◄UNIT 3:EXTERNAL LANDFORM PROCESSES AND RELATED FEATURESUNIT 5 : ROCKS AND MINERALS►

UNIT 4 : WAVE EROSION AND DEPOSITION

  • UNIT 4 : WAVE EROSION AND DEPOSITION

     Key Unit competence
     By the end of this unit, I should be able to categorise different features 
    resulting from the wave action and their relationships with the human 

    activities

     Introductory activity

    Use the pictures provided below and answer the following questions: 

    1) Identify the coastal landforms found on figure above. 
    2) Explain the factors for formation of the coastal landforms identified 

    on the figure.

     4.1. Coastal landforms: Definition of key terms and types of waves
     1. Make a research and show the difference between the following 
    terms related to coastal landforms:
     a. Coast
     b. Shore
     c. Wave
     d. Longshore drift

    2.  Mention the type of waves 
    Learning activity  4.1
     4.1.1. Definition of key terms
     The following are definitions of some terms related with coastal landforms: 
    • Coast: A coast refers to the land that borders the sea or the ocean.  
    It is a narrow zone where the land and the sea overlap and directly 
    interact. Some coasts are made up of broad sandy beaches, while 
    others form rocky cliffs or low-lying wetlands. The shape of the coastline 
    is determined by factors such as the types of rocks present, the forces 

    of erosion, and the changes in sea level. 

     • Shore: This is the area where land meets the sea or ocean. Different 
    features are found in this area resulting from erosion and deposition of 
    sediments, ocean or sea waves, as well as the effects of rivers as they 
    join the sea. It is also called coastline.

    • Waves:     Waves are defined as undulations of sea/lake water 
    characterized by well-developed crests and troughs .Waves are created 
    by the transfer of energy from the wind blowing over the surface of 
    the sea or from submarine shock waves by earthquakes or volcanic 
    activities (e.g. Tsunami).
    • When waves appear with high frequency they demonstrate the short 

    wavelengths. 

    Structure of wave
     Longshore drift, often used interchangeably with beach drifting, is a general 
    term for sediment transport parallel to shore in the nearshore zone due to 
    incomplete wave refraction. In this process sediments transported by the river 
    systems are moved by ocean waves and ocean currents to form beaches. 

    Beach drifting,     also called littoral drifting, is a process in which waves breaking 
    at an angle to the shoreline move sediment along the beach in a zigzag fashion 

    in the swash zone.  Both processes are illustrated on figure below.

     A wave approaching a straight coastline at a large angle will have velocity 
    progressively decreasing. This will cause the wave to swing around, but it may 
    not have enough time to conform fully to the shape of the shoreline before 

    breaking, leading to littoral drifting.

     4.1.2. Types of waves
     There are two main types of waves: constructive waves and destructive waves.
     1. Constructive waves: These are waves whose swash is more powerful 
    than backwash. They are depositional in nature.
     2. Destructive waves: These are waves whose backwash is more powerful 
    than swash. They are erosional in nature. 

    Other types of waves
    – Breaking waves: Formed when the wave collapses on top of itself. 
    There are four types of breaking waves: spilling, plunging, collapsing, and surging.
     Spilling Waves 
    Spilling waves are waves that are produced when the ocean floor has a gentle 
    slope. As the wave approaches the shore, it slowly releases energy, and the 
    crest gradually spills forward down its face until it is all whitewater. These waves 
    take more time to break than any other wave. Surfers usually call these waves, 
    “mushy waves.”

    Plunging Waves
     Plunging waves are formed when the incoming swell hits a steep ocean floor or a 
    sea bottom with sudden depth changes. As a result, the wave’s crest curls over 
    and explodes on the trough. The air under the lip of the wave is compressed, 
    and a crashing sound is often heard. Plungers are more common in offshore 
    wind conditions.
     
    Surging Waves 
    Surging waves are produced when long period swells arrive at coastlines with 
    steep beach profiles. The base of the wave moves fast and does not allow the 
    crest to evolve. As a result, the wave almost doesn’t break, and there is little 
    whitewater. Surging waves look friendly, but can be quite deadly because of the 
    backwash associated with them.

    Collapsing Waves
     Collapsing waves are a blend between surging and plunging waves. The crest 
    never completely breaks, and the bottom face of the wave gets vertical and 
    collapses, resulting in whitewater.
    – Deep water waves/Swell waves: Are made up of a number of waves 
    of different lengths superimposed on each other. They are straight and 
    long, powerful, and travel great distance.
    – Inshore waves: These waves drain the beach as a backwash.
    – Internal waves: Formed due to the disturbances found between two 
    water masses of different density. They are high and become turbulent 
    currents when they hit a landmass.
    – Kelvin waves: Formed due to lack of winds in the Pacific Ocean. They 
    are high and wide waves, warmer than the surrounding water.
    – Progressive waves: Move with a steady speed, so they are called 
    Progressive Waves. They are of two types:
    – Capillary waves: Formed when wind creates pressure over capillarity, 
    the binding force that holds the water molecules of the ocean surface 
    together.
    – Orbital progressive waves: Formed at the boundary of two liquids 
    with different density.
    – Refracted waves: Travel in shallow water when they approach the 
    shore. The shallowness decreases the power of the wave and causes 
    a curve. These are usually seen near headlands and bays.
    – Seiche waves: Caused due to the movement within a confined space. 
    These have long wavelengths and rarely result in any damage as their 
    height is generally short.
    – Shallow water waves: Move in shallow waters. They are of two kinds:
     • Tidal waves: Formed due to the gravitational pull of the sun and 
    the moon on the ocean.
     • Seismic Sea Waves/tsunami: Caused due to earthquakes 
    beneath the ocean. They travel extremely fast in open water, have 
    significant height in shallow water, and are very dangerous and 
    devastating.
    – Swell waves/Surging waves    : Intense waves generating from the 
    center of a storm where the winds are strong. These expel little energy, 

    travel long distance, and break on distant shores.

