2.3.1 - Coastal Enquiry question: What is the coastal zone and how is it influenced by geology and marine processes?
processes
The coast, and The littoral zone consists of backshore, nearshore and offshore zones and is a dynamic
wider littoral zone of rapid change.
zone, has The Litt oral Zone
distinctive Littoral zone: the coastal zone in which sediment is moved around between land, sea and beach; the wider
features and coastal zone, including adjacent areas and
landscapes shallow parts of the sea offshore; it is divided
into subdivisions: the backshore, foreshore,
nearshore and offshore.
Offshore: the open sea
Nearshore: shallow water areas close to land
where intense human activity often occurs.
The nearshore also forms part of the physical
system of the coastline through transfers of
sediment by currents close to the shore.
Foreshore: where wave processes mostly occur between high and low tide marks.
Backshore: above high tide level and is only affected during exceptionally high tides (spring tides) and
major storms.
The littoral zone is a zone of dynamic equilibrium due to the wide range of natural processes that interact
within it.
- Inputs of sediments from the sea and currents from rivers flowing off the land.
- Weathering and mass movement occur on the backshore
- Constructive and destructive waves occur on the foreshore and nearshore, causing deposition and
erosion.
- The tidal range affects all parts by determining where wave action takes place.
- Offshore currents and longshore drift may move sediments some distance along a coast.
- The coastal system operates in a state of dynamic equilibrium because the input and outputs of
sediment in a sediment cell are in a constant state of change but remain in balance.
The zone is constantly changing because of the dynamic interaction between the processes operating in
the seas, oceans and on land. It varies because of:
- Short-term factors (such as individual waves, daily tides and seasonal storms)
- Long-term factors (such as changes to sea levels and climate change)
,Coasts can be classified by using criteria such as geology (resistant/less resistant), high or
low energy, changes of sea level, relief (cliffed or coastal plains) and sediment type.
Valenti n’s Classifi cati on
- Describes the range of coastlines that can occur.
- An advancing coastline may be due to the land emerging or deposition being the prominent
process.
- A coastline may be retreating due to the land submerging or erosion becoming the prominent
process.
Geology
More resistant
- More resistant rock erodes at a slower rate, producing areas of rock that protrudes (headlands)
Less resistant
- Erodes at a faster rate, causing the coast to recede at a faster rate than the surrounding areas of
more resistant rock.
High and low energy coastlines
High Energy Low Energy
Waves: powerful, destructive waves; occur where Waves: constructive waves, short fetches
there are long fetches. Processes: deposition and transport
Processes: erosion and transport Landforms: depositional landforms (beaches, spits,
Landforms: erosional landforms (wave-cut salt marshes, sand dunes, bars, mudflats)
platforms, arches, sea caves, stacks, rocky General location: sheltered form larger waves,
headlands) lowland coasts, coastal plain landscape.
General location: exposed to largest waves, rocky Example locations: Mediterranean Sea coasts, East
landscape, highland and lowland coasts Anglian coast.
Example locations: Atlantic coasts of Norway and
Scotland; Pacific coasts of Alaska and Canada.
Changes in sea level
Emergent Coastlines
Emergent coastline: a coastline that is advancing relative to the sea level at the time.
- Features: relict cliffs, raised beaches
- Emergent coasts are commonly the result of isostatic rebound.
- In Scandinavia, isostatic uplift is still 2 cm per year.
- Once removed from marine processes, wave erosion, transport and deposition cease, and shaping
by terrestrial processes (e.g., wind, rain, rivers and subaerial processes) takes over.
- A raised beach is a former beach now above the high tide line. Some raised beached consist of
several different levels, each indicating a different stage of uplift.
- A relict cliff is a near-vertical slope initially formed by marine processes but is now some distance
inland.
Submergent Coastlines
,Geological Geological structure is responsible for the formation of concordant and discordant coasts.
structure
influences the Geological structure
development of Geology: the study of the structure of the Earth, together with its origin and development, especially by
coastal the examination of rocks.
landscapes Lithology: the physical properties of a rock.
Geological structure is separated into three key elements:
Strata: the different layers of rock within an area.
Deformation: the degree to which rock units have been deformed, either by tilting or folding, by tectonic
activity.
Faulting: significant fractures that have displaced rocks from their original position.
Geological coastlines (concordant and discordant)
Concordant coastlines Discordant coastlines
Strata are parallel to the coast. Strata are perpendicular to the sea so less
The rock type normally consists of bands of more resistant rock is eroded at a faster rate than more
resistant and less resistant rock. The more resistant rock.
resistant rocks (or upfolds) form elongated islands, Alternating hard and soft rock strata create
while the less resistant rock (or downfolds) form headlands of more resistant rock and bays of less
long inlets or coves. resistant rock.
Coves - the outer hard rock is punctured, allowing Features: headlands, bays, wave cut platforms,
the sea to erode the softer rocks behind. beaches.
E.g., the eastern coast of the Adriatic Sea, the E.g., the east-facing coast of Dorset, Swanage Bay.
south-facing coast of Dorset.
Haff coastline
- Form in low energy environments where there is deposition of sediment.
- Large lagoons are found behind the deposits parallel to the shoreline.
- E.g., the Baltic Sea coastline of Poland with features such as the Vistula Haff - a large lagoon that
has formed behind Vistula spit and had deltaic sediments building up within it.
Geological structure (jointing, dip, faulting, folding) is an important influence on coastal
morphology, erosion rates and the formation of cliff profiles and the occurrence of micro-
features such as caves.
Coastal morphology: the study of natural processes ongoing at the shoreline and of the impact due to
, human interventions within the coastal zone.
Jointi ng
- Fractures in rocks created without displacement. They occur in most rocks, often in regular
patterns, dividing rock strata up into blocks with a regular shape.
- Jointing increases erosion rates by creating fissures which marine erosion processes, such as
hydraulic action, can exploit.
- Example: Stair Hole - the Purbeck limestone is intensely folded, which has created heavy jointing,
making the limestone erode much more rapidly than the adjacent Portland limestone.
Faulti ng
- Major fractures in rock created by tectonic forces, with displacement of rocks either side of the
fault line. They are often large scale, extending many kilometres.
- Faulting significantly increases rate of erosion since large forces are involved in faulting and
displacing rock, so the rocks on either side of the fault line are often heavily fractured and broken,
which is easily exploited by marine erosion.
- Example: Bantry Bay in Cork, Republic of Ireland - a major fault runs down the centre of the bay.
The carboniferous limestone was significantly weakened by faulting, allowing rapid fluvial erosion.
The resulting river valley was inundated to form a ria when sea levels rose. Faulting also allowed
more rapid marine erosion than in unfaulted Old Red sandstone, contributing to the headland and
bay coastal morphology.
Dip
Dip: the angle between the maximum slope and the horizontal.
Horizontal dip
- Vertical or near vertical profile with notches reflecting strata that can be easily eroded.
Seaward dip (high angle)
- Sloping, low angle profile, with one rock layer facing the sea, vulnerable to rockslides down the dip
slope.
Seaward dip (low angle)
- Profile may exceed 90°, producing areas of overhanging rock, which is very vulnerable to rock falls.
Landward dip
- Steep profiles of 70-80°, producing very stable cliffs with reduced rock falls.
