % I n S e p t e m b e r the t e m p e r a t u r e in the fissures h a d risen to a m a x i m u m of 135 %
T h e N o r t h East C r a t e r has almost continuously effused lava since I 9 i i, only ceasing w h e n outbursts of lava occurred elsewhere on the m o u n t a i n . D u r i n g the I964 eruption the N o r t h East C r a t e r was d o r m a n t , b e c o m i n g active again in J a n u a r y 1966, eighteen m o n t h s after the 1964 eruption. T h e m a r k e d increase in t e m p e r a t u r e 15 m o n t h s after the i971 eruption suggests t h a t m a g m a is once again rising in fissures below the N o r t h East C r a t e r a n d t h a t activity here could re-occur in the n e a r future.
ACKNOWLEDGEMENTS. This work is part of an Italian and British collaborative programme co- ordinated by the Isistuto Internazionale di Vulcanologia (Catania) and the Royal Society of London, to study Mount Etna. The field work was financed by the Royal Society. The author is particularly grateful to Mr. J. H. Foster and Mr. A. Baker for assistance in the field; and also to Drs. R. Romano and C. Sturiale with whom the author is collaborating in geological mapping on Mount Etna.
References
GUEST, J. E. 1973. The Summit of Mount Etna prior to the x97I eruption, Phil. Trans. R. Sot., London. (in Press) 274.
& SK~J.HORN, R. R. (Eds.) x973. Mount Etna and the x97x Eruption. Phil. Trans. R. Sot., London. (in Press) 274.
LE GUERN, F. x973. Collection and analysis of gases. Phil. Trans. R. Soc. London. (in Press) 274. MACDONArI), G. A. I972. Volcanoes, Prentice-Hall, New Jersey.
RITTMAN, A. 197I. Mount Etna Volcanic activity. Smithsonian Center for short-lived Phenomena.
Event 33-7 I, card 1294.
ROMANO, R. & STURIALE, C. x97I. L'eruzione etnea dell'april-guigno x97 x, Atti della Accadamia Givenia di Scienze Naturoli in Catania, III, 29p.
RosE, W. I. & STOmER, R. E. I969. The I966 Eruption of Izalco volcano, E1 Salvador. Journ. Geophys. Res. 74, 3I x9--3x3 o.
Received I8 November x972.
JOHN EDWARD GUEST, University of London Observatory, Mill Hill Park. London NW7 2QS.
