Therefore, landslides are one major natural hazard that is as likely, or perhaps more likely, to occur in eastern Montana as in the mountainous western part of the state. Two criteria are typically used to classify landslides - types of movement and types of material. The types of movement are falls, topples, slides, spreads, flows, and combinations or two or more of these. Bedrock and soils are considered the principle material types. The soils type is further divided into debris and earth.
Landslides occur when the shearing of forces exceed the resisting forces of earth materials.
Several factors contribute to either increased shear stress or reduced shear strength, thus initiating the landslide process. Loading by natural or human means. Loading may occur with the added weight of rain, hail or snow, the accumulation of loose rock fragments or volcanic material, stockpiles of ore or rock, or the weight of buildings. Vibrations caused by earthquakes, blasting, machinery, traffic or possibly thunder. Material alteration through weathering and other physio-chemical reactions.