The definition of a volcano is discussed, and a new encompassing version is provided. The discussion focuses on the observations that volcanism is a self-similar process that ranges many orders of magnitude in space and time scales, and that all kinds of geologic processes act on volcanoes. Former definitions of volcano , such as that from the Glossary of Geology (1997, p. 690)—“a vent in the surface of the Earth through which magma and associated gases and ash erupt” or “the form or structure, usually conical, that is produced by the ejected material” are clearly insufficient. All definitions that we encountered tend to consider volcanoes from the point of view of a single discipline, each of them neglecting relevant aspects belonging to other disciplines. For the two cases mentioned above a volcano is seen only from the point of view of eruptive activity or of morphology. We attempt to look at volcano holistically to provide a more comprehensive definition. We define a volcano as a geologic environment that, at any scale, is characterized by three elements: magma, eruption, and edifice. It is sufficient that only one of these elements is proven, as long as the others can be inferred to exist, to have existed, or to have the potential to exist in the future.

During flow, lava is subjected to a combination of compressional, extensional, and shearing forces that may result in folds and fractures on the lava surface, depending on the rheologic and material properties of the lava. A series of analog experiments were performed to examine the partitioning of strain associated with shear stresses and the formation of surface fractures. Experiments simulated a range of flow conditions (increasing, decreasing, and constant effusion rates) and different surface crust rheologies (viscous, brittle, and rigid). Strain in experimental flows displayed a combination of pure shear (α) and simple shear (γ) components, which vary in space and time. Pure shear was largely controlled by changing effusion rate. Flows with decreasing effusion rates exhibited pure shear consistent with extension (α > 1) due to progressive flow thinning. In contrast, flows with increasing effusion rates experienced pure shear associated with compression (α < 1) due to thickening and increasing flow velocity. Flows with constant effusion rates displayed simple shear (α ∼ 1) and had stable flow lengths and velocities. Simple shear in the fluid part of the flows was produced by friction along the flow base and, in the experiments with a rigid surface crust, friction below the surface crust. In experiments with brittle crusts, fracture patterns were similar to those observed in natural lava flows (crease structures and tension gashes). As these fractures spread, they were filled by isostatic upwelling of the viscous flow interior. Flows with brittle surface crusts also produced basal breccia due to rolling of the flow front.

Simple experiments were carried out to study the fracturing of the outer crust of lava domes during emplacement. Analog magma was injected vertically from a reservoir into a feeder conduit and flows on a rigid planar base. A cohesive mixture of sand and flour poured on the dome simulated the lava dome crust generated by cooling. Results showed that two opposite end members have to be considered: symmetrical versus asymmetrical deformation for thin and thick brittle shells, respectively. Thin crusts produce gently dipping slopes with mainly radial fractures. In contrast, thick crusts generate steeply dipping flank slopes on which deformation is restricted. Between the end members, there is a general change from one style to the other. For natural domes, the experiments indicate that a certain thickness of crust is necessary to produce explosive activity: thick crust will cause more violent events.

Abstract The process by which magma ascends into and deforms a volcanic edifice is studied by analogue modelling. A control experiment is conducted with a wooden piston moving vertically into a sand cone. This reveals a well-defined fault pattern that makes it possible to draw the main compressive stress trajectory within the cone during the ascent of the piston. This makes it possible to show that the deformational process is that of indentation of the cone by the rigid piston. Experiments with an indenter that is viscous, as in nature, show that the motion of the viscous body is controlled by the first fault created in the cone. This fault serves as a structural guide, making the viscous body deviate from the vertical and resulting in deformation of the flank of the cone, which bulges out. Other major shear faults that were observed in the control experiment are then inhibited and do not form. This result emphasizes that the structural evolution of an indentation process within a brittle cone and at low rate depends on the rheology of the indenter.