Hydrothermal Manifestations Research Articles

Hydrogeochemistry of thermal and mineral water springs of the Azores archipelago (Portugal)

Mineral and thermal water chemistry from the Azores archipelago was investigated in order to discriminate among hydrochemical facies and isotopic groups and identify the major geochemical processes that affect water composition. A systematic geochemical survey of mineral and thermal water chemistry was carried out, incorporating new data as well as results from the literature. The Azores are a volcanic archipelago consisting of nine islands and samples were collected at São Miguel, Graciosa, Faial, São Jorge, Pico and Flores islands. Hydrothermal manifestations show the effects of active volcanism on several islands. Discharges are mainly related to active Quaternary central volcanoes, of basaltic to trachytic composition, but also some springs are related to older dormant or extinct volcanoes. Multivariate analysis – principal component and cluster analysis – enables classification of water compositions into 4 groups and interpretation of processes affecting water compositions. Groups 1 and 2 discharge from perched-water bodies, and mostly correspond to Na–HCO 3 and Na–HCO 3–Cl type waters. These groups comprise of cold, thermal (27 °C–75 °C) and boiling waters (92.2 °C–93.2 °C), with a wide TDS range (77.3–27, 145.7 mg/L). Group 3 is made of samples of dominated Na–SO 4 from very acid boiling pools (pH range of 2.02–2.27) which are fed by steam-heated perched-water bodies. Group 4 is representative of springs from the basal aquifer system and corresponds to Na–Cl type fluids, with compositions dominated by seawater. Results are used to further develop a conceptual model characterizing the geochemical evolution of the studied waters. Mineral and thermal waters discharging from perched-water bodies are of meteoric origin and chemically evolve by absorption of magmatic volatiles (CO 2) and by a limited degree of rock leaching. Existing data also suggest mixture between cold waters and thermal water. Water chemistry from springs that discharge from the basal aquifer system evolves by mixing with seawater; although, processes such as absorption of magmatic volatiles (CO 2), rock leaching and mixture with hydrothermal waters are not excluded by the data because the actual composition of these waters deviates from that expected considering only conservative mixing between fresh and seawater.

Volcanological and geochemical evolution of the Diamante Caldera–Maipo volcano complex in the southern Andes of Argentina (34°10′S)

Two main stages are distinguished in the evolution of the Diamante Caldera–Maipo volcano complex. The ‘Diamante stage’ corresponds to the emplacement of large volume rhyolitic ignimbrites associated with a 20×16 km diameter collapse caldera. This ∼0.45 Ma catastrophic event was followed by the ‘Maipo stage,’ which represents the last 100 ka of the complex's lifetime. Fieldwork and Ar/Ar data support an eruptive record within the Maipo stage that consists of at least four pre-last glacial maximum (pre-LGM) large volume events and three postglacial small volume events. Historic activity remains uncertain. At present, neither fumarolic activity nor hydrothermal manifestations are detected. Dominantly effusive, the Maipo stage includes blocky and flow-banded lavas, a ring-fault dome, and subordinate pyroclastic deposits that define a high-K, calc-alkaline suite ranging in silica from 53 to 68% and encompassing two pyroxene with minor olivine andesites and two pyroxene and hornblende-bearing dacites. Within the volcanic complex, a general geochemical trend toward more evolved, potentially higher explosive products is recognized. Differentiation is strongly controlled by fractional crystallization, but disequilibrium textures suggest periodic magma mixing must be significant in the production of cyclic chemical variations.

Hydrothermal ore manifestations of Logachev and Rainbow fields (Mid-Atlantic Ridge) -- a new type of ocean rift hydrothermal deposits

Hydrothermal ore manifestations of Logachev and Rainbow fields (Mid-Atlantic Ridge) -- a new type of ocean rift hydrothermal deposits

Isotope geochemistry of Pantelleria volcanic fluids, Sicily Channel rift: a mantle volatile end-member for volcanism in southern Europe

