Let's kick things off by understanding what mass movements are. These are large-scale shifts of the Earth's surface that don't have a chaperone like a river, glacier, or ocean wave. They could be:

Very slow movers: Like soil creep (Think of soil as a shy creature that inches downwards when no one is watching. Kind of like your little brother sneaking into the kitchen for a midnight snack!).

Lightning speed runners: Avalanches (Imagine a wall of snow deciding to sprint downhill all at once).

Dry tumblers: Rockfalls (Rocks playing a game of 'King of the Hill' and tumbling down).

Fluid dancers: Mudflows (Mud and water performing a ballet, sliding down slopes).

These movements differ in terms of their size, frequency, and scale. Some are large and occur infrequently (rockfalls), while others are smaller and more continuous (soil creep).

Why does Earth play this shuffle game? This has a lot to do with a slope's "safety factor". This is the measure of the slope's strength or resistance versus the force that's trying to move it.

Three key factors determine movement:

Gravity (acts as a tug of war master, pulling material downslope but also sticking particles to the slope)

Slope angle (the steeper the angle, the faster the rush)

Pore pressure (water that sneaks into spaces between particles, forcing them apart, and making it easier for the material to move)

Essentially, slope failure occurs when the internal resistance of the slope decreases or when the forces attempting to pull a mass downslope increase, or both.

Now let's dive into some key reasons behind increased shear stress:

Removal of lateral support (rivers eroding or glaciers gnawing at the base of the slope)

Removal of underlying support (rivers and waves undercutting or subsurface solution)

Loading of slope (extra weight due to water, vegetation, or debris)

Lateral pressure (water in cracks, freezing in cracks, swelling)

Transient stresses (earthquakes, tree swaying in the wind)

And factors contributing to reduced shear strength:

Weathering effects (disintegration of rocks, hydration of clay minerals, chemical solutions)

Changes in pore water pressure (saturation, softening of material)

Changes in structure (creation of fissures in clays)

Organic effects (animal burrowing, decay of roots)