Publications des scientifiques de l'IRD

Roche Olivier. (2015). Nature and velocity of pyroclastic density currents inferred from models of entrainment of substrate lithic clasts. Earth and Planetary Science Letters, 418, p. 115-125. ISSN 0012-821X.

Titre du document
Nature and velocity of pyroclastic density currents inferred from models of entrainment of substrate lithic clasts
Année de publication
2015
Type de document
Article référencé dans le Web of Science WOS:000352665200011
Auteurs
Roche Olivier
Source
Earth and Planetary Science Letters, 2015, 418, p. 115-125 ISSN 0012-821X
Deposits of pyroclastic density currents (PDCs) often contain accidental lithic clasts of typical size of 0.1-1 m captured from an underlying substrate by the parent flows at distances up to several tens of kilometers from the eruptive vent. In order to gain insights into the nature of PDCs, this study investigates the conditions required for entrainment of particles from a granular substrate by a gas-particle density current, with special emphasis to ignimbrite-forming currents whose dynamics are controversial. The two types of physics of emplacement of PDCs proposed in literature are considered. The first model deals with a hydraulically rough, dilute turbulent PDC of bulk density of similar to 1-10 kg/m(3) and considers that entrainment through both traction and saltation is controlled by a Shield criterion at high (>10(4)) particle Reynolds number. The second model considers entrainment by a PDC consisting of a dense basal flow of bulk density of the order of 10(3) kg/m(3) and with high interstitial gas pore pressure. This model involves uplift of substrate particles, caused by an upward pressure gradient at the flow-substrate interface, and then transport and deposition on the aggrading basal deposit of the flow as demonstrated by recent laboratory experiments. Results show that a dilute PDC can entrain blocks of maximum size of similar to 10-15 cm (for a block density of 2000-3000 kg/m(3)) if maximum current velocities up to similar to 100 m/s are taken into account. This, in turn, suggests that larger (heavier) blocks found in deposits were captured by PDCs if these had a dense basal flow. The dense flow model predicts that PDCs have the potential to entrain metric blocks, whose maximum size (up to similar to 2-5 m) decreases with decreasing atmospheric pressure (i.e. increasing altitude). Application of the model considering published data on the characteristics of accidental blocks in several well-studied ignimbrites indicates that the velocity of the parent dense PDCs was up to similar to 25-30 m/s.
Plan de classement
Sciences fondamentales / Techniques d'analyse et de recherche [020] ; Géologie et formations superficielles [064] ; Géophysique interne [066]
Localisation
Fonds IRD [F B010064109]
Identifiant IRD
fdi:010064109
Contact