Atlantic Ocean Islands, Coastal Geomorphology
With only a few exceptions such as Bermuda and Sable, it is clear from the spatial distribution of almost all of the other individual islands or island groups in the Atlantic Ocean that most owe both their location and coastal morphology to a turbulent tectonic and often volcanic past. The tectonic opening of the Atlantic Ocean is the key factor in this history. Commencing in the Late Jurassic some 140 million years ago and fully open some 65 million years ago (Hansom and Gordon 1998), the Atlantic basin is characterized by the westward movement of both American plates and the eastward movement of the African and Eurasian plates. Basaltic magma upwelling into the crustal gap in the mid-ocean spreading center has over time produced a long submarine ridge composed of a series of fissure volcanoes known as the mid-Atlantic Ridge. In places the volume of upwelling magma has been sufficient to allow the construction of volcanoes that extend to the ocean surface and form the individual volcanic islands and island groups of Bouvet, Gough, Tristan da Cunha, St. Helena, Ascension, Macronesia, Kelard, and Jan Mayen. Elsewhere, where more than two plates and spreading directions are involved, this relatively simple mid-Atlantic geometry is more complex. For example, the development of the Scotia Arc in the South Atlantic was contingent on the opening of the Drake Passage and the eastward movement of several micro-plates along a progressively elongating arc. The boundaries of the Scotia Arc are marked in the south by the South Shetland and South Orkney Islands, in the north by South Georgia and in the east by the still-extending volcanic island arc of the South Sandwich Islands (Hansom and Gordon 1998). As a result, the rocks of South Georgia, South Shetlands, and South Orkney are mainly composed of fragments of older rocks and intruded volcanics, most of which were created elsewhere and subsequently transported to their present sites. By contrast, 70% of the rocks of the South Sandwich Islands are recently extruded basalts (Baker 1990).
The Scotia Arc
The coastline of the Falkland Islands, like the islands themselves, is low, flat, and reminiscent of the coastline of the Outer Hebrides of Scotland. Although hundreds of smaller islands exist, the main island group comprises West and East Falkland separated by Falkland Sound. The intricate and crenulate nature of the coastline is probably more related to the submergence and faulting of the underlying Devonian, Carboniferous, and Permian limestones and sandstones, rather than to the efficiency or otherwise of marine erosion and deposition. In spite of the majority of the coastline being rocky, steep and high cliffs are mainly absent except in the extreme west where 100–200 m cliffs occur at Beaver Island. Everywhere and especially within the broad embayments and inlets, there are innumerable small islets and skerries whose detailed morphology and outline appear to be structurally controlled. A notable feature of the rocky coastline is the abundance of dense stands of giant kelp in the nearshore. On the western flank of the fault that controls Falkland Sound, narrow coast-parallel outcrops of hard and softer rock have been eroded to produce an intricate series of small headlands composed of more resistant rock separated by small arcuate bays cut into less resistant beds. The result is an essentially linear coastline stretching the entire length of Falkland Sound, broken by regular and uniform bays where the outer rocks have been breached. The crenulate nature of the coastline and extensive areas of nearshore shallow water means that a wide range of sheltered locations exists for potential beach development. Streams, although abundant and rarely dry, do not appear to contribute significantly to beaches, since there is a lack of beach development adjacent to the mouths of the creeks and streams entering tidal waters. Beaches of sand and gravels tend to be located in either outer coast sites where ocean waves gain access or within inlets where more open aspects allow wind-generated local waves to produce beaches. The high proportion of shell-sand of the beaches points to an important biological input from nearshore shallow waters. Significant beach development on the outer coast occurs at Bull Point, Lively Island, and at Bertha’s Beach where sandy spits and tombolos connect small islets to mainland East Falkland. Paloma Beach, Elephant Beach, and Concordia Beach in the north of East Falkland are all examples of large open coast sandy beaches where strong winds have resulted in blown sand spreads over inland areas. In West Falkland, large beaches occur within the inlet of Whitsand Bay. Unfortunately, as a result of indiscriminate mine-laying during the Falklands War, several of the beaches of East Falkland are now unsafe and access is restricted.
