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Thermal anomalies in lanzarote (Canary Islands)
Geothermics
(i973) - VoL. 2, N o . 2
Thermal Anomalies in Lanzarote (Canary Islands) V. ARA~A *, R. ORTIZ ** AND J. YUGUERO***
ABSTRACT About a hundred thermometers have been installed 3 m deep in an area of about 20 km ~ in the ~ Montafias de Fuego~ (island of Lanzarote). The temperatures measured oscillate between 16oC and 350"C. In other wells 150 to 250 m deep a gradient of 0.2°C/m has been measured. This gradient is always found in a region of about 200 km '2 around the zones of the highest superficial anomalies. The local surface anomalies are related to the outflow of hot fluids through tectovolcanic fractures. At great depths these fluids probably form a convective system under impervious layers (hyaloclastites?). The heat transfer takes place by means of convection in a deep reservoir and in the superficial levels, but an intermediate zone with impervious layers must exist where heat is transmitted by conduction.
Introduction
M o s t of t h e s e m a t e r i a l s a r e d u e to t h e h i s t o r i c e r u p t i o n s a n d t h e i r t h i c k n e s s r a n g e s b e t w e e n Io a n d 6o m . U n d e r t h e s e m a t e r i a l s o t h e r b a s a l t i c r o c k s o u t c r o p locally b e s i d e o l d c o n e s of p y r o c l a s t s w h i c h h y d r o t h e r m a l p r o c e s s e s h a v e w e l d e d . I t is i n t e r e s t i n g to n o t e t h a t t h e t h e r m a l a n o m a l i e s a r e p r i n c i p a l l y l o c a t e d in t h e s e o l d c o n e s or a t p o i n t s w h e r e t h e 173o-36 m a t e r i a l s a r e n o t very thick.
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T h e i m p o r t a n c e of h e a t in g e o l o g i c a l p r o c e s s e s a n d t h e r e c e n t d e v e l o p m e n t a n d u s e of g e o t h e r m a l r e s o u r c e s h a v e i n s t i g a t e d t h e r e s e a r c h of t h e r m a l l y a n o m a l o u s reg i o n s . O n e of t h e m o s t s p e c t a c u l a r t h e r m a l a n o m a l i e s is located on the island of Lanzarote (Canary Archipelago) w h e r e t e m p e r a t u r e s of m o " C a r e r e a c h e d o n t h e s u r f a c e a n d a b o v e 3oooC a t d e p t h s of a few- m e t e r s . I n t h i s c a s e the sources of heat have a clear magmatic origin, but the p r o b l e m is t h a t of u n d e r s t a n d i n g t h e m e c h a n i s m s of h e a t transIer. The anomalies are dispersed over an elongated area of a b o u t 2o k m 2 c a l l e d , M o n t a f i a s d e F u e g o , (Fire M o u n t a i n s ) . T h i s r e g i o n is l o c a t e d a few k i l o m e t e r s f r o m t h e w e s t e r n c o a s t of L a n z a r o t e a n d its h e i g h t s r a n g e bet w e e n 25o a n d 5oo m ( F i g u r e I). T h e g e o t h e r m a l p h e n o m e n o n d r e w t h e a t t e n t i o n of t h e first c o l o n i z e r s of t h e i s l a n d in t h e i 4 t h c e n t u r y . L a t e r t h e r e w a s a n o t a b l e v o l c a n i c e r u p t i o n in t h i s r e g i o n f r o m t h e y e a r 173o till i 7 3 6 a n d a n o t h e r of lesser i m p o r t a n c e in 1824. T h i s v o l c a n i s m h a d as its a x i s a c l e a r t e c t o v o l c a n i c line w i t h w h i c h t h e a c t u a l t h e r m a l a n o m alies a r e a s s o c i a t e d . D u r i n g t h e l a s t e r u p t i o n a g r e a t v o l u m e of s t e a m and boiling water was emitted, but at the present time t h e r e is no f u m a r o l i c a c t i v i t y n o r t h e r m a l s p r i n g s o n t h e i s l a n d , w h o s e r a i n f a l l is v e r y low ( a v e r a g i n g less t h a n I5o l / m 2 p e r y e a r ) . I n t h e a r e a s t u d i e d t h e r e a r e no soils n o r a n y t y p e of i m p e r v i o u s c o v e r . T h e s u p e r f i c i a l m a t e r i a l s a r e b a s a l tic flows of , a a , a n d . p a h o e h o e , t y p e s , e x t r e m e l y p e r v i o u s , w h i c h a l t e r n a t e w i t h m o r e o r less w e l d e d p y r o elastic deposits.
