Sea caves are formed by the power of the ocean (or in some cases, lakes) attacking zones of weakness in coastal cliffs. The weak zone is usually a fault, or fractured zone formed during slippage. Another type of weak zone is formed where dissimilar types of rocks are interbedded and one is weaker than the other. Typically this is a dike, or intrusive vein of more easily eroded rock found within a stronger host rock. Yet a third instance is in sedimentary rocks where a layer of softer rock is interbedded between harder layers.
The cave may begin as a very narrow crack into which waves can penetrate and exert tremendous force, cracking the rock from within by both the weight of the water and by compression of air. Sand and rock carried by waves produce additional erosive power on the cave's walls.
Sea caves rarely have formations like solution caves or lava tubes. Occasionally some flowstone or small stalagmites are seen, formed much as in solution caves. Typically these occur in caves formed in sandstone, limestone, or sometimes even in basalt.
Sea caves are found all over the world and may be one of the most numerous types of caves. Areas known for large concentrations of sea caves include the Pacific coast states of the USA (Alaska, Washington, Oregon, and California, and especially California's Channel Islands); the Na Pali coast of Kauai; the Greek Isles; the British Isles, and many other places with cliffs of good solid rock to host the caves. Of special interest is New Zealand, which has both the world's two largest sea caves in terms of their length, and also the world's largest in terms of volume. See below for details on these amazing caves.
Arches & Sea Stacks
Regions where sea caves form are often associated with arches and sea stacks. Often these are remnants from collapses of sea caves, and in some cases may line up along what had been a fault line in a rock unit that has been carved by the sea into isolated stacks of rock. Both arches and stacks are relatively ephemeral and numerous arches depicted on postcards from the early 1900s have already collpased. A database of sea arches is maintained by the National Arch and Bridge Society.
Sea caves may form some very large chambers, especially where multiple passages converge and the rock is attacked from multiple points. In time, the ceiling of this chamber may collapse and form a large littoral sinkhole. Often these are oval shaped, giving rise to the punchbowl name. At high tides, punchbowls may appear as churning cauldrons of confused seas. Examples occur worldwide but a well known example is the Devil's Punchbowl on the Oregon coast shown at left.
A relatively uncommon phenomenon associated with sea caves is the blowhole. These tend to be very dependent on tide level and are basically of two types. On the left is a classic blowhole, the Spouting Horn on the Oregon coast. Here waves enter a sea cave and seal off the main entrance, compressing air inside that shoots water violently out a crack in a ceiling. On the right is the blowback sort of blowhole, where a single entrance becomes submerged by a swell, with compressed air shooting out water as the entrance is exposed by the receding swell.
The Entrance Zone
Sea caves may be explored in several ways: with kayaks or other small boats; by swimming in; or in some caves, by wading or walking if the cave empties out at low tide. When entering a cave where the surf is active, it's best to wear a helmet and study conditions carefully before entering. Remember that the power of waves and swell will be amplified in the cave interior!
Inside a Sea Cave
Inside, a sea cave may be dry or wet, depending on the tide, time of year, or the locale. On the left is a long cave formed along a fault, visible along the sloping wall on the right. The white material on the walls is calcite, deposited by water percolating through the rock. On the right is a sea cave floored with just sand, having emptied out temporarily at low tide. Colorful marine algaes adorn the ceiling.
Life in a Sea Cave
Life in a Sea Cave
Sea caves may abound with life, both on their walls and floors. Besides the kind of critters seen in normal tidepools, such as anemones, starfish, and sponges, sea caves with dark zones may harbor organisms not commonly seen in shallow water. . In California, the Giant Anemone is normally green because of an algae that lives inside of it; but in sea caves with dark zones, like the one middle left, these anemones may be white because the green algae doesn't get enough sunlight to grow. Similarly, sponges like that in the same image are generally always white in the darker recesses of sea caves, as opposed to the orange ones in the upper right image.
Gooseneck barnacles (upper left) are common on sea cave walls in the intertidal zone.
In seacaves with deep water, sharks like this small leopard shark may be found. And on the lower right, some harbor seals in a California sea cave. Seals and sea lions ofen congregate in sea caves on offshore islands.
Finally, in the lower left an aptly named batstar gives an interesting tie-in to caves inhabited by mammalian bats, typically solution and sometimes lava caves.
