We compare identification, delineation and recording of freshly erupted deposits around active volcanoes from very high resolution optical images with that done by traditional geologic mapping. Object-oriented classification (OOC) and normalized difference spectral indices of vegetation, moisture and soil redness (NDVI, NDWI and NDRSI) have been applied to sub-metric GeoEye-1 and Pléiades images to identify and map pyroclastic and lahar deposits and trace their spatio-temporal evolution over two years, following the 2010 eruption of Merapi Volcano, Indonesia. We could identify several categories of pyroclastic depositional areas, and also map the damaged forest and destroyed cultivated terraces and settlements in the Gendol and Opak River basins on the south flank of the volcano. More than 75% of erupted deposits were delineated semi-automatically unlike the ground geological map based on photo-interpretation of the 2010 GeoEye image and field observations. A temporal image analysis, using bivariate scatter diagrams of the three spectral indices between 2010 and 2012 and a combination of NDWI and NDVI, separated areas affected by surges from unscathed vegetation. Use of NDRSI and NDWI allowed us to differentiate overbank Pyroclastic Density Current deposits from wet lahar deposits. NDRSI values close to 0 refer to scoria-rich pyroclastic deposits.
About 40% of the devastated upper catchment was recolonized by vegetation between 2010 and 2012. The recovery also took place in the forested valley margins affected by ash-cloud surges. The morphometric analysis of the initial drainage network, digitized from the 2011–2012 images, demonstrated (1) the resurfacing of pristine 2010 PDC deposits by runoff and (2) incision or remobilization by lahars. It took two years following the eruption in the rugged upper catchment devastated by high-energy surges to fully develop the hydrographic network. It is, however, still rudimentary on gently sloping fans created by overbank PDC deposits in the middle valley, thus suggesting the importance of slope gradient, grain size, permeability and thickness of deposits. As much as 35% of the 2010 PDC deposits, emplaced in the vicinity of the river channels, were remobilized by lahars over the two post-eruption rainy seasons and also by constant mining activities. Studies on the erosion of the pyroclastic deposits after 2012 need to concentrate on the upper reach of the catchment on the south flank.