Secondary “distal lahars” after the ~4.2 ka BP Cerro Blanco Plinian eruption, Argentina: implications for source parameter estimation and volcanic hazard assessment

Abstract

Mass flows can remobilize ash-rich tephra deposited tens to hundreds of kilometers from its source during large Plinian eruptions, forming secondary “distal lahars”. Despite its significance, this process has received little attention, particularly from a hazard perspective. Recognizing remobilization processes in distal tephra is crucial for accurately estimating eruptive source parameters (e.g., volume) from pyroclastic deposits and ensuring the reliable use of these deposits for tephrochronological purposes. In this study, we present a facies analysis of distal tephra deposits associated with the ~4.2 ka BP Cerro Blanco Plinian caldera-forming eruption (Northwest Argentina). Although previous studies have classified this eruption as a Volcanic Explosivity Index 7 event, many distal deposits appear to have been remobilized, raising questions about that classification. Our observations indicate that secondary “distal lahars” were the dominant remobilization mechanism in distal areas in the aftermath of the eruption. These findings have implications for both hazard assessment and the estimated volume of the Cerro Blanco eruption, which may warrant revision.