Continental shelf

A continental shelf is a portion of a continent, submerged under an area of shallow water known as a shelf sea. Much of these shelves has been exposed during interglacial periods; the shelf surrounding an island is known as an insular shelf. The continental margin, between the continental shelf and the abyssal plain, comprises a steep continental slope, surrounded by the flatter continental rise, in which sediment from the continent above cascades down the slope and accumulates as a pile of sediment at the base of the slope. Extending as far as 500 km from the slope, it consists of thick sediments deposited by turbidity currents from the shelf and slope; the continental rise's gradient is intermediate between the gradients of the shelf. Under the United Nations Convention on the Law of the Sea, the name continental shelf was given a legal definition as the stretch of the seabed adjacent to the shores of a particular country to which it belongs; the width of the continental shelf varies – it is not uncommon for an area to have no shelf at all where the forward edge of an advancing oceanic plate dives beneath continental crust in an offshore subduction zone such as off the coast of Chile or the west coast of Sumatra.

The largest shelf – the Siberian Shelf in the Arctic Ocean – stretches to 1,500 kilometers in width. The South China Sea lies over another extensive area of continental shelf, the Sunda Shelf, which joins Borneo and Java to the Asian mainland. Other familiar bodies of water that overlie continental shelves are the North Sea and the Persian Gulf; the average width of continental shelves is about 80 km. The depth of the shelf varies, but is limited to water shallower than 100 m; the slope of the shelf is quite low, on the order of 0.5°. Though the continental shelf is treated as a physiographic province of the ocean, it is not part of the deep ocean basin proper, but the flooded margins of the continent. Passive continental margins such as most of the Atlantic coasts have wide and shallow shelves, made of thick sedimentary wedges derived from long erosion of a neighboring continent. Active continental margins have narrow steep shelves, due to frequent earthquakes that move sediment to the deep sea.

The shelf ends at a point of increasing slope. The sea floor below the break is the continental slope. Below the slope is the continental rise, which merges into the deep ocean floor, the abyssal plain; the continental shelf and the slope are part of the continental margin. The shelf area is subdivided into the inner continental shelf, mid continental shelf, outer continental shelf, each with their specific geomorphology and marine biology; the character of the shelf changes at the shelf break, where the continental slope begins. With a few exceptions, the shelf break is located at a remarkably uniform depth of 140 m; the continental slope is much steeper than the shelf. The slope is cut with submarine canyons; the physical mechanisms involved in forming these canyons were not well understood until the 1960s. The continental shelves are covered by terrigenous sediments. However, little of the sediment is from current rivers. Sediments become fine with distance from the coast; these accumulate 15–40 cm every millennium, much faster than deep-sea pelagic sediments.

Shelf seas refer to the ocean waters on the continental shelf. Their motion is controlled by the combined influences of the tides, wind-forcing and brackish water formed from river inflows; these regions can be biologically productive due to mixing caused by the shallower waters and the enhanced current speeds. Despite covering only about 8% of the Earth's ocean surface area, shelf seas support 15-20% of global primary productivity. While the North Sea is one of the better studied shelf seas, it is not representative of all shelf seas as there is a wide variety of behaviours to be found. Indian Ocean shelf seas are dominated by major river systems including the Indus rivers; the shelf seas around New Zealand are complicated because the submerged continent of Zealandia creates wide plateaus. Shelf seas around Antarctica and the shores of the Arctic Ocean are influenced by sea ice production and polynya. There is evidence that changing wind and regional ocean currents in a warming ocean, is having an effect on some shelf seas.

Improved data collection via Integrated Ocean Observing Systems in shelf sea regions is making identification of these changes possible. Continental shelves teem with life because of the sunlight available in shallow waters, in contrast to the biotic desert of the oceans' abyssal plain; the pelagic environment of the continental shelf constitutes the neritic zone, the benthic province of the shelf is the sublittoral zone. The shelves makes up less than ten percent of the ocean, a rough estimate suggest that only about 30% of the continental shelf sea floor receives enough sunlight to allow benthic photosynthesis. Though the shelves are fertile, if anoxic conditions pre


Drogheda is one of the oldest towns in Ireland. It is located on the Dublin–Belfast corridor on the east coast of Ireland in County Louth but with the south fringes of the town in County Meath, 49 km or 30 miles north of Dublin. Drogheda has a population of 41,000 inhabitants, making it the eleventh largest settlement by population in all of Ireland, it is the last bridging point on the River Boyne. The UNESCO World Heritage Site of Newgrange is located 8 km west of the city. Drogheda was founded as two separately administered towns in two different territories: Drogheda-in-Meath and Drogheda-in-Oriel; the division came from the twelfth-century boundary between two Irish kingdoms, colonised by different Norman interests, just as the River Boyne continues to divide the town between the dioceses of Armagh and Meath. In 1412 these two towns were united, Drogheda became a'County Corporate', styled as'the County of the Town of Drogheda'. Drogheda continued as a County Borough until the setting up of County Councils through the enactment of the Local Government Act 1898, which saw all of Drogheda, including a large area south of the Boyne, become part of an extended County Louth.