     Application activity 4.1
     1. Differentiate a constructive wave from a destructive wave.
     2. If you find an occasion to visit the ocean coast as an East Africa 
    person, describe the coastal features you would be interested to 

    discover and explain why.

     4.2. Factors determining the strength of waves and wave 
    action processes
     Learning activity  4.2
     1. Analyse the factors that determine the strength of waves on the coast

     2. Explain how waves can cause erosion along the coast.

     4.2.1. Factors determining the strength of waves
     The following are the major factors determining the strength of waves.
    – Wind strength: Wind must be moving faster than the wave crests for 
    energy transfer to continue; 
    – Wind duration: Winds that blow for a short time will not generate 
    large waves;
    – Fetch: The uninterrupted distance over which the wind blows without 
    changing direction;
    –     Depth of water or roughness of sea bed: As waves enter shallow 
    water, their speed, wavelength and height increase. Therefore waves 
    tend to break in shallow water, for example over a bar at the entrance 
    to a harbor; 
    – Direction and speed of tide: If the tide direction is against the wind, 

    this will also increase wave height and decrease wavelength.

     4.2.2. Wave action processes
     The wave action includes erosion, transportation and deposition. 
    • Erosion: Several mechanical and chemical effects produce erosion of 
    rocky shorelines by waves. Depending on the geology of the coastline, 
    nature of wave attack, and long-term changes in sea-level as well as 
    tidal ranges, erosional landforms such as wave-cut, sea cliffs, and even 
    unusual landforms such as caves, sea arches, and sea stacks can form.

    They erode in four ways as:
    1) Solution: it is also called corrosion. It is common on coasts composed 
    of soluble rocks such as limestone and rock salt.
    2) Corrosion or abrasion: this is a type of wave erosion in which the load 
    already weathered down and hence being transported drag itself on the 
    bed of the coast and hence wears away some rock particles.
    3) Attrition: this is a process of wave erosion which involves the reduction 
    in size of eroded particles by themselves.
    4) Hydraulic action: this is the direction of breaking waves that push water 
    on a cliff. As this water retreats during a backwash, pressure is suddenly 
    released and this generates shock waves that weaken rock particles and 
    make them easily eroded by a backwash. 
    • Transportation: Waves are excellent at transporting sand and small 
    rock fragments. These, in turn, are very good at rubbing and grinding 
    surfaces below and just above water level in a process known as 
    abrasion. Longshore drift, longshore currents, and tidal currents in 
    combination determine the net direction of sediment transport and 
    areas of deposition.
    • Deposition: Sediments transported by the waves along the shore 
    are deposited in areas of low wave energy and produce a variety of 
    landforms, including spits, tombolo, beaches, bars and barrier 
    islands. Different types of pediments are deposited along a coast, 
    sometimes in the form of an accumulation of unconsolidated materials 

    such silt, sand and shingle.

     Application activity 4.2
     1) Wave erosion is done in four ways, differentiate them

     2) Explain the impact of wind and tides on the strength of the waves.

    4.3. Factors for Formation of coastal landforms and 
    landforms produced by wave and their importance
     Learning activity  5.3
     In section 4.2, we have defined key terms related to coastal landforms. 

    Observe carefully the following figure and answer the following questions:

     1) Identify the landforms produced by wave erosion on the figure above.

     2) Explain the factors that result in the formation of coastal landforms.