Chemical and isotopic ratio (He, C, H and O) analysis of hydrothermal manifestations on Pantelleria island, the southernmost active volcano in Italy, provides us with the first data upon mantle degassing through the Sicily Channel rift zone, south of the African–European collision plate boundary. We find that Pantelleria fluids contain a CO 2–He-rich gas component of mantle magmatic derivation which, at shallow depth, variably interacts with a main thermal (∼100°C) aquifer of mixed marine–meteoric water. The measured 3He/ 4He ratios and δ 13C of both the free gases (4.5–7.3 R a and −5.8 to −4.2‰, respectively) and dissolved helium and carbon in waters (1.0–6.3 R a and −7.1 to −0.9‰), together with their covariation with the He/CO 2 ratio, constrain a 3He/ 4He ratio of 7.3±0.1 R a and a δ 13C of ca. −4‰ for the magmatic end-member. These latter are best preserved in fluids emanating inside the active caldera of Pantelleria, in agreement with a higher heat flow across this structure and other indications of an underlying crustal magma reservoir. Outside the caldera, the magmatic component is more affected by air dilution and, at a few sites, by mixing with either organic carbon and/or radiogenic 4He leached from the U–Th-rich trachytic host rocks of the aquifer. Pantelleria magmatic end-member is richer in 3He and has a lower (closer to MORB) δ 13C than all fluids yet analyzed in volcanic regions of Italy and southern Europe, including Mt. Etna in Sicily (6.9±0.2 R a, δ 13C=−3±1‰). This observation is consistent with a south to north increasing imprint of subducted crustal material in the products of Italian volcanoes, whose He and C (but also O and Sr) isotopic ratios gradually evolve towards crustal values northward of the African–Eurasian plate collision boundary. Our results for Pantelleria extend this regional isotopic pattern further south and suggest the presence of a slightly most pristine or ‘less contaminated’, 3He-richer mantle source beneath the Sicily Channel rift zone. The lower than MORB 3He/ 4He ratio but higher than MORB CO 2/ 3He ratio of Pantelleria volatile end-member are compatible with petro-geochemical evidence that this mantle source includes an upwelling HIMU–EM1-type asthenospheric plume component whose origin, according to recent seismic data, may be in the lower mantle.

Groundwater radon measurements in Algeria

Radon contents of groundwater sources have led to a great interest in hydrological, hydrogeological and geological engineering. The most interesting applications are: The determination of the fluctuations of the piezometric levels in groundwater to evaluate hydrogeological resources, the study of recent hydrothermal manifestations, the study of oil- and gas-bearing regions, the estimation of uranium deposits and the study of the relationship between the radon concentration and the degree of stress of the earth's crust at different stages of seismic activity. Waters from springs and deep wells in the plateau of Tassili (southeast Algeria) were sampled, measured and radon quantified. Radon measurements were performed using two different methods. The first method, active, based on the use of a Lucas-type scintillation chamber in conjunction with a portable monitor (model Pylon AB-5); the second method, passive, using an electret ion chamber with a 4 l glass analysis bottle. The aim of this work is to develop a method for sampling, detecting, evaluating and measuring the 222 Rn in groundwater using the scintillation cell method. A comparison of the two methods was carried out and both were found to be useful under environmental conditions in Algeria.

Volcanogenic pollution by acid water discharges along Ciwidey River, West Java (Indonesia)

Active and recently extinct volcanoes surrounding the catchment area of the upper Citarum River (West Java) are the source areas of surface waters with high concentrations of natural contaminants. Ciwidey River, one of its tributaries, originates on the flanks of Patuha, a Quaternary volcano which contains an acid (pH<1) crater lake having high contents of sulphur and chlorine, and where various other hydrothermal manifestations are active. We present the results of a geochemical study carried out in 1995–1996 to assess the impact of volcanogenic pollution along this river, whose natural flow is disturbed by the use of water for irrigation and other purposes. Concentrations of dissolved major and trace elements in the Ciwidey River show peak levels associated with the influx of acid streams. Downstream decreasing trends are attributable to dilution and other attenuation processes commonly observed in acid mine drainage. The main contaminating streams originate from two different sources: (1) a flank spring producing water which is suspected to contain a portion of leaking acid lake water; and (2) artificial sulphur-mud deposits which are the remains of sulphur production from crater-lake sediments that were exploited in the first half of this century. Fluctuations in pH and dissolved metals observed along the river during repeated sampling campaigns are correlated with seasonal successions of wet and dry periods, but the interference in the drainage pattern has modified the original natural variations. The river water shows lowest pH values and remains acid along the greatest distance during the rainy season. Downstream decreases in acidity, which are largely induced by dilution with neutral water, determine to a significant extent whether potentially harmful elements remain dissolved or are removed by precipitation or sorption onto solid phases. The seasonal variability in discharges and acid input, combined with human interference in the river flow thus creates highly dynamic conditions for mobilisation and fixation of the volcanogenic pollutants. Among the elements studied, aluminium, arsenic, boron, iron and manganese reach concentrations which deserve attention in view of the potential consequences for the human environment. In general, the volcanogenic pollutants return to baseline levels before the confluence with the Citarum River at some 30 km from the focal point of the acid discharges. It is expected that only the most conservative dissolved elements could cause detectable changes resulting from the influx of the Ciwidey River water. However, ephemeral peak discharges cannot be excluded, and may have an impact which remained unnoticed during our sampling campaigns.

