A sea cave, also known as a littoral cave, is a type of cave formed primarily by the wave action of the sea. The primary process involved is erosion. Sea caves are found throughout the world, actively forming along present coastlines and as relict sea caves on former coastlines. Some of the largest wave-cut caves in the world are found on the coast of Norway, but are now 100 feet or more above present sea level. These would still be classified as littoral caves. By contrast, in places like Thailand's Phang Nga Bay, solutionally formed caves in limestone have been flooded by the rising sea and are now subject to littoral erosion, representing a new phase of their enlargement.
Some of the best-known sea caves are European. Fingal's Cave, on the Scottish island of Staffa, is a spacious cave some 70 m long, formed in columnar basalt. The Blue Grotto of Capri, although smaller, is famous for the apparent luminescent quality of its water, imparted by light passing through underwater openings. The Romans built a stairway in its rear and a now-collapsed tunnel to the surface. The Greek islands are also noted for the variety and beauty of their sea caves. Numerous sea caves have been surveyed in England, Scotland, and in France, particularly on the Normandy coast. Until 2013, the largest known sea caves were found along the west coast of the United States, the Hawaiian islands, and the Shetland Islands. In 2013 the discovery and survey of the world's largest sea cave was announced. Matainaka Cave – located on the Otago coast of New Zealand's South Island – has proven to be the world's most extensive at 1.5 km in length. Also in 2013, Crossley reported a newly surveyed complex reaching just over a kilometer in survey at Bethells Beach on New Zealand's North Island.
Littoral caves may be found in a wide variety of host rocks, ranging from sedimentary to metamorphic to igneous, but caves in the latter tend to be larger due to the greater strength of the host rock. However, there are some notable exceptions as discussed below.
In order to form a sea cave, the host rock must first contain a weak zone. In metamorphic or igneous rock, this is typically either a fault as in the caves of the Channel Islands of California, or a dike as in the large sea caves of Kauai, Hawaii’s Na Pali Coast. In sedimentary rocks, this may be a bedding-plane parting or a contact between layers of different hardness. The latter may also occur in igneous rocks, such as in the caves on Santa Cruz Island, California, where waves have attacked the contact between the andesitic basalt and the agglomerate.
The driving force in littoral cave development is wave action. Erosion is ongoing anywhere that waves batter rocky coasts, but where sea cliffs contain zones of weakness, rock is removed at a greater rate along these zones. As the sea reaches into the fissures thus formed, they begin to widen and deepen due to the tremendous force exerted within a confined space, not only by direct action of the surf and any rock particles that it bears, but also by compression of air within. Blowholes (partially submerged caves that eject large sprays of sea water as waves retreat and allow rapid re-expansion of air compressed within) attest to this process. Adding to the hydraulic power of the waves is the abrasive force of suspended sand and rock. Most sea-cave walls are irregular and chunky, reflecting an erosional process where the rock is fractured piece by piece. However, some caves have portions where the walls are rounded and smoothed, typically floored with cobbles, and result from the swirling motion of these cobbles in the surf zone.
True littoral caves should not be confused with inland caves that have been intersected and revealed when a sea cliff line is eroded back, or with dissolutional voids formed in the littoral zone on tropical islands. In some regions, such as Halong Bay, Vietnam, caves in carbonate rocks are found in littoral zones, and being enlarged by littoral processes but were originally formed by dissolution. Such caves have been termed as hybrid caves.
Rainwater may also influence sea-cave formation. Carbonic and organic acids leached from the soil may assist in weakening rock within fissures. As in solutional caves, small speleothems may develop in sea caves.
Sea cave chambers sometimes collapse leaving a “littoral sinkhole”. These may be quite large, such as Oregon’s Devil’s Punchbowl or the Queen’s Bath on the Na Pali coast. Small peninsulas or headlands often have caves that cut completely through them, since they are subject to attack from both sides, and the collapse of a sea cave tunnel can leave a free-standing “sea stack” along the coast. The Californian island of Anacapa is thought to have been split into three islets by such a process.
Life within sea caves may assist in their enlargement as well. For example, sea urchins drill their way into the rock, and over successive generations may remove considerable bedrock from the floors and lower walls.
Most sea caves are small in relation to other types. A compilation of sea-cave surveys as of July 2014 ( Long sea caves of the world ) shows 2 over 1000 meters, 6 over 400 meters, nine over 300 meters, 25 over 200 meters, and 108 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. Most of the large sea caves on the Western U.S. coast and Hawaii 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. However, the world's largest sea cave has formed in the heavily fractured Caversham sandstone (Barth, 2013) changing our understanding of which host rocks can form large sea caves.