     4.3.1. Factors influencing the formation of coastal landforms
     The following are the major factors influencing the formation of coastal landform:
    – Tides: Tides are greatly influencing forces of coastal landforms. 
    They are commonly semi-diurnal (12-hour cycle). The rise and fall of 
    water levels produce oscillating currents known as tidal streams. Tidal 
    currents can transport large quantities of sediments, especially at the 
    mouths of estuaries. The tidal amplitude also determines the sediments 
    deposition or erosion and keeps redefining the shoreline of coastal 
    landforms.
    – Nature of rocks at the coast: Wave erosion is more pronounced on 
    areas that are weak and soluble e.g. jointed and consolidated rocks. 
    Rocks which are strong and highly consolidated are hard to erode. The 
    hard and resistant rocks stand as headlands while easily eroded rocks 
    become bays.
    – Openness of the shore to wave attack: Coasts which are totally 
    exposed to wave attack are easily undermined by wave attack while 
    those which are sheltered by coastal reefs and islands are protected 
    from direct wave attack and are hence less eroded.
    – Waves: Waves contribute to the erosion of shore. The greater the wave 
    action, the higher is the erosion and sediment movement. Where the 
    shoreline is long and flatter, the wave energy gets dispersed. Wherever 
    there are rock formations, cliffs and short shore area, the wave energy 
    is high. Strong waves can pick up sediments from deeper waters and 
    make them available for transportation by the coastal currents. The 
    larger the wave, the larger the particle it can move. Storm waves can 
    even move boulders. Even small waves can lift the sediments and 
    deposit along the coastal shoreline.
    – Abundance and size of loads which is used as an abrasive 
    tool:     When materials e.g.; boulders, sands, etc. are in abundance, the 
    coast line will be easily eroded through corrosion. In the absence of 
    such materials, wave erosion becomes meager.
    – Longshore currents: Parallel movement of water is known as 
    longshore current and it extends up to the zone of breaking waves 
    from the coastal shoreline. As the long shore currents are formed by 
    refracting waves, the direction of flow will depend upon the angle of 
    the wave which in turn depends upon the wind directions. If the wind 
    direction is balanced, the sediment movement is also balanced. If the 
    wind movement and resultant wave action dominate in one direction 
    great volumes of sediment may be moved in one direction.
    – Weather elements: The elements of climate, such as wind, rainfall and 
    temperature play an important role in formation of coastal landforms. 
    Winds are directly related to the intensity of waves. Landforms like 
    coastal dunes are created by wind action. Temperature is required for 
    physical weathering of sediments. Rainfalls provide runoff for producing 
    and transporting sediments from land to seashore.
    – Gravity: Gravity is an important factor for the development of coastal 
    landforms. Gravity is indirectly involved in the movement of wind and 
    waves as well as in downward movement of sediments. 
    – Nature of coastal rocks: Soft rocks are easily eroded hence forming 
    erosional features like bays while hard or resistant rocks lead to the 

    formation of headlands.

    4.3.2. Landforms produced by wave erosion (destructive wave)
     The coastal features formed due to marine erosion by sea waves and other 
    currents and   solution processes include cliffs, caves, geo, stacks, blowhole, 
    arch, wave-cut platforms.
    – Cliffs    : A cliff is a steep rocky coast rising almost vertically above 
    sea water. Cliffs are very precipitous with overhanging crest. The 
    steepness of vertical cliffs depends on the following: lithology of the 
    area, geological structure, weathering, erosion of cliff faces and marine 
    erosion of cliff base.
    – Wave-cut platform:     Rock-cut flat surfaces in front of cliffs are called 
    wave-cut platforms or simply shore platforms. They are slightly concave 
    upward. The origin and development of wave-cut platforms is related to 
    cliff recession. The plat-form is composed of bare rock or it may contain 

    a temporary deposit or rock debris, pebbles or sand.

    – Sea caves: A sea cave is a natural cavity or chamber which develops 
    along the coast due to gradual erosion of weak and strongly jointed 
    rocks by up rushing breaker waves (surf currents). Sea caves are more 
    frequently formed in carbonate rocks (limestone and chalks) because 
    they are eroded more by solution processes. However, sea caves are 
    not permanent as they are destroyed with time.

    – Headland:     Is a projection of land into the sea or lake. Where alternate 
    hard and soft rocks occur at the coast, the weak material is eroded to 
    form a bay while the harder rock resists erosion and remains extending 
    out into the water as a headland.

    – Blowhole:     This is a vertical shaft linking the cave to the surface. It is 
    formed when wave action attacks the back part of the roof of the cave. 
    At the same time, weathering by solution acts on the line of weakness 

    from the surface downwards to form a blowhole

    – Geo: Wave erosion may continue on the roof of the cave along the 
    blowhole. Hence, the roof of the cave may collapse to form a long and 

    narrow sea inlet known as Geo.

    – Stack/ Column/Pillar: A stack is an isolated rock monolith or pillar 
    rising steeply from the sea. It is a former part of the adjoining land that 
    has become isolated from it by wave erosion, probably after having 

    formed part of a marine arch.

    – Sea arch:  A sea arch is a natural opening through a mass of rock 
    limestone or boulder clay. It is most commonly seen on the sea coast 
    where waves have cut through a promontory. When the keystone of the 

    marine arch collapses, the feature will become a stack.

     4.3.3. Landforms produced by wave deposition (constructive wave) 

    Sediments transported along the shore are deposited in areas of low wave energy. 
    They produce a variety of landforms, including spits, tombolo, beaches, 
    bars and barrier islands. Different types of pediments are deposited along 
    a coast, sometimes in the form of an accumulation of unconsolidated materials 
    such as silt, sand and shingle.
     • Spits: A spit is an embankment composed of sand and shingle attached 
    to the land on one end and projecting seaward. It may form parallel to 
    the coast and stretch several kilometers. It may also grow at an angle 
    across an estuary. Spits are formed when materials are transported and 
    deposited by the long shore drift, mostly where the orientation of the 

    coast changes.

    •

    Tombolo: It is a spit which grows seawards from the coast and 

    joints to an island.

     • Beaches: A beach is located on a wave-cut platform of solid rock and 
    is generally of a low gradient with a gently concave platform. Beaches 
    may extend for hundreds of kilometers. Beaches are classified into: 

    sand beach, shingle beach, and boulder beach.