Bouvet, Gough, Tristan da Cunha, St. Helena and Ascension
Bouvet Island (Fig. 1) stands at the southern end of the mid-Atlantic Ridge, the slopes of the central cone terminating on all sides in precipitous cliffs that descend abruptly to sea level. Probably a complex of volcanic cones, the island rises almost symmetrically to a flat ice-covered dome 935 m high. The ice covers the eastern side of the island where it reaches the sea in an ice cliff 122 m high. The north and west sides are free of ice but are much steeper. Between 1955 and 1958, a new 0.2 km2 rock platform emerged at 25 m above sea level as a result of either eruption or rock falls associated with tremors (Stonehouse 1972). Since little low-level ground was previously to be found around the coast of Bouvet, the new addition has become a prime-breeding site for penguins, petrels, and seals. Gough Island (Fig. 1) lies 2,300 km to the east of the Cape of Good Hope, South Africa and is the eroded summit of a Tertiary volcano. Although the island is mountainous, rising to 910 m above sea level, the slopes are cut by deep gullies or gulches and the coastline is characterized by steep cliffs that appear to have undergone erosion to produce a variety of narrow boulder beaches at the foot of the cliffs. There are also numerous islets, stacks, and skerries that mostly lie within 100 m of the coast. Tristan da Cunha lies 350 km to the northwest of Gough. It is a circular island of 98 km2 with four other small islands close by. The base of the central volcanic peak lies 3,700 m deep on the seabed and the summit rises to almost 2,100 m above sea level. The lower slopes are almost entirely lava and form high cliffs that surround the island. Emerged beaches, platforms, and caves occur at 5 m above sea level on Tristan but 12 m above sea level on the small adjacent islands, indicating differential tectonic uplift in the area (Bird 1985b). In some places, small and inaccessible beaches occur, some of which are sandy. A small plateau at the foot of the main cliffs on the northwest side at Herald Point provides the site for Edinburgh, the only inhabited part of the island. The cliffs are lower along the Edinburgh coast, allowing access to the sea via small gullies. A narrow gravel beach has developed along this stretch of coast, fed by material eroded from the cliffs. In the months following the 1961 eruption at Big Point, on the updrift section of this coast, rapid recession of the cliff was noted (10 m in 8 weeks). This contributed to accretion of the downdrift gravel beaches which formed a spit enclosing a small lagoon (Bird 1985b).
The island groups comprising Macronesia stretch lie in a long line that extends north between the coast of West Africa and the Portuguese coast (Fig. 1). All of the island groups are volcanic in origin and many remain active volcanoes. Four main stratovolcanoes occur in the arid Cape Verde Islands, Fogo, Brava, Santo Antao, and San Vicente. Fogo is 2,829 m high yet only 24 km wide and so the steep seaward slopes end abruptly in retreating cliffs. The volcano has erupted 10 times between 1500 and 1995 each one sending long streams of lava down the flanks into the ocean and altering the coastal geometry. A similar picture characterizes the other islands of the group, each being surrounded by steep cliffs cut by deep gullies at the foot of which occur infrequent gravelly beaches. Emerged features have been noted at six levels up to 100 m above sea level. Some bays contain fringing algal reefs and in some locations boulders have been moved inland by wave activity by up to 200 m (Bird 1985b). The Canary Islands comprise seven main islands, six of which host volcanoes. Some of these have erupted as recently as 1971. The coastline of the Canaries resembles that of the other mid-ocean volcanic islands in as much as the central volcanic spine of the islands falls steeply to a mainly rocky and cliff coast cut by deeply incised ravines. For example, in the northwest of Tenerife at Acantilado de Los Gigantes, sub-vertical cliffs reach 500 m. The occurrence of such cliffs has constrained some of the tourist-related expansion that many of the coastal towns and villages have undergone in recent years. Some cliffs have been formed in sediments brought down by torrents in the ravines. These deposits have since been tectonically uplifted together with the boulder beaches that once fronted them. The north coast of Gran Canaria has good examples of such cliffs, together with well-developed shore platforms a few meters above sea level. However, the volcanism of the Canaries has in places produced several long and low craggy volcanic peninsulas that have allowed beach development to occur. Fuertoventura, Lanzarote, Tenerife, and Gran Canaria all have sandy beaches some with extensive sand dunes. The long white sand beach at San Andres, on the southeast coast of Tenerife, has been artificially nourished with sand brought from the Sahara Desert. Between Maspalomas and Playa del Ingles, in the south of Gran Canaria, 328 ha of fine shell sand have accumulated at the mouth of the Fataga ravine to enclose a freshwater lagoon. On account of the relative aridity, most dunes in the Canaries are sparsely vegetated and the unfixed dunes at Maspalomas advance east towards the lighthouse at a meter per year. Some of the dunes have succumbed to tourist development, such