1.1.2 How are coastal landforms developed?

Weathering

the breakdown of rock in situ. Increases rate of recession, making rocks more vulnerable to mass movement and cliff retreat. Weathering is a very slow process.

Physical weathering

application of force to physically fragment rock into smaller pieces called clasts. Does not involve any chemical change.

Freeze-thaw weathering (P)

Process:

• water seeps into rocks

• water freezes, expands by 10% and there is tensional force that widens the rock

• this allows more water in, and repeats until pebbles and fragments come off

• in regards to porous rock, this may mean the water inside the pores freezes and pries off rock grains and creating small sand sized sediment

• this is often in areas with rock that already has cracks and fissures in it

• freezing is uncommon on UK coastlines

Salt crystal growth (P)

Process

• seawater penetrates small cracks or pores in rock at high tide, evaporated at low tide which leaves precipitated salt crystal rocks

• repeated tidal cycles lead to their growth until the sides of the cracks are pressed up too greatly and therefore causing pressure

• angular fragments of the rock then break off, or just individual grains come off

• common at all coasts due to salt content in sea

• lesser breaking force than with the freeze-thaw

• greater effect in hot climates due to evaporation and precipitation of salt crystals

• foreshore and backshore zone affected by wave spray from destructive waves

Wetting and drying (P)

found in rocks with clay minerals e.g. clay or shale

• at high tide, minerals on the rock surface become soaked with sea water and expand

• at the low tide, minerals dry and shrink

• this continues until the expansion and contraction caused the rock to fragment and crumble

Chemical weathering

chemical reactions attack individual minerals in the rock, which break bonds and create new compounds

Carbonation (C)

• attacks calcium carbonate in limestone

• rainwater mixes with CO2 form a weak carbonic acid

• acid rain mixes with calcium carbonate to form a soluble calcium bicarbonate solution

• new minerals dissolve into the solution and the rock is disintegrated

• previously cemented clasts are released after this

Hydrolysis (C)

• breakdown of minerals to form new clay minerals, due to water and dissolved CO2

• rocks vulnerable to this are: igneous and metamorphic which contain feldspar and silicate materials

• attacks the feldspar in granite for example (which is what makes it pink)

• the bonds between the feldspar and the quartz means that the quartz is released as sand-like material

Oxidation (C)

• oxygen being added to a material (often iron) which increases the volume due to formation of iron oxides and lead to mechanical breakdown

• the wetting of Devonian sandstone for example causes the iron oxide to become red-coloured iron oxide due to interaction with seawater

• this releases the cemented clasts

• sandstone, shale and silt stones are often vulnerable to this due to iron content

• more effective in seawater or water with impurities

Biological weathering

the breakdown of rock in situ by living or once-living organisms. Speeds up mechanical or chemical weathering through the actions of plants, bacteria or animals

Tree root weathering (B)

• seeds fall into cracks and germinate, and then thanks to rainwater and nutrients blown across by wind, the plant will grow

• the roots will expand and thicken, exerting enough pressure to widen the crack

• this causes angular fragments of rock to break away as cobble or boulder sized sediment

Seaweed acid (B)

• kelp has sulphuric acid inside

• as the cells break, the sulphuric acid attacks minerals like calcium carbonate and leads to a chemical reaction not unlike carbonation

Rock boring (B)

• clams and mollusks bore into rock and secrete rock-dissolving chemicals

• piddocks live in the foreshore zone and drill depressions into rocks by rotating their shell which has sharp edges

• they live in these depressions, and filter feed to protect themselves from high-energy waves

• less resistant rock is targeted more, such as sedimentary rock

Mass movement

downhill movement of material under the influence of gravity. Can range from incredibly slow (1cm per year) to very fast. This is very common and the main causes are weak geology and rainwater weight

Soil creep

• extremely slow, individual soil particles

• particles rising towards the surface due to wetting or freezing and returning diagonally to the bottom due to gravity as the soil dries/thaws

• zig-zag motion (similar to lsd) and cannot be seen in action, but instead through shallow terraces and tree trunk bending

Mud flows

• involves earth and mud flowing downhill, over unconsolidated and weak bedrock e.g. clay after heavy rainfall

