Sea Caves and Coastal Karst
Sea caves form where wave energy focuses on weaknesses in coastal rock. Unlike karst caves they form by mechanical erosion, not chemical dissolution, and can occur in any rock type — though hybrids with karst dissolution produce some of the most spectacular coastal caves.
How sea caves form
Waves attack faults, joints, softer rock bands, or contacts between rock types of different hardness. Repeated impact and hydraulic action (compressed air shattering rock at the cave back) deepens the cavity. Sea caves typically form in hard rocks — granite, basalt, quartzite — that resist dissolution but fracture along structural weaknesses.
Hybrid karst-sea caves
The most spectacular coastal caves combine sea-cave entrances with karst-dissolution interiors. Smoo Cave in Sutherland, Scotland, is a sea cave at the entrance (massive triangular arch) opening into karst chambers formed by groundwater dissolution in Durness limestone. Capri's Grotta Azzurra is similar.
Tidal effects
Sea caves change with the tide. Cave swims, kayak entries, and boat tours all schedule around low tide for maximum chamber volume. At high tide many sea caves are completely submerged.
Notable sea caves
Fingal's Cave (Staffa, Scotland) — hexagonal basalt columns. Sea Lion Caves (Oregon, USA) — largest sea cave in the US, home to Steller sea lions. Painted Sea Caves (Apostle Islands, USA) — Lake Superior sandstone sea caves toured by kayak. Algarve sea caves (Portugal) — including the iconic Benagil.
Risks
Tidal-cave swimmers can be trapped by rising tides; boats can be forced against walls in surge; rockfall from cave roofs is common. Always go with local guides who know the cave's behaviour.
Photography
Sunlight through low cave entrances or skylights produces the classic illuminated sea-cave images. Mid-morning to early afternoon is optimal for blue light. Polariser filters can manage glare from the water surface.
Explore on the map
See sea caves on the interactive map.