Magma–water interactions

Abstract

Overview Magma–water interaction is an unavoidable consequence of the hydrous nature of the Earth’s crust, and may take place in environments ranging from submarine to desert regions, producing volcanic features ranging from passively effused lava to highly explosive events. Hydrovolcanism is the term that describes this interaction at or near the Earth’s surface, and it encompasses the physical and chemical dynamics that determine the resulting intrusive or extrusive behavior, and the character of eruptive products and deposits. The development of physical theory describing the energetics and the hydrodynamics (dynamics of fluids and solids at high strain rates) of magma–water interaction relies on an understanding of the physics of water behavior in conditions of rapid heating, the physics of magma as a material of complex rheology, and the physics of the interaction between the two, as well as detailed field observations and interpretation of laboratory experiments. Of primary importance to address are the nature of heat exchange between the magma and water during interaction, the resulting fragmentation of the magma, and the constraints on system energetics predicted by equilibrium and non-equilibrium thermodynamics. Taken together, these approaches elucidate the relationships among aqueous environment, interaction physics, and eruptive phenomena and landforms. Introduction: magma and the hydrosphere The vast majority of volcanic eruptions take place under water because most volcanism concentrates at mid-oceanic ridges where new oceanic crust is produced. By definition, every kind of extrusive subaqueous volcanism on Earth is hydrovolcanic since some degree of water interaction must take place. The hydrosphere also exists in continental areas, as the consequence not only of lakes and rivers, but also of groundwater and hydrous fluids that circulate in joints and faults in the upper crust and fill pore space in sedimentary rocks. Such locations are typically referred to as geohydrological environments. As a consequence, subaerial volcanism is commonly influenced by magma–water interaction. Chapter 12 describes deep-sea eruptions in greater detail, whereas this chapter focuses on magma–water interaction in surface and near-surface environments.