The Earth’s coastlines are dynamic environments, constantly shaped by the relentless energy of the sea. Marine erosion, driven by powerful waves, carves unique features into rock formations that extend into the ocean. Among the most recognized and striking of these coastal landforms is the sea arch, a spectacular geological structure demonstrating the immense force of nature.
Defining the Sea Arch
A sea arch is a natural rock formation that has been completely perforated by the sea, allowing water to pass directly through it. This feature develops when wave action erodes a headland, which is a section of resistant rock extending into the water. The structure consists of a horizontal span or roof connecting two supporting vertical pillars.
The Step-by-Step Formation Process
The genesis of a sea arch begins when the ocean’s energy exploits natural weaknesses in a headland, such as fault lines or areas of less resistant rock. Waves concentrate their power on these weak points through hydraulic action and abrasion. Hydraulic action involves the force of water compressing air trapped within rock cracks, causing the fissure to expand and break off rock pieces. Abrasion occurs as waves throw loose sediment, like sand and pebbles, against the cliff face, grinding away the rock.
Over time, this focused erosion transforms the initial crack into a small notch at the base of the headland, which deepens to form a sea cave. Since waves often refract around a headland, erosion is concentrated on both sides, allowing two separate caves to form simultaneously. The caves grow progressively larger until they meet and penetrate the headland completely, forming the completed sea arch.
The Arch’s Life Cycle and Eventual Collapse
The arch marks a temporary stage in the landform’s life cycle, as erosional forces continue to attack the remaining structure. The base of the arch’s pillars is continually undercut by the waves, while the unsupported span is simultaneously weakened by processes like freeze-thaw and biological weathering. This weakening eventually makes the structure unstable, leading to the collapse of the roof into the sea. This collapse leaves behind a detached column of rock known as a sea stack, which is gradually worn down until it becomes a low-lying rock platform called a sea stump.
Global Examples of Sea Arches
Sea arches are globally distributed, forming in areas composed of resistant rock types such as limestone, chalk, or granite. Durdle Door, located on the Jurassic Coast in Dorset, England, is one of the world’s most recognizable examples. This impressive feature is composed of durable Portland limestone. Another famous example was the Azure Window on the island of Gozo in Malta, which was also formed from limestone. The Azure Window stood for centuries before its inevitable collapse in 2017 during a heavy storm, leaving a sobering reminder of the ephemeral nature of these structures.