Works Of The Sea And Development Of The Related Landforms For PresentationPresentation Transcript
Work of the sea and development of the related landform Parin Shah LA 8808
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
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.
Tides are periodic changes in the elevation of the ocean surface at a specific location.
Tides are affected by
The Earth’s rotation
The great continental obstructions met with during their circuit of the globe.
Friction against the sea floor , especially in the shallow seas.
When the sun and moon are at right angles relative to the Earth , the moon produces high tides where the sun produces low. The tides are then less high and low than usual and are called neap tides .
When the Earth, moon and sun fall along the same straight line , the tide raising forces of sun and moon help each other, and tides of maximum, known as spring tides , result.
Relationship of the moon and sun to the earth during Spring tides Relationship of the moon and sun to the earth during Neap tides
Waves Wave Measurements
Oceanic waves are confined to the surface at the air-water interface, and particles of water undergo a circular displacement.
Characteristics of a wave
Wave length - the horizontal distance from crest to crest – or from trough to trough.
Wave height - the vertical distance from the trough to the crest of a wave.
Wave base - the maximum depth to which wave motion extends, generally 1/2 of the wave length distance.
Wave period - the time interval between the passage of successive crests at a stationary point.
Larger waves are created when there is a longer distance (fetch) over which the wind can blow, and when the winds are stronger . Typically the largest waves are formed by hurricanes or other storms .
Tsunamis are formed by large movements under the ocean, frequently earthquakes, but also underwater "land slides " ; they are typically much larger than other types of waves.
The waves seen on the sea are almost entirely due to the sweeping of winds over the surface of the water.
A wave is the transmission of energy through a medium : it is not the transmission of material .
Therefore, when a wave passes, materials are temporarily displaced, but they soon return to their original position . Such waves are known as waves of oscillation .
Image showing example of waves of oscillation Formation of a Tsunami wave
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
Beach Drift and long-shore currents
Although waves are refracted, most still reach the shore at an angle. Consequently, the up-brush of water from each breaking wave is oblique.
The effect of this pattern of water movement is to transport particles of sediment in a zigzag pattern along the beach, this movement is called beach drift .
Beach drift, can transport sand and pebbles hundreds or even thousands of metres each day.
Oblique waves also produce currents with the surf zone that flow parallel to the shore , this currents are known as long-shore currents .
Beach drift, caused by the upbrush of water from oblique waves.
The sea operates as an agent of erosion in four different ways:
By hydraulic action – picking up of loose material by currents and waves,
will transfer the materials through waves.
By corrasion – When sediments are driven shoreward by breaking waves.
unconsolidated sediments can simply be washed away,
but the entrained particles can also abrade solid bedrock over time .
By attrition – the fragments or tools are themselves worn down by impact and friction .
By corrosion – solvent and chemical action , on limestone and rocks with a calcareous cement.
Wave cut platforms and cliffs
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.
Welcombe Mouth in North Devon
Blow-hole or gloup
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.
Sea-arch and stack
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.
Example of Stacks
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.
Determinants of wave erosion
There are three things that determine the amount of erosion:
Degree of exposure . Coastline that are fully exposed suffer the most erosion.
Tidal range . The smallest tidal range concentrates more wave energy and erodes more.
Composition of bedrock . Sandstone and shale are more easily eroded than igneous rocks.
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.
Spits Hooked spit, New Zealand
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.
A bar connecting an island to the mainland or to another island is called a tombolo .
The offshore hill or stack bends the incoming waves around it so that their energy sweeps sand onto the tombolo from both sides.
Once the stack erodes down the waterline, the tombolo will disappear. Stacks don’t last long, and that’s why tombolos are uncommon.
Tombolo Stockton Island Ashland County
Example of Tombolo Tombolo
A sand bar that completely crosses a bay, sealing it off from the open ocean.
Such a feature tends to form across bays where currents are weak, allowing a spit to extend to the other side.
Baymouth barrier, Grand Beach, Lake Winnipeg, Manitoba
Baymouth bar A spit north of Gdansk and two smoothly curved bay-mouth bars enclosing broad lagoons along the south-east Baltic coast of Poland and Lithuania.
Offshore bars and barrier islands
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
Emergent coasts develop either because of an area uplifted or as a result of a drop in sea level .
Land formed under water that becomes dry land, characterized by the presence of extensive elevated wave-cut platforms, terraces or platforms and wave cut cliffs.
Emergent coasts Classification of coast An emerging coast due to post-glacial isostatic uplift, Finland
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.