10.2: Mass Wasting Categories

Mass Wasting Categories

Mass wasting can be subdivided by type of movement (falls, flows and slides), the overall geologic material (bedrock, debris, or earth) (Table 10.1), and the average rate of movement (Table 10.2). Mass wasting types often share common morphological features observed on the surface: head scarps commonly seen as crescent shapes on a cliff face; hummocky or uneven surfaces; accumulations of talus Links to an external site. (loose rocky material falling from above); and toe of slope, which covers existing surface material.

Table 10.1: Major categories and subcategories of mass wasting. ​(CC-BY 4.0; Chloe Branciforte, own work)

Category of Movement	Subcategory of Movement	Material	Relative Speed
Falls	Rock Fall	Rock	Extremely rapid
	Rock/Block Avalanche	Rock	Very rapid
	Topple	Rock, soil, debris	Extremely rapid
Flows	Debris flow	Rock, soil, debris, water	Extremely rapid to rapid
	Mudflow	Soil (mud & silt), water	Extremely rapid to rapid
	Earthflow	Soil and debris	Moderate to slow
	Creep	Soil	Extremely slow
Slides	Translational slide	Rock, soil, debris	Extremely rapid to very slow
	Rotational slide (slump)	Soil and debris	Extremely slow to moderate

 

Table 10.2: Qualitative and quantitative comparison of the rates of movement for mass wasting categories. (CC-BY 4.0; Chloe Branciforte, own work)

Extremely rapid to very rapid	3 m (10 ft) per second
Rapid	0.3 m (1 ft) per minute
Moderate	1.5 m (5 ft) per day
Slow	1.5 m (5 ft) per month
Very slow	1.5 m (5 ft) per year
Extremely slow	0.3 m (1 ft) per 5 years

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Falls

Falls are abrupt movements of masses of geologic materials, such as rocks and boulders, that become detached from steep slopes or cliffs (Figure 10.2). Separation occurs along natural breaks, like fractures, joints, and bedding planes, and movement occurs by free-fall, bouncing, and rolling. Falls are strongly influenced by gravity, mechanical weathering, and water. Yosemite National Park Links to an external site. is well known for its impressive rock falls.

Table 10.3: Examples of falls. (CC-BY 4.0; Chloe Branciforte, own work)

Subcategory of Movement	Material	Relative Speed
Rock fall	Rock	Extremely rapid
Rock/Block Avalanche	Rock	Very rapid
Topple	Rock, soil, debris	Extremely rapid

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Flows

Flows are rapidly moving mass wasting events in which the loose material is typically mixed with abundant water, creating long run outs at the slope base (Figure 10.3). Depending on the type of material involved and the amount of water, flows can be separated into debris flow Links to an external site. (coarse material), mudflow Links to an external site. (fine material, faster), and earthflow Links to an external site. (fine material, slower). Creep Links to an external site. is the imperceptibly slow downward movement of material caused by a regular cycle of nighttime freezing followed by daytime thawing in unconsolidated material such as soil. During the freeze, expansion of ice pushes soil particles out away from the slope, while the next day following the thaw, gravity pulls them directly downward. The net effect is a gradual movement of surface soil particles downhill. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges. A special type of soil creep is solifluction Links to an external site., which is the slow movement of soil lobes on low-angle slopes due to soil seasonally freezing and thawing in high-latitude, typically sub-Arctic, Arctic, and Antarctic locations.

Table 10.4: Examples of flows. (CC-BY 4.0; Chloe Branciforte, own work)

Subcategory of Movement	Material	Relative Speed
Debris flow	Rock, soil, debris, water	Extremely rapid to rapid
Mudflow	Soil (mud & silt), water	Extremely rapid to rapid
Earthflow	Soil and debris	Moderate to slow
Creep	Soil	Extremely slow

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Slides

A translational slide Links to an external site. is rock, soil or debris that moves along a sloping surface, similar to sliding down a playground slide (Figure 10.4, right). In most cases, the movement occurs along a plane of weakness and may run parallel to a fracture, bedding, or metamorphic foliation plane. Rotational slides (or slumps) Links to an external site. commonly show slow movement along a curved rupture surface (Figure 10.4, left).

Table 10.5: Examples of slides. (CC-BY 4.0; Chloe Branciforte, own work)

Subcategory of Movement	Material	Relative Speed
Translational slide	Rock, soil, debris	Extremely rapid to very slow
Rotational slide (slump)	Soil and debris	Extremely slow to moderate

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