Divulgazione e didattica
Sei qui: Home > News > Thermal transient PDC behavior induced by topographic drops: A test case at Mt. St. Helens, USA

Thermal transient PDC behavior induced by topographic drops: A test case at Mt. St. Helens, USA

Earth and Planetary Science Letters, Trolese et al. 2024 - Figura1
Schematic cartoon summarizing the transient process induced by a topographic drop. A recirculation zone with a typical transient time (τ) forms when the size of the current (Η) is comparable with the drop height (J). Such configuration alters the PDC internal flow structure, as exemplified by temperature (Τ) and velocity profiles (U) before, over, and after a topographic drop. After a time greater than τ and a distance comparable to J, the flow recovers its “normal” internal stratification, as the topographic control on the PDC dynamics is reduced. The deposit thickness (S) built during the transient sedimentation regime is Sτ = , where D is the average sedimentation rate of the current. Our main findings are valid in the regime where τ is significantly smaller than the typical time needed to fill a topographic drop (τd). In such case, the transient is not affected by the infilling of the topographic drop induced by sedimentation, and the topography can be considered as fixed. A full description of these parameters is available in the Methods, Results, and Discussion sections, where considerations concerning the other regimes are given.

Trolese M., M. Cerminara, G. Giordano, B.D. Brand, N. Pollock, F. Cifelli, M. Mattei, 2024.
Earth and Planetary Science Letters, 627, https://doi.org/10.1016/j.epsl.2023.118441


Topography plays an important yet uncertain role in modulating the temporal and spatial evolution of the internal structure of pyroclastic density currents (PDCs). Understanding such changes is critical to characterize PDC transport regimes and their hazard. Here we combine paleomagnetic data from PDC deposits of the 18 May 1980 Mt. St. Helens eruption with numerical outcomes to capture spatio-temporal temperature variations induced by topography. We show that emplacement temperatures along the northwest flank of the volcano are ≃100°C colder than those recorded along the northeast flank in response to proximal topographic drops. We further report that such vertical drops lead to an initial transient regime where the PDC internal temperature, velocity, and concentration stratification is altered for periods of time that are proportional to the ratio between the drop height and the square root of the current thickness. The topographic control on PDC dynamics is attenuated moving away from the drops or when a stationary phase is attained. Collectively, our results highlight that topographic regions promoting the flow separation/reattachment process are associated with vigorous entrainment of ambient air in the lower portion of PDCs. Low temperature variability is observed in the absence of such topographic irregularities. Based on our findings, we propose a local sedimentation rate of ≃150 ± 100 mm s−1 for PDC deposits in a proximal reattachment region. This investigation demonstrates the importance of transient processes in PDC dynamics, introducing a new methodology to measure sedimentation rates, and highlighting that flow-topography feedbacks should be considered to assess hazards.