Generally the anomalous zones present a hydrotherm a l a l t e r a t i o n a n d s a l t d e p o s i t s . Also d u r i n g t h e f l u i d sampling the pipes used are rapidly obstructed by the saline deposition. For this reason small self-sealing pockets of h o t f l u i d s a r e f r e q u e n t l y f o u n d a t a v e r y s h a l l o w depth.
* Department of Petrology and Geochemistry, University of Madrid, Spain. ** Department of Physics of the Cosmos, University of Madrid, Spain. *** Department of Thermology, University of Madrid, Spain.
Many scientists visiting Lanzarote have dealt with this phenomenon expressing various opinions. On several o c c a s i o n s s u r v e y s h a v e b e e n c a r r i e d o u t to e x a m i n e t h e p o s s i b i l i t y of e x p l o i t i n g t h i s e n e r g y . A m a p of superficial i s o t h e r m s h a s also b e e n p r e p a r e d (GONZALEZ GARCIA A . , pers. c o m m . i972).
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73
To t h e a u t h o r s k n o w l e d g e t h e o n l y specific p a p e r on L a n z a r o t e is t h a t of CALAMAI a n d CERON (I970) w h i c h wilt be discussed below.
Geothermal prospecting T h e d i s t r i b u t i o n a n d d e p t h of t h e t h e r m o m e t e r s were decided a f t e r a v a s t superficial s u r v e y w h i c h s h o w e d t h e r e l a t i o n s h i p b e t w e e n a n o m a l i e s a n d o u t c r o p s of specific m a t e r i a l s . T h e i n s t a l l a t i o n of t h e r m o m e t e r s 3 m deep is n o t effective in t h e l a v a flows since t h e r e is a free air c i r c u l a t i o n to a g r e a t e r d e p t h . On t h e cont r a r y in t h e welded p y r o c l a s t s t h e possibility of cooling b y air c i r c u l a t i o n d i m i n i s h e s or disappears, a n d t h e t h e r m o m e t e r s located 3 m deep are n o t affected b y t h e daily a n d seasonal t e m p e r a t u r e v a r i a t i o n s . H o w e v e r a f t e r a h e a v y r a i n f a l l t h e t e m p e r a t u r e s increase, as will be seen below. T h e r m i s t o r s were used, a l t h o u g h a t some p o i n t s t e m p e r a t u r e checks were carried o u t w i t h p l a t i n u m t h e r m o m eters. I n t h e a r e a of t h e h i g h e s t a n o m a l y a c o n t i n u o u s t e m p e r a t u r e recorder was installed. I n t h e s a m e area as well as in p e r i p h e r a l zones, wells i5o-2oo m deep were used to m e a s u r e t h e g r a d i e n t . I n this case a series of 5 p l a t i n u m t h e r m o m e t e r s was employed. N o n e of t h e s e wells coincided w i t h t h e area of superficial a n o m a l i e s . A decrease in t e m p e r a t u r e in t h e first 2o m could b e seen. A t g r e a t e r d e p t h s a g r a d i e n t of o.2oC/m was o b s e r v e d in a n area of more t h a n ioo kin2. A n a i r b o r n e i n f r a r e d s u r v e y (WILLIAMS I97I ) detected s u b m a r i n e c u r r e n t s of w a r m w a t e r in t h e coastal areas n e a r t h e ~ Montafias de F u e g o )~.
E v a l u a t i o n of d a t a A n i r r e g u l a r d i s t r i b u t i o n of t h e a n o m a l i e s was ded u c e d from t h e a v a i l a b l e d a t a , w i t h v a r i a t i o n s of o v e r ioooC b e t w e e n p o i n t s v e r y n e a r to one a n o t h e r . T h i s m a y be e x p l a i n e d b y t h e v e r y low t h e r m a l c o n d u c t i v i t y of t h e m a t e r i a l s a n d b e c a u s e t h e h o t m a n i f e s t a t i o n s are rel a t e d to c o n v e c t i v e processes associated w i t h m o r e or less p r o n o u n c e d fissures. T h e fluids t r a n s p o r t i n g h e a t , flow u p u n d e r v e r y little pressure due to t h e f a c t t h a t t h e y c i r c u l a t e v e r y slowly t h r o u g h t h e cap rocks w h e r e t h e y m i x w i t h t h e air. T h e p a r t i a l a n a l y s e s of t h e d i s c h a r g e d fluids along w i t h t h o s e of t h e salts d e p o s i t e d b y t h e same, p e r m i t t h e s u p p o s i t i o n t h a t such fluids o r i g i n a t e in t h e e v a p o r a t i o n of sea water, a l t h o u g h t h e i r c o n t a m i n a t i o n b y m a g m a t i c gases c a n n o t b e r e j e c t e d (ARA~A ET AL. I973). G e n e r a l