Coastlines occur where the land meets the sea, and they are formed by marine and land-based processes. These processes result in either erosion or deposition, creating different types of coastal landforms. The formation of the coastal landscape depends on many factors, including the type of rock these processes are acting on, how much energy is in the system, sea currents, waves, and tides. When you next visit the coast, look out for these landforms and try to identify them!
Coastal landforms - definition
Coastal landforms are those landforms found along coasts that have been created by coastal processes of erosion, deposition, or both. These typically involve some interaction between the marine environment and the terrestrial environment. Coastal landforms differ substantially according to latitude due to differences in climate. For example, landscapes shaped by sea ice are found at high latitudes, and landscapes shaped by coral are found at low latitudes.
Types of Coastal Landforms
There are two main types of coastal landforms- erosional coastal landforms and depositional coastal landforms. Let's take a look at how they are formed!
How are coastal landforms formed?
Coastlines emerge or subside from the ocean through long-term primary processes such as climate change and plate tectonics. Climate change can involve global warming, where ice caps melt and sea level rises, or global cooling, where ice masses grow, ocean levels shrink, and glaciers press down on the land surface. During global warming cycles, isostatic rebound occurs.
Isostatic rebound: Process whereby land surfaces rise or 'rebound' from lower levels after ice sheets melt. The reason is that ice sheets exert massive force on the land, pushing it downward. When ice is removed, the land rises, and the sea level drops.
Plate tectonics affects coastlines in many ways.
In volcanic 'hotspot' areas of the oceans, new coastlines are formed as new islands arise from the sea or lava flows create and reshape existing mainland coasts.
Under the ocean, seafloor spreading adds volume to the ocean as new magma enters the ocean environment, displacing water volume upward and raising the eustatic sea level. Where tectonic plate boundaries are the edges of continents, such as around the Ring of Fire in the Pacific; for example, in California, active coastlines are created where tectonic upheaval and submergence processes often create very steep headlands.
After global warming or cooling stabilises along passive coastlines where tectonic activity is not occurring, the eustatic sea level is reached. Then, secondary processes occur that create secondary coastlines that include many of the landforms described below.
The geology of the parent material is critical in the process of coastal landform creation. The characteristics of rock, including how it is bedded (its angle in relation to the sea), its density, how soft or hard it is, its chemical composition, and other factors, are all important. What type of rock lies inland and upstream, reaching the coast transported by rivers, is a factor for some coastal landforms.
In addition, the contents of the ocean -- local sediment as well as material transported long distances by currents -- contribute to coastal landforms.
Mechanisms of erosion and deposition
Ocean currents
An example is a longshore current that moves parallel to the coastline. These currents happen when waves are refracted, meaning they slightly change direction when they hit shallow water. They 'eat' away at the coastline, eroding soft materials such as sand and depositing them elsewhere.
Waves
There are several ways that waves erode material:
| Ways that waves erode material |
|---|
| Erosion way | Explanation |
| Abrasion | Coming from the verb 'to abrade,' meaning to wear down. In this case, the sand that the wave is carrying wears away at the solid rock, like sandpaper. |
| Attrition | This is often confused with abrasion. The difference is that with attrition, particles hit eat other and break apart. |
| Hydraulic action | This is the classic 'wave action' whereby the force of the water itself, as it smashes against the coast, breaks rock apart. |
| Solution | Chemical weathering. Chemicals in the water dissolve certain types of coastal rocks. |
| Table 1 |
Tides
Tides, the rise and fall of sea levels, are regular movements of water that are influenced by gravitational forces from the moon and sun.
There are 3 types of tides:
- Micro-tides (less than 2m).
- Meso-tides (2-4m).
- Macro-tides (more than 4m).
The former 2 help in the formation of landforms by:
- Bringing in massive quantities of sediment that erode the rock bed.
- Changing the depth of the water, shaping the shoreline.
Wind, rain, weathering and mass movement
Wind not only can erode material but also is crucial in determining wave direction. This means that wind has both a direct and indirect effect on coastal formation. Wind moves the sand, resulting in beach drift, whereby sand literally migrates toward the prevailing coastal winds.
Rain is also responsible for erosion. Rainfall transports sediments when it runs down to and through the coastal area. This sediment, along with the current from the water flow, erodes anything in its path.
Weathering and mass movement are also known as 'sub-aerial processes.' 'Weathering' means that rocks are eroded or broken down in place. Temperature can affect this as it can influence the state of the rock. Mass movements refer to the movement of material downslope, influenced by gravity. An example is a landslide.
Gravity
As mentioned above, gravity can influence the erosion of materials. Gravity is important in coastal processes because it not only has an indirect impact on wind and wave movements but also determines the downslope movement.
