Ground deformation caused by magma ascent in an open conduit

This study examines magma ascent processes of three basic types prior to intermittent explosions such as Vulcanian or Strombolian types: specifically their relations to volcano ground deformation. Such repetitive explosions eject magma from an open conduit at short time intervals. Consequently, magma pressure is expected to decrease at the conduit and/or reservoir, and thereby deflate the volcano. Then, magma within the conduit rises, exerting normal and shear stresses on the conduit wall, thereby inflating the volcano. Diffusive mass transfer of water molecules from the melt to bubbles might occur subject to a sudden depressurization by eruption when magma includes numerous small gas bubbles. Such gas bubble expansion lifts the magma in the conduit. Calculations of ground deformation on a semi-infinite medium for such rising magma show that vertical and radial displacements and tilt motions recorded in the far fields are proportional to the 1.5 power of time. For low-viscosity magma, gas bubbles might rise in the melt because of buoyancy force. The gas bubbles expand rapidly with time because of decreased ambient pressure, pushing the magma upward in the conduit. Consequently, the volcano slowly inflates initially; then the rate of deformation increases over time, eventually engendering a rapid inflation immediately before eruption. These temporal changes in ground deformations contrast against cases in which magma does not include gas bubbles. In such cases, amplitudes of ground deformation increase almost linearly or even decrease with time at far fields. These differences, which are recognized as basic characteristics of temporal changes of ground deformation, enable us to know the driving forces of magma in an open conduit and to evaluate the gas bubble behavior quantitatively in magma before eruption.