l y t h e t h e r m a l a n o m a l i e s were f o u n d in h i g h areas w h e r e a s in valleys a n d c r a t e r b o t t o m s t e m p e r a t u r e s are n o r m a l . T h i s is due to t h e colluvial a c c u m u l a t i o n w h i c h o b s t r u c t s t h e fissures in t h e low-lying areas. O n l y a t t h e b o t t o m of one c r a t e r t h e t e m p e r a t u r e was o v e r i5ooc b u t this was d u e to t h e presence of a n i m p o r t a n t fracture. In r e c e n t years a d i s p l a c e m e n t of some a n o m a l i e s h a s b e e n o b s e r v e d , p r o v o k e d b y t h e closing of fissures b y self-sealing processes. Therefore t h e h o t fluids seek o t h e r o u t l e t s a n d t h e old o u t l e t s cool r a p i d l y . S i m i l a r l y a f t e r h e a v y rainfall new areas reveal h i g h t e m p e r a t u r e s , as s h o w n b y t h e r m o m e t e r s installed a t a d e p t h of 3 m. This could b e due to a n increase in t h e h e a t flow caused 74
b y c o n v e c t i o n , as t h e q u a n t i t y of fluid which o p e r a t e s t h e h e a t t r a n s f e r is increased b y rainfall, a n d to t h e f a c t t h a t s t e a m circulates t h r o u g h d u c t s which in d r y periods are n o t i n v o l v e d in h e a t transfer. W i t h regard to t h e g r a d i e n t of o . 2 " C / m which is m a i n t a i n e d in a g r e a t p a r t of t h e island, t h e a u t h o r s believe t h a t it is w o r t h y of b e i n g s t u d i e d s i m u l t a n e o u s l y w i t h the s t r o n g superficial a n o m a l i e s , a l t h o u g h such a g r a d i e n t has o n l y a relative v a l u e since it has b e e n measured in wells t h a t o n l y reach sea level, where t h e watert a b l e is a t 2T'C. This l i m i t a t i o n obliges t h e discussion of different h y p o t h e s e s t h a t m a y b e c a p a b l e of e x p l a i n i n g t h e h e a t i n g of t h e w a t e r table. In t h e h y p o t h e s i s t h a t will b e discussed now i t m u s t be n o t e d t h a t i m p e r v i o u s layers (hyaloclastites?) exist a t some 2oo m below sea level. T h e y h a v e b e e n d e t e c t e d b y a g e o p h y s i c a l s u r v e y (ARA~A, FUSTER 1973)According to t h e s i m p l e s t h y p o t h e s i s t h e sea w a t e r circulates to g r e a t d e p t h where it is h e a t e d a n d l a t e r rises b y c o n v e c t i o n . T h u s t h e source of h e a t m a y be loc a t e d m a n y kilometers deep a n d it would be possible to find t e m p e r a t u r e s less t h a n 3ooC u n d e r t h e w a t e r table. This s i t u a t i o n is f r e q u e n t l y f o u n d in coastal regions u n d e r w h i c h a r e c e n t a n d deep p l u t o n i c i n t r u s i o n exists. How ever a s u b v o l c a n i c i n t r u s i v e b o d y a t g r e a t d e p t h does n o t easily explain t h e s t r o n g superficial a n o m a l i e s . A second h y p o t h e s i s (Figure 2a) considers t h e presence o£ a shallow m a g m a t i c c h a m b e r , as occurs in Hawaii (EA'rON, MURATA i96o). T h i s fact could e x p l a i n t h e superficial h o t m a n i f e s t a t i o n s , b u t in order to m a i n t a i n t h e a c t u a l h e a t flow for zoo years t h e i n t r u s i v e b o d y m cont a c t w i t h t h e w a t e r m u s t h a v e a v o l u m e e q u i v a l e n t to a c y l i n d e r w i t h radius of one k i l o m e t e r a n d h e i g h t of half a k i l o m e t e r (RuBIA E'l" AL. I970). T h i s model is theoretically possible, b u t an i n t r u s i o n of such m a g n i t u d e has
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not been d e t e c t e d a t shallow levels b y g r a v i m e t r i c prosp e c t i n g (MAcFARLANE, RIDLEY I969). A t h i r d h y p o t h e s i s (Figure 2b) foresees i m p e r v i o u s layers s e p a r a t i n g the w a t e r table from a d e e p e r aquifer fed b y sea water. A m a g m a t i c c h a m b e r u n d e r the aquifer m a y h e a t it, giving place to f o r m a t i o n of s t e a m which circulates c o n v e c t i v e l y in a v e r y f r a c t u r e d zone. A small p a r t of t h e c o n v e c t i v e fluids, possibly m i x e d with magmatic gases, escapes directly to t h e surface t h r o u g h the main