Erosional coastal landforms
The erosional landscape is dominated by destructive waves in high-energy environments. A coast formed of more resistant material such as chalk leads to coastal landforms such as arches, stacks, and stumps. A combination of hard and soft materials leads to the formation of bays and headlands.
Examples of erosional coastal landforms
Below is a selection of the most common coastal landforms that you might encounter in the UK.
| Coastal landform examples |
|---|
| Landform | Explanation |
| Bay | A bay is a small body of water, recessed (set back) from a large(r) body of water such as an ocean. A bay is surrounded by land on three sides, with the fourth side connected to the large(r) body of water. A bay is formed when the surrounding soft rock, such as sand and clay, is eroded. Soft rock erodes easier and more quickly than hard rock, such as chalk. This will cause sections of land to jut out into the large(r) body of water called headlands.  Fig. 1 - An example of a bay and headland in St. Sebastian, Spain.
|
| Headlands | Headlands are often found near bays. A headland is usually a high point of land with a sheer drop to the body of water. Headland characteristics are high, breaking waves, intense erosion, rocky shores, and steep (sea) cliffs.  Fig. 2 - Sydney Heads in Sydney, Australia, is an example of a headland.
|
| Cove | A cove is a type of bay. However, it is small, circular, or oval and has a narrow entrance. A cove is formed by what is called differential erosion. The softer rock is weathered and worn away quicker than the harder rock surrounding it. Further erosion then creates the circular or oval-shaped bay with its narrow entrance.  Fig. 3 - Lulworth Cove in Dorset, UK, is an example of a cove.
|
| Peninsula | A peninsula is a piece of land that, similar to a headland, is almost entirely surrounded by water. Peninsulas are connected to the mainland via a 'neck'. Peninsulas can be large enough to hold a community, city, or entire region. However, sometimes peninsulas are small, and you often see lighthouses situated on them. Peninsulas are formed by erosion, similar to headlands.  Fig. 4 - Italy is a good example of a peninsula. Map data: © Google 2022
|
| Rocky coast | These are landforms made up of igneous, metamorphic, or sedimentary rock formations. Rocky coastlines are shaped by erosion through marine and land-based processes. Rocky coastlines are areas of high energy where destructive waves make up the majority of erosion.  Fig. 5 - El Golfo Beach in Lanzarote, Canary Islands, Spain, is an example of a rocky coast.
|
| Cave | Caves can form in headlands. Waves cause cracks to form where the rock is weak, and further erosion leads to caves. Other cave formations include lava tunnels and glacially carved tunnels. 
Fig. 6 - A cave on San Gregoria State Beach, California, US, is an example of a cave. |
| Arch | When a cave forms on a narrow headland and erosion continues, it can become a complete opening, with only a natural bridge of rock at the top. The cave then becomes an arch.  Fig. 7 - Arch on Gozo, Malta.
|
| Stacks | Where erosion leads to the collapse of the arch's bridge, separate pieces of free-standing rock are left. These are called stacks.  Fig. 8 - The Twelve Apostles in Victoria, Australia, are examples of stacks.
|
| Stumps | As the stacks erode, they become stumps. Eventually, stumps wear away below the waterline. |
| Wave-cut platform | A wave-cut platform is a flat area in front of a cliff. Such a platform is created by, as the name suggests, waves that cut (erode) away from the cliff, leaving behind a platform. The bottom of a cliff often erodes the most quickly, resulting in a wave-cut notch. If a wave-cut notch becomes too large, it can result in cliff collapse.  Fig. 9 - Wave-cut platform at Southerndown near Bridgend, South Wales, UK.
|
| Cliff | Cliffs get their shape from weathering and erosion. Some cliffs have a gentle slope because they are made of soft rock, which erodes quickly. Others are steep cliffs because they are made from hard rock, which takes longer to erode.  Fig. 10 - The White Cliffs of Dover
|
| Table 2 |
Depositional coastal landforms
Deposition refers to the laying down of sediment. Sediments such as silt and sand settle when a body of water loses its energy, depositing them on a surface. Over time, new landforms are created by this deposition of sediments.
Deposition occurs when:
- Waves enter an area of lesser depth.
- Waves hit a sheltered area like a bay.
- There is a weak wind.
- The amount of material to be transported is in good quantity.
