2.6 Sub-aerial Processes

What is weathering?

The breakdown of rocks

What zones of the coast are affected by weathering?

The backshore and foreshore parts of the littoral zone

Where in the sediment cell is weathering most active?

The source zone

What is physical weathering?

The application of force to physically fragment rock into smaller pieces, not involving any chemical change

What are the three types of physical weathering?

Freeze-thaw weathering, salt crystal growth and wetting and drying

What is freeze-thaw weathering?

Water seeps into cracks in rocks, expands by 9% when frozen, widening the rock through pressure until eventually it is broken apart

What is salt crystal growth?

Mineral crystal development forces a rock apart

What is wetting and drying?

Rocks containing clay are soaked and expand in volume, then contract when dry; repetition of this causes rocks to eventually break apart

What is chemical weathering?

Where chemical reactions attack individual minerals in rocks

What is carbonation?

Where waves attack calcium carbonate in limestones, dissolving it and weakening the rock

What is biological weathering?

The breakdown of rocks by living/once-living organisms

What is tree root weathering?

Where seeds fall into rock cracks and germinate, growing until they become too big and force the rock apart

What is rock boring?

Species of shellfish bore into rocks and secrete chemicals that dissolve them

What is seaweed acid?

Kelp contains pockets of sulphuric acid which can dissolve some rock minerals

How does weathering affect rate of recession?

Weathering increases the rate of recession by weakening rocks, making them more vulnerable to mass movement and cliff retreat

What climate encourages weathering?

A hot, wet climate

What is mass movement?

The downslope movement of material under the force of gravity, occuring when the downslope gravitational force exceeds the resisting forces of friction & internal rock cohesion

What type of mass movement happens with unconsolidated material?

Slumping

What type of mass movement happens with consolidated material?

Sliding

What is rockfall?

Occurs on \>40° slopes, where a rock fragment breaks away and either drops vertically or bounces downslope

How is rockfall initiated?

Via mechanical weathering (freeze-thaw/crystal growth) or marine erosion (hydraulic action/abrasion/cliff undercutting)

What are the properties of a cliff prone to rockfall?

Cliffs prone to rockfall have a geological structure with many joints, faults & bedding planes, steep/near vertical dip of strata, and often are in earthquake-prone areas

How fast does rockfall occur?

Rockfall is very rapid, taking only a few seconds to occur

What is an example of a real life rockfall case study?

In April 2013, there was a large rockfall in St. Oswald's Bay on the Dorset Coast where an 80m section of chalf cliff was detached overnight

What is rotational slumping?

Rotational slumping involves rock failure & movement along a curved rock plane; slumping material usually moves intact as a single mass

How long does rotational slumping take to occur?

Slumping often occurs in slow motion & can take minutes, hours, days or even years to occur

What are the properties of rocks prone to rotational slumping?

Rotational slumping occurs in weak rocks, unconsolidated material, & rocks with complex geology

What facilitates rotational slumping?

Rotational slumping is facilitated by the presence of water, which adds weight as well as lubricating the rock, reducing friction

What is an example of a real life rotational slumping case study?

At Christchurch Bay, in Barton-on-Sea, unconsolidated sands overlie clay; the bedding plane between sand & clay dips seawards

What is a landslide?

The downslope movement of discrete blocks of rock down a flat/linear slip plane, maintaining contact with the cliff surface throughout

What facilitates a landslide?

Discrete blocks are released by mechanical weathering of well-jointed rocks; gravity then pulls the loosened block down the relatively flat slip plane of the joint or bedding plane

What are the properties of rocks prone to landslides?

Landslides occur in consolidated rocks with joints or bedding planes sloping seawards

What are flows?

Flows occur when unconsolidated fine grained sediment mixes with large volumes of water; weak rocks become saturated, lose their cohesion & flow downslope

What are the properties of rocks prone to flows?

Weak rocks such as clay or unconsolidated sands

What is a talus scree slope?

A fan shaped mound of material

How is a talus scree slope formed?

When angular blockfall debris accumulates at the cliff foot; when cliffs are undercut the creation of wave-cut notches can lead to large falls of material

What is the slope angle of a talus scree slope?

34-40°

What is a rotational scar?

A fresh, curved, unweathered & unvegetated rock surface on the cliff face, formed by rotational slumping

What is a terraced cliff profile?

Where the cliff profiles is stepped, formed by rotational slumping