Unveiling changing eruptive style after sector collapses in basaltic, arc stratovolcanoes: Examples from the eastern Pacific

Abstract:

Basaltic volcanism can produce a range of complex geomorphologies. High eruption rates typical of many basaltic volcanoes build massive edifices, but internal (e.g. magmatic intrusions) and external structural weakening from rapid growth may make them gravitationally unstable, triggering landslides and sector collapses. Moreover, sudden unloading of the volcanic edifice caused by a sector collapse can lead to the low-viscosity magma to produce a violent volcanic response in the short term, and migration of the main vent towards the depressurized area in the medium term. Understanding whether sector collapses in basaltic systems are produced by or trigger the eruption is a fundamental task for volcanic hazard assessment. In this contribution we review and investigate three different sector collapses in basaltic volcanoes and their responses to this sudden decompression, two of them located in the Southern Andes Volcanic Zone of Chile (SAVZ) and one in Guatemala: (1) Planchón I volcano (SAVZ) collapse at ca. 11 ka, with a massive ~10 km³ volume debris avalanche deposit (DAD) followed by an basaltic andesite lava flow on top; (2) Antuco volcano (SAVZ) where the radiocarbon age of a 5 km³ DAD agrees well with an unprecedented Plinian tephra fallout dispersed eastwards about 4 ka, and westward stratigraphy includes massive pyroclastic density currents and subsequent lava flows, and: (3) Pacaya volcano (Guatemala) ~1 ka sector collapse (DAD >0.7 km³) representing an ongoing explosive eruption interrupted and then enhanced by flank failure. We present new information on distribution, sedimentology and facies of their DAD deposits, in addition to stratigraphic investigations and geochemical data, to help constrain the nature of each one of these events and provide foundational data to support numerical models, allowing a comprehensive view of the effects of lithostatic pressure loss on shallow basaltic magmatic systems.