Mass Movements

Every slope is a contest between gravity, which pulls material downhill, and resisting forces like friction and the internal strength of the rock or soil, which hold it in place. A hillside can stand for decades looking perfectly stable because that contest is close but resistance is winning. Add a heavy rain and the contest tips. Water soaks into the pore spaces between soil grains and adds weight, so gravity's pull grows. At the same time that water pushes grains apart, raising pore pressure and cutting the friction between them, so resistance shrinks. Both effects point the same way, which is why slopes so often fail right after a storm rather than during a dry spell when the same slope has sat untouched for years.

This is also why slope angle alone does not decide the outcome. A steep slope cut into solid granite can be far more stable than a gentle slope cut into loose, saturated clay, because stability depends on the ratio of driving force to material strength, not angle by itself. Weak, waterlogged material can fail at a gentle angle while strong rock holds a steep one. Vegetation matters for the same reason: plant roots physically bind soil particles together and also draw water out of the ground, so clearing a hillside removes both that mechanical reinforcement and that drying effect, leaving the slope weaker and wetter at once. This topic connects directly to groundwater, since infiltration and pore pressure are the same water that feeds aquifers, and to erosion and weathering, which slowly produce the loose material that mass movements eventually send downhill.