Lecture Notes in Earth System Sciences Lev Eppelbaum Izzy Kutasov Arkady Pilchin Applied Geothermics 162 4 Temperature Anomalies Associated with Some Natural Phenomena Park’s Norris Geyser Basin, which is the world’s highest currently active geyser with major eruptions of water reaching a height of up to 90 m, was dormant for a period of 60 years (Scott 1995). One of the most graphic examples of the effect of some tectonic features on the regime of hot springs and geysers is the story of the formation and destruction of the Waimangu geyser in New Zealand (Jones 2006). The Waimangu geyser was created by a large volcanic eruption in 1888 and began erupting in 1900 with up to 490 m high jets of water; it erupted for 4 years before being destroyed by a landslide (Jones 2006). A rapid expansion of the geothermal field took place after the 2000 eruption of the Usu volcano in northeast Japan (Saba et al. 2007). Lardarello, the largest dry-steam field in Italy, which was active for over 100 years has only negligible subsidence; by contrast, in New Zealand both the Wairakei and Ohaaki fields have experienced significant subsidence over a relatively short period of time (DiPippo 2007). There are numerous similar cases all over the world. This means that data on volcanoes, geysers, and hot springs can change at any time and some published data may become outdated very quickly. For instance, the parameters of a number of geysers in Yellowstone National Park published in Allen and Day ( 1935) are quite different from those published in Jones (2006) and some other publications. The same can be said of data published in Barth ( 1950) on the hot springs and geysers of Iceland. Similarly, the first production well HV-1 drilled in 1974 in Husavik had hot water temperatures of 399 K in free flow (Georgsson et al. 2005), but later the temperature of the water dropped to 397 K (Hjartarson et al. 2003). In the Hatchobaru geothermal field of Japan, well H-4 was capable of generating 18 MW in 1973, but at the time it was connected to the Hatchobaru power plant in 1977 it was generating *13 MW. During the first three years of plant operations (between 1977 and 1980) the output declined dramatically to 1 MW; over the same time period, temperatures at the feed zone fell by about 50 K (DiPippo 2007). For these reason, we have attempted to use and reference the most recent information available on the Internet for key thermally active regions. 4.1 Thermal Waters, Hot Springs, Geysers and Fumaroles The behavior of thermal waters, like any other ground waters, is described by hydrogeology and hydrology. For this reason, we will first take a look at some definitions and primary characteristics used in hydrogeology in Sect. 4.1.1 . Thermal waters is a very broad term for all kinds of waters with high and elevated temperatures, which are usually divided into more specific kinds of waters such as hot springs, geysers and fumaroles in land areas, and different kinds of vents (including black and white smokers) in oceanic areas. The main difference between them is the temperature and state of the water, and its behavior upon discharge. Geysers differ from hot springs by the presence of a significant amount of steam along with water, and the way the water discharges. Fumaroles differ 4.1 Thermal Waters, Hot Springs, Geysers and Fumaroles 163 from hot springs and geysers in that they have a vent that emits a mixture of steam and other volcanic gases. The difference between various vents is both in tem- perature and the fact that in some cases (mostly for black smokers) water may be in critical [have either temperature or pressure above their critical conditions; T [ 647 K and P [ 21.8 MPa (Hall 1995; Pilchin and Eppelbaum 2009; Pilchin 2011)], supercritical (both temperature and pressure above their critical condi- tions) or subcritical (the water temperature is above its boiling point but below the critical temperature) conditions. Thermal waters are extremely important features in geothermics, since they constitute one of the main causes of the distribution and redistribution of heat flow within the crust (mostly the upper crust) and one of the main mechanisms involved in the cooling of the upper part of the crust and the lithosphere as a whole. 