OCEANS, SEAS AND THEIR COASTS
OCEANS SEAS AND THEIR COASTS An ocean is a large and extensive body of saline water occupying a basin between continents while a sea is a large body of saline water on the margins of continents.
Nature of Ocean Water
- Ocean water is salty
- Due to abundant sodium chloride which rivers dissolved from land, from rocks that the water is in contact with and volcanic materials on the ocean floor?
- Ocean water has high salinity in areas where there is addition of little water and high rate of evaporation leading to high salt concentration e.g. Dead Sea and lower where there is low temperatures and addition of fresh water from rivers, rain or snow melts e.g. Baltic Sea.
- Surface water is warmer than that at the bottom except in Polar Regions where a thin layer of cold water may overlie warmer water.
- Ocean water is a habitat for living organisms
Planktons are plants and animals occupying ocean surface.
- a) Phytoplankton are ocean plants e.g. algae.
- b) Zooplankton are ocean animals e.g. lobsters, jelly fish, crabs, etc.
- Nektons are all forms of fish.
- Benthos are ocean creatures which live only at the bottom of margins of continents where sunlight reaches Sea floor e.g. snails, starfish and sea anemones.
- Ocean water is pollutedg. by industrial effluents, pesticides and herbicides carried by rivers and runoff to the sea.
- Ocean topography is composed of several features
- Continental shelf- Relatively flat part of the continent covered by ocean water.
- Continental slope- Steeply dipping surface between continental shelf and the ocean basin proper.
- Abbysal plain- Almost level area of the ocean where sediments are deposited.
- Mid ocean ridges- Range of hills which are submerged formed by volcanic and seismic activities.
- Sea Islands– pieces of land surrounded by water.
- i) Continental islands– Ones rising from continental shelf.
- ii) Oceanic islands–Ones which rise from the sea floor e.g. Canary and Cape Verde.
iii) Coral islands-Ones made of coral.
- Deep sea trenches – narrow steep sided submarine valleys on the ocean floor.
- Guyots– submerged atolls forming an under water mountain.
- Sea mount– a volcano which doesn’t rise above the sea floor.
- A portion of ocean water moves
There are two types of movements namely:
- Movement of ocean water from surface to bottom and vice versa.
How they occur
- Cold polar water sinking before moving horizontally towards equator.
- Ocean currents converge
- When ocean water sinks at lower depths after ocean currents converge.
- When ocean water rises to the surface in a process called upwelling.
Significance of vertical movements
- Carries nutrients for sea animals by upwelling.
- Oxygenation of water vital for fish survival.
It occurs in the following ways:
- Ocean Currents
An ocean current is a large mass of surface ocean water which is moving in a particular direction e.g.
- Mozambique- warm
- Canaries -cold
- Atlantic drift-warm
- Gulf stream drift-warm
Factors that influencing formation of ocean currents
- Wind by blowing over water causing a mass of surface ocean water to move in its direction forming drift currents.
- Rotation of the earth by causing deflection of ocean currents.
- Shape of land mass by influencing current direction and causing it to flow following the coastal outline.
- Differences in temperature by causing cold polar water which is dense due to low temp moves towards the equator passing on the ocean floor and warm water of the tropics to move towards the poles passing on the surface.
- Periodic rise and fall in the level of ocean and other large water bodies.
- Occurs when the moon and to some e the sun exert gravitational pull on the water bodies on the earth.
- Moons gravitational pull is exerted on the earth causing the water on that side A to bulge resulting in high tide 1
- Some water flows from sides C and D to side B to occupy space created by the moons pull resulting in high tide 2 and low tides 1 and 2 at C and D.
Rotation of the Earth
- It brings any longitude under the influence of 2 high and 2low tides in a lunar day.
- Similar tides occur at an interval of 12hrs 26 minutes.
- A lunar day is time taken by the earth to complete one rotation with respect to the moon (24 hrs 52 min)
- Lunar month is time taken by the moon to complete one revolution around the earth (27.3 days)
- The moon is always ahead of the earth by 52 minutes due to its revolution e.g. if Nairobi is opposite the moon at 6pm the following day the high tide will be at 6.52pm.
Tidal range is the difference between the highest level reached by high tide and lowest level reached by low tide.
Types of tides
- Caused by relative positions of the moon and the sun from the earth.