as at Playa de las Canteras in the north of Gran Canaria. In the easternmost part of the islands, the supply of beach sediment from the nearshore and the inland ravines is augmented from a more exotic source. Fuertoventura and Lanzarote are affected by the summer scirrocco, a hot wind from the Sahara 90 km to the east, which carries large quantities of dust and desert sand to the islands. Locally known as the kalima, it turns the day into twilight and regularly covers surfaces with a thin layer of sediment. Over the centuries, this sediment has been an important source of sand for the beaches of the easternmost islands. In the south, Fuertoventura narrows at the Pared isthmus where sandy beaches extend eastward to connect the original island of Jandia to the main island by what is now a low and narrow peninsula. On the eastern shore, the sands of the 26 km-long Playa de Sotavento (Sp: leeward) are protected from the dominant westerly waves. On the western side, the Playas of Cafete, Pared and Barlovento (Sp: windward) are more exposed with strong currents and undertows. Nearby Lanzarote is a similarly dramatic landscape of lava fields and steep cliffs with intervening sand beaches, such as the sweeping Puerto del Carmen beach on the east coast. Accretion is common at many of the valley-mouth inlets.
Maderia comprises the main island itself, together with the smaller island of Porto Santo, the nearby Islas Desertas, together with the uninhabited Selvagens to the south. All of these islands are volcanic in origin but have not been active in the last 1.5 million years. As a result the, main volcanic core of Madeira, together with its plateau-like lateral subsidiary vents, has become eroded to produce deeply incised valleys and gorges running down to the sea. Between the mouths of the river valleys are high cliffs of vertical columns of basalt with layers of red and yellow tufa exposed in places. The 580 m plunging cliffs of Cabo Giroa, west of Funchal, are among Europe’s highest, but high cliffs are found everywhere on the coast of Madiera. In the north, fragments of shore-platform occur as well as small islets and skerries, particularly near Faial. There are several well-developed stacks near Ponta do Sao Lourenco in the northeast. Small beaches of rounded gray basalt gravel occur at several places, particularly where river mouths occur. The only sandy beach is at Prainha, on the extreme east of the island where the sand is mainly basalt. Shell sand occurs on the low plateau area in the extreme east of the island. The nearby island of Porto Santo is 14 km long and 5 km wide, its generally low volcanic profile veneered by thin deposits of sand, calcareous sandstone and clay. The cliffs of the north coast reach 100 m but the south coast is dominated by an 8 km long white sand beach fronted by a shallow sandy-floored bay backed by low cliffs of cemented sands. Protected from the main force of southwesterly storms by Madeira, from the west by the small island of Baixo and from the north by its own cliffs, the south coast of Porto Santo is relatively sheltered. It appears to have been subject to sediment accumulation over a substantial period of time derived both from a combination of shell sand from shallow nearshore surfaces, aeolian sand blown west on the scirrocco from the Sahara to the east and from topsoil erosion caused by early deforestation. The Selvagens are composed of limestone capped by lava and ash and are cliffed to 100 m in the north but with a gentler beach-fringed coast in the south. Emerged beaches and dune calcarenites occur on Pleistocene marine terraces at 3 and 7 m above sea level (Bird 1985b).
The most northerly of the islands of Macronesia, the nine islands of the Azores are a widely separated group of mountainous but fertile islands which share the steep nature of many of the mid-oceanic islands but also have long beaches and many fishing harbors. The coast of the largest island, Sao Miguel, is a microcosm of the Azores coastline. Where the coast is backed by higher volcanic peaks, it is characterized by steep cliffs fronted by patchy low shore platforms, offshore stacks and islets, such as at Mosteiros. Emerged beaches and platforms occur at various heights up to 60 m (Bird 1985b). However, the highly indented coast also has short sandy beaches between cliff headlands. Where the hinterland is lower, longer sand beaches occur such as at Praia dos Moinhos and at Populo where small pine-clad sand dunes occur. Several coastal features of note occur on the other islands. Pico Island is 42 km long and dominated by a 2351 m cone-shaped strato-volcano of the same name. Various historical lava flows have extended the coastline and now have been eroded back to form low cliffs. In places sheltered from the dominant westerly waves, smaller volcanic forms survive on the coast, such as at Barca on the island of Graciosa where a volcanic cone has been sectioned by marine erosion to form bays fringed by crumbling cliffs of ash that cascade onto boulder beaches below. The 1957/58 eruption at Capelhino on Faial showered the adjacent coast in ash and contributed to accretion of the beach. At Porto Pim, also on Faial, a substantial sandy spit has developed (Bird 1985a). Such is the power of the surf reaching the Azores that Pico has been chosen as the site of a pilot plant to produce energy from waves.
