PAST AND FUTURE HAZARDS AT VALLES CALDERA, NM: ERUPTION DYNAMICS OF EL CAJETE PYROCLASTIC FALL

Caldera-forming eruptions produce some of the most violent geologic hazards on Earth. Smaller ‘post-caldera’ or ‘inter-caldera’ eruptions, also produce significant hazards associated with effusive lava dome emplacement to explosive eruptions consisting of pyroclastic falls and flows. Importantly, these smaller events define the potential hazards for future activity. The Valles caldera, NM, most notably erupted two colossal eruptions 1.6 and 1.2 m.y, each emitting roughly 300 – 400 km³ DRE (dense rock equivalent) of magma. Post-caldera activity mostly consisted of lava dome eruptions along a ring fracture, however, after ~460 ka without eruption, the Valles caldera most recently produced an explosive episode ~74 ka, which included emplacement of the El Cajete pyroclastic beds. These deposits are petrographically and compositionally distinct from previous eruptions and are thought to represent a new batch of magma beneath the caldera. Any future eruption at Valles will likely be similar to the 74 ka eruptions, however, the stratigraphy, and therefore eruption characteristics are poorly resolved, making future hazard assessments difficult.

This study examines the two uppermost and most recent El Cajete pyroclastic fall units through detailed stratigraphic and granulometric analysis to quantify and better understand the eruption characteristics of the youngest explosive phases from the system. We report preliminary estimates here: the volume erupted was 0.38-0.47 km³ per eruption phase; the eruption column height was 20-26 km for both phases; and mass eruption rates (MER) were 7.5x10⁷ kg/s and 1x 10⁸ kg/s, respectively. These findings indicate that a future eruption at Valles caldera could affect cities >100 km from the eruption vent due to widespread ash fall, while communities in the immediate surrounding valleys could be threatened by pyroclastic flows. Continued refinement of these estimates is necessary in order to provide a robust assessment of this most recent eruption as well as forecasting future hazards in the vicinity of the caldera.