Rheological transitions in high‐temperature volcanic fault zones

Abstract

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 900°C 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 × 10À3 m s À1 and shear strain rates of 0.014-1.16 s À1. 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 900°C, the fault showed viscous deformation at a normal stress of 10 MPa, while frictional sliding was predominant at 800°C. 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.

Controls on explosive-effusive volcanic eruption styles

“…Positive feedbacks occur when faster ascent enhances bubble nucleation, which in turn produces smaller bubbles and reduced permeability 83 , and shear deformation that causes heating and vesiculation 71 . Negative feedbacks include the sealing of melt, dome and country rock fractures as a result of gas loss 59 , 128 ; heating of the wall rock to create a viscous 'brake' by inhibiting frictional slides 109 , 129 ; deformation during ascent that increases permeability and gas loss by promoting bubble coalescence; and crystallization driven by gas loss that increases magma viscosity and slows ascent. The combination of positive and negative feedbacks is one way to generate episodicity or even periodicity in eruption rate 130 .…”

On the kinematics and dynamics of crystal‐rich systems

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Physical characteristics of scoriae and ash from 2014–2015 eruption of Aso Volcano, Japan

“…Given the undeformed bubbles in the scoriae observed in Fig. 6, we consider that the measured stress originates from the frictional force between the scoria and the Inconel plate rather than from the viscous deformation of the scoriae, as has been reported for rhyolite (e.g., Okumura et al 2015). When the imposed shear rate increases at 300 s, the normal and shear stresses temporarily decrease and then increase.…”