Faeroes and Jan Mayen
The coastline of Jan Mayen is dominated by the volcanic bulk of Beerenberg, a 2,277 m stratovolcano that comprises the northern half of the 54-km-long island (Norsk Polarinstitutt 1959). The coastline of Nord-Jan is steep and rugged, comprising 200–400-m-high cliffs eroded in ash, lava and tephra as well as into the ice walls of tidewater glaciers that spill down from the central crater. The coastline shows signs of recent advance as a result of lava tongues from eruptions in 1970 and 1985, which have extended into the ocean. These recent eruptions and flows are an extension of the mode of construction of the island over an estimated 700,000 years. Such additions to the coastline have resulted in the base of preexisting cliffs becoming buried by newer lavas and it is onto this platform that more recent glacier terminal moraines have been deposited, such as at Smithbreen in the northeast of the island (Kinsman and Sheard 1962). At Kroksletta, near the northern cape, 4,000-year-old moraines that rest on top of an emerged shore platform and beach at 8–10 m above sea level have been buried by subsequent lava flows. The protruding seaward edge of the Kroksletta lava is now cliffed to a height of 5–13 m (Kinsman and Sheard 1962). The south part of Jan Mayen is a hilly ridge of scoria craters and mounds and trachyte domes whose lower elevations and older age has produced a coastline of low gradient rocky platforms connected by gravelly beaches. In several places around the Sud-Jan coastline a prominent rock platform is present upon which is sited emerged beach gravels. This feature is particularly well developed on the north coast of Sud-Jan at Sorbukta, Engelsbukta, and Haugenstranda. Sediment supply to the central section coastline that joins Sud-Jan to Nord-Jan appears to be healthy both from erosion of the lava cliffs to the west and from shallow water sources: the 10 m depth contour lies about 0.7 km offshore. This has resulted in the construction of a substantial barrier beach enclosing a lagoon along an 11-km-long stretch of south-central coast at Lagunevollen, with a smaller barrier and lagoon at Stasjonsbukta on the north coast. The composition of these barrier beaches is not known but assumed to be composed of mixed sands and gravels transported from the west. Such a large beach complex is unusual for a small mid-oceanic island. However, the full impact of Atlantic westerly waves and swell at Sud-Jan is modified by a large circular submarine reef (probably an eroded volcanic cone), which reaches to within 11 m of the sea surface some 10 km south of Sud-Jan. Although the north coast is subject to variable amounts of sea ice for an average of four months, the south coast is mainly free of ice (Gloerson et al. 1992). Elsewhere, the coastline of Sud-Jan is steep and rocky but the cliffs are not as high as in Nord-Jan and small gravelly cliff-foot beaches fed by rockfall and wave erosion of the adjacent lava occur.
Bermuda and Sable
Sable Island (Fig. 1), on the continental shelf to the east of Nova Scotia, Canada, is a low and wind-swept series of sandy islands famous for its sandy shoals and shipwrecks. The development of the Sable Island Bank, on which the islands sit, is related to the proximity of the continental shelf-break and the maximum ice positions of the Late Wisconsin and later readvances. These ice movements deposited a thick suite of glacigenic tills and superficial sandy material, that was subsequently subject to transgression, modification, and transport (King 2001). The island has spectacular but desolate sandy beaches backed by sand dunes that reach up to 30 m high and cover 40% of the island’s surface (Byrne and McCann 1995). The intertidal zones of the long sandy beaches are characterized by prominent shoreface attached ridges with intervening depressions that reach up to 12 m deep (Dalrymple and Hoogendoorm 1997). Strong alongshore currents cause eastward migration of these bars alongshore at rates of up to 50 m a−1 and at angles of up to 50° to the coastal orientation (Dalrymple and Hoogendoorm 1997). The dunes of Sable consist of primary dunes that have developed in situ together with secondary dunes that have migrated across the island. The resultant coastal morphology represents a mix of both natural processes and anthropogenic disturbance. For example, the constantly changing coastal outline has resulted in the relocation of the lighthouse and is thought to be partly due to dune mobilization and reduced vegetation cover under the enhanced grazing pressure of introduced ponies (Owens and Bowen 1977). Sable Island has undergone 14.5 km of eastward migration but, in spite of this, its 30 km2 has been maintained over the past 200 years (Cameron 1965). Sable Island thus seems to be subject to a regime of deposition that appears roughly balanced by an equivalent amount of erosion.