Examples of depositional coastal landforms
Below you will see examples of depositional coastal landforms.
| Depositional coastal landforms |
|---|
| Landform | Explanation |
| Beach | Beaches are made up of material that has eroded somewhere else and has then been transported and deposited by the sea/ocean. For this to happen, the energy from the waves has to be limited, which is why beaches are often formed in sheltered areas such as bays. Sandy beaches are most often found in bays, where the water is more shallow, meaning that the waves have less energy. On the other hand, pebble beaches are most often formed below eroding cliffs. Here, the energy of the waves is much higher.  Fig. 11 - Aerial view of Bondi Beach in Sydney is one of the most known beaches in Australia.
|
| Spits | Spits are extended stretches of sand or shingle that protrude into the sea from the land. This is similar to a headland in a bay. The occurrence of a river mouth or a change in landscape shape leads to the formation of spits. When the landscape changes, a long thin ridge of sediment is deposited, which is the spit.  Fig. 12 - Spits at Dungeness National Wildlife Refuge in Washington, US.
|
| Bars and tombolos | A bar forms where a spit has grown across a bay, joining 2 headlands together. The tombolo is the small isthmus that forms between an offshore island and the mainland. Shallow lakes called lagoons can form behind tombolos and bars. Lagoons are often short-term bodies of water as they can be filled up again with sediments.  Fig. 13 - A tombolo connecting the islands of Waya and Wayasewa in Fiji.
|
| Saltmarsh | A salt marsh can be formed behind a spit, creating a sheltered area. Due to the shelter, the water movements slow down, which causes more materials and sediments to be deposited. These are found along submergent, meaning party submerged coastlines, often in estuarine environments.  Fig. 14 - Saltmarsh at the Heathcote River Estuary Salt Marsh in Christchurch, New Zealand.
|
| Table 3 |
Coastal Landforms - Key takeaways
- Geology and the amount of energy in the system affect the coastal landforms that occur along a coastline.
- Erosional landscapes result from destructive waves in a high-energy coastal environment where the coast is formed of a material such as chalk leading to coastal landforms such as arches, stacks, and stumps.
- Coastal landforms can be formed by erosion or deposition. In other words, it can either take materials away (erosion) or drop materials (deposition) to create something new.
- Erosion can happen by sea currents, waves, tides, wind, rain, weathering, mass movement, and gravity.
- Deposition occurs when waves enter an area of lesser depth, waves hit a sheltered area like a bay, there is a weak wind, or the amount of material to be transported is in good quantity.
References
- Fig. 1: Bay St Sebastian, Spain (https://commons.wikimedia.org/wiki/File:San_Sebastian_aerea.jpg) by Hynek moravec/Generalpoteito (https://commons.wikimedia.org/wiki/User:Generalpoteito) Licensed by CC BY 2.5 (https://creativecommons.org/licenses/by/2.5/deed.en)
- Fig. 2: Sydney Heads in Sydney, Australia, is an example of a headland (https://en.wikipedia.org/wiki/File:View_from_North_Head_Lookout_-_panoramio.jpg) by Dale Smith (https://web.archive.org/web/20161017155554/http://www.panoramio.com/user/590847?with_photo_id=41478521) Licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
- Fig. 5: El Golfo Beach in Lanzarote, Canary Islands, Spain, is an example of a rocky coast (https://commons.wikimedia.org/wiki/File:Lanzarote_3_Luc_Viatour.jpg) by Lviatour (https://commons.wikimedia.org/wiki/User:Lviatour) Licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
- Fig. 7: Arch on Gozo, Malta (https://commons.wikimedia.org/wiki/File:Malta_Gozo,_Azure_Window_(10264176345).jpg) by Berit Watkin (https://www.flickr.com/people/9298216@N08) Licensed by CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en)
- Fig. 8: The Twelve Apostles in Victoria, Australia, are examples of stacks (https://commons.wikimedia.org/wiki/File:Twelve_Apostles,_Victoria,_Australia-2June2010_(1).jpg) by Jan (https://www.flickr.com/people/27844104@N00) Licensed by CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en)
- Fig. 9: Wave-cut platform at Southerndown near Bridgend, South Wales, UK (https://commons.wikimedia.org/wiki/File:Wavecut_platform_southerndown_pano.jpg) by Yummifruitbat (https://commons.wikimedia.org/wiki/User:Yummifruitbat) Licensed by CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5/deed.en)
- Fig. 10: The White Cliffs of Dover (https://commons.wikimedia.org/wiki/File:White_Cliffs_of_Dover_02.JPG) by Immanuel Giel (https://commons.wikimedia.org/wiki/User:Immanuel_Giel) Licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
- Fig. 11: Aerial view of Bondi Beach in Sydney is one of the most known beaches in Australia (https://en.wikipedia.org/wiki/File:Bondi_from_above.jpg) by Nick Ang (https://commons.wikimedia.org/wiki/User:Nang18) Licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en)
- Fig. 12: Spits at Dungeness National Wildlife Refuge in Washington, US (https://commons.wikimedia.org/wiki/File:Dungeness_National_Wildlife_Refuge_aerial.jpg) by USFWS - Pacific Region (https://www.flickr.com/photos/52133016@N08) Licensed by CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en)
- Fig. 13: A tombolo connecting the islands of Waya and Wayasewa in Fiji (https://en.wikipedia.org/wiki/File:WayaWayasewa.jpg) by User:Doron (https://commons.wikimedia.org/wiki/User:Doron) Licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
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