Geochemical exploration of the Chipilapa geothermal field, El Salvador

Results of the geochemical exploration of the Ahuachapán-Chipilapa area are presented. The procedure for interpreting the chemical composition of very dilute thermal waters is emphasized. Three groups of thermal waters are described, two with a geothermal brine component and one resulting from steam condensation. The model for one of the groups (Type 2) leads to predictions of temperature and chloride concentration that are reasonably close to those of the geothermal liquid feeding Chipilapa well CH-7B. In particular, it was predicted correctly that the salinities in the Chipilapa area would be considerably lower than those in the Ahuachapán field.It is shown that the simultaneous modeling of the carbon dioxide concentration and isotopic composition of fumarole steam allows discrimination between primary and secondary steam. The composition of all fumarole samples is described as steam originating from a single reservoir fluid at 250°C, and composition δ 18O = −4.1, δD = −46, CO 2 = 5 × 10 −5 molar fraction.The total discharge composition of CH-7B confirms the trend observed in the Ahuachapán field of decreasing reservoir salinities towards the east. Postulating the existence of a “deep” reservoir brine in the eastern (Chipilapa) section of the system, with lower salinity but otherwise similar temperature and isotopic composition to the Ahuachapán brine, allows for the generation of relatively simple models that explain the formation of the CH-7B brine, and the three groups of thermal waters. Type 1 waters are noteworthy in the sense that they result from a ternary mixture of meteoric water, geothermal brine and high-temperature steam condensate.The possibility that the east-west trend in salinity results from a process of dilution of brine with condensate from steam separated at very high temperatures is discussed. The distribution of the different types of hydrothermal manifestations delineates a lateral discharge system, with the steam upflow zone to the south of the study area, on the northern slope of the volcanic range, and thermal water discharges several kilometers to the north. It is deemed reasonable that exploratory drilling should be directed towards the southern edge of the geothermal system, as far as topography and the indicators of probable secondary permeability permit.

Application of multi-element geochemistry in Au-phosphate—bearing lateritic crusts for identification of their parent rocks

Serra do Pirocaua is a plateau formed on mature lateritic rocks (Eocene), partially covered by earthy soil. Gold in the region around the serra (plateau) has been mined from the 17th Century to the present. Laterites extend not only over the entire plateau, but also in the neighboring flat and low-lying area. Outcrops of fresh rocks are rare and none has been found as yet on the plateau. Systematic geochemical and mineralogical studies were carried out on samples taken from crusts and surface soils of the plateau. Initial results show that these materials are practically identical both in terms of mineralogy as well as geochemistry indicating that the soils have a physical origin, that is, they are immature. Multi-element geochemical data have identified three main geochemical signatures both for the crusts as well as the soils, representative of three mineral groups: hematite + goethite, Al phosphates and anatase. A lithological map of the substrata was developed, based on the distribution of the anomalous levels of elements in each association. The data suggest that the substrata are formed of volcano-sedimentary rocks of mafic to intermediate composition and probably phosphorite beds. Both volcanic and non-volcanic mafic-ultramafic rocks may be present. Hydrothermal manifestations such as As-bearing tourmaline-quartz veins were suspected because of high levels of As and Au as well as dravite aggregates in Fe crusts and soils.

Light hydrocarbons in hydrothermal and magmatic fumaroles: hints of catalytic and thermal reactions

Volcanic gaseous mixtures emitted from active volcanoes frequently show variable amounts of saturated (alkanes), unsaturated (alkenes) and aromatic volatile hydrocarbons. Three major patterns of distributions can be recognized, apparently related to the chemical-physical environment of formation of the gas exhalations: alkane-rich, low-temperature gas emissions from recently active volcanic areas; aromatic-rich hydrothermal manifestations; and alkene-rich, ‘magmatic’ fumaroles on active volcanoes. Thermodynamic data, together with theoretical and practical findings from the petroleum industry, point to two main types of reactions occurring in these volcanic environments: cracking and reforming. Cracking processes, mainly caused by thermal effects, occur when hydrocarbon-bearing hydrothermal fluids enter and mix with a hot and dry, rapidly rising magmatic gas phase. The most probable products are light alkenes with carbon numbers decreasing with increasing reaction temperatures. The presence of aromatic species in hydrothermal fluids can be linked to reforming processes, catalysed by several possible agents, such as smectites and zeolites, generally present in the hydrothermally altered volcanic terranes, and facilitated by long residence times in a hydrothermal envelope.

