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Magma–Water Interactions Ken Wohletz, Bernd Zimanowski and Ralf Büttner

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Magma–Water Interactions Ken Wohletz, Bernd Zimanowski and Ralf Büttner Chapter 11 Magma–water interactions Ken Wohletz, Bernd Zimanowski and Ralf Büttner energetics predicted by equilibrium and non- Overview equilibrium thermodynamics. Taken together, these approaches elucidate the relationships Magma–water interaction is an unavoidable con- among aqueous environment, interaction phys- sequence of the hydrous nature of the Earth’s ics, and eruptive phenomena and landforms. crust, and may take place in environments ran- ging from submarine to desert regions, producing volcanic features ranging from passively effused lava to highly explosive events. Hydrovolcanism 11.1 Introduction: magma and is the term that describes this interaction at or the hydrosphere near the Earth’s surface, and it encompasses the physical and chemical dynamics that deter- The vast majority of volcanic eruptions take mine the resulting intrusive or extrusive behav- place under water because most volcanism ior, and the character of eruptive products and concentrates at mid-oceanic ridges where new deposits. The development of physical theory oceanic crust is produced. By definition, every describing the energetics and the hydrodynam- kind of extrusive subaqueous volcanism on ics (dynamics of fluids and solids at high strain Earth is hydrovolcanic since some degree of rates) of magma–water interaction relies on an water interaction must take place. The hydro- understanding of the physics of water behav- sphere also exists in continental areas, as the ior in conditions of rapid heating, the physics consequence not only of lakes and rivers, but of magma as a material of complex rheology, also of groundwater and hydrous fluids that cir- and the physics of the interaction between the culate in joints and faults in the upper crust and two, as well as detailed field observations and fill pore space in sedimentary rocks. Such loca- interpretation of laboratory experiments. Of pri- tions are typically referred to as geohydrologi- mary importance to address are the nature of cal environments. As a consequence, subaerial heat exchange between the magma and water volcanism is commonly influenced by magma– during interaction, the resulting fragmentation water interaction. Chapter 12 describes deep-sea of the magma, and the constraints on system eruptions in greater detail, whereas this chapter Modeling Volcanic Processes: The Physics and Mathematics of Volcanism, eds. Sarah A. Fagents, Tracy K. P. Gregg, and Rosaly M. C. Lopes. Published by Cambridge University Press. © Cambridge University Press 2012. 9780521895439c11_p230-257.indd 230 8/2/2012 9:50:39 AM MAGMA–WATER INTERACTIONS 231 focuses on magma–water interaction in surface formalized by Abraham Werner). Initial ideas and near-surface environments. about the role of ground and surface water in vol- The explosive intensity of volcanic eruptions canism developed during the last century. These depends on the extrusion rate of magma and on perceptions resulted largely from observations the coupling of its thermal energy (i.e., heat flux) of unusually explosive periods of Hawaiian vol- to the surroundings. Because the thermal con- canism, during which groundwater entered rifts ductivity of magma is very low, hydrovolcanic along which normal lava fountaining had previ- heat flux is mainly governed by the size of the ously occurred (Jaggar, 1949), as well as through interfacial area between magma and water and examination of fragmental basalts found where its growth rate during interaction. Thus, the key lava had entered water (Fuller, 1931). Three well- process that determines the thermal energy flux documented eruptions during the late 1950s and from the magma to its surroundings is magma early 1960s brought an increased awareness of fragmentation. explosive hydrovolcanism: Capelinhos, Azores Water is the predominant thermodynamic (Tazieff, 1958; Servicos Geologicos de Portugal, working fluid on Earth, and practically every 1959), Surtsey, Iceland (Thorarinsson, 1964), power plant in the world uses water for con- and Taal, Philippines (Moore et al., 1966). Fisher verting thermal energy into mechanical energy and Waters (1970), Waters and Fisher (1971), and finally into electricity. Where rising magma and Heiken (1971) expanded the characteriza- contacts water (in contrast to rocks or atmos- tion of phreatomagmatic eruptions to include pheric gases) the major effect is an increase in steam-rich eruption columns, base surges, and thermal energy flux and, by analogy to commer- typical landforms such as maars, tuff rings, and cial power production, in the efficacy of heat tuff cones. As a result of this work, numerous and power generation. For this reason there is twentieth century phreatomagmatic eruptions a rich technical literature in science and engin- are now recognized, many of which formed eering that can be applied to understanding maar-like craters (e.g., Self et al., 1980). After magma–water interactions; this is the intent of cinder cones, phreatomagmatic constructs (tuff this chapter. rings, tuff cones, and maars) are perhaps the most abundant terrestrial volcanic landform. However, magma–water interaction is certainly not limited to explosive phreatomagmatic 11.2 Hydrovolcanism: from pillow eruptions – the hydrologic environment plays lava to maar tephra a major role in determining the kind of inter- action that can occur. 