Mass Movement

Name the 3 types of mass movement

Blockfall, rotational slumping, landslides

Define mass movement

The downslope movement of material under the force of gravity

When does mass movement occur?

When downslope gravitational force exceeds resisting forces

What is meant by consolidated material?

Rock that is hard, compacted and cemented together over time.

What is meant by unconsolidated material?

Loose sediment that has not been compacted

When does blockfall occur?

On slopes greater than 40 degrees

What is blockfall?

Breaking away of rock fragments

What 2 processes is blockfall initiated by?

Mechanical weathering and marine erosion

Duration of blockfall

Only a few seconds

Example of blockfall

April 2013 - St Oswald’s Bay, Dorset

April 2013 - St Oswald’s Bay, Dorset - what happened?

80m section of the chalk cliff detached

What is rotational slumping?

Rock failure and movement along a curved rock plane

How does material move in rotational slumping?

As a single mass

Effect of water on rotational slumping

Increases gravitational force and reduces friction

Example of rotational slumping

Christchurch Bay, Barton-on-Sea

Process of rotational slumping

• Soil above sand dries and cracks

• Water flows into cracks and soaks into permeable sand layer

• Water builds up in sand - cannot flow through impermeable clay underneath

• Increases water pressure and lubricates boundary between sand and clay

• Creates a slip plane

• Waves erode base of cliff - create a wave-cut notch

• Causes cliff to collapse in a rotational slump

What is a landslide?

Downslope movement of discrete blocks of rock down a slip plane

How are the discrete blocks released?

Mechanical weathering of weak jointed rocks