Classification of Coasts

Classifying Coasts

Coasts evolve depending on long-term processes that take millions of years, as well as short-term processes that are constantly happening.

Long-term Processes: Overview

Over millions of years, the location of coastlines fluctuates based on the volume of water in the oceans, the size of the ocean basins themselves and the movement of tectonic plates.

Tectonic processes such as seafloor spreading can change the size of ocean basins by adding magma along the mid-ocean ridges. From land, sediment is constantly being added to the world's oceans, and over millions of years, the dying off of coral reefs also adds material.

Another way that the eustatic sea level can change is based on the water's salinity (salt content), as this affects water density.

During episodes of global cooling, much of the world's water is locked up in ice caps as freshwater, with less water in the oceans as a result. During interglacials such as during the last 11,000 years, ice caps melt, and sea levels increase. This cycle has been repeated many times, with dramatic effects on coastlines.

A long-term process called isostatic rebound (glacial isostatic adjustment) occurs during interglacials, as the massive weight of ice caps is removed and compressed land surfaces emerge from the sea.

During the last several hundred thousand years, sea levels have risen and fallen tens and even hundreds of metres on numerous occasions. When sea levels fall, land bridges can form, such as between Australia and Tasmania. When sea levels rise, land bridges can be destroyed, see the deep dive below for an example.

Fig. 1 - The hypothetical extent of Doggerland.

Short-term Processes: Overview

Coasts are built with material from the ocean, local material (beach sand, rock from cliffs and platforms), and material coming from inland.

Sediment from inland source areas such as mountain ranges is transported to coastal areas via river systems. Depending on local conditions, this sediment may be deposited as new coastline (for example, in bird's-foot deltas), removed to the continental shelf, or even flow off the shelf through submarine canyons.

The ocean itself transports sediment perpendicularly to the coast, depositing it every tidal cycle on the beach and sometimes well inland through tidal bores up estuaries. During storm events and tsunamis, water and its contents are often deposited many meters or even kilometres inland, depending on the slope of the coastal landscape and the amount of energy involved.

The ocean also transports sediment parallel to the coast via longshore currents, which are related to local wind directions involved in the constant movement of sand.

Coastlines include many areas of sand, mud, and other sediment transported from inland or laterally along the coast. They also include igneous, metamorphic, and sedimentary rock formations in the process of being eroded by salt water and wind and perhaps also being uplifted or depressed at the same time.

Saltwater corrodes surfaces rapidly, particularly when becoming aerosolised as a salt spray or when hitting solid surfaces through wave action. Wave action is heightened during storms, which often occur seasonally and by tides.

Types of Coastal Waves

Many geographical factors influence waves that break on coasts. These include winds, tidal cycles, storm activity, the shape of the terrain underwater and on land, etc. Waves then take on different characteristics that determine how their potential energy is converted into kinetic energy.

Two helpful terms are swash and backwash. The swash is the water of the wave as it breaks on the shore, heading up the beach, and the backwash is the same water travelling back to the sea.

Constructive Waves

These are typically small and far apart, with stronger swashes than backwashes. Sediment that is carried up to the beach is left there, and net deposition on the coast occurs.

Destructive Waves

These are relatively tall and close together, and their backwash is more powerful than their swash. Therefore, the backwash carries sediment back into the ocean, resulting in net erosion of the coastal landscape.

Types of Coasts

There are several complementary ways to characterise coastal landscapes and landforms. These include primary and secondary coasts, emergent and submergent coastlines, erosional versus depositional, low-energy and high-energy, and active vs passive. These different classification schemes are related to the dominant long-term and/or short-term forces that happen to be the primary contributors to a coastline when it is being characterised.

Primary vs Secondary Coasts

As described above, primary coasts are those formed by long-term processes such as plate tectonics and climate cycles. Once tectonic and climatic stability is reached in a coastal system--such as after the end of rapid warming following an ice age--then coastlines are subject to secondary processes. Primary processes may still be occurring, but secondary processes are dominant.

In the current "Anthropocene" era, secondary coastal development appears to be ceding to primary evolution, caused not only by the rapid sea-level rise attributed to human-caused global warming but also by the vast and devastating effects of human coastal infrastructure development that is destroying barrier islands, sheltering reefs, estuarine zones, and even cutting off natural processes like beach drift.

Emergent vs Submergent Coastlines

Emergent coastlines are those that are rising relative to sea level, whereas submergent coastlines are those where the sea level is rising relative to the coast.

Emergent Coastlines

Secondary coastal landscapes where the sea level has dropped relative to the land (because the land is rising, the sea is falling, or both) are quite distinctive and include emergent coastal landforms such as raised beaches and marine platforms.

