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PART VI Volcano Interactions  A magma body that resides at shallow levels beneath a volcano is not static; it is in dynamic interaction with its surrounding environment. As a result, many fascinating phenomena are observed which include volcanic gases, hydrothermal systems on land and under the oceans, and volcanic lakes. The interactions may be hazardous, releasing gases dangerous to human health, draining crater lakes catastrophically, and causing explosive eruptions. Yet these interactions also are beneficial to humans, since geothermal energy can be harnessed to produce electricity, and ore deposits can be mined for our natural resource needs.Many volcanoes release billowing clouds of gas, not only during an eruption but also when a volcano is seemingly quiet between eruptions. The opening chapter of this section, Volcanic Gases, examines the different types of gases which are to be found at volcanoes. The gases can be emitted at high temperatures from fumaroles in active crater areas, and at low temperatures on the flanks of volcanoes. The compositions of the gases are controlled by factors such as their solubilities in magma, the temperature and pressure at which they reside, the infiltration of surface waters, and the tectonic setting of the volcano. Volcanoes are favorable sites for geothermal systems, which are discussed in the following chapter. As groundwaters percolate through a faulted, permeable volcanic edifice, they encounter a heat source, which is normally magma and hot rocks. The interaction of the water and the magma creates convection cells whereby hot water is circulated within a permeable zone. The hot water reacts with country rocks, dissolving the rock and adding various chemical elements to the hydrothermal fluid. Geothermal systems are found not only at active volcanoes but also in other regions of high heat flow, typically along tectonic plate boundaries. Geothermal systems can be exploited commercially for electric production, and this aspect is discussed further in Part IX, in the chapter Exploitation of Geothermal Resources. Geothermal and hydrothermal systems created by interaction of waters with magma and hot rocks form impressive manifestations at the surface; the chapter Surface Manifestations of Geothermal Systems delves into the details of these manifestations, which may include spectacular fumaroles, acid crater lakes, acidic hot springs and even acid streams. High-temperature hydrothermal systems at volcanoes may be dominantly liquid, vapor, or a combination of the two. Lower-temperature systems also are found at volcanic centers; these may be cooling and decaying systems which were once at high temperature. These systems may form certain types of ore deposits (as discussed later in this section in the chapter Mineral Deposits Associated with Volcanism). Not only does water-magma interaction occur on land, it also occurs in the oceans. The chapter Deep Ocean Hydrothermal Vents examines the remarkable hydrothermal systems which are formed in regions of active submarine volcanism. Most of our knowledge regarding these systems is based on discoveries in the past twenty years. As two tectonic plates form and move apart at mid-ocean ridges, magma is generated which serves as a heat source. Seawater percolates into the fractured oceanic crust and is heated by the magmatic source, setting up hydrothermal convection cells. In fact, submarine hydrothermal circulation is a principal means by which Earth loses its heat. As the heated water circulates, it dissolves rock. The chemically charged water transports metals upward, forming ore deposits when pressure-temperature conditions are reduced. Researchers have even observed and filmed spectacular hydrothermal chimneys, termed "black smokers", venting at the sea floor. In the vicinity of the hydrothermal vent, unique forms of life are nurtured by the hot mineral-rich water. Back on land, the next chapter, Volcanic Lakes, shows that lakes can form on volcanoes when surface waters are available in large amounts. These lakes typically are situated in craters at the tops of volcanoes and can be hot, containing large amounts of dissolved solids. In some cases, the lakes are actually concentrated mixtures of sulfuric and hydrochloric acid! Some lakes may host molten sulfur bodies, while others have high levels of carbon dioxide dissolved at the bottom. Acidic crater lakes are another surface manifestation of interaction between groundwaters and a degassing magma; they also can be considered as the uppermost part of a magmatic-hydrothermal system which may be forming an ore deposit at deeper levels. Volcanic lakes can be hazardous, since they are associated with hydrovolcanic eruptions (see the chapters Phreatomagmatic Fragmentation; Surtseyan and Related Phreatomagmatic Eruptions; Pyroclastic Surges and Blasts), gas bursts of CO2 (see Hazards of Volcanic Gases), and lahars (see Lahars; Lahar and Jökulhlaup Hazards). We have seen above how volcano interactions potentially can form mineral deposits. These ore deposits are explained in the final chapter of this section, Mineral Deposits Associated with Volcanism. The ores may form directly from the magma, from magmatic fluids when the magma becomes saturated in volatiles, or from hot groundwaters which circulate within a magmatic-hydrothermal system. The type of ore deposit which is formed depends on its tectonic environment. Above mantle plumes, flood basalt and komatiite eruptions host nickel-copper ores and platinum-goup deposits. At mid-ocean ridges, black smokers form massive sulfide ore deposits by precipitation of metallic minerals as the hot hydrothermal solution is quickly cooled by seawater. This type of deposit also is formed at oceanic subduction zones. Subduction-related volcanoes on land having hydrothermal systems may host large deposits of gold, copper, and other metals.
Hazel Rymer | |