- Baker PE (1990) The south sandwich islands. In: LeMasurier WE, Thomson JW (eds) Volcanoes of the Antarctica plate and southern oceans. American Geophysical Union, Washington, DC, pp 361–395Google Scholar
- Bird ECF (1985a) Coastline changes: a global review. Wiley, Chichester, p 219Google Scholar
- Bird ECF (1985b) Atlantic ocean islands. In: Bird ECF, Schwartz ML (eds) The World’s coastline. Van Nostrand, New York, pp 1035–1039Google Scholar
- Cameron HL (1965) The shifting sands of Sable Island. Geogr Rev 44:363–376Google Scholar
- Clapperton CM (1971) Geomorphology of the Stromness Bay-Cumberland Bay area, South Georgia. British Antarctic Surv Rep 70:25Google Scholar
- Clapperton CM, Sugden DE, Birnie RV, Hansom JD, Thom G (1978) Glacier fluctuations in South Georgia and comparison with other island groups in the Scotia Sea. In: Van Zinderen Bakker EM (ed) Antarctic glacial history and world palaeoenvironments. A.A. Balkema, Rotterdam, pp 95–104Google Scholar
- Dalrymple RW, Hoogendoorm EL (1997) Erosion and deposition on migrating shoreface-attached ridges, Sable Island, Eastern Canada. Geosci Can 24(10):25–36Google Scholar
- Gloerson P, Campbell WJ, Cavalieri DJ, Comiso JC, Parkinson CL, Zwally HJ (1992) Arctic and Antarctic Sea Ice, 1978–1987. Satellite passive-microwave observations and analysis (NASA SP-511). Scientific and Technical Information Program, NASA, Washington DCGoogle Scholar
- Hansom JD (1981) Storm surging in South Georgia. British Antarctic Surv Bull 53:141–146Google Scholar
- Hansom JD (1983b) Ice-formed intertidal boulder pavements in the sub-Antarctic. J Sediment Petrol 53(1):0135–0145Google Scholar
- Hansom JD, Gordon JE (1998) Antarctic environments and resources: a geographical perspective. Addison Wesley, Longman, p 402Google Scholar
- Hansom JD, Kirk RM (1989) Ice in the intertidal zone: examples from Antarctica. Essener Geographische Arbeiten 18:211–236Google Scholar
- John BS, Sugden DE (1971) Raised marine features and phases of glaciation in the South Shetland Islands. British Antarctic Surv Bull 24:45–111Google Scholar
- King CAM (1972) Beaches and coasts. St. Martins press, New YorkGoogle Scholar
- King EL (2001) A glacial origin for Sable Island: ice and sea-level fluctuations from seismic stratigraphy on Sable Island Bank, Scotian Shelf, offshore Nova Scotia. In: Geological survey of Canada, Current research. D19, 18 pGoogle Scholar
- Norsk Polarinstitutt (1959) Topografisk Kort over Jan Mayen. Blad 1 & 2, 1;50 000, OsloGoogle Scholar
- Owens EH, Bowen AT (1977) Coastal environments of the maritime provinces. Marit Sed 13:1–32Google Scholar
- Simkin T, Siebert L (1994) Volcanoes of the World. Geoscience Press, TucsonGoogle Scholar
- Stonehouse B (1972) Animals of the Antarctic: the ecology of the far south. Peter Lower/Eurobook, LondonGoogle Scholar
- Trenhaile AS (1997) Coastal dynanmics and landforms. Clarendon Press, OxfordGoogle Scholar
- Vacher L (1973) Coastal dunes of younger Bermuda. In: Coates DR (ed) Coastal geomorphology. George Allen & Unwin, London, pp 355–391Google Scholar