Heat flow in the Kenya rift zone

An understanding of the processes of continental rifting is fundamental to understanding the evolution of the continents. Considerable evidence exists to suggest that continental rift zones are associated with high heat flow and elevated lithospheric geotherms, but direct heat-flow measurements from young rifts do not clearly define surface heat-flow anomalies associated with deep-seated thermal processes in these rifts. The first detailed compilation of heat-flow data from the Neogene Kenya rift is presented here. Heat-flow data are presented from traditional heat-flow determinations in water drill-holes, from bottom-hole-temperature measurements in oil wells, and from heat-flow estimates from groundwater silica data. These data define generally low heat flow on the flanks of the Kenya rift, with high, but variable heat flow on the rift floor. There is a spatial association among high heat-flow values, Quaternary volcanism and faulting, and hydrothermal manifestations on the rift floor. We interpret these results to suggest that any deep-seated thermal anomaly associated with the Kenya rift has not yet been conducted to the surface. The high heat-flow values are interpreted to result from heat advected into the axial rift zone with local redistribution of this heat by hydrothermal convection. Normal to moderately high heat flow was measured in eastern Kenya between the rift zone and the coast. The regional heat flow in eastern Kenya is interpreted to be normal, with local shallow modification by groundwater flow eastward from the Kenya dome. These interpretations support a model of relatively young evolution of the asthenospheric anomaly beneath the Kenya rift zone, with the age of heating of the mantle at the Mono no older than about 10 Ma.

Preliminary results of a magneto-telluric survey over a geothermal anomaly in Portugal

During summer 1990 a magneto-telluric (MT) survey was conducted over a geothermal anomaly discovered a few years ago in south central Portugal. This anomaly, known as the Alentejo Geothermal Anomaly (AGA), exhibits heat flow density values (greater than 160 mW m 2) at its centre. The area covered by the MT survey was about 2500 km 2, and 34 MT sites were occupied. The preliminary results indicate that the central area of the AGA coincides with a region of high electrical resistivity which continues to depths greater than 10 000 m. As no intense hydrothermal manifestations are known in the study area, it is suggested that the heat flow density anomaly is the result of high heat production in some of the geological formations that outcrop in the region. Two major faults cross the area, intersecting at approximately a right angle. Low-resistivity zones appear to be associated with the fault zones, and a possible interpretation is that these conductive zones may be related to fluids contained in fractures surrounding the faults.

La Primavera caldera (Mexico): Structure inferred from gravity and hydrogeological considerations

La Primavera geothermal field (Mexico) is associated with a Pleistocene rhyolitic caldera. This gravity study was conducted to assist its development and explotation. Digital processing of the gravity data (upward and downward continuations, vertical derivatives) enabled delineation of the main features of the caldera’s subsurficial structure. A 3-D structural model was established, which could be supported by gravity modeling (2-D and 3-D forward modeling). Accordingly, the caldera is featured by an asymmetric subsurface structure: a major depression in its northern half, and a boomerang‐shaped structural high to the south. Lineaments reflecting the regional northwest‐southeast and northeast‐southwest structural fabric were observed. The basal volcanics units are affected by lineaments of the northwest‐southeast system, whereas the northeast‐southwest system affects only the shallower units. The structural high has a northwest‐southeast trend at the western and southwestern portion of the caldera. From its middle part eastward, it has a northeast‐southwest direction. The actual geothermal production zone is located above this structural high, on the portion where it changes orientation. Correlation with hydrogeological and geochemical data enabled interpreting the different geologic structures in the context of the hydrothermal system: at depth the northwest‐southeast structures seem to control lateral fluid migration, and connect areas of enhanced permeability (i.e., the central production zone and the hydrothermal manifestations located at the caldera’s western rim). Enhanced zones of fracturing favorable for entrapping hydrothermal fluids and structural accidents that may act as conduits (respectively as barriers) for fluids are delineated. In particular, a new target zone, where the production of geothermal fluids may extend, has been identified to the south of the production zone. The structural image elaborated here constitutes a geologic frame for the prevailing hydrogeological conceptual model. This structural information is also useful for the tasks of selecting sites for the reinjection of geothermal brines.

