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Table of contents
Introduction
Coastal areas, the meeting points of land and sea, hold a special place in our experiences with the ocean. But have you ever stopped to ponder why a coast exists where it does or why it takes on its unique shape? These beautiful junctions between land and sea are shaped by a combination of marine and terrestrial processes.
Waves, tides, sea level changes, biological activities, and tectonic forces all contribute to shaping the coast. In this article, we will explore the factors that influence the location and formation of coasts, as well as the impact of sea level changes on coastal landscapes.
Understanding the Coast
The shore, where the ocean meets the land, is the immediate boundary between the two. However, the term “coast” encompasses a larger area that includes not just the sandy beach but also the marshes, sand dunes, cliffs, sandbars, and troughs near the shoreline. In total, the world’s coastline stretches for approximately 440,000 kilometers (273,000 miles).
Being in close proximity to both the ocean and the land, the coast is greatly influenced by natural events and processes that occur in both realms. It serves as a dynamic battleground where wind waves break and release their energy, tides ebb and flow, rivers deposit their sediments, and storms from the ocean batter the continents.
The location and shape of a coast are primarily determined by global tectonic activity and the volume of water in the ocean. Various processes such as uplift and subsidence, erosion, and sediment transport contribute to the formation of a coast’s distinctive features.
Plate Tectonics and Coasts
Plate tectonics, the scientific theory explaining the movement of the Earth’s lithospheric plates, has revolutionized our understanding of coasts. Geologists began classifying coasts based on their tectonic positions in the 1960s and discovered fundamental differences between active coasts near converging plate boundaries and passive coasts near diverging plate boundaries.
By considering plate movements, we gain a better understanding of the shapes, compositions, and ages of coasts. However, the slower forces of plate movement often remain overshadowed by the more immediate actions of waves, erosion, and sediment transport.
Sea Level Changes
One crucial factor influencing coasts is long-term sea level change. Sea level can be affected by various factors, three of which cause eustatic changes—variations in sea level observed worldwide:
1. Fluctuations in the amount of water in the world’s oceans: During global glaciation periods (ice ages), sea level is lower due to less water being present in the ocean. Conversely, during warm periods, when glaciers are smaller, sea level rises. Volcanic outgassing can also add water to the ocean and raise sea level.
2. Changes in the volume of the ocean’s “container”: High rates of seafloor spreading cause the expansion of oceanic ridges, displacing the ocean’s water and elevating sea level along the edges of the continents. Sediments deposited by rivers during periods of rapid erosion can decrease ocean basin volume and raise sea level. Additionally, changes in seawater volume due to temperature variations contribute to sea level changes, with global warming causing seawater to expand and raise sea level.
3. Local changes in sea level: Tectonic motions and isostatic adjustments can alter the height and shape of a coast. Coasts may experience uplift as continental plates converge or be weighed down by ice masses during glaciation periods. As the ice melts, the continents slowly rise. Other factors such as wind, currents, storm surges, El Niño or La Niña events, and water motion also influence local sea levels.
Over the past 2 million years, global sea level has varied from about 6 meters (20 feet) above to about 125 meters (410 feet) below its present position, with the most recent low point occurring approximately 18,000 years ago during the last glaciation period. It is important to note that coastlines have not yet fully adjusted to the modern sea level, and future sea-level rise is expected to accelerate. These changes in sea level significantly impact the position and nature of coastlines, particularly in areas with gradual continental slopes or areas experiencing uplift or subsidence.
Classification of Coasts
Given the multitude of factors influencing coasts, a useful classification scheme is based on the predominant processes occurring in a specific area: erosion and deposition. Erosional coasts are characterized by the removal of coastal material, while depositional coasts are relatively stable or growing due to sediment accumulation or the presence of living organisms, such as corals.
For example, the rocky shores of Maine are erosional, experiencing more erosion than deposition, whereas the sandy coastline from New Jersey to Florida is typically depositional, with sediment deposits protecting the shore from erosion. In the United States, approximately 70% of the coastline is erosional, while the remaining 30% is depositional. Throughout this article, we will use this erosional-depositional classification scheme to explore different coastal processes.
Erosional Processes and their Impact
Both land erosion and marine erosion play significant roles in shaping rocky coasts. On land, erosion occurs through various processes, including stream erosion, wind-driven abrasion, freezing and thawing of water in rock cracks, plant root probing, glacial activity, rainfall, dissolution by acids, and slumping.
