From the total area of the earth 71% is represented by the ocean and marginal seas. The remaining 29% is represented by the continents . That is the reason the earth is sometimes called ‘the water planet’.
The continents and oceans are not evenly divided between Northern and Southern hemispheres. Nearly 61% of the surface is water in Northern hemisphere , while about 39% is land.
In the Southern hemisphere 81% of the surface is water and only 19% is land. That is the reason Northern hemisphere is called the Continental(land) hemisphere and the Southern hemisphere the Oceanic (water) hemisphere .
Distribution of land and water over the Earth’s surface. I. Continental Hemisphere II. Oceanic Hemisphere
The drag exerted by winds blowing steadily across the ocean causes the surface layer of the water to move.
Marine climates are influenced by ocean currents. Warm ocean currents carry warm water from the tropics toward the poles. Cold currents bring cold water from the polar zones to near the equator. The surface of the water warms or cools the air above it. The warmed or cooled air then moves to nearby land. So a warm current brings warm air to the land it touches. A cold current brings cool air.
Deep-ocean circulation is governed by gravity and driven by density differences . Density is in turn dependent on the temperature and salinity of the water. Sea water becomes denser with decreased temperature and increased salinity.
Because of this dependence, deep water circulation is sometimes referred to as thermohaline circulation .
Thermohaline circulation is very slow. After leaving the surface of the ocean, waters will not reappear at the surface for an average of 500-2000 years.
A schematic showing the ocean "conveyor belt", where surface waters sink, enter deep water circulation, then resurface after slowly flowing through the deep ocean.
As long as a wave is in deep water it is unaffected by water depth . When a wave approaches the shore the water becomes shallower and influences wave behavior . The wave begins to ‘feel bottom’ at water depth equal to about one-half its wave length.
As the wave continues to advance toward the shore, the slightly faster waves catch up decreasing the wave length. As the speed and length of the wave diminish, the wave steadily grows higher.
Finally a critical point is reached when the steep wave form is unable to support the wave, and it collapses, or breaks. What had been a wave of oscillation now becomes a wave of translation in which the water advances up the shore. The turbulent water created by breaking waves is called surf .
Movement of water particles with the passage of a wave Changes that occur when wave moves onto shore
Since the waves reach the shallow water in front of the headland sooner than they do in adjacent bays, they are bent more nearly parallel to the protruding land and strike it from all three sides.
This wave refraction causes headlands to be eroded and coves to receive deposition. Over a period of time the effect of this process is to straighten irregular coastlines.
Most waves approach a shoreline at an angle . When they reach shallow water of a smoothly sloping bottom, they are bent and tend to become parallel to the shore . Such bending of waves is called refraction.
Wave refraction along an irregular shoreline.
Wave refraction Wave refraction at Sitges, Barcelona The impact of the rock islets at Sitges: refraction has led to beach erosion
The destructive impact of breakers against obstructions is often far greater than generally realized. Cracks and crevices are quickly opened up and extended. Water, often in the form of high – pressure spray, is forcibly driven into every opening, tightly compressing the air already confined within the rocks. As each wave recedes, the compressed air suddenly expands with explosive force, and large blocks as well as small become loosened and are sooner or later blown out bodily, by pressure from the back. The combined activity of bombardment and blasting is most effective as a quarrying process on rocks that are already divided into blocks by jointing and bedding, or otherwise fractured along faults and crush zones.
When rock is exposed at the shore, the first erosional feature to form is a wave-cut notch , caused by the breaking of waves against the rock . As this notch is undercut more deeply into the rock, large pieces of the rock are undermined and may fall off into the ocean. This forms a sea cliff and provides larger fragments to abrade the rock.
Wave cut plat-form and sea cliff Wave cut notch and platform, Old Red Sandstone
As the notch is excavated into the rock, behind it is left the wave-cut platform , which is cut at wave base. The sand and other sediments created by this process may be washed out to sea to the point that the water is deeper than wave base. In this area particles settle out below wave base and accumulate into a wave-built platform.
Wave cut platform
Wave-cut platform, a geological feature caused by the sea's erosion of cliffs, seen atSoutherndown, near Bridgend, South Wales.
By subsequent falling – in of the roof of a sea cave and removal of the debris, long narrow inlets are developed. Tidal inlet of this kind is called ‘geo’. The roof of a cave at the landward end of a geo – or indeed of any sea cave – may communicate with the surface by way of a vertical shaft which may be some distance from the edge of the cliff.
A natural chimney of this kind is known as a blow-hole or gloup .
The Cannon blowhole south of Esha lighthouse is unusual in that in high seas water is blown out sideways from the cliff with a loud report.
When two caves on opposite sides are of a headland unite, a natural arch results, and may persist for a time known as Sea-arch .
Later the arch falls in and the seaward portion of the headland then remains as an isolated stack . Eventually it too will be consumed by the action of the waves.
Minor erosional features along a rocky coast, seen at low tide. Surf hollows out sea cave in more erodible part of bedrock. Cave through headland becomes a sea arch. Surf tears away parts of bedrock, leaving isolated stack as an ‘island’ on wave-cut platform.