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. Matainaka Cave in New Zealand has 12 separate entrances into which waves can penetrate and numerous joints along which intersecting passages have developed.
A Sea Cave Near Lisbon is an 1896 British short silent actuality film, directed by Henry Short, featuring a view looking out to sea through the Boca do Inferno (Hell's Mouth) cave near Lisbon, with waves breaking in. The film was popular with audiences and received positive reviews.Arniston, Western Cape
Arniston is a small seaside settlement on the coast of the Overberg region of South Africa, close to Cape Agulhas, the southernmost tip of Africa. Prior to the wreck of Arniston, it is also known as Waenhuiskrans, an Afrikaans name meaning literally "Wagon house cliff", after a local sea cave large enough to accommodate a wagon and a span of oxen.Blue Grotto (Capri)
The Blue Grotto (Italian: Grotta Azzurra) is a sea cave on the coast of the island of Capri, southern Italy. Sunlight, passing through an underwater cavity and shining through the seawater, creates a blue reflection that illuminates the cavern. The cave extends some 50 metres into the cliff at the surface, and is about 150 metres (490 ft) deep, with a sandy bottom.Boathoist Cave
Boathoist Cave (or Boat Hoist Cave), also known as Bulman's Cave, is a huge sea cave on the south eastern flank of the British Overseas Territory of Gibraltar.Bullers of Buchan
The name Bullers of Buchan refers both to a collapsed sea cave and to the adjacent village, situated about 6 miles (9.7 km) south of Peterhead in Buchan, Aberdeenshire, Scotland.Cave
A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word cave can also refer to much smaller openings such as sea caves, rock shelters, and grottos, though strictly speaking a cave is exogene, meaning it is deeper than its opening is wide, and a rock shelter is endogene.Speleology is the science of exploration and study of all aspects of caves and the cave environment. Visiting or exploring caves for recreation may be called caving, potholing, or spelunking.Coptic Cave
Coptic Cave is a sea cave in the British Overseas Territory of Gibraltar. The cave was intended to be used as a decoy to protect Operation Tracer. This was a plan to leave behind spies should the British lose control of the Rock of Gibraltar in World War II.Craighead Caverns
Craighead Caverns is an extensive cave system located in between Sweetwater and Madisonville, Tennessee. It is best known for containing the United States' largest and the world's second largest non-subglacial underground lake, The Lost Sea. In addition to the lake, the caverns contain an abundance of crystal clusters called anthodites, stalactites, stalagmites, and a waterfall.Deerness
Deerness (Old Norse: Dyrnes) is a quoad sacra parish (i.e. one created and functioning for ecclesiastical purposes only) and peninsula in Mainland, Orkney, Scotland. It is about 13.5 kilometres (8.4 mi) south east of Kirkwall. Deerness forms a part of the civil parish of St. Andrews and Deerness.It consists chiefly of the peninsula, but also takes in its surrounding islets of Copinsay, the Horse of Copinsay and Corn Holm. The Brough of Deerness is the site of an early Christian monastery near the north eastern tip of the peninsula. The Gloup is a sea-cave approximately 40 metres (44 yd) long and 25 metres (82 ft) deep just south of the Brough.Edwin Muir was born in Deerness in 1887.
The Covenanter's Memorial at Deerness, commemorating the loss in a shipwreck of 200 Covenanters en route to the New World of America (as a punishment), was largely paid for by Robert Halliday Gunning.Dongyin, Lienchiang
Dongyin Island (Chinese: 東引島; Dĕ̤ng-īng-dō̤) is an island in the Taiwan Strait off the coast of Fujian. The Dongyin Township (Chinese: 東引鄉; pinyin: Dōngyǐn Xiāng; Dĕ̤ng-īng-hiŏng), composed of Dongyin Island and Siyin Island, is a township of the Lienchiang County, Fukien Province, Republic of China.The township was originally part of the Loyüan County before the ROC government evacuated to Taiwan in 1949 following the Chinese civil war.
