Introduction
A blowhole, sometimes called a marine geyser, is a coastal landform in which seawater is expelled through a surface opening that links to an underlying sea cave. They form as wave erosion enlarges caves and progressively extends them upward and inland until one or more vertical conduits connect the cave chamber to the surface, creating a direct vent for water and air. During wave surges, incoming water and trapped air are driven into the confined cave space and forced up these shafts, producing jets or sprays at the surface; the intensity of discharge depends on the degree of hydraulic compression achieved within the cavity. The geometry of the system — the dimensions, shape, alignment and any constrictions of the cave, shaft and port — governs pressure buildup and thus the velocity and volume of ejected water, with narrower or more confined passages generally producing stronger, higher sprays. Tidal elevation and sea-state conditions (wave height, period and direction) control how much water enters the cave and how much energy is available to drive the discharge, so spray height and frequency vary predictably with these factors. Blowholes are characteristic of coastlines subject to persistent wave attack where rock type and structural weaknesses permit cave formation and upward breaching; their presence therefore reflects an interaction among lithology, relative sea level and sustained wave energy. Although sometimes conflated with so-called breathing caves, blowholes are distinguished by the hydraulic expulsion of seawater through a surface aperture rather than primarily by cyclic atmospheric exhalation or minor air exchange.
Mechanics
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Blowholes are coastal erosional vents formed where energetic ocean waves force seawater and air into pre‑existing cavities in shorerock, producing abrupt, high‑pressure expulsions of spray and sound. They occur on rocky coasts worldwide, especially where structural weaknesses such as intersecting faults, joint systems or lava tubes concentrate marine attack, and on windward shores that receive greater open‑ocean swell. Their spectacle often makes them focal points for tourism.
Functionally a blowhole is the surface expression of a littoral cave system that comprises three linked components: a wave‑receiving inlet, an enclosed compression chamber in which hydraulic and pneumatic pressures accumulate, and a narrow outlet that vents the pressurized mixture. The geometry, size and orientation of these elements govern discharge intensity and the relative proportions of air and water ejected; rapid pressure changes within the cave can also drive strong reverse drafts, with locally measured gusts approaching 70 km h−1.
Blowholes develop only after littoral caves have formed. Cave initiation and enlargement are controlled by wave energy (including frequency and incident direction) and by the rock’s susceptibility to marine and subaerial weathering; enlargement proceeds landward and upward along existing joints and weaknesses. Lithology dictates the dominant cave‑forming mechanism: carbonate coasts tend to evolve by solutional (karst) processes, whereas igneous shores produce pseudokarst cavities through mechanical and thermal breakdown—differences that affect cave shape, porosity and rates of enlargement. Continued roof attenuation or collapse creates a surface aperture and may evolve into a steep‑walled inlet, altering local wave focusing, sediment transfer and subsequent erosion patterns.
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A well‑known example is La Bufadora on the Punta Banda Peninsula (Baja California), a littoral cave with a narrow surface port that produces eruptions every ~13–17 seconds and can throw water columns approaching 30 m above sea level.
Images: Representative blowholes and their settings
Blowholes are coastal venting features that form where concentrated wave energy penetrates sea caves or fissures in rocky shorelines, compressing air and water and forcing intermittent vertical jets of spray and water through subaerial shafts. Their occurrence and intensity depend on lithology and structural weakness of the coast, the morphology of cliffs and cavities, the magnitude and direction of incoming waves, tidal stage and storm surge. They are characteristic of high-energy shorelines in both tropical and temperate regions and are geomorphic indicators of active marine erosion as well as localized coastal hazards.
Several well-documented examples illustrate how these controls combine in different oceanic contexts. Alofaaga Blowholes on Savai’i (Samoa) develop where volcanic coastal rocks, strong South Pacific swell and island-scale shore geometry concentrate wave power into focused vents, producing hazardous surges and spray while marking ongoing erosion of the volcanic shoreline. The cluster of blowholes on Barbados’s north coast occurs on a windward, high-energy Atlantic margin where persistent swell, bedrock exposures and interactions with reef–lagoon systems have carved cavities that vent spray and jets above the shore. Hummanaya in Sri Lanka’s Southern Province occupies a monsoon-influenced Indian Ocean setting; seasonal shifts in wave direction and intensity, together with hard coastal rock and nearshore bathymetry, modulate the blowhole’s periodicity and force.
In the central Pacific, Hālona Blowhole on Oʻahu exemplifies blowhole formation on volcanic basalt cliffs exposed to long Pacific fetch and steep shorelines, where the resulting sea‑cave geometries and hydraulic pressures yield powerful intermittent spouts. Kiama Blowhole on the New South Wales coast demonstrates similar processes in a temperate setting: Tasman Sea swell acting on resistant headland rock has produced a prominent vent that is both a feature of coastal evolution and a focal point for tourism and hazard management. La Bufadora at Ensenada, Baja California, on the northeastern Pacific margin, is another case in which long‑fetch swell, steep coastal relief and regional tectonic/volcanic lithologies produce a pressurized sea‑cave system with regularly occurring, high-energy discharges.
Together these sites underscore the common physical prerequisites for blowhole development—suitable rock and structure, focused wave energy, and appropriate cavity and shaft geometries—while also highlighting how regional wave climate, tidal and storm conditions, and local coastal form determine the character, frequency and hazard potential of individual blowholes.