Factors Influencing Size
Most sea caves are small in relation to other cave types. A current compilation of sea-cave surveys Long sea caves of the world shows three over 300 meters, 15 over 200 meters, and 85 over 100 meters in length. In Norway, several apparently relict sea caves exceed 300 meters in length. There is no doubt that many other large sea caves exist but have not been investigated due to their remote locations and/or hostile sea conditions.
Several factors contribute to the development of relatively large sea caves. The nature of the zone of weakness itself is surely a factor, although difficult to quantify. A more readily observed factor is the situation of the cave’s entrance relative to prevailing sea conditions. At Santa Cruz Island, the largest caves face into the prevailing northwest swell conditions—a factor which also makes them more difficult to survey. Caves in well-protected bays sheltered from prevailing seas and winds tend to be smaller, as are caves in areas where the seas tend to be calmer.
The type of host rock is important as well. All of the largest sea caves are in basalt, a strong host rock compared to sedimentary rock. Basaltic caves can penetrate far into cliffs where most of the surface erodes relatively slowly. In weaker rock, erosion along a weaker zone may not greatly outstrip that of the cliff face.
Time is another factor. The active littoral zone changes throughout geological time by an interplay between sea-level change and regional uplift. Recurrent ice ages during the Pleistocene have changed sea levels within a vertical range of some 200 meters. Significant sea caves have formed in the California Channel Islands that are now totally submerged by the rise in sea levels over the last 12 000 years. In regions of steady uplift, continual littoral erosion may produce sea caves of great height — Painted Cave is almost 40 m high at its entrance. On the Norwegian coast there are huge sea caves now uplifted 30 or more meters above sea level. Sediment dating in the largest of these (Halvikshulen in Osen, 340 m long)shows that it was formed over a period of at least a million years. It may well be the longest wave-cut cave in the world. The largest cave by volume is Rikoriko Cave in the Poor Knights Islands in New Zealand with 221,494 m3.
Finally, caves that are larger tend to be more complex. By far the majority of sea caves consist of a single passage or chamber. Those formed on faults tend to have canyon-like or angled passages that are very straight. In Seal Canyon Cave on Santa Cruz Island, entrance light is still visible from the back of the cave 189 m from the entrance. By contrast, caves formed along horizontal bedding planes tend to be wider with lower ceiling heights. In some areas, sea caves may have dry upper levels, lifted above the active littoral zone by regional uplift.
Sea caves can prove surprisingly complex where numerous zones of weakness—often faults—converge. In Catacombs Cave on Anacapa Island (California), at least six faults intersect. In several caves of the Californian Channel Islands, long fissure passages open up into large chambers beyond. This is invariably associated with intersection of a second fault oriented almost perpendicularly to that along the entrance passage. When caves have multiple entrances, they are exposed to more wave action and hence may grow relatively faster. There is an exceptionally large cave underlying the Fogla Skerry, an islet off the coast of Papa Stour, in the Shetland Islands. Though unsurveyed, estimates place it at almost 500 m of passage--meaning it could actually be the world's longest sea cave.
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