Gas emission and hydrothermal manifestation in the Lake Nyos area: January 5, 1987, January 26, 1987, June 20, 1987

Gas emission and hydrothermal manifestation in the Lake Nyos area: January 5, 1987, January 26, 1987, June 20, 1987

Subduction of Atlantic aseismic ridges and Late Cenozoic evolution of the Lesser Antilles island arc

A great amount of data concerning the Lesser Antilles arc and the near Atlantic has been reviewed: nature, timing and spatial evolution of the Oligocene to Recent volcanic sequences; the location and scale of uplifted areas; facturation of the arc ridge; the structural pattern of the Central Atlantic oceanic crust; and characteristics of the subduction beneath the Caribbean plate. These data allow us to lay stress on the role of the interaction of underthrust oceanic aseismic ridges with the evolution of the overriding arc. Depending on the degree of gravimetrical compensation (buoyancy) of these features, this interaction can induce effects of the first and second orders of magnitude. With the onset of the Late Oligocene, a buoyant ridge of anomalous mid Cretaceous crust reached the front of the northern half of the Lesser Antilles arc basement. It momentarily stopped the subduction along the whole arc, bringing the arc volcanism (Older arc) to a standstill as it was underplated beneath the eastern rim of the Caribbean lithosphere, and triggering the westward tilting of this part of the arc, with locally dramatic uplifting (La Désirade). After a gap of 9–10 m.y., the volcanic activity resumed in the early Burdigalian (Recent arc), with a westward jump of the volcanic axis north of Martinique (Inner arc, corresponding to the northern part of the Recent arc). Later, the subduction of non-buoyant (gravimetrically uncompensated) ridges (Barracuda, Tiburon and St. Lucia ridges) exerted significant effects, but of a lower extent with regard to their spatial incidence and intensity, on the arc north of St. Lucia. These effects induce a sudden transverse shift of the volcanic front, of only some kilometers in length (forward and backward), centrifugal (longitudinal) migration of the eruptive centers on either side of the arc segment above the ridge, gentle uplifts, and enhance seismic and hydrothermal ( sensu lato) manifestations. An inverse approach of reasoning leads us to infer the existence of a ridge in the southern part of the subducted slab, the “Grenadines crypto-ridge”. The latter might be in its final stage of absorption by the subduction. Emphasis is put on the role played by the aseismic ridges in the volcanic and tectonic evolution of convergent margins, and particularly their control of the thickness of the subducted sediments, thought to be the necessary condition for triggering the usual arc volcanism.

Deep seismically active fracture zones in Ecuador and northern Peru

A system of 8 seismically active fracture zones was delineated on the basis of the distribution of earthquake foci in the continental lithosphere of Ecuador. The position and width of the outcrop, thickness, dip and maximum depth of the individual fracture zones were estimated and correlated with surface geological and tectonic phenomena, volcanism and hydrothermal manifestations. The existence and strike of the fracture zones was independently confirmed by the occurrence of historical disastrous earthquakes.

Tourmalinites in the Aphebian Mistassini Group, Quebec

Tourmalinites, or fine-grained sedimentary rocks containing 15–20% or more tourmaline, occur within the Papaskwasati Formation of the Aphebian Mistassini Group in central Quebec. The formation, which is largely composed of fluvial sand-stones and conglomerates with minor argillite beds, was deposited in a restricted embayment close to the transition into marine carbonates. Concentrations of tourmaline are limited to fine-grained sediments and intraclasts, and fine sedimentary structures are preserved within the resulting tourmalinites. This suggests that the tourmalinites formed from preexisting boron-rich muds. It is unlikely that the muds were formed from sea-floor hydrothermal emanations, as no hydrothermal manifestations or spenecontemporaneous vulcanism is known to be associated with this formation. An evaporitic origin for the tourmalinites is consistent with both the sabkha conditions postulated for the origin of nearby carbonates and the limited areal extent of the sedimentary basin.