- Sometimes the moon and the earth are nearer or farther from each other due to their elliptical orbits.
- In which the highest and lowest tides occur.
- Occurs when the sun, moon and the earth are in a line (syzygy position) and pulling in the same plane causing pulling force to be greatest.
- In which high tide is lower than normal and low tide is higher than normal.
- Occurs when the sun, moon and earth form a right angle and pulling water to themselves.
- In which tidal range is 20% higher than normal.
- Occur when the moon is nearest to the earth (perigee position) causing pulling force to be greatest.
- In which tidal range is lower than normal.
- Occur when moon is farthest from the earth (apogee position) causing pulling force to be weakest.
- 1H1L in a lunar day
Semi Diurnal Tides
- 2H2L in a lunar day which may rise or drop at the same level.
- Occur in most of Pacific Ocean.
- 2H2L in a lunar day where one pair may fluctuate in level while the other remains constant.
- A wave is a moving ridge of water on the sea.
- It’s formed when wind blows over an open water body causing oscillation of water particles.
Parts of a wave
Crest – the top of a wave.
Trough – the bottom of a wave.
Wavelength – horizontal distance between two successive crests.
Height – difference in height between crest and trough.
- When a wave reaches the shore, the water particles below the surface start touching the ocean floor causing it to break.
- There is forward movement of water to the beach which is called swash/send.
- There is backward movement of water to the sea due to gravity called
- The rest flows at the bottom back into the sea in a water current called undertow.
Types of waves
- a) Constructive Waves
- Waves in which swash is stronger than backwash resulting in deposition.
- b) Destructive Waves
- Waves in which swash is weaker than backwash resulting in erosion.
Processes of Wave Erosion
- Scratching of ocean floor by materials carried by the back wash.
- Hurling of pebbles and rock fragments against the rocks causing some particles to break off.
- Rock fragments dragged up and down by the swash and backwash hitting against each other becoming smaller in size. It provides tools for abrasion and corrosion.
- Removal of materials from the coast by action of the force of moving water.
Direct wave force
- Large amounts of wave water crush against a rock face weakening and eventually breaking of the rock.
Compressed air action
- Waves crush against a rock.
- The force of water pushes air into cracks compressing it and exerting pressure causing them to widen.
- Wave retreats causing trapped air to expand resulting in sudden pressure release causing cracks to expand further.
- The process is repeated several times causing the rocks to shatter.
- Some soluble minerals in rocks dissolve directly in water and are carried away in solution leaving cavities in rocks.
- Some minerals such as limestone reacting with sea water which has dissolved carbonic acid.
Factors influencing wave erosion
- Waves must have strong backwash and a weak swash
- Slope –The coast that slopes steeply into the sea favours erosion.
- Load-large amount provides more abrasive tools. Angular shaped load is more effective in abrasion.
- Amount of water in a wave – the larger the amount the greater the hydraulic force.
Features Resulting From Wave Erosion
- a) Cliff and Wave Cut Platform
Cliff – A steep rock face which borders the sea.
Wave Cut Platform – A fairly flat part of the shore formed when a cliff retreats inland.
- Breaking waves erode rock surface of a steep coast cutting a notch.
- Erosion continues causing the base of the rock to be undercut resulting into an overhanging rock.
- Undercutting continues causing the overhanging rock to eventually collapse forming a cliff.
- The process is repeated and a fairly flat part of the shore is formed between the new and the former cliff.
Bays and Headlands
Bay – Piece of sea water jutting into the land or a curved inlet of sea.
Headland – a piece of land jutting into the sea.
- At first there is a coast with hard and soft rocks.
- Soft rocks are eroded more by wave action to form sea inlets called bays.
- Resistant rocks called headlands are left sticking into the sea. A big bay is called a gulf.
- c) Caves, Blow Hole and Geos
Cave – Natural cylindrical tunnel like chamber extending into the cliff or into the side of a headland.
- A small hollow form on a weak area of the cliff after limestone is acted upon by carbonation.
- Corrosion and direct dissolving act on the hollow extending it into the cliff forming a cave.
Blow Hole/ Gloup – Vertical hole formed on the side of cliff bordering the land.
- Formed when a cave reaches the surface some distance inland as a vertical pit.
It’s called a blow hole because when the waves break water is forced out of the hole.
