Volcano Deformations Preceding Intermittent Eruptions From an Open Conduit

Recent geodetic measurements at active volcanoes have revealed small volcano inflations that precede each small explosion of Strombolian or Vulcanian eruptions. These geodetic data must give us some important information on the magma ascent process in an open conduit, which are, for example, gas bubble growth and out-gassing processes that control the volcanic explosivity. In the present study, some basic processes of magma ascent in an open conduit are examined with their relations to the volcano deformation that are quite useful and widely used to monitor the volcano activity. Since the magma in the uppermost part of an open conduit is subject to a sudden depressurization by a short duration of intermittent eruptions, gas bubble growth due to diffusion mass transfer of the water molecules from melt to gas bubbles may occur in the magma. In such case, increase of the total volume of magma can rise magma itself up in the conduit to exert stresses on the conduit wall. Calculations show that vertical and horizontal displacements and tilt increase with time, being proportional to the 1.5 power of time. When magma is basaltic with a low viscosity, gas bubbles can rise up in melt by buoyancy force. Since ambient pressure decreases, ascent of gas bubbles leads to gas bubble expansion that pushes up magma surface level in the conduit. In this case, it is predicted that the volcano edifice is slowly inflated at the beginning, and is gradually accelerated toward an eruption. On the other hand, when magma does not accompany gas bubble growth, the magma upward migration in an open conduit can be approximated as a Poiseuille flow driven by pressure difference caused between the top of magma and deeper parts in the conduit. The ground deformation almost constantly increases or even decreases with time. Such no accelerated temporal changes are different from those for the ascent of magma that accompanies gas bubble growth. These theoretical considerations strongly suggest that temporal changes of volcano deformations can be used for understanding the mechanism of magma rise in an open conduit and for estimating some physical parameters of micro-scale phenomena in magma which controls the explosivity of eruptions.