4.1.1 Key Definitions of Hydrogeology and Related Characteristics Any process involving water is a part of the water cycle, also known as the hydrologic cycle or H 2O cycle (Berner and Berner 1987), and plays an important role in the process of cooling of the Earth’s lithosphere and Earth itself. From this point of view, processes involving ground waters (water infiltration and its underground flow; see Fig. 4.1) are related to the collection of heat energy by ground water and the delivery of this energy to the surface and surface water reservoirs. Infiltrating water (meteoric water; usually low-temperature rainwater or water from melting snow; etc.) in a recharge zone enters an aquifer and flows through it to the discharge zone (an artesian aquifer), where it exits the aquifer to the surface or surface water reservoir. However, while it is moving through the aquifer it is heated to higher temperatures than it initially had, and at the same time it reduces the temperature of the sedimentary layer by absorbing some of the heat from the layers confining the aquifer. In some cases infiltrating water moves through aquifers in regions of ongoing magmatic activity or past young magmatic activity. Under such circumstances, these ground waters may be heated to very high temperatures and can deliver significant amounts of heat to the surface or water reservoirs (i.e. ocean by vents) which assists in cooling the rock layers in contact with magma more quickly. In the case of magmatic activity in a region, significant amounts of magmatic waters (or juvenile waters), existing within and in equilibrium with the magma or water-rich volatile fluids related to the magma are released into the atmosphere during either a volcanic eruption or hydrothermal fluid release during the late stages of magmatic crystallization within the Earth’s crust. It is clear that in all these cases some amount of heat energy is absorbed from rocks of different layers of the crust and delivered to the surface. Depending on the conditions governing each individual case, the location of the discharge of groundwater or thermal water is characterized by thermal anomalies of different 164 4 Temperature Anomalies Associated with Some Natural Phenomena Fig. 4.1 Schematic structure of underground water systems magnitudes, because heat transfer by circulating water is much more intensive than heat transferred by conduction. This thus leads to the cooling of crustal layers and the lithosphere. It should be taken into account that ground waters mostly cool upper layers of the crust, which generate an increased gradient between the upper crustal layers and the lower crustal/lithosphere layers, hence increasing heat flow by heat con- duction, which leads to quicker cooling of the lithosphere. In this section, we only discuss processes related to groundwater and thermal water activity. Any manifestation of thermal waters (springs, geysers, fumaroles, etc.) on land or the sea bottom (vents, black and white smokers, etc.) is a hydrogeological process that follows the laws of hydrogeology and hydrology. Hydrogeology is the area of geology that deals with the distribution and movement of groundwater within the Earth’s crust (mostly its sedimentary layer). Groundwater is water that fills pores and fractures in the ground. Let us first take a look at some key defi- nitions in hydrogeology (e.g., Harter 2008). An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (e.g., sand and gravel or fractured rock) that transmits groundwater (i.e., lets water flow through it). An aquitard is a rock layer with very low permeability (silt, clay, etc.) within the Earth that restricts the flow of groundwater from one aquifer to another (a water barrier). An impermeable aquitard is usually called an aquiclude. Aquitards and/or aquicludes confine an aquifer (from its top and bottom) in a groundwater/hydrologic system. The bot- tommost aquiclude is known as the bedrock. If the impermeable or low-permeable layer overlies an aquifer, it is called a confined aquifer. If an aquifer is not capped by either an aquitard or an aquiclude (generally the topmost aquifer), it is known as an unconfined aquifer. Such an aquifer is in direct contact with the surface through porous space and, depending on the conditions, it may be unsaturated with water. The level to which water rises within an unconfined aquifer is the water table (the top of ground water). The water table is normally used to characterize the water 4.1 Thermal Waters, Hot Springs, Geysers and Fumaroles 165 level on land, because in sea and ocean areas it is represented by the sea level. In most regions in California for instance, the water table is between 3 and 30 m below the land surface, though it is as deep as 90 m in some southern California desert basins (Harter 2008). An unconfined aquifer usually has porous pressure below the hydrostatic pressure, which creates abnormally low stratum pressure and abnormally low porous pressure (ALSP/ALPP) in it. The entire layer above the first aquitard or aquiclude is characterized by low hydrostatic pressure, which can explain the presence of a low velocity zone near the surface known in continental areas from seismology. Only below the first aquitard or aquiclude can the pressure in the second aquifer be higher than or equal to the hydrostatic pressure (AHSP/AHPP), which may also indicate the beginning of an abnormally high stratum pressure or abnormally high porous pressure (AHSP/AHPP) zone.