<P> Bathymetry is the mapping and study of the topography of the ocean floor . Methods used for measuring the depth of the sea include single or multibeam echosounders, laser airborne depth sounders and the calculation of depths from satellite remote sensing data . This information is used for determining the routes of undersea cables and pipelines, for choosing suitable locations for siting oil rigs and offshore wind turbines and for identifying possible new fisheries . The Earth is composed of a magnetic central core, a mostly liquid mantle, and a hard rigid outer shell (or lithosphere), which is composed of the Earth's rocky crust and the deeper and mostly solid outer layer of the mantle . The crust below land is known as continental while that under the abyssal sea is called oceanic . The latter is composed of relatively dense basalt and is some five to ten kilometers (three to six miles) thick . The relatively thin lithosphere floats on the weaker and hotter mantle below and is fractured into a number of tectonic plates . In mid-ocean, magma is constantly being thrust through the seabed between adjoining plates to form mid-oceanic ridges and here convection currents within the mantle tend to drive the two plates apart . Parallel to these ridges and nearer the coasts, one oceanic plate may slide beneath another oceanic plate in a process known as subduction . Deep trenches are formed here and the process is accompanied by friction as the plates grind together . The movement proceeds in jerks which cause earthquakes . Heat is also produced and magma is forced up, creating underwater mountains, some of which grow into volcanic islands . Near some boundaries between the land and sea, the slightly denser oceanic plates slide beneath the continental plates and more subduction trenches are formed . As they grate together, the continental plates are deformed and buckle causing mountain building and seismic activity . </P> <P> The Earth's deepest trench is the Mariana Trench which extends for about 2,500 kilometers (1,600 mi) across the seabed . It is near the Mariana Islands, a volcanic archipelago in the West Pacific . Though it averages just 68 km (42 mi) wide, its deepest point is 10.994 kilometers (nearly 7 miles) below the surface of the sea . An even longer trench runs alongside the coast of Peru and Chile, reaching a depth of 8,065 m (26,460 ft) and extending for approximately 5,900 km (3,700 mi). It occurs where the oceanic Nazca Plate slides under the continental South American Plate and is associated with the upthrust and volcanic activity of the Andes . </P> <P> The zone where land meets sea is known as the coast and the part between the lowest spring tides and the upper limit reached by splashing waves is the shore . A beach is the accumulation of sand or shingle on the shore . A headland is a point of land jutting out into the sea and a larger promontory is known as a cape . The indentation of a coastline--especially between two headlands--is a bay; a small bay with a narrow inlet is a cove and a large bay or bay - shaped sea may be referred to as a gulf . Coastlines are influenced by a number of factors including the strength of the waves arriving on the shore, the gradient of the land margin, the composition and hardness of the coastal rock, the inclination of the off - shore slope, and the changes of the level of the land due to local uplift or submergence . Normally, waves roll towards the shore at the rate of six to eight per minute . These are known as constructive waves as they tend to move material up the beach and have little erosive effect . Storm waves arrive on shore in rapid succession and are known as destructive waves, as their swash moves beach material seawards . Under their influence, the sand and shingle on the beach is ground together and abraded . Around high tide, the power of a storm wave impacting on the foot of a cliff has a shattering effect as air in cracks and crevices is compressed and then expands rapidly with release of pressure . At the same time, sand and pebbles have an erosive effect as they are thrown against the rocks . Along with other weathering processes such as frost, this tends to undercut the cliff . Gradually, a wave - cut platform develops at the foot of the cliff and this has a protective effect, reducing further wave - erosion . </P> <P> Material worn from the margins of the land eventually ends up in the sea, where it is subject to attrition as currents flowing parallel to the coast scour out channels and transport material away from its place of origin . Sediment carried to the sea by rivers settles on the seabed causing deltas to form in estuaries . All these materials move back and forth under the influence of waves, tides, and currents . Dredging removes material and deepens channels but may have unexpected effects elsewhere on the coastline . Governments make efforts to prevent flooding through building breakwaters, seawalls, and other defenses against the sea . In Britain, the Thames Barrier protects London from storm surges, while the failure of the dykes and levees around New Orleans during Hurricane Katrina created a humanitarian crisis in the United States . Land reclamation in Hong Kong permitted the construction of Hong Kong International Airport through the leveling and expansion of two smaller islands . </P>

Which of the following is not generally a factor in wave generation in the open ocean