(Edited by: Tatsuhiko Kawamoto, Stephen Kirby, Junichi Nakajima, Bruno Reynard, Hiroaki Toh)

Almost all physico-chemical characteristics of earth-forming materials are influenced by the presence of H₂O. As N. L. Bowen stated in 1928, H₂O plays the role of Maxwell’s demon—it does just what a petrologist may wish it to do [p. 282, The evolution of the igneous rocks (Bowen, 1928). In the following decades, this has been proven to be the case not only in petrology but in every field of solid Earth science.

H₂O is the most abundant fluid in the Earth, except for liquid iron alloys present in the outer core. Volcanoes emit magmas and volatiles, which include COH ± S ± N species (chemical compounds composed of Carbon, Oxygen, Hydrogen, ± Sulfur, and ± Nitrogen), halogens (F, Cl, I, Br), rare gases (He, Ne, Ar, Kr, Xe), fluid-mobile elements such as alkali elements (Li, Na, K, Rb, Cs), B, possibly Pb and U, and less likely Th. In the Earth’s interior, these volatile components exist as geofluids, affecting various phenomena and acting as effective tracers for the respective phenomena. Seawater and atmosphere are geofluids that have accumulated on the surface of the Earth, and they hydrate and carbonate lithosphere through chemical reactions and depositions. Geofluids are released from subducting lithospheres, migrate upward, and play vital roles in various subduction-zone phenomena, such as magma genesis, seismic activity, rock deformation, and electromagnetic response. Geofluids also affect mantle dynamics, including global material circulation and chemical differentiation, and the transportation of mainly C-O-H fluids into the Earth’s interior. This special issue is a collection of 30 studies on such geofluid processes.