Close view of the source area for the landslide high on the south flank of Casita Volcano. The landslide scar exposes rocks and deposits that form Casita. The light-colored rocks in the uppermost part of the source area were relatively unaltered by weathering or hydrothermal activity, but many fractures cut through the rocks; low-grade alteration was concentrated along the fractures because groundwater had preferentially moved along the cracks. The dark purple rock is hydrothermally-altered basaltic scoria. As the landslide moved down the volcano, it increased in size as it eroded highly altered rocks of the volcano's lower flank.