Sea cave
Caves formed by wave erosion along coastlines worldwide.
A sea cave, or littoral cave, is a type of cave formed primarily by the wave action of the sea through the process of erosion. These caves are found worldwide, actively forming along present coastlines and also existing as relict caves on former coastlines. For instance, some of the largest wave-cut caves are located on the coast of Norway, now situated 100 feet or more above current sea level, yet they remain classified as littoral caves. In contrast, places like Thailand’s Phang Nga Bay contain limestone caves originally formed by dissolution that have been flooded by rising seas and are now subject to littoral erosion, representing a new phase of enlargement. Notable European examples include Fingal’s Cave on Staffa, Scotland, a spacious 70-meter cave in columnar basalt, and the Blue Grotto of Capri, famous for its luminescent water caused by light passing through underwater openings, where the Romans built a stairway and a now-collapsed tunnel. The Greek islands are also known for their variety of sea caves. Until 2013, the largest known sea caves were along the west coast of the United States, Hawaii, and the Shetland Islands. In 2013, Matainaka Cave on New Zealand’s South Island was discovered and surveyed, proving to be the world’s most extensive at 1.5 kilometers in length, with another complex just over a kilometer at Bethells Beach on the North Island. Littoral caves form in various host rocks, from sedimentary to igneous, with larger caves typically found in stronger igneous or metamorphic rock. Formation requires a weak zone, such as a fault, dike, or bedding-plane parting. Wave action drives erosion, widening fissures through hydraulic force, compressed air, and abrasive sand. Blowholes, which eject seawater sprays, demonstrate this process. Rainwater can also assist by leaching acids that weaken rock, and small speleothems may develop. Collapse of sea cave chambers can create littoral sinkholes or sea stacks, as seen with Oregon’s Devils Punch Bowl or the splitting of Anacapa Island in California. Marine life like sea urchins can also enlarge caves by drilling into rock. Most sea caves are small, but surveys show over 45 exceed 200 meters in length, with several in Norway surpassing 300 meters. Factors influencing size include the nature of the weak zone and the cave entrance’s orientation to prevailing sea conditions.
- type
- Geological formation
- primary_process
- Erosion by wave action
- host_rock_types
- Sedimentary, metamorphic, igneous
- largest_known_by_length
- Matainaka Cave, New Zealand, 1.5 km
- notable_locations
- Norway, Scotland, Italy, Greece, California, Hawaii, New Zealand
Lore & Background
Sea caves, also called littoral caves, are erosional features shaped primarily by wave action along coastlines. They form when the sea exploits weak zones in the host rock—such as faults, dikes, or bedding planes—widening and deepening fissures through hydraulic pressure, air compression, and abrasion by suspended sand and rock. Blowholes, which eject sprays of seawater as waves retreat, demonstrate the force of compressed air within these caves. Most sea-cave walls are irregular and chunky, reflecting piecemeal fracturing, though some sections are smoothed and rounded by swirling cobbles. Rainwater carrying carbonic and organic acids may assist in weakening rock, and small speleothems can develop inside. Marine life, such as sea urchins, can also enlarge caves by drilling into the rock over successive generations. Sea caves occur worldwide, both actively forming on present coastlines and as relict caves on former coastlines; for instance, some of the largest wave-cut caves are found in Norway, now elevated over 100 feet above sea level. Host rocks vary from sedimentary to metamorphic to igneous, with larger caves typically forming in stronger igneous rock. Notable examples include Fingal’s Cave in Scotland, formed in columnar basalt, and the Blue Grotto of Capri, known for its luminescent water. In some regions, such as Thailand’s Phang Nga Bay, solutionally formed limestone caves have been flooded by rising seas and are now undergoing littoral erosion. Sea caves can also collapse, creating littoral sinkholes or leaving free-standing sea stacks.
Reader's Guide
Sea caves are significant as dynamic coastal features that illustrate the power of marine erosion. Factors influencing size include the nature of the weak zone, exposure to prevailing seas, host rock strength, and time. The world's largest sea cave by length, Matainaka Cave in New Zealand, formed in heavily fractured sandstone, challenging prior assumptions. Relict sea caves in Norway, uplifted over 100 feet above sea level, demonstrate long-term geological change. Hybrid caves, such as those in Thailand's Phang Nga Bay, combine dissolutional and littoral processes. Sea caves can collapse into littoral sinkholes or form sea stacks when tunnels cut through headlands.
Did You Know?
- Matainaka Cave on New Zealand's Otago coast is the world's most extensive sea cave at 1.5 km in length.
- Some relict sea caves in Norway are now 100 feet or more above present sea level.
- The Blue Grotto of Capri has a Roman-built stairway and a now-collapsed tunnel to the surface.
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