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MASS MOVEMENT

Landslides

Mass Movement

Mass Movement: Movement of earth materials (bedrock, sediment, soil) downslope by the force of gravity.

Erosion: Movement of earth materials by a medium such as wind, water or ice.

(Differs slightly from book which includes both weathering and mass movement in definition of Erosion)

Mass Movement

Causes of Slope Instability (leading to Mass

Movement)

Gravity, Friction, Slope Angle

Slope Composition

• Solid rock – deformation, weathering, daylighted beds,

solubility, jointing, rock cleavage, foliation.

• Unconsolidated Materials – angle of repose.

• Vegetation.

• Water- reduces friction and cohesion, adds weight,

saturation.

Causes of Slope Instability

Relationship to Gravity

Mass Movement occurs when gravity that is directed downslope (Gd) overcomes friction and gravity (Gp) that resists downslope movement.

A component of friction is weight of the material

Steeper the slope, the more instability

Cutting away the toe makes slopes steeper

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Causes of Slope Instability

Slope Composition (Cohesion)

Bedrock

Unconsolidated materials

Vegetation

Water

Bed Rock Unconsolidated Materials

Vegetation

• Native vegetation stabilized slope. • Non-native tree destabilized, along with heavy rainfall.

Vegetation

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Water Water

Triggering Mass Movement

Natural Earthquakes

Volcanic eruptions - Lahars

Heavy Rainfall

Erosion of slopes

Human Steepening slopes

Loading

Saturating

Cutting vegetation

Loud Noise

Earthquake Triggered

Madison Mountain, Earthquake Lake; Montana

A large eruption can produce a mudflow known as a lahar. These occur when the volcanic ash and debris become water-saturated.

A lahar developed (Toutle River, WA) when Mt. St. Helens erupted. The volcano contained large amounts of water in the form of snow and glaciers that melted nearly instantaneously during the major eruption.

The resulting debris flow moves down valleys and can be very destructive.

Triggered by Volcanic Eruption

In 1985, 20,000 people were killed in Colombia by a lahar

when Nevado del Ruiz erupted.

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Triggered by Heavy Rainfall Triggered by Erosion of Slope

Triggered by Erosion of Slope Slope Condition – Day-lighted Beds

Triggered by Human Activity

• Road cut • Avocado Trees, bare ground • Irrigation Pipes

Triggered by Human Activity

• House at top of slope • Graded slope for

room to build lower houses

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Types of Mass Movement

Creep & Solifluction

Falls

Slides

Slumps

Flows

Mudflows

Debris flows

Creep – Soil Creep

Gradual downhill movement of soil and regolith. Trees are forced to bend to compensate for movement of soil.

Solifluction

Solifluction, Tien Shan Mountains, Kyrgyzstan

Rock Fall– Just outside Yosemite, CA

Rock Fall Lands End – San Francisco

TALUS SLOPES – ROCKY MOUNTAINS

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TALUS SLOPES – ROCKY MOUNTAINS

The Gross Ventre Slide – Sheep Mountain

Slump

Small Slump

Mudflow

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Debris Flow

Debris Flows act as a liquid but contain large particles such as trees and boulders. They are very fast and powerful.

Debris Flow

Debris Flow