Mass Wasting:

• Mass wasting is the downslope movement of regolith and masses of rock under the pull of gravity

•Mass wasting is a basic part of the rock cycle. Weathering, mass-wasting, and other aspects of erosion constitute a continuum of interacting processes

• the most widespread geological hazard• does not account for many deaths • cost in damages is over three times that of all other catastrophes combined

Mass Movement Hazards

Role of Water in Mass Movement

1. Reduces internal friction in regolith

• If sand, silt, or clay becomes saturated with water, and the fluid pressure of this water rises above a critical limit, the fine-grained sediment will lose strength and begin to flow

Mass Movement Hazards

Classification of Mass Movement

Mass-movement events are classified on the speed and manner in which materials move downslope:

Slow mass movement

1. Creep is a very slow type of granular flow measured in millimeters or centimeters per year.

• Rates higher on steep slopes than on gentle slopes• Rates tend to increase as soil moisture increases.• Alternating freezing and thawing cause Creep

2. Solifluction occurs in arctic regions as consequence of freeze and thaw cycle – melted top layers slides over bottom

frozen layer.

Mass Movement Hazards

Example of Creep

Rapid Mass Movement

1. Falls: rock and debris drop from steep slopes

2. Slides move downslope along preexisting plane of weakness as a single, intact mass

3. Slumps move downslope along a concave slip surface - both downward and outward motion

4. Flow: mixture moves downslope as a viscous fluid

Types of Rapid Mass Movement

• Rock falls require fractured bedrock exposures.

• Slides commonly originate along contacts between layers of rock, soil, etc. with contrasting mechanical properties.

• Major areas of concern are where surface vegetation cover is destroyed by fires, followed by rainfall.

• Roadcuts or other areas of altered or excavated slopes are common targets (this includes sites of old landslides).

• Volcanoes are common targets for such mass wasting because of the wealth of fine, unconsolidated sediment.

• All of these are made more prevalent by seismic activity, that can act as a trigger mechanism.

Optimal Conditions for Falls, Slides, Flows

Rockfall is the free falling of detached bodies of rock, common in precipitous mountainous terrain, where slopes are to steep for loose material to remain on the surface.

A debris fall is similar to a rockfall, but it consists of a mixture of rock and weathered regolith, as well as vegetation.

Mass Movement Hazards

A rock fall in this area in July, 1996,

had rather amazing consequences.

A single granite block weighing

~200 tons broke free and

disintegrated upon impact.

The burst of air from the impact

flattened some 2000 trees and the

area was showered with dust from

the obliterated boulder.

Only one person was killed.

The debris at the base of a steep slope such as this is called talus.

Yosemite, CAYosemite, CA

A slump is slope failure in which a downward and outward rotational movement of rock or regolith occurs along a curved concave-up surface. - Associated with heavy rains or sudden shocks, such as earthquakes.

The top of the displaced block usually is tilted backward, producing a reversed slope.

Mass Movement Hazards

Slumping and Scarps in Dorset

Mass Movement Hazards

Slides

Mass Movement Hazards

commonly originate along contacts between layers of rock, soil, etc. with contrasting mechanical properties

Landslide

This is a typical landslide.

Note that materials hold together

in more-or-less singular blocks.

As is common, when the slide

blocks reach a lower slope, they

break up (becoming a debris

flow in this case).

The La Conchita landslide, near Santa Barbara, CA, Spring, 1995.

Mass Movement Hazards

Flow Types

A. Flows: A movement of water-saturated sediment which move downslope as a thick fluid.

1. MudflowDef. – A quick movement of material that results from a heavy downpour of rain or rapid snowmelt.

After heavy rain in a mountain canyon, a mudflow can start as a muddy stream that becomes a moving dam of mud and rubble.

Mudflows produce sediments fans at the base of mountain slopes.

Mass Movement Hazards

Mass Movement Hazards

Mudflow in Sarno, Italy, 1998

Mass Movement Hazards

1. Earthflows – relatively slow mvts. • most common mass-wasting process.

• At the top of a typical earthflow is a steep scarp.

• A special type of earthflow called liquefaction occurs in wet, highly porous sediment consisting of clay to sand-size particles weakened by an earthquake.

– An abrupt shock increases shear stress and may cause a momentary buildup of water pressure in pore spaces which decreases the shear strength.

– A rapid fluidization of the sediment causes abrupt failure.

Mass Movement Hazards

Figure 13.16

Earthflow

Mass Movement Hazards

3. Debris avalanche

• a huge mass of falling rock and debris that breaks up, pulverizes on impact, and continues to travel downslope

•The flanks of steep stratovolcanoes are especially susceptible to collapse that can lead to the production of pyrolclastic debris avalanches

Mass Movement Hazards

Source: Lloyd S. Cluff

Before the Yungay Avalanche

Mass Movement Hazards

Source: Lloyd S. Cluff

After the Yungay AvalancheMass Movement Hazards

Natural triggers of mass-movement hazards include:

• torrential rainstorms• earthquakes• volcanic eruptions• Oversteepened slopes

Human triggers of mass movement hazards include:

• excessive irrigation• clear-cutting of steep slopes• slope oversteepening or overloading• mining practices

Mass movement Hazards To Life And Property In the United States alone, landslides in a typical year cause more than $1 billion in economic looses and 25 to 50 deaths

Careful planning can often reduce or even eliminate the impact of mass-wasting processes on human environment

Slopes subject to creep can be stabilized by draining or pumping water from saturated sediment