Coastal Mass MovementCoastal Mass Movement(Landslides)(Landslides)

Higher GeographyHigher GeographyLithosphereLithosphere

IntroductionIntroduction

• Mass movement occurs when Mass movement occurs when weathered material moves downhill weathered material moves downhill under the force of under the force of gravitygravity (often (often lubricated by water).lubricated by water).

• Landslips in coastal areas occur Landslips in coastal areas occur because of erosion from the waves, because of erosion from the waves, physical (freeze-thaw) and chemical physical (freeze-thaw) and chemical (solution) weathering.(solution) weathering.

A classification of mass movement

After Carson and Kirkby

4 star hotel in Scarborough,

North Yorkshire (1993).

Black Ven, Dorset Coast

Black Ven, Dorset Coast

Osmington Coastal Path, Dorset Coast

How Landslips work (1)How Landslips work (1)

• Rain water sinks through the porous rocks Rain water sinks through the porous rocks but once it reaches the underlying clays it but once it reaches the underlying clays it can sink no further. The water builds up can sink no further. The water builds up along the junction between the rock layers along the junction between the rock layers and seeps out of the cliffs as a series of and seeps out of the cliffs as a series of springs.springs.

• After periods of prolonged rainfall, the After periods of prolonged rainfall, the build up of water increases the weight of build up of water increases the weight of the cliff top. Increased poor pressure the cliff top. Increased poor pressure reduces the friction and allows large reduces the friction and allows large sections of the cliff top to break away. sections of the cliff top to break away.

How Landslips work (2)How Landslips work (2)

• As the cliff top block subsides, it As the cliff top block subsides, it rotates along the slip plane within rotates along the slip plane within the cliff, resulting in the flat surface the cliff, resulting in the flat surface tipping back towards the cliff. tipping back towards the cliff.

• The main type of landslide that The main type of landslide that occurs at the coast is called occurs at the coast is called slumpingslumping..

Slumping (also landslips)Slumping (also landslips)

Slumping can occur after heavy rainfall

or earthquakes

Back tilted slopes

Large blocks break away Sliding surface is concave

Soft boulder clay cliffs can be undercut by the sea and slumps are

common.

Slumps occur in weaker rocks and Slumps occur in weaker rocks and involve some rotational movement. involve some rotational movement.

Landslides in DorsetLandslides in Dorset

• The Dorset and East Devon coast The Dorset and East Devon coast contains a wide range of landslides, contains a wide range of landslides, both large and small. both large and small.

• The coast is unstable because there The coast is unstable because there are numerous situations where porous are numerous situations where porous strata (eg: chalk and sandstone) lie strata (eg: chalk and sandstone) lie over impermeable clays. over impermeable clays.

Old Harry Rocks

Another viewAnother view

Sliding surface is concave

Clay

Sandstone

Impermeable Clay

Permeable Sandstone

Rotational Slumping

Sheer Face

Your turnYour turn

• Make notes on Make notes on Landslides and Landslides and slumps from page slumps from page 87.87.

• Copy Rotational Copy Rotational sliding diagram sliding diagram from page 88.from page 88.

Copy Fig 4.13 from page 87.