The erosional force that acts alone to produce avalanches and landslides is gravity.

Here's why:

* Avalanches: Snow avalanches are triggered by the force of gravity pulling down a mass of snow that has become unstable due to factors like heavy snowfall, weak snowpack, or steep slopes.

* Landslides: Landslides are caused by gravity pulling down unstable slopes of soil, rock, or debris. This can be triggered by factors like heavy rainfall, earthquakes, or human activities like construction.

While other forces like wind and water can contribute to erosion and instability, they are not the primary drivers of avalanches and landslides. Gravity is the fundamental force that overcomes the friction holding these masses in place, leading to their rapid descent.