<p>We study the physical mechanisms that operate in impact zones of collapsing fountains during explosive volcanic eruptions. To this end, we simulate the formation of pyroclastic density currents in analogue laboratory experiments by releasing a mixture of glass particles of different sizes from a hopper, which impact the base of a horizontal channel. Acoustic attenuation measurements and high-speed imaging are used to monitor the particle concentration and velocity of falling mixtures just before impact. Over the course of the experiments, we identify two successive distinct regimes primarily governed by the particle concentration of the falling mixtures, which decreases over time. The first is an early decoupled regime, where particles settle in the impact zone to form a concentrated granular flow overlain by a dilute particle cloud. The second is a late coupled regime, in which particles remain entrained with the carrier air, resulting in a dilute suspension. At the transition between the regimes, we find a power-law relationship between particle concentration and the Stokes number, suggesting that these two parameters play an important role in controlling impact zone processes. We compare our results to recent numerical simulations on pyroclastic fountains and discuss relevant parameters that control the impact dynamics of pyroclastic mixtures.</p>