A sea arch is a natural opening, resembling a bridge, that forms within a headland or cliff face along a coastline. This geological feature is created by the relentless energy of ocean waves wearing away rock over long periods of time. The formation of these arches is one of continuous removal, making the sea arch a fundamentally destructive landform. The entire sequence of events is driven by coastal erosion, which is the breakdown and carrying away of rock material.
Defining Destructive Forces
Coastal landforms are classified as either constructive, involving the deposition of material to build features like beaches, or destructive, involving the removal of material through erosion. Sea arches fall into the destructive category because they are sculpted by high-energy waves, which have a powerful backwash that pulls sediment away from the shore. The erosion that carves the arch is accomplished by a combination of mechanical and chemical forces.
One primary mechanical force is hydraulic action, where crashing waves compress air trapped within the rock’s cracks and fissures. As the wave retreats, this compressed air expands, exerting pressure that weakens and eventually dislodges pieces of the rock. Another element is abrasion, which occurs when sediment carried by the waves is hurled against the cliff face, grinding it down. Chemical weathering, often called corrosion, also contributes by dissolving minerals in certain rock types, such as limestone, further weakening the structure.
Stages of Sea Arch Formation
The formation of a sea arch begins when waves focus their energy on a headland, a piece of resistant rock jutting out into the sea. Wave refraction causes energy to concentrate on the sides of the headland and at pre-existing weaknesses in the rock, such as faults or joints. These points of vulnerability allow hydraulic action and abrasion to attack the rock face more aggressively than the surrounding material.
Over time, this focused erosion gradually exploits the weaknesses, hollowing out a cavity at the base of the cliff called a sea cave. The concentrated wave action continues to deepen the cave, often simultaneously attacking the headland from both sides. When the two caves meet, or when the cave erodes through the headland’s back wall, a natural bridge of rock is left spanning the opening, which is the fully formed sea arch. This process illustrates continuous material removal, reinforcing the arch’s classification as an erosional landform.
The Final Landforms: Stacks and Stumps
A sea arch represents only a temporary stage in the destructive cycle of coastal erosion. Once formed, the arch remains vulnerable to continued wave erosion at its base and sub-aerial weathering processes, like wind and rain, on its roof. The constant undermining and weakening of the arch’s supports eventually makes the spanning rock section unstable.
Inevitably, gravity and continued weathering cause the arch’s roof to collapse into the sea. This collapse leaves behind an isolated, vertical column of rock known as a sea stack, which stands separated from the mainland. The stack is then subject to the same erosional forces that created it. Eventually, the stack is worn down further until it becomes a low platform of rock, often only visible at low tide, which is the final stage known as a sea stump.