11.2.1 History of hydrovolcanism Magma–water interactions occur deep within the 11.2.2 Hydrovolcanic environments Earth (hydromagmatism) as well as at or near its The wide variety of hydrovolcanic phenom- surface (hydrovolcanism). These terms are roughly ena suggests that interaction between water synonymous because in many cases the realm and magma or magmatic heat may occur in where interaction initiates and later manifests any volcanic setting and geohydrological envir- may span from deep within the Earth’s crust onment, and is not restricted to a particular to the surface. For this reason the term phreato- magma composition. From the formation of magmatic is used to designate interaction within pillow lava and lineated, folded, and jumbled the phreatic realm of the Earth’s surface, which sheet flows in deep water, to the intrusion of includes the zone of saturation where ground- breccia in dikes and sills deep in the crust, to water and surface water exist. the eruption of plumes of fine ash in desert, The topic of magma–water interaction may tropical, and shallow water environments, be considered to have its roots in the eighteenth- magma–water interaction includes both pas- Century Neptunists’ theory about the origin sive and dynamic phenomena. During ascent of basaltic rocks in oceans (which was later to the surface, magma commonly encounters 9780521895439c11_p230-257.indd 231 8/2/2012 9:50:39 AM 232 KEN WOHLETZ, BERND ZIMANOWSKI, AND RALF BÜTTNER Figure 11.1 Schematic illustration of common environments of hydrovolcanism (adapted from Wohletz, 1993). 0.1–0.5 0.3–1.5 km km 0–0.2 PLUME 0.1–2.0 km 20–50 HEIGHT km 10–40 0.5–3.0 km km km subaqueous hawaiian and strombolian plinian columns phreatoplinian surtseyan surtseyan lava flows ERUPTIVE ballistic scoria tephra fall column/collapse ash fall cypressoid jets (pillow, lineated, PHENOMENA tephra fall PDCs ashfall, PDCs lateral surges falls and surges folded, jumbed, sheet) mm- to m-size fragments m- to mm-size fragments DOMINANT cm-size fragments µ condensing steam superheated steam PRODUCTS liquid water ROLE OF EXTERNAL Little/none Optimum Excessive WATER Increasing water abundance Figure 11.2 Relationship of typical eruptive phenomena and products to water abundance (adapted from Sheridan magma fragments is called phreatic (Ollier, 1974) and Wohletz, 1983). Deposit features indicative of magma– or hydrothermal (Muffler et al., 1971; Nairn and water interaction may include planar to duneform bedding, Solia, 1980). Subglacial volcanism (Noe-Nygaard, impact sags or slumps and, particularly for greater water 1940; see Chapter 13) is best known from its abundances, accretionary lapilli, soft sediment deformation structures, and cohesive, altered, or vesiculated tuff. products, including massive floods jökulhlaups( ), table mountains (tuyas or stapi), and ridges (tin- dars or mobergs). In all these environments, a major factor determining the expression of groundwater, connate (entrapped depositional) hydrovolcanism is the abundance of water water, marine, fluvial, or lacustrine water, ice, available to interact with the magma. Not only or rain water (Fig. 11.1). From this diversity are the eruptive phenomena affected by water of hydrovolcanic environments comes a wide abundance, but so are the characteristics of the range of terminology. For example, the subaque- interaction products, their dispersal, and the ous environment includes all activity beneath resulting landforms (Fig. 11.2). a standing body of water (Kokelaar, 1986); products of this activity have been called sub- 11.2.3 Hydrovolcanic eruption styles aquatic (Sigvaldason, 1968), aquagene (Carlisle, A wide variety of eruption styles result from 1963), hyaloclastite (for deep marine; Bonatti, magma–water interactions, ranging from pas- 1976), hyalotuff (for shallow marine; Honnorez sive lava effusion to highly energetic thermo- and Kirst, 1975), and littoral (Wentworth, 1938). hydraulic explosions, depending on ambient Volcanism that heats groundwater to produce conditions and the proportions of water and steam explosions that do not eject juvenile magma interacting (Fig. 11.2). At the highest 9780521895439c11_p230-257.indd 232 8/2/2012 9:50:40
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