From the sea, powerful storm surges generate immense pressures, with waves forcing air and water into tiny rock crevices. The repeated buildup and release of pressure weaken and fracture the rocks. Additionally, the impact of waves, carrying sand, gravel, and stones, contributes to the erosion of the coast. Water dissolves minerals in coastal rocks, particularly in easily soluble rocks like limestone.
Even the activities of marine organisms further contribute to erosion. The rate of coastal erosion depends on factors such as rock hardness, resistance, wave intensity, and local tidal range. Hard rocks like granite or basalt erode slowly, whereas soft sandstone or other weak materials erode at a much faster rate. Marine erosion is most rapid in high-energy coasts, which are exposed to large, powerful waves.
Such coasts are commonly found in stormy ocean regions with extensive fetch or along the eastern edges of continents prone to tropical storms. In contrast, low-energy coasts are less frequently exposed to powerful waves. Protected locations, like the Gulf of Mexico, experience a low-energy coast, except during hurricanes. Erosion primarily occurs near average sea level, where waves impact the coast directly.
Coasts with little tidal variation erode quickly as wave action persists at a single level for an extended period. However, low-energy coasts protected by offshore islands erode more slowly, as do areas below the low-tide line. While waves cause some erosion below the surface due to water motion, their erosive effect diminishes beyond approximately 15 meters (50 feet) below average sea level. Cliffs above the shore are subject to wave pounding, either directly or from rocks hurled by waves.
Features of Erosional Coasts
Erosional forces can create wave-cut shores with several distinctive features. Sea cliffs, sloping abruptly from land to the ocean, are often formed through the collapse of undercut notches. These cliffs mark the shoreward limit of marine erosion.
The action of waves carves sea caves into the cliffs at zones of weakness in the rocks. Most sea caves are accessible only at low tide. Blowholes can form when erosion follows a weakness upward to the top of the cliff, resulting in a fissure that releases spray when waves crash into the cliff. Offshore features of rocky coasts include natural arches, sea stacks (isolated pillars of rock), and smooth wave-cut platforms near the shore, which indicate the submerged limit of rapid marine erosion.
During the formation of these structures, debris removed from cliffs may be deposited in calmer waters farther offshore or accumulate as exposed beaches at the base of the cliffs. It is worth noting that broad beaches are often characteristic of depositional coasts.
Selective Erosion and Coastline Straightening
Marine erosion tends to intensify the irregularity of newly exposed coastlines initially. Coastal rocks vary in composition over horizontal distances, with some being more resistant to erosion than others. Consequently, erosion may affect hard rocks differently from soft rocks, leading to the formation of stacks, arches, and sea cliffs.
However, over time, erosion tends to smoothen coastal irregularities. Wave refraction plays a crucial role in this process, as wave energy concentrates on headlands and is dispersed away from bays. Sediment eroded from headlands is then deposited in the calmer bay areas, forming beaches. As erosion continues, these deposits may protect the base of the cliffs from further wave action.
Consequently, with the passage of time, coastal irregularities tend to be smoothed out. This straightening process occurs more rapidly in high-energy coasts.
Coastal Shaping by Land Erosion and Sea-Level Change
Land erosion and sea-level change significantly contribute to the shaping of coasts. During periods of lower sea levels, rivers carve across the land, creating coastal river valleys. When sea levels rise again, these valleys may become submerged, resulting in drowned river mouths.
Examples of drowned river mouths include Sydney Harbor, Chesapeake Bay, and the Hudson River valley. Glaciers also impact coastal landscapes when they form in river valleys, especially in high-latitude regions. Glaciers erode valleys into deep, U-shaped troughs, leading to the formation of narrow, deep bays known as fjords. Fjords can be found in places like British Columbia, Greenland, Alaska, Norway, and New Zealand.
Volcanism and earth movements also play a role in shaping coasts. Volcanic islands, such as the Hawaiian Islands, feature lobed lava flows extending into the ocean. Volcanic craters can collapse and fill with seawater. These geological processes contribute to the diversity of coastal formations worldwide.
Conclusion
Coasts are the dynamic and ever-changing boundaries where land and sea interact. Shaped by a combination of marine and terrestrial processes, coasts undergo continuous rearrangement due to waves, tides, sea level changes, biological activities, and tectonic forces. Understanding the factors that influence the location and formation of coasts is crucial for comprehending their unique features.
With the ongoing rise in sea levels and the intricate interplay between erosion and deposition, the future of our coasts remains a subject of constant change. By studying and appreciating these intricate processes, we can develop sustainable strategies for coastal management and ensure the preservation of these captivating environments for future generations.
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