Dome growth and coulée spreading controlled by surface morphology, as determined by pixel offsets in photographs of the 2006 Merapi eruption

At many explosive volcanoes viscous domes extrude, which are destroyed by complete or partial collapses. Although the growth and development of silicic domes and the associated flow and collapse mechanisms are of vital importance for understanding the occurrence and scale of pyroclastic flows, quantitative measurements of dome deformations are limited. We report on a sequence of photographs taken of a growing and deforming dome. A sequence in 2006 featuring the Merapi dome taken from similar camera positions allow the application of a digital image correlation algorithm, the aim being to detect and explore the temporal evolution of pixel offsets. The results suggest that the dome underwent deformation in two regions between September and October 2006: (i) dome growth and spreading at the volcano summit and (ii) coulée flow through a narrow canyon. The latter is associated with strain localization and flow acceleration, which indicates that the displacements and flow velocities at silicic domes are governed by the topographic structure into which the flows develop. The downslope motion of the distal parts of the flow and apron slumps continued during episodes of dome extrusion by gravitational spreading. An analysis of the 2006 Merapi dome and coulée displacement also provides insights into processes that may occur along the newly established southerly eruption direction, which also may have influenced pyroclastic flows associated with the 2010 eruption.