With the passing of the County of Louth and Borough of Drogheda Provisional Order, 1976, County Louth again grew larger at the expense of County Meath. The boundary was further altered in 1994 by the Local Government Regulations 1994; the 2007–2013 Meath County Development Plan recognises the Meath environs of Drogheda as a primary growth centre on a par with Navan. The city was selected to host Fleadh Cheoil na hÉireann in 2018; the town is situated in an area which contains a number of archaeological monuments dating from the Neolithic period onwards, of which the large passage tombs of Newgrange and Dowth are the best known. The density of archaeological sites of the prehistoric and early Christian periods uncovered in the course of ongoing developments, have shown that the hinterland of Drogheda has been a settled landscape for millennia. Despite local tradition linking Millmount to Amergin Glúingel, in his 1978 study of the history and archaeology of the town John Bradley stated that "neither the documentary nor the archaeological evidence indicates that there was any settlement at the town prior to the coming of the Normans".

The results of a number of large-scale excavations carried out within the area of the medieval town appear to confirm this statement. One of the earliest structures in the town is the motte-and-bailey castle, now known as Millmount Fort, which overlooks the town from a bluff on the south bank of the Boyne and, erected by the Norman Lord of Meath, Hugh de Lacy sometime before 1186; the wall on the east side of Rosemary Lane, a back-lane which runs from St. Laurence Street towards the Augustinian Church is the oldest stone structure in Drogheda, it was completed in 1234 as the west wall of the first castle guarding access to the northern crossing point of the Boyne. The earliest known town charter is that granted to Drogheda-in-Meath by Walter de Lacy in 1194. In the 1600s, the name of the town was spelled "Tredagh" in keeping with the common pronunciation, as documented by Gerard Boate in his work Irelands' Natural History. In c. 1655 it was spelled "Droghedagh" on a map by William Farriland.

Drogheda was an important walled town in the English Pale in the medieval period. It hosted meetings of the Irish Parliament at that time. According to R. J. Mitchell in John Tiptoft, Earl of Worcester, in a spill-over from the War of the Roses the Earl of Desmond and his two youngest sons were executed there on Valentine's Day 1468 on orders of the Earl of Worcester, the Lord Deputy of Ireland, it came to light, that Elizabeth Woodville, the queen consort, was implicated in the orders given. The parliament was moved to the town in 1494 and passed Poynings' Law, the most significant legislation in Irish history, a year later; this subordinated the Irish Parliament's legislative powers to the King and his English Council. The town was besieged twice during the Irish Confederate Wars. On the second occasion an assault was made on the town from the south, the tall walls breached, the town was taken by Oliver Cromwell on 11 September 1649, as part of the Cromwellian conquest of Ireland and it was the site of a massacre of the Royalist defenders.

In his own words after the siege of Drogheda, "When they submitted, their officers were knocked on the head, every tenth man of the soldiers killed and the rest shipped to Barbados."The Earldom of Drogheda was created in the Peerage of Ireland in 1661. The Battle of the Boyne, 1690, occurred some 6 km west of the town, on the banks of the River Boyne, at Oldbridge. In 1790, Drogheda Harbour Commissioners were established, they remained in place until 1997 when the Drogheda Port Company a commercial enterprise replaced them. In 1825, the Drogheda Steam Packet Company was formed in the town, providing shipping services to Liverpool. In 1837, the population of Drogheda area was 17,365 people. Drogheda's coat of arms features St. Laurence's Gate with three lions, a ship emerging from either side of the barbican; the town's motto Deus praesidium, mercatura decus translates as "God our strength, merchandise our glory". The star and crescent emblem in the crest

Bruce Field (athlete)

Bruce William Field is an Australian former athlete who competed at the 1972 Summer Olympics and won a silver medal at the 1974 British Commonwealth Games. He specialised in sprint races and the long jump. Field was trained at St. Stephens Harriers in Mount Waverley. At the 1972 Summer Olympics he took part in both 400 metres hurdles; the only Australian in the long jump competition, he finished in 15th position with a leap of 7.76m, finishing 3 cm short of a place in the final. He was in the fourth heat for the 400 metres hurdles, won by eventual gold medalist John Akii-Bua. Field was eliminated from the competition. In 1974 he was the national champion in the 400 metres hurdles and in the same year won a silver medal in the 400 metres hurdles event, behind England's Alan Pascoe, at the British Commonwealth Games in Christchurch, his time of 49.32 set in Christchurch remained a national record for 20 years. He had top five finishes in the 400 metres and long jump. While competing in athletics, Field studied for a PhD at the University of Melbourne.

He is now an Associate Professor in the Monash University Department of Mechanical Engineering. Bruce Field at Sports Reference