Famous Sea Caves
Famous Sea Caves
Probably the world's two most famous sea caves are the Blue Grotto on the Italian island of Capri (left), and Fingal's Cave on the British island of Staffa (formed in columnar basalt). While spacious inside, they are only moderate in length, neither of them exceeding 250 feet from entrance to end.
World's Longest Sea Cave
A list of the world's longest sea caves was originally compiled by Dave Bunnell and Bob Gulden and is now maintained by Paul Burger on cave-exploring.com. Paul's site gives info on long and deep records in all manner of caves.
Matainaka Cave, on the Otago coast of New Zealand' South Island, has been verified by surveys as of October 2012 as the world's largest sea cave by length, an amazing 1.54 km or 5,051 feet, not quite a mile. But if the rate of its formation outpaces that of its collapse, it could yet reach that milestone in centuries to come.
Many fractures in the host sandstone have resulted in a cave with numerous interesecting passages and a dozen entrances in the sea cliff.
Although the largest by length, it is probably not the largest by volume, although a volumetric survey wasn't produced.
World's Largest Sea Caves
Purple Cathedral is another large cave system on the Otago Coast of New Zealand recently surveyed to a length of 404m or 1325 feet. Unlike its neighbor Matainaka, it consists primarily of one long passage with a single entrance. The purple on the walls is a red algae exposed at low tide.
Sea Lion Caves
Sea Lion Caves, a show cave operation in Florence, Oregon claims to be "America's largest sea cave." As of Nov 2018 this claim is true, as its length has been verified by a laser survey, which yielded a total distance of 1315 feet of traversible passage in the cave. Over half of this distance is in a tunnel not entered on the tours, but visible from the viewing platform.
Incidentally, there is one other sea cave open to public tours, in La Jolla, California, know as Sunny Jims. It is much smaller in size.
Painted Cave is on California's Santa Cruz Island. It is 1227 feet long and large enough to take a 40-foot boat inside. On the left, looking out the 130-foot-high entrance. On the right are two views of the very dark inner chamber. The top image shows a sharp edge to the right of the red kayak, where the two faults along which the chamber eroded intersect. Sea lions inhabit the ledges in the back of the chamber much of the year. Click here to see a detailed map of the cave.
Scotland's Holl o Boardie
Holl o Boardie is a cave on Papa Stour, one of the Shetland Islands, off the northern tip of Scotland. It hasn't been officially surveyed, but can be estimated fairly accurately since it is a tunnel passing through a headland. A local expert puts it at about 330m (1082 feet) long. This makes it somewhat longer in linear extent than any but Purple Cathedral, so by that yardstick is one of the world's longest. It is not surprising that the longest sea cave passage should be a two-entrance cave, as this has allowed the sea to attack it at both ends. Eventually, the tunnel's roof will collapse and leave a sea stack.
Just to the southwest off the coast of Papa Stour is the Fogla Skerry, a small islet which is riddled with cave passages that intersect , with multiple entrances. One local kayaker told me that you can probably cover over 1500 feet of cave inside of it, which could make it the world's second largest sea cave by length. However, it has not been properly surveyed.
Norway's Relict Sea Caves
Some of the world's largest sea caves are relict or raised sea caves on the coast of Norway. These are caves formed by wave action during the ice ages that have been uplifted above the littoral zone by isostatic rebound once the glaciers had melted. They now range from 100 to 384 feet above the present sea level, so none are currently enlarging by wave action. They formed over a very long period of time, perhaps more than a million years in some cases according to dating of sediments within. No doubt this is why they are so much bigger than caves at current ocean level. The largest are formed in granitic rocks.
Swedish caver Rabbe Sjoberg provided this list of the 10 longest based on accurate surveys:
Halvikshulen, Osen 340 m (1,115 feet)
Lispingdalskyrka, Nordgutvik : 325m
Trollhole nr. 2, Reksten: 300m
Harbakkshulen, Stocksund: 200m
Rephelleren, Varö: 188m
Dolsteinshulen, Sandön: 180m
Tonneshulen, Melfjord : 170m
Torghatshullet, Brönnöy: 160m
Gaupehulen, Bjugn: 150m
Rosvikshule, Solstad: 150m
In the photos on the right are two of these long caves. Note the people for scale in the middle photo. That entrance is 722 feet wide and probably the largest sea cave entrance in the world.