fissures t h a t cross t h e i m p e r v i o u s cap. CALAMAI and CERON (I97 o) q u o t e a t e m p e r a t u r e of over 7oooC a t 27 m deep, p r o b a b l y because t h e y h a v e a c c e p t e d an old unreliable m e a s u r e m e n t . T h e y explain t h a t the effluents in t h e area of h o t m a n i f e s t a t i o n s consist exclusively of air. So air p e n e t r a t e s into a n d circulates inside t h e (, M o n t a f i a , until it reaches a buried spine of m o l t e n lava. Here, air is s u p e r h e a t e d to a t e m p e r a t u r e of several h u n d r e d degrees and rises to t h e surface, b y d r a u g h t , t h r o u g h preferential fractures. This t h e o r y is not c o n s i s t e n t with t h e actual d a t a a b o u t t h e chemical c o m p o s i t i o n of t h e effluents a n d also, as has been seen, a buried spine near the surface c a n n o t maintain a m o l t e n s t a t e for 20o years. Also in a b r o a d hole i2 m deep one can see t h a t c o m b u s t i o n of wood is almost i n s t a n t a n e o u s near t h e surface, b u t v e r y difficult a t hotter and deeper levels, because air does not circulate a t these d e p t h s . The possibility has also been considered of t h e h e a t h a v i n g an exogenous origin, b u t t h e theoretically possible models, such as t h e reaction CaO + H.,O m Ca(OH)z + + 15,54o cal, would d e m a n d the previous f o r m a t i o n of too g r e a t a volume (to 6 m3/year) of CaO from carbonares, which does not seem possible in this area.
Conclusions A l t h o u g h t h e r m o m e t r i c d a t a do not t h e m s e l v e s perm i t the d e t e r m i n a t i o n of a model of t h e g e o t h e r m a l syst e m causing the anomalies of L a n z a r o t e , it is i n t e r e s t i n g to note the t h i r d h y p o t h e s i s which supposes t h e existence of a reservoir with c o n v e c t i v e circulation of fluids.
r h i s t h e o r y is the only effective one with respect to the e x p l o i t a t i o n of g e o t h e r m a l resources a n d could be confirmed w h e n the p l a n n e d e x p l o r a t o r y wells reach the impervious layer. F i n a l l y one of the objectives of this p a p e r is to show how, in specific cases (when the t y p e s of anomalies a n d the geological p a t t e r n p e r m i t it), it is possible to realize an i n e x p e n s i v e t h e r m o m e t r i c s t u d y a t little d e p t h with useful results. Also it is interesting to note the value of investigating the g e o t h e r m a l resources on islands of recent volcanism where the presence of shallow m a g m a t i c c h a m bers can be supposed as well as impervious layers capat~le of storing sea w a t e r a t some d e p t h . Acknowledgements
The data used in this paper were obtained in the program for the evaluation of geothermal resources in the Canary Islands. The authors thank Prof. J. M. F0STEt, Director of the Project, for his help during the elaboration of this work. REFERENCES ARaRA V., FUSTER J. M. 1973 - - Nora previa sobre la prospecci6n geotdrmica en Las Montafias de Fuego (Lanzarote). Etudios Geoldgicos, (in press). ARaRA V., HERNam)Ez-PacHECO A., GARCIALUIS A. 1973 - - Quimismo de los fluidos y dep6sitos hidrotermaies de Las Montafias de Fuego (Lanzarote). Estudios Geol6gicos, (in press). CaLaMAI A., CERON P. 1970 - - Air Convection within ~ Montafia del Fuego >~ (Lanzarote Island, Canarian Archipelago). U.N. Symp. on Develop. and Utiliz. o/ Geoth. Resources, Pisa, Geothermics, sp. issue 2, v. 2, p. 1. FUSTER J. M., FERNANDEZSaNTIN S., SaGREDOJ. 1968 - - Geology and Volcanology of the Canary Islands. Lanzarote. lnstituto Lueas Mallada, C.S.I.C. Madrid. MAcFARLANE D. J., Rn)LEY W. I. 1969 - - An interpretation of gravity data for Lanzarote (Canary Islands). Earth Plan. Sc. Letters, v. 6, 431. RUBIA J., DoRIa J., OSORm A, CaLLEJa J., Ataiqa V. 1970 - - Distribuci6n transitoria de temperaturas en las proximidades de una eapa magm~itica intrusiva en el proceso de enfriamiento. Estudios Geo16gicos, v. 26, 273. WILHAMS R. S. 1971 - - Geologic Analysis of Aerial - thermography of the Canary Islands, Spain. U.S. Geological Survey. memo-report. EATON J. P., MURATt~ K. J. 1960 - - How Volcanoes Grow. Science, v. 132, 3432.
