Blowholes

Definition

A blowhole is a crack or fissure in coastal rock through which air and spray is expelled when waves break on the shore.

Blowholes are a feature where large swell impacts coasts, on which the rock contains fractures. Weaknesses, such as joints or fault lines, are preferentially widened, primarily through wave action but also through other processes such as solution of reef limestone. In many cases this can result in a sea cave. Coasts that experience strong swell are impacted by trains of regular period waves generated by remote storms and which have travelled across the ocean. Successive waves trap air into the fissure or sea cave and compress it as the crest of the wave fills the cavity. This pneumatic pressure is released in a spectacular fashion with a deep hiss and an upward spray or blast of water through the cracks Figure 1.

Blowholes. Figure 1

Several adjacent blowholes on the southern shore of Tongatapu, Kingdom of Tonga.