The coast represents one of the most complex environments on Earth, serving as the interface where the land meets the sea. It is not a fixed, single line, but a geographical feature constantly shaped by the energy transfer between the marine and terrestrial realms. This zone is characterized by continuous movement, where processes of erosion and deposition reorganize sediments and reshape geological structures.
Defining the Coastal Zone
The coast is defined as a broad zone that encompasses both the landward and seaward areas directly influenced by marine processes. The most immediate part of this system is the shore, which is the area exposed between the lowest and highest tidal marks. This transitional space experiences daily cycles of inundation and exposure caused by the rhythmic rise and fall of the ocean.
Moving seaward, the nearshore zone begins at the low-tide mark and extends out to where waves first begin to break, a region sometimes called the breaker zone. In this shallow area, friction between the incoming wave and the seabed starts to distort the wave’s shape, releasing significant energy. Beyond this is the offshore zone, the deeper water area where the seafloor is no longer shallow enough to dramatically affect wave movement. These zones delineate the full physical extent of the coast.
Forces Shaping the Coast
The shape and composition of the coastal zone are determined by three interconnected forces: waves, tides, and changes in sea level. Waves, primarily generated by wind blowing across the ocean surface, are the most significant agents of instantaneous change, driving both erosion and deposition. When a wave approaches the shore, its energy can either scour material away from the land, contributing to cliff retreat, or deposit sediment, building up beaches.
Tidal forces, which result from the gravitational pull of the Moon and the Sun, create a periodic rise and fall of sea level that dictates where wave energy is focused. The regular movement of the tide causes the shoreline to migrate landward and seaward, influencing where sediment is accumulated or removed on a daily cycle.
Water level changes, both short-term like storm surges and long-term like eustatic sea-level rise, profoundly affect coastal morphology. A higher water level allows waves to act on parts of the coast that are typically sheltered. This increases the potential for erosion and alters the equilibrium profile of the shore.
Categorizing Coasts
Scientists classify coasts based on their dominant characteristics and the processes responsible for their formation. One major classification distinguishes coasts based on whether their features were initially formed by non-marine or marine processes. Primary coasts are those whose present shape is dominated by terrestrial processes, such as tectonic activity, volcanism, or the drowning of river valleys. The Chesapeake Bay is an example of a drowned river valley known as a ria coast.
Secondary coasts, in contrast, are shaped mainly by marine processes like wave erosion or sediment deposition, often after sea level has stabilized. Barrier islands, which are long, narrow accumulations of sand separated from the mainland by a lagoon, are a prime example of a secondary coast built by marine deposition.
Another useful dichotomy sorts coasts into erosional and depositional types, reflecting the dominant process at work. Erosional coasts, such as the rocky shorelines of the western United States, are characterized by a net loss of sediment, resulting in features like sea cliffs and wave-cut platforms. Conversely, depositional coasts are areas where sediment supply is abundant, leading to a net gain of material and the formation of extensive features like sandy beaches and river deltas.