A review of electromagnetic approaches to the seafloor spreading ridge system

Electrical conductivity (or its reciprocal resistivity) is sensitive to temperature, component, and phase of materials, and the degree and form of interconnection of material in matrix for two or more materials’ system. Revealing electrical conductivity structures of the crust and the upper mantle of the seafloor spreading ridge system is useful to place constraints on current states and processes of the system, such as thermal structure, a region of partial melting and the amount of melt existing within the region, the amount of water in source mantle, mantle upwelling, and hydrothermal circulation. Marine geophysical electromagnetic explorations by using natural and artificial sources (Magnetotellurics, Magnetometoric resistivity method, Controlled-source electromagnetic method, and so on) have been normal to reveal electrical conductivity structures of the system thanks to recent development and increase of observational instruments. We review field experiments of electromagnetic approaches for exploring mid-ocean ridges and back-arc spreading ridges in various tectonic settings, focusing on natural-source experiments conducted in recent years at Indian, EPR, Mariana, and Lau ridge systems. Ridge systems investigated have diverse characteristics in seafloor spreading rate and crustal production rate, chemistry and temperature of source mantle, and the proximity to subducted plate. Electrical conductivity structures revealed from the field data provide an opportunity to understand dynamics of individual ridge system as well as to obtain geophysical knowledge on the systematics of the global seafloor spreading ridges.