Have you ever gone for a stroll on a beach? Swam in the ocean? Gone surfing? Or built a sand castle next to the waves? If you have, then you've been to a coast and walked along a coastline. The Earth has approximately 620,000 kilometres (390,000mi) of coastlines and they are not just a barrier between land and water; they are also important natural ecosystems. Let's take a look at some of the different types of coastlines and their role in our geopolitics and global climate.
Coastline definition in geography
Within geography, the definition of a coastline is the area where land meets water. The water, with an endless supply of waves, whether pounding or gentle ripples, is constantly changing coastlines around the globe.
How coastlines are made and shaped
The extent to which the shape of a beach or coast is created or altered depends mainly on the action of waves upon it. Waves can be gentle and infrequent or more significant, more frequent, and more powerful.
Waves interact with land through three major marine processes: erosion, transportation, and deposition. Over time, waves beating against the shore will wear it down (or erode it). Transportation is the movement of material from a coastline--such as sand and gravel--while deposition is the addition of material to a coastline. These processes are happening constantly and usually work in conjunction to produce coastal features.
Types of coastlines
There are four major different types of coastlines:
- Emergent coastlines
- Submergent coastlines
- Concordant coastlines
- Discordant coastlines
Below, we will go into more detail on all of these different types of coastlines.
Emergent coastlines
Emergent coastlines happen when either the water level has fallen or the land has risen. Either way, there is now a (bit of) coastline that is no longer submerged under water. Emergent coastlines can pop up after tectonic activity, where the land is being pushed up by the tectonic plates.
Many emergent coastlines have existed since the glacial phases of the Pleistocene Epoch, which lasted from about 2,580,000 and 11,700 years ago because the sea levels were much lower than they are today.
Emergent coastlines can have features such as marine terraces, relict sea cliffs, sea stacks, and raised beaches. They are, for the time being, out of reach of the current wave action and will therefore not be affected by them.
Submergent coastlines
In contrast with emergent coastlines, submergent coastlines are coastlines submerged under water due to rising sea levels. Many of these types of coastlines were actually formed toward the end of the Last Glacial Period (LGP). The LGP encompasses the period of c. 115,000 to c. 11,700 years ago. During this time, glaciers and ice sheets were retreating, causing a rise in global sea levels and localised changes in land height.
Coastlines are constantly changing, with sea levels rising or falling, and it is no different with submergent coastlines. The rise in sea level can be the result of an increase in water volume or the sinking of the land's surface. The latter can happen when tectonic forces lowered the land level or when river deposits and compaction of alluvial (river) sediments occur.
There are two special types of submerged coastlines: ria coasts and fjord coasts.
Coastlines: Ria coasts
A ria is a drowned river valley that leads out to the sea. Where once there was a simple river with well-defined banks, now there is a sprawling river that has submerged most of the terrain around it. A ria coast hosts multiple rias. There are rias throughout the world, including Poole Harbour in Dorset.
Coastlines: Fjord coasts
Fjords are created when glaciers cut through valleys and are submerged. The results are deep, lengthy, thin inlets surrounded by high cliff faces. A fjord coast hosts multiple fjords. While there are fjords all over the world, the most renowned fjord coasts are in Norway--and in fact, "fjord" is a Norwegian word.
Discordant coastlines
Discordant coastlines occur when bands of different types of rocks run perpendicular (at 90 degrees) to the coast. These bands of rocks alternate between soft rock and hard rock, all eroding at different rates and in varying ways. Because of this difference in erosion resistance, discordant coastlines are home to headlands, due to eroding hard rock, and bays, due to eroding soft rock.
The coastline between Durlston Head and Studland Bay in Dorset, UK, is a great example of a discordant coastline. There are different bands of rock that have shaped this discordant coastline, namely:
| Area | Type of rock |
| Durlston Head | limestone (hard rock) |
| Swanage Bay | clay and sand (soft rock) |
| Ballard Point | chalk (hard rock) |
| Studland Bay (and beyond) | clay and sand (soft rock) |
Fig. 1 - Approximate locations on the coastline between Durlston Head and Studland Bay, map data: © 2022 Google
The image below (figure 2) is taken at Durlston Head, which shows the bay (yellow) and the headland (red).
Fig. 2 - Durlston headland and bay
Fun Fact: if you are interested in dinosaurs and dinosaur fossils, Durlston Bay is a renowned site for Early Cretaceous fossils. The Early Cretaceous, sometimes known as Lower Cretaceous, is the period stretching from 145 million years ago to 100.5 million years ago.
Concordant coastlines
While discordant coastlines have bands of different rock types running perpendicular to the coast, concordant coastlines have bands of similar rock types running parallel (alongside) to the coast. The difference in rock types between discordant and concordant coastlines means that there are differences in erosion. As mentioned earlier, discordant coastlines form headlands and bays; on the other hand, concordant coastlines form coves. These coves are formed by waves breaking through an outer layer of hard rock, such as limestone, and then, over time, the waves sweep away the soft rock further inland, like sand and clay, creating a cove.
A concordant coastline can take one of the following two landform types:
| Landform type | Explanation |
| Dalmatian type | Named after the Dalmatia region on the Adriatic Sea. Long offshore islands and coastal inlets are running parallel to the coastline. |
| Haff type | These are found in Haffs, also known as lagoons, on the Baltic Sea's southern shores. Long spits of sands run parallel to the low coast, enclosing the coast. |
An example of a concordant coastline is Lulworth Cove, again, in Dorset, UK (figure 3). This cove is located near the village of West Lulworth and is one of the best examples of a concordant coastline.