C- and O-isotope and fluid inclusion studies of carbonates from pyrite and polymetallic ore deposits and associated country rocks (Southern Tuscany, Italy)

A total of 42 C- and O-isotopic analyses on 35 samples of carbonates from pyrite and polymetallic deposits of southern Tuscany and associated country rocks are presented. Hydrothermal calcites and dolomites in the ore deposits show a wide range of isotopic compositions, which probably accounts for multiple sources of carbon and oxygen. Calculated isotopic compositions for COH species in equilibrium with calcites are similar to those measured in present-day geothermal fluids of the nearby Larderello-Travale field, suggesting a common link for all late- to post-Apenninic hydrothermal manifestations in the area. The isotopic compositions of country rocks range from typical marine values to lower values, the isotopic depletion likely accounting for metamorphic and hydrothermal effects related to the Mio-Pliocene Apenninic event. In contrast, 10 samples of similar rocks from non-mineralized areas of northern Tuscany show no isotopic evidence of metamorphic and hydrothermal alteration. Data from fluid inclusions indicate important relationships between calcite-forming fluids and sphalerite-forming fluids in the ore deposits here studied.

C- and O-isotope and fluid inclusion studies of carbonates from pyrite and polymetallic ore deposits and associated country rocks (Southern Tuscany, Italy)

A total of 42 C- and O-isotopic analyses on 35 samples of carbonates from pyrite and polymetallic deposits of southern Tuscany and associated country rocks are presented. Hydrothermal calcites and dolomites in the ore deposits show a wide range of isotopic compositions, which probably accounts for multiple sources of carbon and oxygen. Calculated isotopic compositions for COH species in equilibrium with calcites are similar to those measured in present-day geothermal fluids of the nearby Larderello-Travale field, suggesting a common link for all late- to post-Apenninic hydrothermal manifestations in the area. The isotopic compositions of country rocks range from typical marine values to lower values, the isotopic depletion likely accounting for metamorphic and hydrothermal effects related to the Mio-Pliocene Apenninic event. In contrast, 10 samples of similar rocks from non-mineralized areas of northern Tuscany show no isotopic evidence of metamorphic and hydrothermal alteration. Data from fluid inclusions indicate important relationships between calcite-forming fluids and sphalerite-forming fluids in the ore deposits here studied.

Hydrothermal history of Piton des Neiges volcano (Reunion Island, Indian Ocean)

Abstract The Piton des Neiges volcano on Reunion Island represents a unique example of an oceanic volcano where the extreme development of amphitheatre-headed valley erosion has led to the formation of three large cirques. They are so large that the island's volcano-structural and petrological history can be traced from its emergence to the latest stages of its sub-aerial evolution (> 2.1 m.y. to 22,000 years ago). The various magmatic series of the Piton des Neiges are, moreover, abundantly invaded by hydrothermal mineralization. It is this post-magmatic feature, represented by the hydrothermal alteration of the series, which is examined here. Mineralogical studies (X-ray, microprobe, scanning electron microscope) reveal a large number of hydrothermal species. Of these, zeolites are the most common and five are described here for the first time in Reunion (gonnardite, levynite, erionite, garronite, herschelite). Six hydrothermal facies characteristic of weak metamorphism are defined: chabazitephillipsite, natrolite-thomsonite, analcime-thomsonite, laumontite-thomsonite, albiteprehnite and prehnite-pumpellyite. The paleo-temperatures covered by these facies range from 0 to 380°C. On the basis of these data and supporting field observations, three main hydrothermal phases were determined and fitted into the known chronostratigraphy. These three phases have succeeded one another over the last two million years in the Piton des Neiges massif. The extent and mineralogical facies of each phase can be related to the volcanotectonic structures. This sequence has been directly linked to the geological evolution of the massif. The progressive restriction with time of the hydrothermal manifestations to the present Piton des Neiges occurred alongside the focusing of volcanism centralized on this same relief. An attempt is made to reconstruct the island's hydrothermal history.

Heat flow map, a review of heat flow data and anomalies for the European plate

Heat flow values are shown in mW/m 2 on a mpa compiled using data from more than 1200 stations. The given area covers the European continental block as far as the Ural folding system, including the Caucasus, the Carpathian mountain system, the East European platform, the Scythian plate and foredeeps and also including such marginal seas as the Meditteranean, Black and Caspian seas, the Alpine lakes, a part of the Atlantic Ocean and Iceland. Heat flow values in the continental areas around the main hydrothermal manifestations of Larderello and Radicofani in Italy, Novoselovskaja in the Crimea, the Pannonian basin in Hungary, the Rhodopean Massif in Bulgaria are discussed. Besides the magmatic and tectonic processes, the hydrothermal systems also affect the parameters of the thermal field in the upper layers of the crust. A number of regional hydrothermal anomalies of high heat flow values on the Earth's surface e.g. Tuscany in Italy, the Pannonian Basin in Hungary, Rhodopean Massif in Bulgaria are related to convective processes of heat transfer.