    Different elements of a beach
     • Bar: A bar is an elongated deposit of sand, shingle or mud occurring 
    in the sea. It is more or less parallel to the shoreline and sometimes 
    linked to it. Bars may be of submerged or emergent embankments of 
    sand and gravel built along the shore by waves and currents. One of 

    the most common types of bars is the spit.

     • Barrier Islands: Barrier Islands are long, offshore islands of sediments 
    tending parallel to the shore. They form long shorelines adjacent to 
    gently sloping coastal plains, and they are typically separated from the 
    mainland by a lagoon. Most barrier islands are cut by one or more tidal 

    waves.

     • Cuspate foreland: This is a large triangular-shaped deposit of sand, 
    mud and shingles projecting seaward. It is a rare feature formed when 
    two adjacent spits growing towards each other at an angle join and 

    enclose a shallow lagoon.

     • Mud flats: These are platforms of mud, silt and river alluvium kept by 
    salt-tolerant plants to form a swamp or marshland. They are formed 
    when tides deposit fine silts along gently sloping coats in bays and 

    estuaries.

     • Coastal dunes: These are low-lying mounds of fine sand, deposited 
    further inland from a wide beach by strong onshore winds. They are 

    common in arid and semi-arid coasts.

    Application activity 4.3
     1) Describe landforms produced by wave deposition.
     2) Explain the factors influencing the formation of coast landforms.
     3) According to you, which landforms are likely to be found around lakes 

    in Rwanda ?

     4.4. Importance of coast landforms produced by wave 
    action and type of coasts
     Learning activity  4.4

     Study the following photograph and answer related questions:

     1. Describe the types of coasts.

     2. Describe the economic activities that can be carried out in this area

    4.4.1. Importance of coast landforms produced by wave
     Coastal landforms produced by wave action are very important in different ways 
    as follows:
    – Many of the world’s major cities are located in coastal areas, and a 
    large portion of economic activities, are concentrated in these cities.
    – There are different activities that take place in coastal zones including 
    coastal fisheries, aquaculture, industry, and shipping.
    – Many of coastal landforms are very favourable for tourism that contributes 
    to the economic development of countries.
    – Marine, estuary and coastal wetland areas often benefit from flows of 
    nutrients from the land and also from ocean upwelling which brings 
    nutrient-rich water to the surface. They thus tend to have particularly 
    high biological productivity.
    – The world’s fish production is dependent on the nature of coastal 
    landforms like bays and headlands.
    – Beaches support leisure, recreation, trade and mining of sand
    – Mud flats and sand dunes have fine silt which attracts mangrove 
    swamps used in crafts industry.
    – Features produced are important in agriculture development
    – Cliffs protect the land from wave attack.
    – These landform features are used in study purposes.

    – Cliffs may produce waterfalls important in generation of power. 

    4.4.2. Types of coasts
     There are two types of coasts: Submerged coasts and Emerged coasts.
     i) Submerged coasts
     Submerged coasts fall into two categories:  Submerged upland coasts and 
    submerged lowland coasts.
     
    A. Submerged upland coasts
     When the margin of an irregular upland area is submerged, a more or less 
    indented coastline is produced. It appears with islands and peninsulas 
    representing the former uplands, and with inlets indicating the former valleys. 
    The following are the three types of submerged coasts: 
    1) Ria coasts: Ria is a Spanish term widely used to describe a submerged 
    coastal valley or estuary resulting from a rise of the sea level. In the case 
    of a Ria coast, hills and river valleys meet the coastline at right angles. 
    The rias are characterized by funnel-shaped which decreases width 
    and depth as they run inland. The head of a stream which is small is 

    responsible for the formation of the valley at the inlet.

     2) Fiord (Fjord) coasts: A long, narrow inlet of the sea bound by steep 
    mountain slopes. These slopes are of great height and extend to 
    considerable depths (in excess of 1,000 m) below sea level. It is formed 
    by the submergence of glacially over deepened valleys due to a rising 
    sea level after the melting of the Pleistocene ice sheets. Fiords occur 
    in western Scotland, Norway, Ireland, Greenland, Labrador, British 
    Columbia, Alaska, Southern Chile and New Zealand. The main reason 
    for their existence is the submergence of deep glacial troughs and that is 

    why fiords have many characteristics of glaciated valleys.

     3) Dalmatian or longitudinal coasts: Dalmatian is a term derived from 
    the Yugoslavia Adriatic in which the coast runs parallel with the lineament 
    of the topography and probably with the underlying geological structure. 
    A rise of sea level (estuary) has drowned the coastal area, resulting in 
    a coastline of narrow peninsulas, lengthy gulfs and channels and linear 

    islands. The Dalmatian coast tends to be straight and regular.

     4) Submerged lowland coasts
     These are formed when a rise in the sea level drowns a lowland coast.  The 
    sea penetrates deep inland along rivers to form estuaries. The rise in base level 
    causes an increase in deposition by rivers leading to formation of mud flats, 
    marshes, and swamps which are visible at low tides. Delta: Is a large, flat and 
    low lying plain of river deposits laid down where a river flows to the sea or lake.