Geos – Narrow sea inlet formed when the roof of a cave between the blow hole and the sea collapses.
- d) Natural Arch, stack and stump
Natural arch – Opening from one side of a headland to the other.
- Formed when a cave extends into the head land to the other side.
- Or when caves which have developed on both sides of headland join each other.
Stack – Pillar of rock left standing on the seaward side.
- Formed when continuous wave erosion causes the roof of the arch to collapse.
Stump – The base of stack left when it collapses as a result of erosion at the base.
Types of load moved by waves are such as shingle, sand, mud and other objects dumped into the sea.
How the sea acquires its load
- Materials brought by rivers and wind.
- ProductsÞ@^ú rivweathering.
- Materials brought by rivers and wind.
- Debris from volcanic eruptions in the sea or on land bordering the sea.
Waves transport load by a process called long shore drift. Long shore drift is progressive dragging of materials along the beach as a result of waves breaking at an angle.
- Waves break at an angle.
- Swash pushes materials up the beach at an angle.
- Backwash brings them back at right angle to the edge of water.
- Process is repeated causing materials to be progressively dragged along the beach.
Factors Influencing Wave Transportation
- Strength of waves
Strong waves carry large quantities of load over a long distance while weak waves carry small quantities of load over a short distance.
Tides cause waves to break farther inland causing materials that were not in contact with breaking waves to be moved about.
- Ocean currents
Ocean currents cause movement of materials from one part of the ocean to another e.g. coconut fruits from southern part of Africa to Gulf of Guinea by Benguela current.
- Gradient of the shore
On gentle coasts transportation of materials is favoured by long shore drift while on a steep coast they bounce off cliffs and remain floating.
- Orientation of coast line.
Transportation by long shore drift is favoured where coast is aligned obliquely to the direction of breaking waves while on transversely aligned coast swash moves materials back and fourth along the same line.
- Nature of the load.
Lighter materials such as sand are carried over long distances while heavy load is transported over a short distance.
- Process in which materials transported by waves are laid down on the shore.
Factors Influencing Wave Deposition
Deposition occurs in selective manner:
- Boulders are deposited at farthest end of land because they are swept towards the land by powerful swash during high tide followed by pebbles.
- Then sand and finally mud because the weak backwash brings them back towards the sea as they are light.
Waves must have a strong swash and a weak backwash in order to cause excess load to be left behind on the shore.
- gradient of the shore
The coast must be sloping to reduce the velocity and hence the energy of waves so that depositing occurs.
- Depth of Water
Deposition takes place where water is shallow for waves to come into contact with ocean floor and break the cyclic motion of water.
Features Resulting From Wave Deposition
- a) Beaches
- Gently sloping mass of accumulated materials such as sand, shingle and pebbles along the coast.
- Formed by constructive waves during a relatively calm weather when backwash is weakest resulting in materials accumulating at the shore.
During storms destructive waves destroy beaches creating other minor features such as: i) Beach cusps
- Horn like projections of sand and gravel which gives the coast line a series of curves.
- Waves break at right angles.
- Powerful swash in form of eddies scour depressions moving coarse materials to either side forming head like projections called cusps leaving finer materials forming bay like inlets.
- ii) Beach Ridges and Beach Berms
Beach Ridges – Low ridges of coarse sand, boulders and shingle deposited roughly parallel to the shore formed by waves approaching the coast at right angles.
iii) Beach Berms
- Narrow terrace of shingle thrown up the beach by storm waves formed where tidal range is high.
- iv) Beach Rock Shells
Masses of sand, shells and pebbles cemented together by calcium carbonate forming projections above the beach.
- b) Spits
- Low lying ridge of sand, shingle and pebbles with one end attached to the coast and the other projecting to the sea.
- Movement of materials by long shore drift is halted causing deposition due to coast changing its direction towards the land e.g. across estuary or entrance of a bay.
- The process continues and the accumulation grows towards the sea.
- Waves carry sand to the inner end of the spit creating a hook like feature e.g. at the mouth of R. Senegal.
- c) Tombolo
- Spit that grows out from the coast into the sea and joins an island e.g. Ras Hafur in Somalia and Ngomeni on Kenyan coast.
- d) Bars
- Ridge of sand, shingles and mud which lies almost parallel to the coast.
- i) Bay bar – Bar which forms across the entrance of a bay.
