A sea arch is a natural, bridge-like opening carved through a mass of rock, typically found along coastlines where land meets the ocean. These formations are sculpted by the powerful energy of waves, representing a temporary stage in the continuous reshaping of a rocky shore. Sea arches are evidence of the sea’s ability to erode and alter the geological landscape over long periods.
The Starting Point: Headlands and Wave Action
The formation of a sea arch begins on a headland, a stretch of resistant rock that juts out into the sea. These features are the primary targets for the concentrated energy of incoming waves. Wave action is intensified by wave refraction, where waves slow down in shallower water and bend, focusing their energy directly onto the sides of the headland.
This focused energy initiates erosion along natural lines of weakness in the rock, such as joints, faults, and bedding planes. The mechanical force of the water, known as hydraulic action, is a significant erosional agent. As waves crash into the cliff, air is compressed into small cracks; the subsequent rapid decompression works to widen the fissures and dislodge rock fragments.
Another powerful mechanism is abrasion, where the sea uses loose sediment, pebbles, and larger rocks as tools to grind and wear away the cliff face. This combination of hydraulic action and abrasion steadily carves out indentations, known as sea caves, at the base of the headland, concentrating the erosive force at sea level. The rock’s geological structure, particularly its resistance and weaknesses, dictates the speed and location of the initial cave formation.
From Caves to Arches
As waves continue to attack the headland, the initial sea caves grow deeper, taller, and wider. This process typically occurs simultaneously on both sides of a narrow headland, exploiting the same zone of structural weakness. The deepening of the caves eventually leads to a breakthrough, where the two hollows meet in the middle of the headland.
This connection creates the sea arch, a curved span of rock standing over the opening. The arch’s span is continually widened by the passage of water and erosion acting on its sides. Even after the arch forms, the structure remains under attack from sub-aerial processes, such as weathering caused by wind, rain, and temperature changes, which weaken the rock surface above the waterline.
The roof of the arch is composed of the more resistant rock that survived the wave attack. However, the arch’s base, or supporting pillars, continues to be undercut by wave action, which increases the height of the opening. The combined forces of marine erosion at the base and weathering on the roof mean the arch is inherently unstable, representing a transitional landform.
The Life Cycle After Breakthrough
The completed sea arch is not a permanent feature; it is a temporary stage in the ongoing cycle of coastal erosion. The weight of the arch’s roof exerts constant stress on the supporting pillars. This stress, combined with weakening from marine erosion at the base and weathering processes above, ultimately leads to the arch’s collapse.
When the arch roof collapses, it falls into the sea, leaving behind an isolated, vertical column of rock known as a sea stack. The stack is a remnant of the original headland, separated from the mainland by the collapsed section. The isolated stack is then subjected to the same erosional processes that created the arch.
Waves continue to erode the base of the stack, gradually making it thinner and more unstable. Over a long period, the stack is worn down until it becomes a mere stub of rock, often only visible above the water at low tide, which is known as a sea stump. The entire sequence, from headland to stump, demonstrates the cyclical nature of coastal geology, where the sea constantly reduces large landmasses to smaller, less resistant features.