Coastlines can be classified into two primary types: submerged and emerged coastlines. Submerged coastlines arise from either rising sea levels or sinking land, while emerged coastlines result from falling sea levels or rising land.

Submerged coastlines, often referred to as submergent coastlines, are characterized by distinct geological formations, including rias, fjords, and estuaries. These features develop when sea levels increase, or when land sinks, causing previously elevated areas to become submerged. Such phenomena can result from a variety of geological processes, including tectonic activity, isostatic adjustment, and climatic changes, such as the melting of ice caps that contribute to rising sea levels. Rias are formed from drowned river valleys, and fjords originate from submerged glacial valleys, both of which serve as evidence of previous land exposure. Estuaries, where freshwater rivers meet the ocean, are also common in submerged coastlines, often resulting in diverse and productive ecosystems.

In contrast, emerged coastlines, or emergent coastlines, occur due to falling sea levels or rising land, which exposes areas that were once submerged. This exposure leads to the formation of features such as raised beaches, marine terraces, and wave-cut platforms. Raised beaches are former beach areas that now lie above the high tide line, while marine terraces are flat or gently sloping surfaces that indicate former sea levels. Wave-cut platforms are flat regions created through wave erosion and become visible as water levels decrease or land elevates. These formations provide clear evidence of the coastline’s historical submersion and subsequent emergence.

In summary, submerged and emerged coastlines result from different geological processes and are defined by distinct geographical features. Submerged coastlines are a consequence of rising sea levels or sinking land, leading to the inundation of previously exposed areas. Conversely, emerged coastlines arise from falling sea levels or rising land, resulting in the re-exposure of areas that were once submerged.