Offshore bar – Bar which forms off a very shallow coast line.
- Wave is forced to break off shore due to water shallowness.
- They throw up a ridge of materials off the coast.
- Between the bar and the coast there will be a shallow lagoon colonised by marsh plants.
- e) Cuspate foreland
- Broad triangular shaped deposits of sand or shingle projecting from the mainland into the sea.
- Two spits grow towards each other at an angle.
- A triangular feature called cuspate barrier develops which encloses a lagoon.
- The lagoon is filled with mud and sand to form the foreland.
- Vegetation grows on the marsh and with time dries up e.g. ‘A’ Laree in Malagasy.
- f) Dune Belts
- Belt of low lying mounds of sand found on extreme landward side of the beach above the high tide level.
- Sand on the beach dries up during the high tide.
- It’s picked by onshore winds and deposited at a distance away from the reach of breaking waves.
- It collects behind obstacles like grass or other vegetation and gradually builds up forming a dune.
- The dunes may be covered with vegetation to form marshes.
- g) Mud Flats and Salt Marshes
Mudflats – Platform of mud consisting of fine silt and alluvium deposited in sea inlets such as bays and river estuaries.
Salt marshes – Vegetation such as grasses and mangrove that grows on a mudflat
- Fine silt and river alluvium are deposited in sea inlets by tides.
- A platform of mud builds up and is colonised by vegetation forming a swamp called salt marsh.
- The dense network of plants roots trap more mud and alluvium causing the mudflats to extend seawards.
Factors Determining the Type of Coasts
- Wave action
Wave erosion makes a coast to have erosion features while deposition causes depositional features.
- Tidal currents
Where tidal range is high more surface area is exposed to wave action.
- Nature of rocks
Weak rocks are eroded to form bays (inlets) while resistant ones are left standing to form headlands.
- Alignment of coast
There is more erosion on exposed coasts while deposition occurs where the coast is obliquely aligned to the breaking waves.
- Change in sea level
Fall in sea level leads to emergence and rise to submergence.
Types of Coasts
According To the Alignment of Coast
- discordant/transverse/irregular coast
- Coast which lies transversely to the coast line.
- Has a large number of inlets and receives heavy rainfall because winds blow onshore e.g. Mombasa.
- Concordant coasts/regular/longitudinal coasts
- One which lies almost parallel to the coastline.
- Almost straight and lacks inlets and receives little rainfall due to winds blowing offshore e.g. Lamu.
According To Features Present
- Submerged Coasts
- Coasts where a part of coastal land lies under the sea.
Causes of submergence
- a) Rise in sea level e.g. when large quantities of melt water were released to the sea causing its level to rise due to climate change at the end of ice age.
- Sinking of coastal land and a part of the sea floor.
- a) Submerged Highland Coasts
- Found where submergence occurs on a coast characterised by steep slopes.
- Characterised by drowned features.
- i) Ria Coast
A Ria is a drowned river mouth.
- Funnel shaped
- U-shaped in cross section.
- Deeper and wider on the seaward side and shallower and narrower inland e.g. the Kenyan coast at Kilindini and Mtwapa.
- ii) Fiord/Fjord coast
A fiord is a submerged glaciated valley.
- Ice deepens and widens glacial valleys until the floor lies below the sea level.
- When the ice retreats sea water flows in drowning the valley forming inlets called fiords.
- Deeper inland.
- Shallower at the sea ward end due to terminal moraine deposited when glacier was melting.
iii) Longitudinal/Dalmatian Coasts
- Coast where ridges and valleys lying parallel to the coast line are drowned.
- Valleys form inlets called sounds while ridges form islands.
- b) Submerged Lowland Coasts
- Found where submergence occurs on a coast characterised by gentle slopes.
- i) Estuarine Coast
- Coast characterised by broad shallow estuaries and mud flats which are visible at low tide.
- Wider and shallower than rias e.g. coastlines of Guinea and Senegal.
- ii) Fjard Coast
- Coast characterised by numerous inlets formed by submergence of glaciated rocky lowland coasts.
- Have numerous islands and are deeper than rias e.g. S.E. coast of Sweden.
- Emerged Coasts
- Coast where part of seafloor has become permanently exposed.
Causes of Emergence
- Decrease in sea level due to decline in the source of water e.g. waters being held up in a glacier instead of it flowing back as rivers to the ocean.
