In humid climates, water-saturated soil, regolith, or weak shale can move down a steep slope in just a few hours. This is an earthflow.

It’s common to see shallow earthflows, affecting only the soil and regolith, on sod-covered and forested slopes that have been saturated by heavy rains.

An earthflow can affect a few square meters, or it may cover an area of a few hectares (several acres).

If the bedrock of a mountainous region is rich in clay, earthflows can involve millions of metric tons of bedrock moving like a great mass of thick mud.

During heavy rains, earthflows can block highways and railroad lines.

The flows aren’t usually a threat to life because they move slowly, but they can often severely damage buildings, pavements, and utility lines that have been constructed on unstable slopes.