Sea arches. Rising sea level is cutting deeply into the sandstone bedrock, etching crevices and arches along weaknesses caused by fractures. The parallel layers of strata and crosscutting fractures are visible. Anse de l'Est, Ile aux Loups, Iles de la Madeleine, Quebec
The outer coast of Shetland includes long sections of spectacular cliff coastline. The form of cliffs is intimately related to rock type and structure, together with the more muted influence of past processes, including glaciation and sea level history.
Sea arch at Hawaii's Volcano National Park, on the Big Island of Hawaii made of igneous rock (basalt), formed by lava from the Kilauea Volcano and the sculpting power of the Pacific Ocean. Sea arch
Apostle Islands sea arch Australia’s Nullarbor plain ends abrutly in limestone cliffs. Rocks pounded today by ocean were laid down on the sea floor 20 million years ago.
The Old Man of Hoy, Orkney isles, Scottland. A stack of Old Red Sandstone, 137mt. High rising from the platform of Devonian lava.
Twelve apostles, Southern Australia, have been carved by thrusting waves and searing winds.
Padestal of calcareous sandstone supported by an undercut pillar of shale on the fore shore at Sheepstone, Yorkshire In the bay of Fundy, Nova Scotia, the tide falls 45ft. Exposing rocks eroded by the water.
Where beach drift and long shore currents are active , several features related to the movement of sediment along the shore may develop.
Elongated ridges of sand that project from the land in the mouth of an adjacent bay.
When waves carrying sand and other sediments are obstructed by currents their velocity is reduced, due to this they deposit the material on the shore. Spit thus tend to migrate landwards, often becoming curved in the process. A spit thus may be developed in to a hook.
The dominant winds and waves are the agents essentially responsible for the curving of spits .
Development of a hooked spit by the refraction of oblique waves Notsuke Hooked Spit, Hokkaido, Japan
Spits Examples of Spit Spurn Head, built by beach drifting in to the estuary of the Humber, in continuation of the Holderness coast, Humberside, England.
Southward drift is very active along the east coast of Norfolk and Suffolk. Ten centuries ago the Yarmouth sands had already spread across the estuary of the Yare, forming an obstruction which deflected the river towards the south . The spit then continued to grow south-wards, hugging the coast as closely as possible, with river confined between itself and the mainland.
By 1347 the end of the spot and the outlet of the river had reached Lowestoft. At Aldeburgh, halfway between Lowestoft and Harwich, the longest spot on the east cost has similarly diverted the outlet of the Alde .
River Yare, Norfolk, England Spits River Alde, Suffolk, England Examples of river deflection in East Anglia by the southerly extension of sand and shingle spits.
The Atlantic and Gulf coasts of the United States are bordered by long stretches of barrier beaches which are separated from the mainland by lagoons or expanses of sea , except where they are locally tied to headlands.
These are known as offshore bars or if they should be discontinuous at both ends, as barrier islands, which are low ridges of sand that parallel the coast.
A bar or barrier beach extends from one headland to another. When the bay inside is completely enclosed, it becomes a marsh, or if it receives streams from the mainland, a shoreline lake.
Barrier island, cross section
Offshore bars and barrier islands Offshore bars and barrier islands with lagoons, along the coast of North Carolina, USA Offshore bars and barrier islands
When waves break against headlands most of their energy dissipates. Therefore the waves inside adjacent bays have lower energy and as a result sediments are deposited in the bays. As headlands erode and bays fill with sediment, an irregular coastline eventually becomes smooth . Development of the Coast-line Development of the Coast-line
Submergent coasts are created when sea level rises or the land adjacent to the sea subsides .
Dry land gets drowned either by land subsidence or a rise in sea level.
Their appearance is controlled by the erosional processes prior to the rise of the sea level .
Submergent coasts Submerging edge of preglacial erosion surface, near New Richmond, Quebec The gently tilted land surface, seen here extending under the sea, along the north coast of Baie des Chaleurs, is an ancient landscape that was created by erosion across the soft rock in the region. This preglacial erosional surface postdates an older erosion surface, which was uplifted, tilted, and eroded, leaving sooth summits of resistant, igneous rock. The juxtaposition of these two ancient landscapes gives Gaspesie a bilevel terrain.
A celebrated coast of submergence: Rio de Janeiro, Barzil Submergent coasts
From a structural point of view, the coasts are classified by Suess as two contrasting types as Atlantic and Pacific.
Coasts of Atlantic or transverse type are determined by fractures and subsidences
that characteristically cut across the strike or ‘grain’ of the folded rock formations ;
they characteristically border relatively young oceans that are widening as a result of sea floor spreading.
Submerged Atlantic or transverse cost. Old red sandstone crops out along anticlines which have remained as uplands or broad ridges that jut out as promontories. Less resistant Carboniferous strata outcrop as synclines in the valleys, which pass seawards into long bays or rias, south-west Eire.
Submerged ‘Pacific’ or longitudinal coast (Dalmatian type), Yugoslavia
Coasts of Pacific or longitudinal type border or lie within mountain chains, including island festoons like those of Asia and follow the general ‘grain’ of the land.
When partially drowned such coasts are said to be of Dalmatian type.