A RHEOLOGICAL INVESTIGATION OF THE 1915 SILICIC LAVAS AT LASSEN PEAK, CALIFORNIA

Arc volcanoes, such as Lassen Peak, can suddenly switch between effusive and explosive styles of eruption. This southernmost active Cascades volcano is located within the Lassen Volcanic Center, where threat from volcanic hazards is considered to be very high. In 1915, Lassen Peak produced two effusive eruptions terminated by a subplinian eruption on May 22. The initial lava, erupted May 14–19, is often classified as a dacite lava dome. A second lava flow, which erupted following a conduit-opening explosive event on May 19, was emplaced within ~60 hours. The common consensus is that the second lava flow was less viscous than its predecessor owing to apparent differences in morphology and its capability to flow greater distance. Yet, no rheological measurements have been conducted.

We studied the three-phase rheology of silicic lavas at Lassen Peak through field interpretations of flow morphology and laboratory experiments. We measured the apparent viscosity of the lavas over timescales similar to their emplacement using a uniaxial parallel-plate viscometer at 958–1013 °C, low shear stress (~0.14 MPa), and low strain-rates. To gather detailed topographic data of the flow surfaces, LiDAR data was collected across a 40 km² swath with a density of 10 points per square meter. Glass compositions appear increasingly rhyolitic as the eruption sequence progressed. Our experiments show that the apparent viscosity of the second lava flow is up to 1.6 log units higher than the initial lava. At 958 °C, experimental samples vertically deformed ~2.5% over 45 hours (similar to emplacement timescale), implying eruption temperatures were very high (> 900 °C) in order to sustain viscous deformation. Rheomorphic and secondary structures, such as spines and joints, respectively, are oriented perpendicular to slope and flow direction creating fracture-bound ridges, implying that the lava flow underwent extension during emplacement. Upon reaching Lassen Peak’s steep-sided slopes, substrate topography likely had a heavier influence on flow rheology versus intrinsic factors such that gravity-driven extension enabled the higher viscosity lava to flow farther. Viscosity data imply both lavas exhibit strain hardening, which we infer to have contributed to plugging of the conduit, hampering gas escape and promoting explosive behavior.