     A delta is a large area covered by river deposits (alluvium) formed at the mouth of a river

     Ii) Emerged coasts
     Emerged coasts comprise emerged highlands coasts and emerged lowland 
    coasts.
     A. Emerged upland coasts
     Raised beaches: when the sea level drops, wave activity also drops to lower 
    levels. The wave deposition will be at a new point of low tide level forming a new 
    beach there, hence leaving the old beach up high at a former point of sea. These 
    types of beaches are usually evident on land that is far away from the present 
    edge of the water. They may have been formed at the head of a bay but they are 
    now isolated on land. Most raised beaches are colonized by vegetation.
     Raised cliffs: this is formed when there is a relative fall in the level of the sea.
     A raised terrace: the drop in sea level produces a wave cut platform down to 
    a new level of the sea leaving the former terrace suspended up to the original 

    level of the sea before emergence.

     B. Emerged lowland coasts
     An emerged lowland coast has been produced by the uplift of part of the 
    neighboring continental shelf. The landward edge of such coastal plain is found 
    in the southern of USA.  It is formed by the fall-line where rivers descend from 
    the Appalachian in a series of waterfalls.  Other examples of emerged lowland 
    costs are: the northern shore of the Gulf of Mexico and the southern shore of 

    the Rio-de-la Plata in Argentina.

    Application activity 4.4
     1. Give five examples of cities located in coastal areas, including at least 
    two cities located in East African Community.
     2. Indicate the type of submerged coast, and describe its characteristics.
     3. Suppose that you live nearby the coast, explain the business 
    opportunities that you may carry out there and the challenges you can face.  

    4.5. Coral reefs: Nature, types and formation of coral reefs
     Learning activity  4.5
     Observe the figure below of a coral reef and answer the following questions: 

    answer the following questions: 



    1. What do you think are the elements that constitute a coral reef?
     2. Analyze the processes in which coral reefs are formed.
     3. What do you think are the problems related to coral reefs formation?
     A coral is a hard limestone rock made up of the skeletons of tiny (very small) 
    marine organisms, known as coral polyps. Also coral reefs are limestone rocks 
    which are formed from dead animals called corals. Corals have a hard shell of 
    calcite, formed by the extraction of calcium carbonate from sea water.

    They are generally attached to submarine platforms or islands submerged under 

    seawater.

    A. Types of coral reefs 
    On the basis of the location of the main types of reefs, we distinguish tropical 
    coral reefs and marginal belt coral reefs. But, by categorizing on the basis of 
    the nature, the shape and the mode of occurrence, we have three types of coral 

    reefs which are: fringing reefs, barrier reefs and atoll.

    1) Fringing reefs (Shore Reefs): These are the coral reefs developed 
    along the continental margins or along the islands. The seaward slope is 
    steep and vertical while the landward slope is gentle. A fringing reef runs 
    as a narrow belt which grows from the deep sea bottom sloping steeply 
    seaward side. It is separated from the main land by a narrow and shallow lagoon.

     2) Barrier reefs: Barrier reefs are extensive linear reef complexes that are 

    parallel to a shore and are separated from it by a deep and wide lagoon. 

    3) Atoll: An atoll is a roughly circular (annular) oceanic reef system 
    surrounding a large and often deep central lagoon. There are three types 
    of atolls, namely, true atolls, island atolls and coral island or atoll islands. 
    – True atolls are characterized by circular reef enclosing a shallow 
    lagoon but without an island;
    – Island atolls have an island in the central part of the lagoon enclosed 
    by circular reefs; 
    – Coral islands or atoll islands do not have islands in the beginning but 
    later on islands are formed due to erosion and deposition by marine waves.

     B. Formation of coral reefs
     1) The process of coral reefs formation
     Coral reefs start to form when the free-swimming coral larvae attach to the 
    submerged rocks or other hard surfaces along the edges of islands or continents. 
    This continues to grow under the influence of coral reefs conditions to grow in 
    any types accordingly. The coral reef formation takes three stages: fringing, 

    barrier and atoll. 

    Concerning the process, a typical fringing reef is attached to or borders the 
    shore of a landmass, while a typical barrier reef is separated from the shore by 
    a body of water. An atoll began as a fringing reef around a volcanic island. Over 
    time, the volcano stopped erupting, and the island began to sink. Over time, 
    coral growth at the reef’s outer edge would push the top of the reef above the 
    water. As the original volcanic island disappeared beneath the sea, only an atoll 

    would remain.

     1) The general conditions influencing coral formation
     • Corals are found mainly in the tropical oceans and seas because they 
    require high mean annual temperature ranging between 20°C and 
    21°C for their survival. They cannot survive in the waters having either 
    very low temperature or very high temperature.
     • Corals do not live in deep waters, that is, not more than 60-77 meters 
    below the sea level.
     • There should be clean sediment-free water because muddy water or 
    turbid water clogs the mouths of coral polyps resulting into their death.
     • Though coral polyps require sediment-free water, fresh water doesn’t 
    allow their growth. This is why corals avoid coastal lands and live away 
    from the areas of river mouths.
     • High salinity is injurious to the growth of coral polyps because such 
    waters contain little amount of calcium carbonates whereas lime is 
    important food of coral polyps. The oceanic salinity ranging between 
    27% and 30% is most ideal for the growth and development of coral polyps.
     • Ocean currents and waves are favorable for corals because they bring 
    necessary food supply for the polyps.
     • There should be extensive submarine platforms for the formation of 
    colonies by the coral polyps. Besides, polyps also grow outward from 
    the submarine platforms.
     • Human activities like deforestation, industrialization cause global 
    warming, which adversely affects corals in their habitats. Corals are 
    more susceptible   to long-term climatic change.  Corals are generally 
    termed as rainforests of the oceans. These cannot survive in extreme 

    warm environment.

