SNOW–ICE INTERACTIONS DURING ERUPTIONS OF ALEUTIAN ARC VOLCANOES AND IMPLICATIONS FOR LAHAR GENERATION

Many volcanoes in the Aleutian arc of Alaska are mantled by glacier ice and perennial snow. Eruptive activity results in a wide variety of snow-ice interactions and generation of variable amounts of melt water. Recent eruptions (Redoubt 2009; Veniaminof 2013; Pavlof 2007, 2013, 2014) highlight the different mechanisms of melt water generation and the resulting magnitude of lahars that eventually developed. Explosive (VEI 3 or greater) pyroclastic-flow producing eruptions, like the 2009 eruption of Redoubt Volcano, can generate significant volumes of melt water and large lahars (10⁷–10⁹ m³) due to the highly dynamic interaction of pyroclastic flows with glacier ice. During periods of lava fountaining associated with smaller magnitude Strombolian eruptions (≤VEI 3), collapse of unstable accumulations of spatter high on the flanks of the volcano often results in the formation of hot pyroclastic avalanches. These avalanches behave like pyroclastic flows (but are typically smaller in volume) and can sweep across ice and snow and generate melt water and lahars of moderate volume (typically <10⁴–10⁵ m³). This is a common process of lahar generation at Pavlof Volcano, and eruptions in 1986, 1996, 2007, 2013, and 2014 generated lahars by this mechanism. Eava producing eruptions that occur in the presence of ice and snow also produce melt water, but volumes are typically small in comparison to those produced by the mechanisms described above. The historical eruptions of Veniaminof Volcano in 1983, 1993, and 2013, and the 1991–92 eruption of Westdahl volcano produced small lava flows that advanced passively over glacier ice; visual observations indicate limited production of melt water from these events. Dynamic emplacement of pyroclastic flows over glacier ice that involves significant erosion of ice and snow and subsequent intermixing with hot pyroclastic debris leads to production of large volumes of melt water. Less dynamic, but still energetic interactions such as those that have occurred at Pavlof Volcano, produce smaller amounts of melt and correspondingly smaller-volume lahars. Lavas flowing onto ice and snow, (e.g., Veniaminof Volcano) result in the smallest amount of melt water production, mainly because the lava-ice interaction is not very dynamic and only a small proportion of the heat flux goes to melt ice.