Jigsaw-fit blocks are highly fractured rocks of up to tens of meters wide, associated to volcanic debris avalanches and debris flows, traveling long distances (km) from the volcano edifice. Despite the mass movement long runout and agitated motion, jigsaw-fit blocks are found on their deposits with no apparent disaggregation and with occasional thin matrix facies filling the jigsaw cracks. The mechanisms behind the fragments constrained disaggregation and matrix infilling remain unclear and are limited to field observations. The rheology of granular flows suggests that segregation mechanisms should prevail and high fragmentation rates result from internal shearing and intense inter-granular collisions, challenging the theorized kinematics associated to the frustrated disaggregation of jigsaw-fit blocks. We study experimentally for the first time the segregation and disaggregation processes of analogue jigsaw-fit blocks within a granular flow as a function of fragmentation patterns and fragments density. We found that disaggregation appears regardless of the fragmentation pattern and its initiation is conditioned by the fragment density, occurring faster in fragments lighter than the moving granular media. Our results demonstrate that jigsaw-fit blocks remain united for a short period of shearing, but separate as a result of the fragments rotation and eventual particle infilling. We predict that this work sets a starting point for reviewing the interpretation of jigsaw-fit blocks in debris avalanche deposits, allowing the inference of kinematic features from the fragments configuration and its level of disaggregation.

We present the late Holocene eruption history of the poorly known Doña Juana volcanic complex, in SW Colombia, which last erupted in the twentieth century. This represents a case study for potentially active volcanism in the rural Northern Andes, where tropical climate conditions and a fragmented social memory blur the record of dormant volcanoes. We reconstructed the volcanic stratigraphy of the central-summit vent area by integrating new mapping at 1:5000 scale with radiocarbon ages, sedimentology analysis, and historical chronicles. Our results revealed cyclic transitions from lava-dome growth phases and collapse to explosive Vulcanian and possibly subplinian phases. Pyroclastic density currents were generated by dome collapse producing block-and-ash flows or by pyroclastic fountain/ column collapse and were rapidly channelized into the deeply incised fluvial valleys around the volcano summit. The pyroclastic density currents were ~4–10 × 10⁶ m³ in volume and deposited under granular flow– or fluid escape–dominated depositional regimes at high clast concentrations. In places, more dilute upper portions reached a wider areal distribution that affected the inhabited areas on high depositional terraces. The coefficient of friction (ΔH/L) is higher for block-and-ash flows and dense lava–bearing fountain/low-column-collapse pyroclastic density currents compared to pumice-bearing, column-collapse pyroclastic density currents. Associated mass-wasting processes included syneruptive and intereruptive debris flows, with the last one documented in 1936 CE.