75
(i973) - VoL. 2, N o . 2
Thermal Anomalies in Lanzarote (Canary Islands) V. ARA~A *, R. ORTIZ ** AND J. YUGUERO***
ABSTRACT About a hundred thermometers have been installed 3 m deep in an area of about 20 km ~ in the ~ Montafias de Fuego~ (island of Lanzarote). The temperatures measured oscillate between 16oC and 350"C. In other wells 150 to 250 m deep a gradient of 0.2°C/m has been measured. This gradient is always found in a region of about 200 km '2 around the zones of the highest superficial anomalies. The local surface anomalies are related to the outflow of hot fluids through tectovolcanic fractures. At great depths these fluids probably form a convective system under impervious layers (hyaloclastites?). The heat transfer takes place by means of convection in a deep reservoir and in the superficial levels, but an intermediate zone with impervious layers must exist where heat is transmitted by conduction.
Introduction
M o s t of t h e s e m a t e r i a l s a r e d u e to t h e h i s t o r i c e r u p t i o n s a n d t h e i r t h i c k n e s s r a n g e s b e t w e e n Io a n d 6o m . U n d e r t h e s e m a t e r i a l s o t h e r b a s a l t i c r o c k s o u t c r o p locally b e s i d e o l d c o n e s of p y r o c l a s t s w h i c h h y d r o t h e r m a l p r o c e s s e s h a v e w e l d e d . I t is i n t e r e s t i n g to n o t e t h a t t h e t h e r m a l a n o m a l i e s a r e p r i n c i p a l l y l o c a t e d in t h e s e o l d c o n e s or a t p o i n t s w h e r e t h e 173o-36 m a t e r i a l s a r e n o t very thick.
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T h e i m p o r t a n c e of h e a t in g e o l o g i c a l p r o c e s s e s a n d t h e r e c e n t d e v e l o p m e n t a n d u s e of g e o t h e r m a l r e s o u r c e s h a v e i n s t i g a t e d t h e r e s e a r c h of t h e r m a l l y a n o m a l o u s reg i o n s . O n e of t h e m o s t s p e c t a c u l a r t h e r m a l a n o m a l i e s is located on the island of Lanzarote (Canary Archipelago) w h e r e t e m p e r a t u r e s of m o " C a r e r e a c h e d o n t h e s u r f a c e a n d a b o v e 3oooC a t d e p t h s of a few- m e t e r s . I n t h i s c a s e the sources of heat have a clear magmatic origin, but the p r o b l e m is t h a t of u n d e r s t a n d i n g t h e m e c h a n i s m s of h e a t transIer. The anomalies are dispersed over an elongated area of a b o u t 2o k m 2 c a l l e d , M o n t a f i a s d e F u e g o , (Fire M o u n t a i n s ) . T h i s r e g i o n is l o c a t e d a few k i l o m e t e r s f r o m t h e w e s t e r n c o a s t of L a n z a r o t e a n d its h e i g h t s r a n g e bet w e e n 25o a n d 5oo m ( F i g u r e I). T h e g e o t h e r m a l p h e n o m e n o n d r e w t h e a t t e n t i o n of t h e first c o l o n i z e r s of t h e i s l a n d in t h e i 4 t h c e n t u r y . L a t e r t h e r e w a s a n o t a b l e v o l c a n i c e r u p t i o n in t h i s r e g i o n f r o m t h e y e a r 173o till i 7 3 6 a n d a n o t h e r of lesser i m p o r t a n c e in 1824. T h i s v o l c a n i s m h a d as its a x i s a c l e a r t e c t o v o l c a n i c line w i t h w h i c h t h e a c t u a l t h e r m a l a n o m alies a r e a s s o c i a t e d . D u r i n g t h e l a s t e r u p t i o n a g r e a t v o l u m e of s t e a m and boiling water was emitted, but at the present time t h e r e is no f u m a r o l i c a c t i v i t y n o r t h e r m a l s p r i n g s o n t h e i s l a n d , w h o s e r a i n f a l l is v e r y low ( a v e r a g i n g less t h a n I5o l / m 2 p e r y e a r ) . I n t h e a r e a s t u d i e d t h e r e a r e no soils n o r a n y t y p e of i m p e r v i o u s c o v e r . T h e s u p e r f i c i a l m a t e r i a l s a r e b a s a l tic flows of , a a , a n d . p a h o e h o e , t y p e s , e x t r e m e l y p e r v i o u s , w h i c h a l t e r n a t e w i t h m o r e o r less w e l d e d p y r o elastic deposits.
