Summary
Highlights
Gravity pulls material downhill. Friction is the force resisting this movement, dependent on material roughness, surface angle, and weight. When the downhill component of gravity overcomes friction, an object moves. This illustrates why steeper slopes increase the likelihood of movement.
Landforms are shaped by tectonic forces, erosional forces, and varying rock resistance. Erosion is the physical removal and transport of rock fragments, while weathering is the breaking down of rock, and deposition is the dropping of rock in a new location. This video focuses on gravity as an agent of erosion.
Mass movement is the downhill motion of weathered rock material. The slowest type is creep, where topsoil or sediment moves gradually downhill due to daily expansion and contraction. Over years, this results in noticeable changes, such as bent trees or fences.
Rock falls are the fastest type, occurring when cracks in rock cliffs cause chunks to break off and fall, forming talus slopes. Rock avalanches happen when many rocks fall simultaneously, riding on a cushion of air, reaching speeds over 200 mph and covering long distances, posing a significant threat to mountain communities.
Slides occur when a layer of rock moves downhill along a planar surface like a bedding plane, fault, or foliation plane. They can be triggered by water reducing friction or by excavation at the base. Slumps involve material moving downhill along a curved surface.
Water significantly increases the likelihood of all mass movement types by reducing friction and adding weight. While a small amount of water can bind particles, too much can cause liquefaction, leading to 'flows' where surface material moves like a fluid.
Several factors influence a hillside's stability: the steepness of the slope (gentler is better), the solidity of the rock (solid rock is better than loose sediment), the geological structure (planes parallel to the hillside are weaker), water drainage (good drainage is crucial), and vegetation (roots help stabilize soil).
Common triggers for mass movement include excavation of the hillside's base (e.g., for construction), added weight to the top (e.g., buildings, piled sediment), heavy rains or water leaks reducing friction and adding weight, and ground shaking (e.g., earthquakes, explosions, heavy foot traffic).
Mass movements cause significant financial and human impact, particularly in California. It is crucial to assess hillside stability when planning construction or trails to mitigate risks.