    Application activity 4.5
     1) Using illustrative graphics, differentiate the types of coral reefs.

     2) Explain the conditions for coral reefs formation.

    4.6. Theories of the origin of coral reefs, Problems facing 
    the development and growth of coral reefs, Impact of coral reefs
         Learning activity  4.6
     1. Using concrete examples, show how coral reefs are important

     2. Mention the theories explaining the formation of coral reefs

    4.6.1. Theories of the origin of coral reefs
     There are three main theories about the origin of coral reefs that are:
     • The subsidence theory by Darwin,
     • Antecedence theory by Murray,

     • Glaciated control theory by Daly.

    A. Darwin’s Theory or subsidence theory
     Darwin, a British naturalist developed his theory as follows:
    – Darwin’s theory starts with a volcanic island which becomes extinct.
    – As the island and ocean floor subside, coral growth builds a fringing 
    reef, often including a shallow lagoon between the land and the main reef.
    – As the subsidence continues, the fringing reef becomes a larger barrier 
    reef further from the shore with a bigger and deeper lagoon inside.
    – Ultimately, the island sinks below the sea, and the barrier reef becomes 

    an atoll enclosing an open lagoon. 

    B.  Murray’s theory or antecedence theory 
    The Antecedent-Platform or uplift theory for the origin of coral reefs stipulates 
    that:
    – Any bench or bank that is located at a proper depth within the circum
    equatorial coral-reef zone is potentially a coral-reef foundation.
    – If ecological conditions permit, a reef may grow to the surface from 
    such a foundation without any change in sea-level. 
    – Reef foundations,  or platforms, are formed by erosion, deposition, 
    volcanic eruption, or earth movement or by combinations of two or 
    more of these processes. 
    – The theory agrees that atoll coral reefs formed when the tops of islands 

    were undergone wave action resulting to a platform.


         C. Daly’s theory or glaciated control theory 
    Daly studied the coral reefs of Hawaii and he was greatly impressed by two 
    things:
    – The reefs were very narrow and there  were marks of glaciations
    – There should be a close relationship between the growth of reefs and 
    temperature.– According to Daly’s hypothesis, in the last glacial period, an ice sheet 
    had developed due to the fall in temperature. This caused a withdrawal 
    of water, equal to the weight of the ice sheet. This withdrawal lowered 
    the sea level by 125-150 m.
    – The corals which existed prior to the ice age had to face this fall in 
    temperature dining this age and they were also exposed to air when 
    the sea level fell. As a result, the corals were killed and the coral reefs 
    and atolls were planed down by sea erosion to the falling level of sea 
    in that period.
    – When the ice age ended, the temperature started rising and the ice 
    sheet melted. The water returned to the sea, which started rising. Due 
    to the rise in temperature and sea level, corals again started growing 
    over the platforms which were lowered due to marine erosion.
    – As the sea level rose, the coral colonies also rose. The coral colonies 
    developed more on the circumference of the platforms because food 
    and other facilities were better available there than anywhere else.
    – Hence, the shape of coral reefs took the form of the edges of submerged 
    platforms, a long coral reef developed on the continental shelf situated 
    on the coast of eastern Australia. Coral reefs and atolls developed on 
    submerged plateau tops. After the ice age, the surface of platforms 
    was not affected by any endogenic forces and the crust of the earth 

    remained 


    4.6.2. Impact of coral reefs
     Coral reef landforms have crucial impact in world economic activities.
    These     are:
    – Tourist attraction:     Coastal features like caves, beaches and arches 
    are tourist attractions.
    – Development of harbors    : Rias and fiords favor the development of 
    deep sheltered harbors.
    – Industrial raw materials    : Coral limestone provides raw materials for 
    the manufacture of cement. This is obtained from raised coral reefs.
    – Fishing grounds    : Fiords contain sheltered waters which are suitable 
    for feeding and development of fishing ports. Continental shelves 
    contain shallow waters which favor growth of planktons. This makes 
    them rich fishing grounds.
    – Habitat for marine life    : Lagoons, mud flats and mangrove swamps 
    are good habitats for marine life. This has promoted the development of 
    research on marine life and establishment of marine parks.
    – Impact on agriculture    : emerged coasts have sand, gravel and bare 
    rock. These inhibit agriculture, especially crop farming.
    – Transport barrier:     coastal features such as sandbars and coral reefs 

    inhibit water transport and development of ports.

