Aging, change in property depending on the elapsed time from preparation, is known to affect the rheological behavior of various materials. Therefore, whether magma ages must be examined to characterize potentially widespread volcanic phenomena related to the transition from rest to flow. To achieve this, we performed rheological measurements and microstructural analyses on basaltic andesite lava from the 1986 Izu-Oshima eruption. The rheology shows an initial overshoot of shear stress during start-up flow that correlates with the duration and the shear rate of a pre-rest time. This indicates that the yield stress of magma and lava increases with aging. The microstructure shows that original aggregates of crystals, which may grow during crystallization, coalesce during the pre-rest period to form clusters without changing the crystal volume fraction. We conclude that the clusters are broken by shear in the start-up flow, which induces the stress overshoot. Thus, aging in magma rheology will impact the understanding of dynamic flow.
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