It is claimed by the People's Republic of China government as part of the Luoyuan County of its Fuzhou prefecture.Fingal's Cave
Fingal's Cave is a sea cave on the uninhabited island of Staffa, in the Inner Hebrides of Scotland, known for its natural acoustics. The National Trust for Scotland owns the cave as part of a National Nature Reserve. It became known as Fingal's Cave after the eponymous hero of an epic poem by 18th-century Scots poet-historian James Macpherson.Gorham's Cave
Gorham's Cave is often mistaken for a natural sea cave, but is in fact a sea level cave, in the British Overseas Territory of Gibraltar. It is considered to be one of the last known habitations of the Neanderthals in Europe. It gives its name to the Gorham's Cave complex, which is a combination of four distinct caves of such importance that they are combined into a UNESCO World Heritage site, the only one in Gibraltar. The three other caves are Vanguard Cave, Hyaena Cave, and Bennett's Cave.It is located at Governor's Beach on the southeastern face of the Rock of Gibraltar. When first inhabited some 55,000 years ago, it would have been approximately 5 kilometres from the shore, but, due to changes in sea level, it is now only a few metres from the Mediterranean Sea.Grotta Bianca
The Grotta Bianca ("white cave") is a sea cave located on the island of Capri, Italy. It derives its name from white incrustations of calcareous matter upon its sides, and from clusters of white stalactites which hang from the roof and fringe the entrance. The cave faces east and is situated near the Punta della Chiavioa. The entry, about 70–80 feet (21–24 m) high, leads into upper and lower caves, of which the former is not easily accessible. The lower cave can be entered by boat for a short distance. Unlike most other caves at the water-level, it is much broader at approximately 6 feet (1.8 m) above the water than actually at the surface. The total height is no more than 24 feet (7.3 m). The upper erosion line is clearly marked near the cave and within it. The upper cave seems to belong to an earlier period.According to the book The Sicilian by Mario Puzo, the Grotta Bianca is where Salvatore Giuliano, the famous Sicilian bandit, stayed during his first nights as an outlaw after shooting dead an armed national policeman. Although the book is dramatized, many believe some information in it is true, with regards to Giuliano.Grotta Verde
The Grotta Verde (Italian: "Green Cave") is a sea cave located on the island of Capri, southern Italy.
Near it, the coast is rugged, being fully exposed to sirocco gales. As wear and tear of the rocks is very rapid, former water line marks are not likely to be well preserved. Consequently, it is difficult to recognize any definite indication of former land levels. All that can be definitely stated is that it is a sea caves, the floor of which, formed when the land was higher, but is now submerged. The roof was eroded when the land was at a lower level. The green light upon the rocks stems to be a composite effect due to blue light reflected and transmitted from the water, playing upon the yellowish-hued sides and roof of the cave. It is best seen in the lower l'assaggio Verde, which may be travelled by boat on a calm day.Grotta delle Felci
The Grotta delle Felci (Italian for "Fern Grotto") is a cave located on the island of Capri, in Campania, Italy.
The cave housed Neolithic men; 549 archaeological findings and fossils have been found internally.Martin's Cave
Martin's Cave is a cave in the British Overseas Territory of Gibraltar. It opens on the eastern cliffs of the Rock of Gibraltar, below its summit at O'Hara's Battery. It is an ancient sea cave, though it is now located over 700 feet (210 m) above the shore of the Mediterranean Sea. It is only accessible because Martin's Path was constructed.Portbraddon Cave
Portbraddon Cave (also spelled Portbradden, Portbraddan) is a relict sea cave located near Portbraddon, County Antrim on the north coast of Northern Ireland. Its location 5 m (16 ft) above the present-day high water mark makes it important archaeologically, as it would have been inhabited as far back as the Mesolithic.In the 1930s the cave was the subject of an archaeological dig by Andrew McLean May, who made a number of notable finds. During the course of the excavations, which went as deep as 2 m (7 ft) below the modern floor level, May encountered: iron objects, including the barrel of a muzzle-loading pistol; pottery and flint tools, some of which showed Mesolithic characteristics; fireplaces and animal bones and ultimately the original water-worn floor of the cave. Most notably May also discovered partial remains of three human females. They were examined by Professor Thomas Walmsley at Queen's University Belfast, who noted that "most of the bones have primitive features which are not likely found together in post-medieval time; and there are some characteristics which suggest an epi-palaeolithic inheritance."The whereabouts of the human bones have since become unknown, and they have not been subjected to modern radiocarbon dating techniques.Smoo Cave
Smoo Cave is a large combined sea cave and freshwater cave in Durness in Sutherland, Highland, Scotland. The cave name is thought to originate from the Norse 'smjugg' or 'smuga', meaning a hole or hiding-place.Vanguard Cave
Vanguard Cave is a natural sea cave in the British Overseas Territory of Gibraltar which is part of the Gorham's Cave complex. This complex of four caves has been nominated as a UNESCO World Heritage Site status in 2016. The cave complex is one of the last known habitations of the Neanderthals, with a period of inhabitation from 55,000 to 28,000 years ago. It is located on the southeast face of the Rock of Gibraltar.
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