A tombolo is a sandy or shingle isthmus. It is a deposition landform by which an island becomes attached to the mainland by a narrow piece of land such as a spit or bar. Once attached, the island is then known as a tied island. The word tombolo is from the Italian , meaning 'pillow' or 'cushion', and sometimes translated incorrectly as ayre (an ayre is a shingle beach of any kind).
Several islands tied together by bars which rise above the water level are called a tombolo cluster.Glossary of Geology and Related Sciences. The American Geological Institute, 1957 Two or more tombolos may form an enclosure (called a lagoon) that can eventually fill with sediment.
Formation
The shoreline moves toward the island (or detached breakwater) owing to the accretion of sand in the lee of the island, where wave energy and longshore drift are reduced and therefore deposition of sand occurs.
Wave diffraction and refraction True tombolos are formed by wave refraction and diffraction. As waves near an island, they are slowed by the shallow water surrounding it. These waves then bend around the island to the opposite side as they approach. The wave pattern created by this water movement causes a convergence of longshore drift on the opposite side of the island. The beach sediments that are moving by lateral transport on the lee side of the island will accumulate there, conforming to the shape of the wave pattern. In other words, the waves sweep sediment together from both sides. Eventually, when enough sediment has built up, the beach shoreline, known as a spit, will connect with an island and form a tombolo.
Unidirectional longshore drift In the case of longshore drift due to an oblique wave direction, like at Chesil Beach or Spurn Head, the flow of material is along the coast in a movement which is not determined by wave diffraction around the now tied island, such as the Isle of Portland, which it has reached. In this and similar cases like Cádiz, while the strip of beach material connected to the island may be technically called a tombolo because it links the island to the land, it is better thought of in terms of its formation as a spit, because the sand or shingle ridge is parallel rather than at right angles to the coast.
Morphology and sediment distribution Tombolos demonstrate the sensitivity of shorelines. A small piece of land, such as an island, or a beached shipwreck can change the way that waves move, leading to different deposition of sediments. Sea level rise may also contribute to accretion, as material is pushed up with rising sea levels. Tombolos are more prone to natural fluctuations of profile and area as a result of tidal and weather events than a normal beach is.
Because of this susceptibility to weathering, tombolos are sometimes made more sturdy through the construction of roads or parking lots. The sediments that make up a tombolo are coarser towards the bottom and finer towards the surface. It is easy to see this pattern when the waves are destructive and wash away finer grained material at the top, revealing coarser sands and cobbles as the base.
Examples
Aupōuri Peninsula, New Zealand Barrenjoey Headland, Pittwater, New South Wales, Australia Beavertail Point, Conanicut Island, Rhode Island, United States Biddeford Pool, Maine, United States Bijia Mountain, China Boquete sandbar (Paniquian Island), Puerto Galera, Philippines Broulee Island, New South Wales, Australia Bruny Island, Tasmania, Australia Burgh Island, Devon, England Cabo Rojo, Puerto Rico Cádiz, Andalucía, Spain Cana Island, Wisconsin, United States Chappaquiddick Island, Martha's Vineyard, Massachusetts, United States Charles Island, Connecticut, United States Chausey, Manche département, France (two features connecting the main island and two smaller outcrops) Chesil Beach, Portland, Dorset, England Cheung Chau, Hong Kong Crimea, Ukraine Depeldet, El Nido, Philippines Eaglehawk Neck, Tasmania, Australia S'Espalmador, Formentera, Spain Fingal Bay, New South Wales, Australia The Rock of Gibraltar Grand Island National Recreation Area, Michigan, United States Grótta, Seltjarnarnes, Iceland Guanarteme, Las Palmas de Gran Canaria, Canary Islands, Spain Gugh, St Agnes, Isles of Scilly, England Gwadar, Pakistan Hakodate, Hokkaido, Japan Howth Head, Dublin, Republic of Ireland Inishkeel Island, Narin, Republic of Ireland Isola Bella (Sicily), Italy Kapıdağ Peninsula, Balıkesir, Turkey Sinop Peninsula, Sinop, Turkey Konet Island, Kuala Sungai Baru, Malacca, Malaysia Kurnell, Sydney, New South Wales, Australia Lake Pomorie, Bulgaria Langness, Derbyhaven, Isle of Man Little Heart's Ease, Newfoundland and Labrador beach, Newfoundland, Canada Llandudno, North Wales Louds Island at Muscongus Bay, Maine, United States Maharees, Dingle Peninsula, Republic of Ireland Mare Island, Vallejo, California, United States Maria Island, Tasmania, Australia Maury Island, Washington, United States McMicken Island State Park, Washington, United States Miquelon, Saint-Pierre and Miquelon, France Monemvasia, Laconia, Peloponnese, Greece Monte Argentario, Tuscany, Italy Mont-Saint-Michel, Normandy, France Mount Maunganui, New Zealand Mount Taipingot, Rota, Northern Marianas Nahant, Massachusetts, United States (a natural tombolo, but connected to the mainland by a causeway) Nissi beach, Ayia Napa, Cyprus Nusa Dua, Bali, Indonesia Ormara, Pakistan Palisadoes, Kingston, Jamaica Pangandaran, West Java, Indonesia Peniche, Portugal Peniscola, Castellon, Spain Presque Isle, Michigan, United States Presqu'ile de Giens, Hyères, France Presqu'ile Provincial Park, Ontario, Canada Quiberon, France Ras Hafun, Somalia Sainte-Marie, Martinique, France Scotts Head, Dominica Seongsan Ilchulbong in Jeju Province, South Korea Sharp Island, Sai Kung District, Hong Kong Silver Strand (San Diego), Coronado, California, United States St Michael's Mount, Cornwall, England St Ninian's Isle, Shetland, Scotland Sveti Stefan, near Budva, Montenegro Tandol tombolo, El Nido, Philippines Tombolo at Dogashima in Japan Vatersay, Scotland – the island comprises two sections connected by a broad tombolo Zhifu Island, Yantai, China
Some of these may be simple isthmuses, and not have the deposition creation that defines a true tombolo.
Image gallery
See also
References
External links
Geology.About.com's page on tombolos (useful for its descriptive photograph) Tombolo in Sainte-Marie, Martinique (useful for its photos and description) further reading on Detached breakwaters from Vlaams Instituut voor de Zee in Belgium further reading on coastal structures from Prof. Leo van Rijn in Holland