Interactions between pyroclastic density currents : insights from analog experiments




O'Donnell, Sean Bryan

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During some explosive volcanic eruptions, multiple pyroclastic density currents have been produced within a short time span of each other and flowed through the same area. This creates the potential for the currents to interact, specifically in a way where a leading current is produced, and then a similar trailing current is produced a short time later and possibly flows into the leading current. The leading current, having changed the ambient surroundings from normal air, may then have an effect on the dynamics and behavior of the trailing current. To examine this effect, we designed scaled experiments to produce an analogue leading current and a trailing current that flows into it. The experiments took place in both an air medium and a water medium. The results of the experiments showed that the behavior of the trailing current may change as a result of interacting with the leading current. After certain intervals of time between currents, the trailing current had a longer final runout distance compared to the leading current it flowed through. This is caused by the presence of a plume created by the leading current when it reverses buoyancy. At intermediate heights above the bed, after moderate amounts of time between currents, the leading plume is less dense than the newly created trailing plume, and the trailing current cannot rise, and the momentum stays in the body of the trailing current. This accelerates the trailing current, which decreases sedimentation rate, and allows the current to runout to a greater distance before lifting off. At low heights and great heights above the bed, the leading plume is denser than the trailing plume, and the trailing plume can then rise without impediment. In natural pyroclastic density currents, the magnitude by which the leading current affects the trailing current depends on the rise time of the leading plume.


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