Possible Role of Internal Erosion in the Development of Ground Fissures around Lake Ziway, Ethiopia

Ground fissures are linear or curvilinear cracks that are common in the Main Ethiopian Rift, which is part of the East African Rift System. They are a unique type of geohazard affecting roads and railway in many parts of the rift valley. The vicinity of Lake Ziway is underlain by lacustrine, pyroclastic, and/or volcaniclastic sediments that are cut by numerous ground fissures, which run for 2–3 km with up to 1–3-m-wide openings. Plausible causes of ground fissure events, as described in previous studies, include extensional movement due to active rift tectonics, hydro-compaction, and piping. However, the exact mechanism of their evolution is poorly understood. Based on field observations and sediment characterization, we propose a strong case for the role played by internal erosion or piping of pumice deposits in the development of ground fissures. Internal erosion causes the formation of subsurface conduits, which grow in size and eventually collapse, forming ground fissures. Typical sediment-erosion–related geomorphologic features such as disappearing streams, sinkholes, blind gullies, and piping mounds have been observed. These features indicate the presence of a network of pipes that connect the groundwater with surface water in a manner very similar to karst hydrology. Pumice deposits in the study area are found to be the most susceptible to internal erosion leading to ground fissuring due to their bimodal grain size distribution and ultralow density (average specific gravity = 0.6).