Coastlines

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:

1. Emergent coastlines
2. Submergent coastlines
3. Concordant coastlines
4. 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.

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:

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:

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.

• Figure 4: https://commons.wikimedia.org/wiki/File:Lulworth.png