Figure 3: Location of Lulworth Cove, Dorset, UK, map data: © 2022 Google
The outer layers of the coastline, those directly on the waterline, are Portland and Purbeck limestone, and they have been eroded away over many years. After the waves broke through, creating an opening, the softer clay after the limestone started to erode away as well, creating a cove (figure 4). The shape of the cove is the result of wave diffraction.
Wave diffraction happens when the narrow opening to the cove causes the waves to bend, creating an arc shape wave.
Fig. 4 - the process of forming Lulworth Cove, Dorset, UK
The image below shows the narrow opening created in the limestone and the cove that formed afterwards.
Figure 5: Lulworth Cove in Dorset, UK, map data: © 2022 Google
Fun Fact: Lulworth Cove is a World Heritage Site and attracts approximately 500,000 visitors a year. Lulworth Cove is located on the so-called Jurassic Coast, spanning 185 million years of geological history, from the Triassic (252 - 201 million years ago), Jurassic (199.6 - 145.5 million years ago), and Cretaceous (145 - 66 million years ago) periods. The Jurassic Coast is a world-renowned site where you can find geological (natural) features, and different types of fossils, such as dinosaurs and a fossil forest.
Did you know? Concordant coastlines are also called 'concordant longitudinal' or 'Pacific type' coastlines.
Facts about coastlines
Okay, now we know what coastlines are. But do they serve as anything more than a place to go for a hike or a tan? Besides hosting a multitude of specialised plants and animals, coastlines are also critical to our economic and political infrastructures, providing people with sources of food and livelihoods and allowing us to determine where our borders actually end.
Coastal ecosystems
Numerous plants and animals have adapted to live along coastlines. If you've been to the beach, you've probably seen some of them: mangrove trees and hermit crabs, penguins and sea oats- organisms that can neither fully survive way out at sea, nor venture too far inland. Large colonies of sea lions and seals sleep and breed along coastlines, entering the seas to hunt. Sea turtles return to coasts to lay their eggs, and birds like gulls and pelicans do much of their hunting near the shore.
Coastal populations
Humans, too, could almost be classified as a coastal species! Approximately 40% of all people live within 100km of a coastline. Many of our major cities have developed along oceanic coasts as well: think of New York City, Tokyo, Istanbul, Dubai, Hong Kong, and Copenhagen, just to name a few! Even London is built along the River Thames, which flows into the North Sea. This is because access to the coast offers the opportunity for harvesting marine resources, especially fish, as well as the ability to conduct international trade via the sea.
Coastlines as national boundaries
Coastlines also help us demarcate international boundaries. This is important in delineating who has legal, economic, and military jurisdiction in areas along the coast.
In 1982, the United Nations held a Convention on the Law of the Sea (UNCLAS), where maritime (sea) boundaries were established. While not every UN member has ratified UNCLAS, most nations abide by it anyway.
The coastline determines everything. Taking the low-water line along the coast (or, the baseline), UNCLAS put forth the following:
| Zone | Distance from baseline | National rights |
| Territorial waters | 12 nautical miles (∼22.2km) | This is considered sovereign national territory, the same as boundaries on land. |
| Contiguous zone | 24 nautical miles (∼44.4km) | Limited law enforcement jurisdiction to prevent crimes related to customs or trafficking. |
| Exclusive economic zone | 200 nautical miles (∼370.4km) | Unique access to harvest all resources within the EEZ, including fishing and fracking. |
Special exceptions apply to areas like straits, where ships can't help but pass through territorial waters. However, overall, access to a coastline can grant a country food supplies and economic resources that land-locked countries cannot acquire without trade.
Coastlines and climate change
As our Earth warms, glaciers melt, causing the sea level to rise. As we mentioned earlier, this tends to shift the coastline further inland. The shifting coastlines can impact freshwater resources near the coasts by creating brackish mixtures, and can also pose an obvious danger to infrastructure built directly along the coast. Many of the major cities built directly along coastlines, such as New York City and Tokyo, will be forced to develop solutions that counter rising sea levels or else abandon waterfront infrastructures and construct further inland.
Additionally, hotter temperatures enable extreme weather events like hurricanes to occur more frequently. As these systems develop at sea, communities along coastlines are the most vulnerable to any potential destruction.
Coastlines - Key takeaways
- There are four major types of coastlines: emergent, submergent, discordant, and concordant.
- Emergent coastlines have emerged from the water; submergent coastlines have submerged below the water.
- Discordant coastlines have bands of different rock types running perpendicular to the coast; concordant coastlines have bands of similar rock types running parallel to the coast.
- The United Nations uses coastlines to help determine international boundaries at sea.
- Coastlines will expand as climate change continues.
References
- Fig. 2: Durlston headland and bay (https://en.wikipedia.org/wiki/File:Durlston_bay_from_durlston_castle.jpg) by Jim Champion (https://commons.wikimedia.org/wiki/User:JimChampion) licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
- Fig. 4: Formation of Lulworth Cove (https://commons.wikimedia.org/wiki/File:Lulworth.png) by Red (https://en.wikipedia.org/wiki/User:Red) licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
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