- Uplift of the coastal land by faulting, folding or isostatic adjustment.
- i) Emerged Highland Coasts
- Found where emergence occurs on a coast characterised by steep slopes.
- Characterised wave action features which are isolated on land e.g. raised beaches, raised cliffs, raised wave cut platforms and raised arches.
- ii) Emerged Lowland Coasts
- Found where emergence occurs on a coast characterised by gentle slopes.
- Characterised by exposed depositional features e.g. spits and offshore bars which are found on land and a coastal plain formed as a result of a part of continental shelf becoming exposed.
- Coral Coasts
- Coasts composed of coral rocks which are exoskeletons of marine organisms called coral polyps.
- They live in colonies/groups, feed on plankton and extract lime from the sea and build shells for protection.
Conditions Necessary for Coral Growth
- warm water(25-29◦C)
- Saline and clear water.
- Sunlight should penetrate at least to a depth of 50m to allow plankton growth.
- Plentiful supply of plankton which they feed on.
- Shallow water.
Types of coral reefs
- i) Fringing Reefs
- Platform of coral formed when coral polyps start building a reef near the shore.
- Flat or concave shaped
- Higher on the seaward side
- Outer edge falls steeply into the sea
- ii) Barrier Reefs
- Platform of coral formed a long distance from the shore.
- Formed when polyps start to grow and extend seawards where conditions are favourable.
- Its coral is joined to the shore.
- Its outer edge falls steeply into the sea.
iii) Atoll Reef
- Coral ring formed around a submerged island.
- Circular in shape.
- Encloses a fairly deep lagoon.
Theories of Formation
- Fringing reef develops around an island.
- The island starts to sink.
- Coral continues to grow upwards to keep pace with rising sea level and seawards because there is more food and water is clear.
- The reef extends great distance away from the land to become barrier reef.
- The island continues to sink becoming completely submerged.
- The barrier reef forms a ring of coral called atolls.
- Fringing reef grows on a submarine hill.
- It disintegrates due to wave attack.
- Coral fragments accumulate on the seaward end.
- Polyps start building on it upwards where there is more food and clear water to form barrier reef.
- The barrier reef forms a ring of coral called atolls.
- During ice age there was withdrawal of water causing global fall in sea level.
- Coral growth was retarded by low temperatures.
- Waves pounded coral reefs and islands and flattened them to the same level as the sea.
- At the end of ice age temperatures began to rise again favouring the growth of coral once again.
- More water was added to oceans causing polyps to continue to grow upwards to keep pace with the rising sea level.
- They were permanently exposed on the surface to form atolls
Significance of Oceans, Coasts and Coastal Features
- Influence climate by contributing the bulk of precipitation, warming effect in cool season and cooling effect in hot season by breezes and ocean currents.
- Used for transport by means of boats, steamers and ferries.
- Tourist attraction by being site for recreation e.g. swimming and sport fishing and marine life in parks such as in Mombasa and Lamu.
- Oceans are a source of fish which is a source of food, income and employment.
- Source of fresh water when its water is distilled.
- Tropical seas have mangrove forests with mangrove trees which provide strong building and fencing posts and tannin for tanning leather and also habitat for marine life which is a tourist attraction.
- Source of salt which crystallizes naturally after water evaporates in constricted bays in hot climates.
- Tsunamis from oceans flood the neighbouring coastal areas causing great loss of life and property.
- Oceans may also flood the neighbouring coastal areas as a result of rise in sea level caused by melting of glaciers caused by global warming.
- Also harbour dangerous animals such as sharks and crocodiles which may attack and hurt or kill people.
- Drowning accidents when there is breakdown of vessels or ship wrecking.
Coasts and Coastal Features
- Fiords, rias and lagoons favour development of deep and well sheltered harbours.
- Fiords are also a good breeding ground for fish as their shallow continental shelf encourage growth of plankton which is food for fish.
- Coral rocks are used locally as building materials.
- Coral rocks are also a source of coral limestone for cement manufacture.
- Features such as coral reefs, caves, cliffs and fiords are a tourist attraction.
- Some emerged coastal lands have infertile soils unsuitable for agriculture for being covered by sand, gravel and bare rock.
- Deposited sands, bars and coral reefs are barrier to transport as they can cause ship wrecking if vessels hit them.