subsurface waters with temperatures of at least 20°C. The 20°C isotherm lies at depths of 1,500–2,000 m in the earth’s crust in regions of permanently frozen rocks, rising to 100 m or higher in subtropical regions; at the boundary with the tropics, it emerges onto the earth’s surface. In artesian basins, waters with temperatures of 70°–100°C or more emerge from fissures that extend to depths of 2,000–3,000 m. In mountainous regions, such as the Alps, the Caucasus, the Tien-Shan, and the Pamirs, thermal waters reach the surface in numerous hot springs, with temperatures of up to 50°–90°C, and in regions of present-day vulcanism they are manifested in the form of geysers and steam vents (in this case, waters with temperatures of 150°–250°C emerge from fissures that extend to depths of 500–1,000 m), which emit steam-water mixtures and vapors upon emergence onto the surface. Examples of this type of thermal waters are Pauzhetka on the Kamchatka Peninsula, Big Geysers in the USA, Wairakei in New Zealand, the soffione at Larderello in Italy, and geysers in Iceland.
In chemical and gaseous composition and mineralization, thermal waters range from fresh and brackish hydrocarbonate and hydrocarbonate-sulfate, calcium, sodium, nitrogen, carbon-dioxide, and hydrogen-sulfide waters to salty and briny chloride, sodium and calcium-sodium, nitrogen-methane and methane, and, in some places, hydrogen-sulfide waters.
Thermal waters have long been used for therapeutic purposes—for example, the thermae in ancient Rome and in Tbilisi. In the USSR, freshwater nitrogen hot springs rich in silicon dioxide are used at such well-known health resorts as Belokurikha in the Altai and Kul’dur in Khabarovsk Krai. Carbon-dioxide thermal waters are used at resorts of the Caucasian Mineral Waters region (Piatigorsk, Zheleznovodsk, and Essentuki), and hydrogen-sulfide waters are used at the Matsesta health resort near Sochi. In balneology, thermal waters are divided into warm, or subthermal (20°–37°C), thermal (37°–42°C), and hyperthermal (above 42°C).
A number of electric power plants that use superheated thermal waters with temperatures higher than 100°C are in operation in regions of contemporary or recent vulcanism in Italy, Iceland, Mexico, the USSR, the USA, and Japan. In the USSR and other countries, such as Bulgaria, Hungary, Iceland, New Zealand, and the USA, thermal waters are also used to provide heat for residential and industrial buildings, hothouse-hotbed combines, and swimming pools and for industrial purposes. For example, the city of Reykjavik, Iceland, is heated entirely by thermal waters. In the USSR, heat is provided for mikroraiony (neighborhood units) in the cities of Kizliar, Makhachkala, Zugdidi, Tbilisi, and Cherkessk; hotbed-hothouse combines are heated on the Kamchatka Peninsula and in the Caucasus. Waters used for heating are classified as slightly thermal (20°–50°C), thermal (50°–75°C), or highly thermal (75°–100°C).
REFERENCESIzuchenie i ispol’zovanie glubinnogo tepla Zemli. Moscow, 1973.
Mavritskii, B. F. Termal’nye vody skladchatykh i platformennykh oblastei SSSR. Moscow, 1971.
B. F. MAVRITSKII