Generally the anomalous zones present a hydrotherm a l a l t e r a t i o n a n d s a l t d e p o s i t s . Also d u r i n g t h e f l u i d sampling the pipes used are rapidly obstructed by the saline deposition. For this reason small self-sealing pockets of h o t f l u i d s a r e f r e q u e n t l y f o u n d a t a v e r y s h a l l o w depth.
* Department of Petrology and Geochemistry, University of Madrid, Spain. ** Department of Physics of the Cosmos, University of Madrid, Spain. *** Department of Thermology, University of Madrid, Spain.
Many scientists visiting Lanzarote have dealt with this phenomenon expressing various opinions. On several o c c a s i o n s s u r v e y s h a v e b e e n c a r r i e d o u t to e x a m i n e t h e p o s s i b i l i t y of e x p l o i t i n g t h i s e n e r g y . A m a p of superficial i s o t h e r m s h a s also b e e n p r e p a r e d (GONZALEZ GARCIA A . , pers. c o m m . i972).
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73
To t h e a u t h o r s k n o w l e d g e t h e o n l y specific p a p e r on L a n z a r o t e is t h a t of CALAMAI a n d CERON (I970) w h i c h wilt be discussed below.
Geothermal prospecting T h e d i s t r i b u t i o n a n d d e p t h of t h e t h e r m o m e t e r s were decided a f t e r a v a s t superficial s u r v e y w h i c h s h o w e d t h e r e l a t i o n s h i p b e t w e e n a n o m a l i e s a n d o u t c r o p s of specific m a t e r i a l s . T h e i n s t a l l a t i o n of t h e r m o m e t e r s 3 m deep is n o t effective in t h e l a v a flows since t h e r e is a free air c i r c u l a t i o n to a g r e a t e r d e p t h . On t h e cont r a r y in t h e welded p y r o c l a s t s t h e possibility of cooling b y air c i r c u l a t i o n d i m i n i s h e s or disappears, a n d t h e t h e r m o m e t e r s located 3 m deep are n o t affected b y t h e daily a n d seasonal t e m p e r a t u r e v a r i a t i o n s . H o w e v e r a f t e r a h e a v y r a i n f a l l t h e t e m p e r a t u r e s increase, as will be seen below. T h e r m i s t o r s were used, a l t h o u g h a t some p o i n t s t e m p e r a t u r e checks were carried o u t w i t h p l a t i n u m t h e r m o m eters. I n t h e a r e a of t h e h i g h e s t a n o m a l y a c o n t i n u o u s t e m p e r a t u r e recorder was installed. I n t h e s a m e area as well as in p e r i p h e r a l zones, wells i5o-2oo m deep were used to m e a s u r e t h e g r a d i e n t . I n this case a series of 5 p l a t i n u m t h e r m o m e t e r s was employed. N o n e of t h e s e wells coincided w i t h t h e area of superficial a n o m a l i e s . A decrease in t e m p e r a t u r e in t h e first 2o m could b e seen. A t g r e a t e r d e p t h s a g r a d i e n t of o.2oC/m was o b s e r v e d in a n area of more t h a n ioo kin2. A n a i r b o r n e i n f r a r e d s u r v e y (WILLIAMS I97I ) detected s u b m a r i n e c u r r e n t s of w a r m w a t e r in t h e coastal areas n e a r t h e ~ Montafias de F u e g o )~.
E v a l u a t i o n of d a t a A n i r r e g u l a r d i s t r i b u t i o n of t h e a n o m a l i e s was ded u c e d from t h e a v a i l a b l e d a t a , w i t h v a r i a t i o n s of o v e r ioooC b e t w e e n p o i n t s v e r y n e a r to one a n o t h e r . T h i s m a y be e x p l a i n e d b y t h e v e r y low t h e r m a l c o n d u c t i v i t y of t h e m a t e r i a l s a n d b e c a u s e t h e h o t m a n i f e s t a t i o n s are rel a t e d to c o n v e c t i v e processes associated w i t h m o r e or less p r o n o u n c e d fissures. T h e fluids t r a n s p o r t i n g h e a t , flow u p u n d e r v e r y little pressure due to t h e f a c t t h a t t h e y c i r c u l a t e v e r y slowly t h r o u g h t h e cap rocks w h e r e t h e y m i x w i t h t h e air. T h e p a r t i a l a n a l y s e s of t h e d i s c h a r g e d fluids along w i t h t h o s e of t h e salts d e p o s i t e d b y t h e same, p e r m i t t h e s u p p o s i t i o n t h a t such fluids o r i g i n a t e in t h e e v a p o r a t i o n of sea water, a l t h o u g h t h e i r c o n t a m i n a t i o n b y m a g m a t i c gases c a n n o t b e r e j e c t e d (ARA~A ET AL. I973). G e n e r a l l y t h e t h e r