NEW CUBIC-KILOMETER-SCALE BASALTIC FISSURE ERUPTION OF TOLBACHIK, KAMCHATKA, RUSSIA

The 1975-76 flank eruption of Plosky Tolbachik (3,085 m) was the largest basaltic eruption in the Kuril-Kamchatka arc during the past 200 years (>1 km³ DRE). On 27 November 2012 a short-lived swarm of shallow (<10 km) earthquakes marked the onset of the new eruption. The 3.5-km-long radial eruptive fissure opened on the south flank between 1500 – 2000 m a.s.l. Lava fountaining from multiple small vents ceased after several days and the eruption has continued from vents at the southern end of the fissure. Almost continuous lava fountains up to 200 m high issued from a small lava lake located inside the broad, open crater of the largest cinder cone. While explosive activity was rather mild, initial discharge of lava was very high (up to 400 m³/s) and by the end of December ‘a’a lava flows had travelled up to 17 km from the vent. SiO₂ concentrations for the plagioclase-phyric lava were 54 wt.%, but have since decreased to 52 wt.%. In January 2013 lava was transported through a system of lava tubes 1 km long and up to 5 m wide. From tube exit points it propagated in the form of channelized lava streams (velocities 1-3 m/s; discharge rates 30-50 m³/s); on lower slopes of the volcano it propagated mostly as ‘a’a flows. Lava channels were frequently dammed by floating clinker and accretionary balls, which caused flooding of proximal areas by ropy/shelly/slabby pahoehoe lavas. Locally small volumes of lava were extruded through the upper surfaces and lateral levees of ‘a’a lava to form very slowly inflating ‘toothpaste’, or pillow-like, lava lobes. Since mid-February the average intensity of the eruption has gradually declined, with sporadic bursts in February and April. By May discharge rates of lava had decreased to approximately 15 m³/s and most of lava started to flow as entrail pahoehoe. By July the volume of erupted products (dominantly lavas) reached approximately 1 km³. As of August 2013 the eruption was ongoing.