The 2012–2013 flank eruption of Tolbachik volcano (Kamchatka) lasted 9 months and produced 0.54 km3of basaltictrachyandesite lava, thus becoming one of the most voluminous historical lava effusions of basic composition in subduction-related environments globally. From March to July 2013, the volcano monotonously erupted lava of constant composition(SiO2= 52 wt%) with a nearly stable effusion rate of 18 m3/s. Despite the uniform eruptive and emplacement conditions, thedominant style of lava propagation throughout that time gradually changed from‘a’a to pahoehoe. We report results of instru-mental field measurements of the‘a’a and pahoehoe flow dynamics (documented with time-lapse cameras) as well as the lavaviscosity determined by flow rate and shear stress (using penetrometer) methods. Maximal propagation velocities of the‘a’afronts ranged from 2 to 25 mm/s, and those of the pahoehoe from 0.5 to 6 mm/s. The flow front velocities of both lava typesexperienced short-period fluctuations that were caused by complex flow mechanics of the advancing flow lobes. Minimalviscosities of lava of the‘a’a lobes ranged from 1.3 × 105to 3.3 × 107Pa s (flow rate method), and those of the pahoehoe fromto 5 × 103to 5 × 104Pa s (shear stress method). Our data include the first ever measured profiles of viscosity through the entirethickness of actively advancing pahoehoe lava lobes. We have found that both the‘a’a and pahoehoe flows were fed by identicalparental lava, which then developed contrasting rheological properties, owing to differences in the process of lava transport overthe ground surface. The observed transition from the dominant‘a’a to the dominant pahoehoe propagation styles occurred due togradual elongation and branching of the lava tube system throughout the course of the eruption. Such evolution became possiblebecause the growing lava field, composed of semisolidified flows, provided an environment for shallow subsurface intrusionsand internal migrations of lava that, with time, developed into branches of the lava tube system. Based on our data, we proposephenomenological models of the‘a’a and pahoehoe flow mechanics.
