Silicic magma experiences shear-induced brittle fracturing during its ascent, resulting in the formation of a magmatic fault at the conduit margin. Once the fault is formed, frictional behavior of the fault controls the magma ascent process. We observed torsional deformation of a magmatic fault gouge in situ at temperatures of 800 and 900C using synchrotron radiation X-ray radiography. The torsional deformation rate was set at 0.1-10 rpm, corresponding to equivalent slip velocities of 2.27 × 10-5-1.74 × 10-3 m s-1 and shear strain rates of 0.014-1.16 s-1. The normal stresses used were 1, 5, and 10 MPa. The magmatic fault showed frictional sliding as well as viscous flow even above the glass transition temperature. The transition between frictional sliding and viscous flow depends on temperature, deformation rate, and normal stress on the fault. At 900C, the fault showed viscous deformation at a normal stress of 10 MPa, while frictional sliding was predominant at 800C. We propose the ratio of timescales of fault healing and deformation as a criterion for transition between frictional sliding and viscous flow. The experimentally calibrated criterion infers that frictional sliding is predominant from ~500 m in depth during explosive eruption; this may explain rapid magma ascent without efficient outgassing. Frictional heating would in turn enhance fault healing, resulting in the reverse transition from frictional sliding to viscous flow, followed by deceleration of magma ascent. Therefore, cyclic transitions between frictional sliding and viscous flow are a possible explanation for the cyclic behavior of lava effusion.
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