m a l a n o m a l i e s were f o u n d in h i g h areas w h e r e a s in valleys a n d c r a t e r b o t t o m s t e m p e r a t u r e s are n o r m a l . T h i s is due to t h e colluvial a c c u m u l a t i o n w h i c h o b s t r u c t s t h e fissures in t h e low-lying areas. O n l y a t t h e b o t t o m of one c r a t e r t h e t e m p e r a t u r e was o v e r i5ooc b u t this was d u e to t h e presence of a n i m p o r t a n t fracture. In r e c e n t years a d i s p l a c e m e n t of some a n o m a l i e s h a s b e e n o b s e r v e d , p r o v o k e d b y t h e closing of fissures b y self-sealing processes. Therefore t h e h o t fluids seek o t h e r o u t l e t s a n d t h e old o u t l e t s cool r a p i d l y . S i m i l a r l y a f t e r h e a v y rainfall new areas reveal h i g h t e m p e r a t u r e s , as s h o w n b y t h e r m o m e t e r s installed a t a d e p t h of 3 m. This could b e due to a n increase in t h e h e a t flow caused 74
b y c o n v e c t i o n , as t h e q u a n t i t y of fluid which o p e r a t e s t h e h e a t t r a n s f e r is increased b y rainfall, a n d to t h e f a c t t h a t s t e a m circulates t h r o u g h d u c t s which in d r y periods are n o t i n v o l v e d in h e a t transfer. W i t h regard to t h e g r a d i e n t of o . 2 " C / m which is m a i n t a i n e d in a g r e a t p a r t of t h e island, t h e a u t h o r s believe t h a t it is w o r t h y of b e i n g s t u d i e d s i m u l t a n e o u s l y w i t h the s t r o n g superficial a n o m a l i e s , a l t h o u g h such a g r a d i e n t has o n l y a relative v a l u e since it has b e e n measured in wells t h a t o n l y reach sea level, where t h e watert a b l e is a t 2T'C. This l i m i t a t i o n obliges t h e discussion of different h y p o t h e s e s t h a t m a y b e c a p a b l e of e x p l a i n i n g t h e h e a t i n g of t h e w a t e r table. In t h e h y p o t h e s i s t h a t will b e discussed now i t m u s t be n o t e d t h a t i m p e r v i o u s layers (hyaloclastites?) exist a t some 2oo m below sea level. T h e y h a v e b e e n d e t e c t e d b y a g e o p h y s i c a l s u r v e y (ARA~A, FUSTER 1973)According to t h e s i m p l e s t h y p o t h e s i s t h e sea w a t e r circulates to g r e a t d e p t h where it is h e a t e d a n d l a t e r rises b y c o n v e c t i o n . T h u s t h e source of h e a t m a y be loc a t e d m a n y kilometers deep a n d it would be possible to find t e m p e r a t u r e s less t h a n 3ooC u n d e r t h e w a t e r table. This s i t u a t i o n is f r e q u e n t l y f o u n d in coastal regions u n d e r w h i c h a r e c e n t a n d deep p l u t o n i c i n t r u s i o n exists. How ever a s u b v o l c a n i c i n t r u s i v e b o d y a t g r e a t d e p t h does n o t easily explain t h e s t r o n g superficial a n o m a l i e s . A second h y p o t h e s i s (Figure 2a) considers t h e presence o£ a shallow m a g m a t i c c h a m b e r , as occurs in Hawaii (EA'rON, MURATA i96o). T h i s fact could e x p l a i n t h e superficial h o t m a n i f e s t a t i o n s , b u t in order to m a i n t a i n t h e a c t u a l h e a t flow for zoo years t h e i n t r u s i v e b o d y m cont a c t w i t h t h e w a t e r m u s t h a v e a v o l u m e e q u i v a l e n t to a c y l i n d e r w i t h radius of one k i l o m e t e r a n d h e i g h t of half a k i l o m e t e r (RuBIA E'l" AL. I970). T h i s model is theoretically possible, b u t an i n t r u s i o n of such m a g n i t u d e has
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not been d e t e c t e d a t shallow levels b y g r a v i m e t r i c prosp e c t i n g (MAcFARLANE, RIDLEY I969). A t h i r d h y p o t h e s i s (Figure 2b) foresees i m p e r v i o u s layers s e p a r a t i n g the w a t e r table from a d e e p e r aquifer fed b y sea water. A m a g m a t i c c h a m b e r u n d e r the aquifer m a y h e a t it, giving place to f o r m a t i o n of s t e a m which circulates c o n v e c t i v e l y in a v e r y f r a c t u r e d zone. A small p a r t of t h e c o n v e c t i v e fluids, possibly m i x e d with magmatic gases, escapes directly to t h e surface t h r o u g h the main fissures t h a t cross t h e i m p e r