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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, so we're just devoting one page to them here in The Virtual Cave. Occasionally some flowstone or small stalagmites are seen, formed much as in solution caves. Typically these occur in caves formed in sandstone or basalt. Sea caves are found all over the world, and may be one of the most numerous type 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 good solid rock to host the caves. The author has explored and surveyed over 500 sea caves, and written two books and an encyclopedia chapter on them. Below are a sampling of some of the photos taken along the way. |
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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! |
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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 at low tide. Colorful marine algaes adorn the ceiling.
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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 lower left, these anemones may
be white because the green algae doesn't get enough sunlight to grow.
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Famous Sea Caves Probabe 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 end to end. |
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World's Longest The world's longest known, actively forming sea cave based on an actual survey is Painted Cave on California's Santa Cruz Island. It is 1215 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 bottom 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. Another huge sea cave and second on the list of the world's longest sea caves compiled by the author is Waiahuakua on Kauai's Na Pali coast, which tapes out at 1155 feet long.. |
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Other contenders for Sea Lion Caves, a show cave operation in Florence, Oregon, claim it to be the "world's largest sea cave." However, this isn't remotely true whether based on length or volume. In terms of length, the author estimates a combined length of no more than 600 feet based on topographic measurement of the peninsula in which the cave has formed. Volumetrically it is smaller than either Painted Cave or Riko Riko, below. To my knowledge no accurate survey of the cave has ever been published. |
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| Holl o Boardie is a cave on Papa Stour, one of the Shetland Islands, off the most northern edge 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 Painted Cave, 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 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 would make it the world's largest sea cave by length. However, it has not been properly surveyed. I am considering an expedition to the island to do just that. |  See a larger version of the Papa Stour image |
Some of the world's longest sea caves are relict or raised sea caves on the coast of Norway. These are caves formed by wave action over a long period that have been uplifted above the littoral zone. They 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) In the photos on the right are 3 of these long caves. Note the people for scale in them. top: Halvikshulen entrance, 722 feet wide and 262 feet high. Probably the largest sea cave entrance in the world. |
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 A 50-foot dive boat in the entrance of RikoRiko, viewed from inside and outside (upper right) |
World's Largest When we ask what is the world's largest sea cave by volume rather than horizontal length, we get a different answer: Riko Riko Cave, on the Poor Knight Islands off the east coast of New Zealand's North Island. Lengthwise, Riko tapes out at only about 500 feet, but the egg-shaped chamber was determined via laser survey to have a volume of about 7,800,000 cubic feet. Painted Cave's volume can only be estimated from its survey, to be roughly 56% that of Riko. Dive! Tutakaka offers boat tours of the Poor Knights that include a visit to Riko Riko. Click here for a detailed article on the 3D laser survey of Riko Riko with more photos, in Adobe Acrobat format. |
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Last updated Feb. 6, 2010 |