Earthquakes, heavy rain, undercutting, overburden
Landslide conditions:
Deforested slope, geologic contact (slip surfaces), steep slopes, mobile material, manmade structures (e.g. poorly constructed roads), chemically altered "rotten" rock, overburden Cecchi and Wyk de Vries discovered flank spreading at Casita Volcano, Nicaragua. The cause was hydrothermal alteration and this weakened the slopes significantly enough to allow the catastrophic landslide and lahar of 1998. Deformation caused by altered rock may be expressed in edifce geomorphology, this allows field surveys and analysis to determine whether slopes are stable or whether a volcano is primed for collapse.