    4.6.3. Problems facing the development and growth of coral reefs
     The following are the major problems facing the development and growth of 
    coral reefs:
    – Overfishing: Increasing demand for food fish and sea tourism has 
    resulted in over fishing of not only deep-water commercial fish, but 
    key reef species as well. This affects the reef’s ecological balance and 
    biodiversity. 
    – Coral disease: coral diseases contribute to the deterioration of coral 
    reef communities around the globe. Most diseases occur in response 
    to the onset of bacteria, fungi, and viruses. 
    – Destructive fishing methods: Fishing with dynamite, cyanide 
    and other methods that break up the fragile coral reef are highly 
    unsustainable. Dynamite and cyanide stun the fish, making them easier 
    to catch. Damaging the coral reef habitat on which the fish rely reduces 
    the productivity of the area.
    – Unsustainable tourism: Physical damage to the coral reefs can occur 
    through contact from careless swimmers, divers, and poorly placed 
    boat anchors. Hotels and resorts may also discharge untreated sewage 
    and wastewater into the ocean, polluting the water and encouraging 
    the growth of algae, which competes with corals for space on the reef.
    – Coastal development: The growth of coastal cities and towns 
    generates a range of threats to nearby coral reefs. Coral reefs are 
    biological assemblages adapted to waters with low nutrient content, 
    and the addition of nutrients favours species that disrupt the balance of 
    the reef communities.
    – Pollution: Coral reefs need clean water to thrive. From litter to waste 
    oil, pollution is damaging reefs worldwide. Pollution from human 
    activities inland can damage coral reefs when transported by rivers into 
    coastal waters.
    – Marine debris: It is any solid object that enters coastal and ocean 
    waters. Debris may arrive directly from a ship or indirectly when washed 
    out to sea via rivers, streams, and storm drains. Human-made items 
    tend to be the most harmful such as plastics (from bags to balloons, 
    hard hats to fishing line), glass, metal, rubber (millions of tires!), and 
    even entire vessels.
    – Dredging operations. They are sometimes completed by cutting a 
    path through a coral reef, directly destroying the reef structure and killing 
    any organisms that live on it.  Operations that directly destroy coral are 
    often intended to deepen or otherwise enlarge shipping channels or 
    canals, due to the fact that in many areas, removal of coral requires a 
    permit, making it more cost-effective and simple to avoid coral reefs if 
    possible.
    – Global Aquarium Trade: It is estimated that nearly 2 million people 
    worldwide keep marine aquariums. The great majority of marine aquaria 
    are stocked with species caught from the wild. This rapidly developing 
    trade is seeing the movement of charismatic fish species across borders. 
    Threats from the trade include the use of cyanide in collection, over
    harvesting of target organisms and high levels of mortality associated 
    with poor husbandry practices and insensitive shipping. Some regulation 
    is in place to encourage the use of sustainable collection methods and 
    to raise industry standards.
    – Alien invasive species: Species that, as a result of human activity, 
    have been moved, intentionally or unintentionally, into areas where they 
    do not occur naturally are called “introduced species” or “alien species”. 
    In some cases, where natural controls such as predators or parasites 
    of an introduced species are lacking, the species may multiply rapidly, 
    taking over its new environment, often drastically altering the ecosystem 
    and out-competing local organisms. 
    – Climate change: Rising sea levels due to climate change requires coral 
    to grow to stay close enough to the surface to continue photosynthesis. 
    Also, water temperature changes can induce coral bleaching in which 
    sea surface temperatures rose well above normal, bleaching or killing 
    many reefs.
    – Ocean acidification: results from increases in atmospheric carbon 
    dioxide. The dissolved gas reacts with the water to form carbonic acid, 
    and thus acidifies the ocean. This decreasing pH is another issue for 
    coral reefs.
    – Coral mining: Both small scale harvesting by villagers and industrial 
    scale mining by companies are serious threats. Mining is usually done 
    to produce construction material which is valued as much as 50% 
    cheaper than other rocks, such as from quarries.  The rocks are ground 
    and mixed with other materials, like cement to make concrete. Ancient 
    coral used for construction is known as coral rag. Building directly on 
    the reef also takes its toll, altering water circulation and the tides which 

    bring the nutrients to the reef.


         Application activity 4.6
     1. Establish the similarities of the subsidence, antecedence and 
    glaciated control theories of coral reefs formation.
     2. Account for the negative impacts of human activities on the coral 
    reefs growth.

     3. Describe the economic importance of coral reefs.


         4.7. Sea level change
     Learning activity  4.7

     Observe the following picture and answer the question that follow:

    1. Find evidence that the level of water on this picture changes.
    2. What do you think can cause that change? 
    3. Describe any three features observed on this picture
     
    4.7.1. Meaning of sea level change and its resulting features 
    The sea level change is the variation and fluctuation of the sea level throughout 
    time. It can contribute to the formation of submergent landforms such as Ria 
    (a river valley that’s been flooded by the eustatic rise in sea level), fjords and 
    dalmatian coastline, and emergent landforms such as raised beaches. These 
    are wave-cut platforms and beaches that are above the current sea level. There 
    are also some old cliffs (relic cliffs) behind these raised beaches with wave-cut 

    notches, arches and stacks along them.

    4.7.2. Types of sea level changes
     There are two types of sea level changes which are:
     • Submergence or Rise of sea level: This is referred to as an increase 
    in global mean sea level as a result of an increase in the volume of water 
    in the world’s oceans. This leads to the formation of coastal features of 
    submergence.
     • Emergence or fall of sea level: This is referred to as the decrease in 
    global mean sea level as a result of a decrease of the world’s oceans. 