• water is trapped in the rock, so the pore water pressure increases, forcing the rock particles apart and the slope fails. This is often sudden and fast flowing

Landslides

• a block of rock moving very rapidly downhill along a planar surface, often a bedding plane that is parallel to the ground surface

• the block of moving materials remains intact and they are often triggered by earthquakes and heavy rainfall due to lubrication of the surface and subsequent reduction in friction

• they are rapid and are a threat to people and property

Rockfall

• sudden collapse or breaking away of individual rock fragments at a cliff face

• commonly associated with steep or vertical cliffs and triggered by mechanical weathering or an earthquake

• the rocks fall and bounce down the slope to form scree and this is an input into the sediment cell

Landslip/slump

• differs from landslide due to having a curved slide surface instead of flat

• weak and unconsolidated clays and sands, permeable rock over impermeable and this builds up pore water pressure

• several of these can lead to a terraced appearance on the cliff face

Cliffs, headlands and bays

• rocks of differing strength are exposed at a coastline

• more resistant rocks such as granite and limestone tend to form headlands and cliffs

• less resistant rocks such as clays and shales are eroded and form bays

• e.g. Ballard Point and Durlston Head and Swanage Bay

Wave cut platforms

• rocks that are gently sloping seawards (less than 4°)

• irregular surface, pitted by rock pools an micro-cliff features

• found in the intertidal zone (between low and high tide) which causes friction for waves

• the foot of the cliffs are then where the waves break and this is the maximum erosion so a wave cut notch will develop

• overhanging cliffs may then collapse and so the cliff retreats and the wave cut platform will lengthen

• e.g. Kimmeridge Bay, Dorset

Caves, arches, stacks and stumps

• occurs where there is a cliff face with a weakness such as joints or bedding planes

• waves open up a prelonged joint, forming a deep and steep sided inlet (geo)

• smaller hollows are then targeted by wave refraction and caves are created

• caves being created on either side of the headland will then erode back, and when they meet they will collapse through and form an arch

• the sea then erodes under the arch and the rock keeping the arch up is eroded and this, in combination with aerial weathering, causes the ‘roof’ to collapse and a stack to be formed

• the stack then further erodes into a stump

• e.g. Old Harry Rock, Dorset

Beaches

• built up with accretion in and across bays of sand and shingle

• e.g. Swanage beach

• can be swash or drift aligned (up and down or lsd motion)

• can be split into 3 zones:

  • offshore - beyond the influence of breaking waves

  • nearshore - intertidal and within breaker zone

  • backshore - above the influence of normal wave patterns, marked at the lower end by berms and may have a storm beach further up

Spit/Tombolo

• long, narrow stretches of sand or shingle that protrude into the sea or across an estuary

• materials are moved along the coast by lsd

• continues in the same direction when the coastline curves and where there is an estuary with a strong current that interrupts material movement, and this projects out into it

• the end of a spit is often curved (creating a series of laterals) where waves are refracted around the end of the spit to more sheltered water behind

• a tombolo is where a spit joins the mainland at one end and an island at the other

• spit : e.g. Spurn Point, Yorkshire (recurved). tombolo : Chesil Beach to Portland Island, Dorset

Bars and barrier beaches

• created when the spit is developed across a bay and therefore there are no strong currents to disturb this, so water is dammed and forms and lagoon

• bars can also form due to storms picking up pebbles and are left in offshore ridges and a barrier beach is formed

• e.g. Slapton Sands, Devon

Offshore bars

• deposits of sand and shingle at a distance from the coastline, laying below sea level and can be seen at low tide

• formation:

  • in shallow seas, the continental shelf being a distance from the shore means that the waves break sooner

  • steep waves may break on a beach, and the strong backwash carries the material back to form a ridge (rip current)

• if the bar comes to be above sea level, it often becomes a barrier beach and a lagoon inland, and ocean on the other side

• e.g. Ras El Bar, Egypt

Sand dunes

• deposited by the sea under low energy conditions

• wind may move the sand to build up dunes further along

• these become colonised by stabilising plants (creating a psammosere)

• e.g. at the Nile Delta/Aberffraw