What is Mass Movement?

Mass movement is the movement of surface material caused by gravity. Landslides and rockfalls are examples of very sudden movements of this type. Of course geological agents such as water, wind and ice all work with gravity to cause a leveling of land.

A block diagram of an earth flow where movement is a combination of slip and flow.

Water aids in the downslope movement of surface material in several ways. Water adds weight to the soil; it fills pore spaces of slope material and it exerts pressure which tends to push apart individual grains. This decreases the resistance of the material to movement. Landslide is a general term that is commonly broken down into the more specialized terms such as slump, rockslide, debris slide, mudflow and earthflow. 

A slump is a downward and outward movement of rock or unconsolidated material moving as a unit or series of units. Large blocks of material move suddenly downward and outward along a curved plane. 

Rockslides are the most catastrophic type of landslide. They involve a sudden rapid slide of bedrock along planes of weakness. Rockslides are very common in the oversteepened canyons and drainages of Idaho, particularly in those areas like the Salmon River Canyon where more than 5,000 feet of elevation may exist between the ridge tops and the canyon bottoms.

 Debris Slide
A debris slide is a small sudden downstope movement of unconsolidated material. This type of slide produces a hummocky surface of low relief.

A mudflow is a mass of saturated rock particles of all sizes. This type of landslide is caused by a sudden flood of water from a cloudburst in semi-arid country or a sudden thaw. The flood waters carry the soil and rocks from a large slope area and washes them to a gulch or canyon. 'Then the water and debris move down the canyon and spread out on the gentle slopes below. Mudflows are very common in the semi-arid areas of southwestern Idaho.

An earthflow is a downslope movement of soil which has been saturated with water to the extent that the debris moves as a fluid. While flowing, either slowly or rapidly, the mass generally remains covered by a blanket of vegetation. Typically a steep scarp is developed where the moving debris has pulled away from the upper slope. A hummocky lobe forms at the toe or front of the earthflow.

A talus slope is developed by an accumulation of rock fragments at the foot of a cliff or ridge. Rock fragments break loose from the cliff above, roll down the slope and pile up in a heap of rock rubble. Individual talus forms as a half-cone with the apex pointing upwards. In most cases a series of half cones coalesce around the base of a mountain.

 Horseshoe Bend Hill Slide Area
Earthflows are a very common occurrence on the Horseshoe Bend Hill area between Boise and Horseshoe Bend. From the highway you can easily see earthflows of less than one year old as well as those much older. The more recent flows show fresh brown crescentric cracks where the fresh earth is exposed. The older flows are more difficult to identify because vegetation has grown over the scarp areas. Most of the slides occur during the spring when the ground surface is saturated with water. Placement of this major north-south highway over an active slide area has resulted in a section of highway constantly deformed and broken by the slowly-moving land surface.

 Warm Springs Mesa Slide
Warm Springs Mesa is situated immediately south of Table Rock in east Boise. The entire Warm Springs Mesa is a 300-acre landslide. The construction of Warm Springs Avenue along the toe of the landslide has caused an oversteepened natural slope. There has been sliding along this oversteepened slope for years and debris is constantly falling on Warm Springs Avenue. Although geologists have long cautioned against development until study of the effect of increased water in the sediment is cornpleted, development of the subdivision has not stopped.

A number of investigators has determined that the combination of the oversteepened slopes coupled with ground water causes the sliding. The additional ground water derived from the new residential uses is also believed to have had an adverse impact on the sliding activity.

The Warm Springs Mesa slide originated in an area next to Table Rock. Perhaps an earthquake suddenly dislodged the material and caused a sudden movement of a large earth mass downslope in a southwest direction some 1,200 feet towards the Boise River. The surface of the landslide is now revegetated but has the typical hummocky rolling topographv of a landslide area. Numerous large boulders of sandstone are exposed chaotically over the surface but are particularly abundant on the oversteepened south slope. The large sandstone boulders are derived from the sedimentary rock (Glenns Ferry Formation) that is now exposed at Table Rock. From an airplane perspective, one can readily envision both the source and the total extent of the fan-shaped slide deposit.

As one drives along Warm Springs Avenue near the toe of the slide, undisturbed river gravels predating the slide are exposed in the road cuts just to the north of the road. These Boise River gravels were overriden by the slide.

Other Idaho Landslides
Landslides are a common sight in the mountainous areas of Idaho. Once you know what to look for they can be readily identified by the presence of a rupture in the vegetative cover exposing fresh earth or by a hummocky lower surface.

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