Submergent Coastlines

Secondary coastal landscapes where the sea level has risen relative to the land (because the land is subsiding, the sea is rising, or both) form some of the world's most spectacular and distinctive coastlines.

Examples are:

Erosional vs Depositional Coasts

Erosional and depositional forces can and usually do occur simultaneously in coastal landscapes.

The marine processes that can cause both erosion and deposition involve two primary types of transportation. These are traction, whereby heavier materials are dragged or bounce along the seafloor, and suspension, whereby smaller particles can float in the currents.

Sub-aerial erosion, mass wasting, and direct runoff also occur on coastlines, as elevated coastal landscapes break down and erode into the sea.

Erosional Landscapes

Headlands, beaches, and other coastal features are typically being eroded constantly by wave action.

Sea cliffs are particularly dynamic locations and, depending on their parent material, may be carved away or bored into by waves, resulting in caves. Rocky areas along coasts are eventually broken apart by hydraulic action, with wave-quarrying forming arches and stacks. An example is the White Cliffs of Dover.

As the sea impacts the land, rocky coasts are affected by:

• corrasion - when rocks hit cliffs

• abrasion - where sand and salt act like sandpaper, wearing down rock

• cavitation - the carving away of weaker rock layers, often forming sea caves

• solution - the dissolution of limestone and other less-resistant rock

• attrition - the erosion of particles in the sea by other particles (rounding of sand grains, for example)

Fig. 4 - The White Cliffs of Dover, UK, is an example of an erosional landscape.

Depositional Landscapes

Beaches are the most famous and typical landscapes of coastal deposition. They are incredibly dynamic places that change with every tidal cycle and every season. They consist primarily of sand (eroded from sandstone) transported laterally by the wind (beach drift), a very important point meaning that sandy beaches migrate sort of like the shorebirds often seen on them. This is why when areas are blocked by seawalls and other human infrastructure, new sand must be trucked in to fill beaches. If sand migration is blocked, natural beaches and other features disappear.

Beaches may also consist of other types of rubble from volcanic rock and near-shore coral reefs.

Various types of depositional sand landforms result from wind and water action. These include:

• Simple and compound spits: these are small peninsulas formed by migrating sand

• Tombolos: an isthmus of sand that forms between a beach and an island

• Offshore bars: areas of usually submerged sand elevated above the seafloor

• Barrier beaches and islands: built from sandbars, when these rises far enough above tide level that terrestrial vegetation can take root and stabilise them. Longshore currents create barrier islands parallel to certain coasts, always highly subject to storm activity and erosion.

• Sand dunes: away from the active tidal zone, storms deposit sand where vegetation can grow on it and stabilise it. Wind action often creates characteristic dune shapes similar to those found in deserts. As coastline emergence occurs, old sand dunes are effectively located farther inland but are still often considered part of the coastal environment.

Estuaries

Along submergent coastlines, rivers often form deltas where they meet the sea. This estuarine zone of saltwater and brackish (salt-fresh mix) produces some of Earth's most biologically rich but environmentally sensitive ecosystems. Salt-tolerant vegetation ranges from grasses to trees anchoring saltmarshes and mangrove swamps inundated during high tide. Mudflats that emerge during low tide can stretch for kilometres and are rich in nutrients and organisms.

Low Energy vs High Energy Coasts

When wave energy is considered the primary factor, coasts can roughly be divided into high-energy and low-energy environments.

Low energy coasts

These tend to be either in a location sheltered from waves and wind (protected by an island or a reef, for example) or in a place where yearly weather fluctuations are minimal and storms infrequent, as in equatorial regions. Low-energy coasts may see much slower change relative to high-energy coasts and are typically depositional environments.

High energy coasts

These are exposed to direct waves, bearing the brunt of storms, and are usually beset by constant and often high winds. They are typically erosional environments.

Tectonic plates and coastlines

The location of a coastline relative to a tectonic plate boundary and what type of plate boundary it is is a strong determinant of coastal evolution. An example is Big Sur.

Fig. 5 - Big Creek Bridge in Big Sur, California, US. Big Sur is an active coastal landscape.

Active Coasts

Active margins along tectonic plates may coincide with coastlines, and where this is the case, the continental shelf is very narrow and gives way quickly to an offshore marine trench, as along the Pacific Ocean's "Ring of Fire." Emergent coastlines result from the tectonic uplift of landmasses in these areas.

Passive Coasts

Wide continental shelves are usually found along passive margins where the land meets the sea somewhere other than the edge of a plate. Most of the coastlines around the Atlantic Oceans are of this type.