Thick lava flows of Karisimbi Volcano, Rwanda: insights from SIR-C interferometric topography

We use a digital elevation model (DEM) derived from interferometrically processed SIR-C radar data to estimate the thickness of massive trachyte lava flows on the east flank of Karisimbi Volcano, Rwanda. The flows are as long as 12 km and average 40-60 m (up to >140 m) in thickness. By calculating and subtracting a reference surface from the DEM, we derived a map of flow thickness, which we used to calculate the volume (up to 1 km(3) for an individual flow, and 1.8 km(3) for all the identified flows) and yield strength of several flows (23-124 kPa). Using the DEM we estimated apparent viscosity based on the spacing of large folds (1.2 x 10(12) to 5.5 x 10(12) Pa s for surface viscosity, and 7.5 x 10(10) to 5.2 x 10(11) Pa s for interior viscosity, for a strain interval of 24 h). We use shaded-relief images of the DEM to map basic flow structures such as channels, sheer zones, and surface folds, as well as flow boundaries. The flow thickness map also proves invaluable in mapping flows where flow boundaries are indistinct and poorly expressed in the radar backscatter and shaded-relief images.