Destructiveness of pyroclastic surges controlled by turbulent fluctuations

Pyroclastic surges are lethal hazards from volcanoes that exhibit enormous destructiveness through dynamic pressures of 100–102 kPa inside flows capable of obliterating reinforced buildings. However, to date, there are no measurements inside these currents to quantify the dynamics of this important hazard process. Here we show, through large-scale experiments and the first field measurement of pressure inside pyroclastic surges, that dynamic pressure energy is mostly carried by large-scale coherent turbulent structures and gravity waves. These perpetuate as low-frequency high-pressure pulses downcurrent, form maxima in the flow energy spectra and drive a turbulent energy cascade. The pressure maxima exceed mean values, which are traditionally estimated for hazard assessments, manifold. The frequency of the most energetic coherent turbulent structures is bounded by a critical Strouhal number of ~0.3, allowing quantitative predictions. This explains the destructiveness of real-world flows through the development of c. 1–20 successive high-pressure pulses per minute. This discovery, which is also applicable to powder snow avalanches, necessitates a re-evaluation of hazard models that aim to forecast and mitigate volcanic hazard impacts globally.

Miocene Volcaniclastic Environments Developed in the Distal Sector of the Bermejo Basin, Argentina

During the Miocene, in the distal sectors of the Bermejo Basin, a complex relationship developed between a floodplain and contemporary volcanic activity. Seven stages of sedimentation are established to interpret this paleoenvironmental relationship. Stage I corresponds to the development of the floodplain previous to pyroclastic activity; in Stage II, pyroclastic activity is manifested by fall deposits and dry pyroclastic surges. A probable calm in the volcanic activity, associated with exceptional rains, generates laharic deposits (Stage III). Stage IV is dry pyroclastic surges that collapse the floodplain. Subsequently, the river system is reestablished (Stage V) under a regime of low to null volcanic activity. During Stages VI and VII, thick deposits of dry and wet pyroclastic surges, which have records of contemporary seismic activity. The presence of deformational structures within the pyroclastic deposits and lahars indicate that the volcanic centers were in distant areas. The volcanism that generated these deposits is probably associated with the migration to the east of the Miocene volcanic arc of the Cordillera de Los Andes or could be associate with the volcanism of the Sierra de Famatina.

Soft sediment deformation in dry pyroclastic deposits at Ubehebe Crater, Death Valley, California

Soft sediment deformation structures are common in fine-grained pyroclastic deposits and are often taken, along with other characteristics, to indicate that deposits were emplaced in a wet and cohesive state. At Ubehebe Crater (Death Valley, California, USA), deposits were emplaced by multiple explosions, both directly from pyroclastic surges and by rapid remobilization of fresh, fine-ash-rich deposits off steep slopes as local granular flows. With the exception of the soft sediment deformation structures themselves, there is no evidence of wet deposition. We conclude that deformation was a result of destabilization of fresh, fine-grained deposits with elevated pore-gas pressure and dry cohesive forces. Soft sediment deformation alone is not sufficient to determine whether parent pyroclastic surges contained liquid water and caused wet deposition of strata.