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EQ1: Why are coastal landscapes different and what processes cause these…
EQ1: Why are coastal landscapes different and what processes cause these differences?
What are the main features in the coastal zone?
Coast = land adjacent to the sea and often heavily populated and urbanised
Backshore = above high tide leveal and only affected by waves during exceptionally high tides and major storms
Foreshore = where wave processes occur between the high and low tide marks
Nearshore = shallow water areas close to land and used extensivly for fishing, coastal trade and leisure
Offshore = the open sea
Types of coasts
:
Primary
= dominated by land-based processes, such as deposition at the coast from rivers or new coastal land formed from lava flows
Secondary
= dominated by marine erosion or depostion processes
Emergent
= where the coast is rising relative to the sea level, e.g. as a result of tectonic uplift
Submergent
= being flooded by the sea either becuase of sea-level rise and/or subsiding land
Littoral Zone
= the wider coastal zone, including coastal land areas and shallow parts of the sea just offshore.
Tidal ranges
:
Microtidal
= tidal range of 0-2m
Mesotidal
= tidal range of 2-4m
Macrotidal
= tidal range greater than 4m
Wave Energy:
Low energy
= sheltered coasts with limited fetch and low wind speeds resulting in small waves
High energy
= exposed coasts facing pervailing winds with long wave fetches resulting in powerful waves
How does the geological structure affect the shape of the coastline?
Geological structure
:
Strata
= the different layers of rock and how they rekate to each other
Deformation
= the degree to which the rocks have been tilted or folded
Faulting
= the presence of fractures that moved rocks from their original position.
Lithology
= the physical characteristics of the rock
Geomorphology
= the study of landforms and how the are formed
Discordant coastlines
:
These are dominated by headlands and bays. Less resistant rocks are eroded from bays whereas more resistant geology remains as headlands protruding into the sea. For example West Cork coast in Ireland. In this area:
Rock strata meet the coast at 90° in parallel bands
Weak rocks have been eroded, creating elongated, narrow bays
More resistant rocks form headlands
Especially resistant areas remain as detached islands, such as narrow bays.
Concordant coastlines
:
These are more complex. Different rock strata (layers of rock folded into ridges) run parallel to the coast, but vary in terms of their resistance to the sea. The outer hard rock provides a protective barrier to the erosion of the soft rock behind it. They have fewer headlands and bays than a discordant coastlines. The most well-known example is Lulworth Cove where:
The hard portland limestone and fairly resistant Purbeck Beds protect much softer rocks landward (the wealden and Gault Beds)
Marine erosion has broken through the resistant beds, and then rapidly eroded a wide cove behind
There is resistant chalk at the back of these coves, which prevents erosion further inland.
How does geology affect cliff profiles?
Cliff profile
= the height and angle of a cliff face, plus its features such as wave-cut notches or chnaged in slope angle.
Dip
= the angle of rock strata in relation to the horizontal. DIp is a tectonic feature. Sedimentary rocks are formed in horizontal layers, but can be tilted by tectonic forces. When this is exposed on a cliffed coastline it has a dramatic effect on the cliff profile.
Horizontal dip
= Vertical or near-vertical profile with notches reflecting strata that are more easily eroded
Seaward dip
(high angle) = Sloping, low-angle profile with one rock layer facing the sea; vulnerable to rock slides down the dip slope
Seaward dip
(low angle) = Profile may exceed 90° producing areas of overhanging rock; very vulnerable to rock falls
Landward dip
= Steep profiles on 70-80° and producing a very stable cliff with reduced rock falls
Other geological features influence cliff profiles and rates of erosion. These include:
Faults
: either side of a fault line, rocks are often heavily fractured and broken and these weaknesses are exploited by marine erosion
Joints
: occur in most rocks, often in regular patterns, dividing rock strata up into blocks with a regular shape.
Fissures
: much smaller cracks in rocks, often only a few centimeters or millimeters long but with a regular shape.
Folded rocks
: often heavily fissured and jointed, meaning they are more easily eroded.
Micro-feature
= small-scale coastal features such as caves and wave-cut notches with form part of a cliff profile.
Fault
= major fractures in rocks produced by tectonic forces and involve displacement of rocks on either side of the fault line
What are the differences between high and low relief coastlines?