    This leads to the production of emergence coastal landform.

    4.7.3. Causes of sea level change
     The sea level changes daily because of the following causes:
    – Eustatic variations in sea leve    l are the effects of external forces. 
    Most experts agree that human induced global warming is the force 
    behind the current global sea-level rise. There are three factors that 
    primarily affect eustatic sea level change that are: thermal expansion of 
    the ocean, melting of nonpolar glaciers, and change in the volume of 
    the ice caps of Antarctica and Greenland.
    – The changes in global temperature     affect the amount of ice stored 
    on land as water, thus changing the sea levels. A rise in temperatures 
    causes the ice caps to melt, and sea levels rise, and vice versa.
    – The changes in sea levels are also affected by the steric effect    . 
    This is where the density of the water increases or decreases as the 
    temperature rises or falls. If the temperature rises the water expands 
    and if it falls the water contracts. It is estimated that sea levels can rise 
    up to 0.4 mm per year.
    – The mass of ice adds weight     to the earth’s crust causing it to sink 
    lower into the mantle resulting into relative rise in the sea-level during 
    glacial period.
    – Isostatic re-adjustment;     at the end of glacial period, the mass of ice 
    melts and the weight is lost from crust which then rises. When the ice 
    melts the land begins to rise as the weight is removed. This process 
    results in a relative fall in sea-level. This is called isostatic re-adjustment.
    – Uplift/mountain building     due to plate movements may also result in 
    a relative fall in sea-level as land rises up.
    –     Pre-glacial erosion of a coastline causes the coast rise and end

    up to the sea level change.

    4.7.4. Evidences of sea level changes
     The following are evidences of sea level changes:
    – The presence of old coastline high above the present sea level: 
    During the Ice Age the sea levels fall leaving the old coastline. Since 
    the end of the Ice Age, sea levels have risen again, but not to their 
    previous levels. The raised beaches continue to be above the present 
    sea level by quite a distance.
    – The estuaries and inlets flooded:  Sea level rise after the last Ice 
    Age caused estuaries and inlets to be flooded. This occurred in South 
    West England, drowning many river valleys around the coasts of Devon 
    and Cornwall, and creating Rias. In other more northern areas, glacial 
    valleys were drowned to create Fjords.
    – Isostatic re-adjustment phenomenon: Some places in Scotland 
    still undergoing isostatic re-adjustment up to 7 mm per year in some areas.

         4.7.5. Effects of the sea level changes
     Rising sea level has many impacts on coastal areas. The following are some of 
    them:
    – Erosion of beaches and bluffs: Beach erosion is the most common 
    problem associated with rising sea level. Depending on beach 
    composition, beaches erode by about 50 to 200 times the rate of sea 
    level rise. That translates a 2-millimeter (0.08-inch) per year increase 
    in sea level eroding from 10 to 40 centimetres (3.9 to 15.6 inches) 
    of coastline per year. Beach erosion has not only a strong ecological 
    impact, but also a profound economic impact;
    – It increases the flooding and storm damage caused by changes 
    in sea level;
    – Contamination of drinking water: as the rising sea crawls farther 
    and farther up the shore, in many places it will seep into the freshwater 
    sources in the ground that many coastal areas rely on for their drinking 
    water. Saltwater is unsafe to drink, and while it is possible to remove 
    the salt from water, doing so is an expensive and complicated process;
    – Interference with farming: Those same freshwater sources we use 
    for drinking also supply the water we use for irrigation. The problems 
    here are the same: The intruding sea could make these groundwater 
    sources saltier. Saltwater can stunt or even kill crops, but creating 
    freshwater from saltwater is a costly and unsustainable practice;
    – Change in coastal plant life: more saltwater hitting the shores 
    changes the soil composition on the coast, meaning the plant life there 
    will most likely change as well;
    – Threating the wildlife population: Many forms of wildlife make their 
    home on the beach. As the rising ocean erodes the shoreline and floods 
    the areas in which coastal animals live, animals like shorebirds and sea 
    turtles will suffer and die and others will migrate;
    – Hurting the economy: the tourism and real-estate industries in 
    coastal areas are likely to take a hit as prime beachfront properties and 
    recreational areas are washed away by rising waters. This is a fact that 

    some involved in these industries are finding hard to swallow.

    Application activity 4.7
     1. Explain the causes of sea level change
     2. According to you, which feature is more attractive to tourism. Defend 
    your view

     3. Explain the environmental effects of sea level changes.

     Skills Lab
     With help of knowledge and skills acquired in this unit, suggest ways 

    beaches may be preserved and more productive.

    End unit assessment
     1) Describe the major features resulting from wave erosion and 
    deposition processes.
     2) Observe the following photographs and answer the questions that follow:
     i) Examine the economic activities that should be carried out in the 
    regions demonstrated on photographs.
    ii) According to you, what are the advantages of coast or shore to 
    people living nearby?
     3) Demonstrate the impacts of sea level change to the environment.

    ◄UNIT 3:EXTERNAL LANDFORM PROCESSES AND RELATED FEATURESUNIT 5 : ROCKS AND MINERALS►