RECONSTRUCTING VOLCANIC CONDUIT PROCESSES: THE COMPLEMENTARY NATURE OF PUMICE/SCORIA AND ASH

Volcanic eruptions reflect the combined effect of conditions in the magma storage region and processes occurring in the conduit. The result is pyroclastic deposits that vary in grain size, density, shape and internal texture. These characteristics are used to categorize eruptive activity, particularly in the absence of eye witness accounts, and are often studied separately, and for discrete size classes. Here we show that a more complete picture of volcanic activity can be obtained by combining a range of data types from different pyroclast size classes. We focus particularly on the complementary information preserved in lapilli (pumice and scoria) and ash particles using examples from (1) the rhyodacite eruption of Mount Mazama that produced Crater Lake, OR, and (2) the second of three consecutive basaltic andesite eruptions of Mount Spurr, AK, in 1992. These examples show that ash particles commonly preserve a much wider textural range than co-erupted pumice/scoria. As a result ash deposits may provide a record of shallow conduit processes – including temporary plug formation – that is not evident in proximal pumice deposits. This raises a broader question of controls on magma fragmentation, particularly ways in which rates of magma decompression and resulting changes in the physical properties of conduit magma (vesicularity and crystallinity) control both the overpressure required for magma disruption and the resulting size distribution of constituent particles.