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Mass Wasting 2

© Geoff Habiger

In my previous article we discussed the various factors that effect mass wasting events. These factors include the effects of gravity, the driving force behind mass wasting, and water, which is often critical in tipping the balance between stable and unstable ground. Mass wasting events can be categorized as flows, slips, or falls depending on the consistence of the moving mass. There are several common types of mass wasting events.

Creep might be a term used to describe an unsavory character, but in geology creep has a different meaning. Creep refers to a slow, continuous, downslope movement of soil or unconsolidated debris. Slow and continuous are the key terms here. Most creep movement occurs on the scale of less than a centimeter per year and requires long periods of observation to record its movement. One may see evidence of this creep through observation of objects or structures that usually stand vertical such as gravestones or telephone poles. Over time, on gentle slopes, these objects will begin to lean as the ground beneath them moves.

Creep is affected by the amount of water in the soil and by the daily freezing and thawing cycles. Most creep occurs in regions that receive above average rain and also where freezing and thawing can occur. The rate of downslope movement can be affected by the presence or absence of grass or other plants, which can stabilize the soil and keep it from moving.

A second type of mass wasting is the debris flow. Debris flow refers to mass wasting in which motion takes place throughout the flow. There are three types of debris flow, earthflow, mudflow, and debris avalanche. An earthflow is where debris flows downslope as a viscous fluid, either with rapidity or slowly. Earthflows typically occur when heavy rains have saturated the ground. The flowing mass usually remains covered with vegetation and forms a vertical scarp at the site where the debris pulled away from the upper slope. Earthflows can be triggered by human activity in the form of overwatering through sprinklers or septic systems. Undercutting the base of the slope can also trigger them.

Mudflows are flowing mixtures of debris and water that typically flow through channels. They typically have the consistency of a thick milkshake and are composed of soil, water, and other debris that flow downslope. Mudflows are typical of places where the debris is not protected by a cover of vegetation. They commonly occur on the slopes of volcanoes or in mountainous regions after forest fires. A very devastating mudflow occurred in Columbia in 1985 after the eruption of the Nevado del Ruiz volcano killing 23,000 people. (See my review of the book No Apparant Danger, which covers this tragic event.) Mudflows can move quickly, but usually slower than a typical water stream, but they are capable of carrying large boulders, trees, and even automobiles.