hot springs A spring is called ‘hot’ or ‘thermal’ if it has a temperature noticeably above the mean annual air temperature in the locality. In Europe the general practice has been to apply the term ‘thermal’ only to springs having temperatures greater than 20 °C.
Hot springs are found throughout the world wherever there are permeable geological structures that (1) channel meteoric water (water derived from rain or snow) into the ground where heating occurs, and (2) channel the heated water back out of the ground at a rate that is fast enough to prevent all the thermal energy from being dissipated by conduction into the surrounding rock during upward flow. Most hot springs with temperatures above 60–70 °C occur in volcanic belts, or where the continental crust has been thinned and faulted as a result of geological processes that pull the crust apart, as in the Basin and Range province of the western United States.
The driving force for the discharge of hot springs is the bouyancy that results from the lower density of hot water compared to cold water. The temperature attained by meteoric water during deep convective flow depends on the Earth's thermal gradient where the flow occurs, and the depth of its circulation. Temperature normally increases by about 25° to 30 °C per kilometer of depth, and most hot spring waters attain temperatures less than 100 °C in the course of circulation to depths of 2 or 3 kilometres. Heating to more than 100 °C can result either from deeper circulation along faults in non-volcanic regions, or by relatively shallow circulation that brings water into contact with anomalously hot rock in volcanic belts. Drilling in search of geothermal resources in volcanic regions has encountered groundwaters that have been heated to temperatures as high as 350 °C at depths as shallow as 2 km. This is near the expected upper temperature limit for circulating meteoric waters because faulting generally does not occur in the continental crust where temperatures are greater than 350–370 °C. Repeated faulting appears to be essential for keeping open channels of flow that otherwise would become clogged by minerals that are deposited from circulating hot waters.
Where the circulating waters attain underground temperatures higher than 100 °C and the rate of upward flow is relatively fast, boiling will occur in response to decompression (the decrease in the weight of the overlying column of water as the upward-flowing water rises to shallower depths). A mixture of boiling water and steam will then be discharged at the Earth's surface. In some places this discharge occurs episodically as geyser eruptions. The depth at which the boiling is initiated in the upward flowing water, and the ratio of steam to water in the fluid that is discharged by a boiling spring, increase in proportion to the maximum temperature attained by the deeply circulating water. The maximum temperature exhibited by a hot spring is independent of the water to steam ratio; it is controlled by the boiling point of water at the elevation where the discharge occurs. At sea level pure water boils at 100 °C. There is a decrease in the boiling temperature by 1 °C for each 303 m increase in altitude. On the other hand, hot springs that issue on to the floors of lakes, or at the bottom of the ocean, can have temperatures much greater than 100 °C because the boiling temperature at the point of discharge is determined by the weight of the overlying column of cold lake or ocean water. Vent temperatures as high as 360 °C have been measured at hot springs deep in the ocean where volcanic activity occurs along plate boundary spreading centres. These springs are called ‘black smokers’ because of the dark colour imparted to their water by precipitating minerals.
The compositions of hot spring waters are determined by the temperatures and types of rocks encountered during underground flow, and, in volcanic regions, by the degree of mixing with waters and gases given off from crystallizing magmas. The types of minerals deposited by hot springs reflect these conditions. Massive travertine (calcium carbonate) deposits occur where hot spring waters issue from limestone after having attained a temperature no higher than about 100–200 °C. Siliceous sinters are deposited where hot spring waters flow relatively quickly to the surface after having attained a temperature greater than about 180 °C. Acidic springs and ‘mud pots’ occur in volcanic regions where upward-flowing steam rich in hydrogen sulphide condenses in perched bodies of groundwater at or near the Earth's surface.
Robert O. Fournier
Bibliography
Waring, G. A. (Revised by R. R. Blankenship and and R. Bentall ) (1965) Thermal springs of the United States and other countries of the world. US Geological Society Professional Paper 492.