v i o u s cap. CALAMAI and CERON (I97 o) q u o t e a t e m p e r a t u r e of over 7oooC a t 27 m deep, p r o b a b l y because t h e y h a v e a c c e p t e d an old unreliable m e a s u r e m e n t . T h e y explain t h a t the effluents in t h e area of h o t m a n i f e s t a t i o n s consist exclusively of air. So air p e n e t r a t e s into a n d circulates inside t h e (, M o n t a f i a , until it reaches a buried spine of m o l t e n lava. Here, air is s u p e r h e a t e d to a t e m p e r a t u r e of several h u n d r e d degrees and rises to t h e surface, b y d r a u g h t , t h r o u g h preferential fractures. This t h e o r y is not c o n s i s t e n t with t h e actual d a t a a b o u t t h e chemical c o m p o s i t i o n of t h e effluents a n d also, as has been seen, a buried spine near the surface c a n n o t maintain a m o l t e n s t a t e for 20o years. Also in a b r o a d hole i2 m deep one can see t h a t c o m b u s t i o n of wood is almost i n s t a n t a n e o u s near t h e surface, b u t v e r y difficult a t hotter and deeper levels, because air does not circulate a t these d e p t h s . The possibility has also been considered of t h e h e a t h a v i n g an exogenous origin, b u t t h e theoretically possible models, such as t h e reaction CaO + H.,O m Ca(OH)z + + 15,54o cal, would d e m a n d the previous f o r m a t i o n of too g r e a t a volume (to 6 m3/year) of CaO from carbonares, which does not seem possible in this area.
Conclusions A l t h o u g h t h e r m o m e t r i c d a t a do not t h e m s e l v e s perm i t the d e t e r m i n a t i o n of a model of t h e g e o t h e r m a l syst e m causing the anomalies of L a n z a r o t e , it is i n t e r e s t i n g to note the t h i r d h y p o t h e s i s which supposes t h e existence of a reservoir with c o n v e c t i v e circulation of fluids.
r h i s t h e o r y is the only effective one with respect to the e x p l o i t a t i o n of g e o t h e r m a l resources a n d could be confirmed w h e n the p l a n n e d e x p l o r a t o r y wells reach the impervious layer. F i n a l l y one of the objectives of this p a p e r is to show how, in specific cases (when the t y p e s of anomalies a n d the geological p a t t e r n p e r m i t it), it is possible to realize an i n e x p e n s i v e t h e r m o m e t r i c s t u d y a t little d e p t h with useful results. Also it is interesting to note the value of investigating the g e o t h e r m a l resources on islands of recent volcanism where the presence of shallow m a g m a t i c c h a m bers can be supposed as well as impervious layers capat~le of storing sea w a t e r a t some d e p t h . Acknowledgements
The data used in this paper were obtained in the program for the evaluation of geothermal resources in the Canary Islands. The authors thank Prof. J. M. F0STEt, Director of the Project, for his help during the elaboration of this work. REFERENCES ARaRA V., FUSTER J. M. 1973 - - Nora previa sobre la prospecci6n geotdrmica en Las Montafias de Fuego (Lanzarote). Etudios Geoldgicos, (in press). ARaRA V., HERNam)Ez-PacHECO A., GARCIALUIS A. 1973 - - Quimismo de los fluidos y dep6sitos hidrotermaies de Las Montafias de Fuego (Lanzarote). Estudios Geol6gicos, (in press). CaLaMAI A., CERON P. 1970 - - Air Convection within ~ Montafia del Fuego >~ (Lanzarote Island, Canarian Archipelago). U.N. Symp. on Develop. and Utiliz. o/ Geoth. Resources, Pisa, Geothermics, sp. issue 2, v. 2, p. 1. FUSTER J. M., FERNANDEZSaNTIN S., SaGREDOJ. 1968 - - Geology and Volcanology of the Canary Islands. Lanzarote. lnstituto Lueas Mallada, C.S.I.C. Madrid. MAcFARLANE D. J., Rn)LEY W. I. 1969 - - An interpretation of gravity data for Lanzarote (Canary Islands). Earth Plan. Sc. Letters, v. 6, 431. RUBIA J., DoRIa J., OSORm A, CaLLEJa J., Ataiqa V. 1970 - - Distribuci6n transitoria de temperaturas en las proximidades de una eapa magm~itica intrusiva en el proceso de enfriamiento. Estudios Geo16gicos, v. 26, 273. WILHAMS R. S. 1971 - - Geologic Analysis of Aerial - thermography of the Canary Islands, Spain. U.S. Geological Survey. memo-report. EATON J. P., MURATt~ K. J. 1960 - - How Volcanoes Grow. Science, v. 132, 3432.
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