Abstract
In the aftermath of pyroclastic density current-dominated eruptions, lahars are the main geomorphic agent, but at the decadal scale, different sets of processes take place in the volcanic sediment cascade. At Unzen volcano, in the Gokurakudani gully, we investigated the geomorphologic evolution and how the topographic change and the sediment change over time is controlling this transition. For this purpose, a combination of LiDAR data, aerial photography and photogrammetry, ground penetrating radar and sediment grain size analysis was done. The results show choking zones and zones of enlargement of the gully, partly controlled by pre-eruption topography, but also by the overlapping patterns of the pyroclastic flow deposits of 1990–1995. The ground penetrating radar revealed that on top of the typical lahar structure at the bottom of the gully, side wall collapses were trapping finer sandy sediments formed in a relatively low-energy deposition environment. This shows that secondary processes are taking place in the sediment transport process, on top of lahar activity, but also that these temporary dams may be a source of sudden sediment and water release, leading to lahars. Finally, the sediments from the gully walls are being preferentially oozed out of the pyroclastic flow deposit, meaning that over longer period of time, there may be a lack of fines, increasing permeability and reducing internal pore pressure needed for lahar triggering. It also poses the important question of how much of a past event one can understand from outcrops in coarse heterometric material, as the deposit structure can remain, even after losing part of its fine material.
Highlights
Introduction and State of the ArtQuantifying landforms by balancing erosion and uplift on mountain and volcanic slopes is a century-old quest: e.g., [1]
The present volcanic geomorphology study at Unzen volcano has demonstrated that: (1) The left and right walls of the Gokurakudani gully evolve at different rates and that the collapsed profiles and maximum sustained slope angles vary by more than 20 degrees
This difference corresponds to layers in the pyroclastic density current deposits either tilting towards the gully or away from it
Summary
Sediment fluxes increase sharply in the aftermath of eruptions. At Merapi Volcano (Indonesia), out of all the 851 lahars produced during the 44 years of existing records, 11.2%, 9.2% and 15.2% occurred immediately after respectively the eruptions of 1969, 1975 and 2011 [5]. The timing of the rainfall and the availability of removable material is crucial This pattern could be observed at Mt. Pinatubo, where broadband seismometers show an increase in high sediment concentration events between 1991 to 1994, corresponding to increased-intensity rainfalls over the years. Because the period 1980–1994 was relatively dry, the decreasing trend in sediment fluxes in the first 14 years after the volcanic collapse suddenly ramped up in 1995–1996, starting a new decreasing trend [11] From these events, it appears that at the decade and multi-decade scale, the sediment fluxes mostly carried by lahars are first controlled by the amount of material available, before being controlled more predominantly by rainfall trends. Despite a relatively rapid (considering the life of a volcanic structure) decrease in lahar frequency, their deposits dominate the material volcanic aprons are made of [8,13,14]
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