Rocky Coasts
:
Many coastlines consist of rocky cliffs that vary in height from a few meters (low relife) to hunders of meters (high relife). High relife cliffs are composed of relatively hard rock. There are two main cliff profile types:
Marine Erosion dominated
= wave action dominates and cliffs tend to be steep, unvegetated and there is little rock debris at the base of the cliff
Subarieal process dominated
= not actively eroded at the base by waves; shallower, curved slope and lower relife; surface runoff erosion, mass movement and weathering (mechanical, biological and chemical) are responsible for the cliff shape.
Coastal Plains
:
Low-lying, flat areas close to the coast are called costal plains. Many contain esturay wetlands and marches, being just above sea level and poorly drained becuase of the flatness of the landscape. Coastal plain form when:
Sea levels fall, exposing the sea bed of what was once a shallow continental shelf sea, e.g. the Atlantic coastal plain in the USA.
Sediment brought from the land by river systems is depositde at the coast causing coastal accretion so coastlines gradually move seaward such as the River delta.
Sediment is moved from offshore courses (sand bars) towards the coast by ocean currents.
Coastal plains are a low-energy environment usally lacking large and powerful waves except rare occasions such as during hurricanes.
Accretion
= the depostion of sediment at a coast that expands the area of land
Subarieal processes
= include weathering processes (mechanical, chemical and biological), mass movements processes (landslides, rock falls) and surface runoff erosion.
What factors affect coastal recession?
Coastal recession
= how fast a coastline is moving inland. This is influenced by many factors, but one is lithology (rock type). There are three major rock types - igneous, sedimentary and metamorphic.
Rock Types
:
Sedimentary
: sandstone, limestone & shale
Moderate to fast erosion rate at 0.5-10 cm per year. Most sedimentary rocks are clastic and eroded faster than crystalline igneous and metamorphic rocks. The age of sedimentary rocks is important, as geologically young rocks tend to be weaker. Rocks with many bedding planes and fractures, such as shale, are often most vulnerable to erosion.
Metamorphic
: slate, schist & marble
Slow erosion reate at 0.1-0.3 cm per year. They are crystalline, and the interlocking crystals make for strong, hard erosion-resistant rock. Rocks such as granite often have few joints, so there are limited weaknesses that erosion can exploit
Igenous
: granite, basalt & dolerite
Very slow erosion rate at less than 0.1 cm per year. They are crystalline, and the interlocking crystals make for strong, hard erosion-resistant rock. Rocks such as granite often have few joints, so there are limited weaknesses that erosion can exploit
Key terms
:
Unconsolidated material
: sediment that has not been cemented to form solid rock, a process known as lithification
Clastic
= these rocks consist of sediment particles cemented together
Crystalline
= are made up of interlocking mineral crystals
Pore water pressure
= an internal force within cliffs exerted by the mass of groundwater within permeable rocks.
factors affecting erosion and weathering resistance:
Whether rocks are clastic or crystalline - the latter are more erosion-resistant
How reactive minerals in the rock are when exposed to chemical weathering
The degree to which rocks have cracks, fractures and fissures, which are weaknesses exploited by weathering and erosion.
How does vegetation help stabilise coastlines?
On coastal dunes, the succession begins with the colonisation of embryo dunes by pioneer species. Embryo dune pioneer plants:
Add dead organic matter to the sand, beginning the process of soil formation.
Reduce wind speeds at the sand surface, allowing more sand to be deposited
Stabilise the mobile sand by their root systems.
On a coast where there is a supply of sediment and deposition takes place:
The end result of plant succession is called a climatic climax community.
Each step in plant succession is called a seral stage
The forms the first stage of plant succession
Pioneer plant species will begin to grow in the bare sand or mud
Some coastlines including coastal sand dunes, salt marshes and mangrove swamps are protected from erosion by the stabilising influence of plants. Vegetation stabilises coastal sediment in a number of ways:
Plants protect sediment from erosion by wind, by reducing wind speed at the surface because of friction with the vegetation.
When submerged, plants provide a protective layer so the sediment surface is not directly exposed to moving water and erosion
Roots bind the sediment together
Plant succession
: the changing structure of a plant community over time as an area of inititally bare sediments.
A similar process of successional development happens on bare mud deposited in estuaries at the mouths of rivers that are exposed at low tide but submerged at high tide. Estuarine areas are ideal for the development of salt marshes because:
They are sheltered from strong waves, so sediment (mud and slit) can be deposited.
Rivers transport a supply of sediment to the river mouth, which may be added to by sediment flowing into the estuary at high tide.
