Groundwater drainage from fissures as a source for lahars

One mechanism for generating lahars at volcanoes experiencing unrest is the disruption of internal aquifers. These disruptions can trigger releases of large quantities of groundwater. An example of such aquifer disruption occurred at Nevado del Huila Volcano, Colombia, during February and April 2007 when large fractures formed across the summit area of the volcano and lahars were emitted from them. Previous work interpreted that lahar volumes could not be accounted for by melted glacial snow or precipitation, and by elimination suggested that the primary water source was groundwater. Conceptual models have been developed for perched, confined aquifers that have been heated and pressurized by magma intrusions, followed by sudden pressure release and water emission during fracture formation. We consider an alternative end member wherein water release from large fissures at volcanoes is driven by simple gravity drainage. We apply numerical modeling to quantify water discharge from the porous medium surrounding a fissure with a low-elevation free exit. If a long fracture with high vertical extent (on the order of hundreds of meters) intersects a highly connected saturated porous medium, large volumes (on order 103 m3/m of crack length) of water may be released within tens of minutes. The drainage rates from the model may be adequate to account for the Nevado del Huila events if the medium surrounding the crack contains a large volume of water and has high horizontal permeability. This simple but poorly understood mechanism can present a hazard on its own or compound other processes releasing water from volcanoes.