Beyond Sedimentary Volcanism: A Hierarchical Classification of Mud, Sand and Related Sediment–Fluid Eruptive Systems

The terms mud volcano and sedimentary volcano can conflate material, process and surface expression. Here, a mud volcano is defined operationally as a present or preserved surface or seafloor expression of focused subsurface sediment–fluid extrusion, with geological evidence for source-to-expression connectivity and a characteristic mud-rich product or matrix. Sand-rich extrusions remain classifiable without being renamed mud volcanoes. A structured critical review, supplemented by selected Chinese-language studies, informs a hierarchical framework. An observational layer records object scale, material, temporal information and evidence provenance. Explicit decision rules distinguish supported sedimentary and hybrid systems, adjacent manifestations, candidates, unresolved alternatives and excluded objects. Five separate axes describe genetic association, environment, morphology, activity and discharge; evidence quality is distinguished from data coverage. The Quaternary bounds the terrestrial application, not the classification logic, and source age is separated from expression and activity age. A literature-derived demonstrator contains 29 cases, with 20 discussed in the main text and 26 terrestrial entries mapped. Source-linked evidence sheets and a guarded worksheet support field, subsurface and planetary applications without replacing conventional geological names. Unknown properties remain unassigned. The framework makes decisions auditable, but geological boundaries and independent inter-rater reliability remain unvalidated.