Influence Of Oceans On Coastal Landforms & Climate PDF

LECTURE FIVE (5) INFLUENCE OF OCEANS ON COASTAL LANDFORMS & CLIMATE Lecture 5 Outline 5.1 Introduction 5.2 Lecture Objectives 5.3 Marine Coastal Landforms & Processes 5.3.1 Geomorphic Agents derived from Ocean Water Movements 5.3.2 Schemes Used to Classi...

LECTURE FIVE (5) INFLUENCE OF OCEANS ON COASTAL LANDFORMS & CLIMATE Lecture 5 Outline 5.1 Introduction 5.2 Lecture Objectives 5.3 Marine Coastal Landforms & Processes 5.3.1 Geomorphic Agents derived from Ocean Water Movements 5.3.2 Schemes Used to Classify Coasts 5.4 Effects of Ocean Currents 5.4.1 Influence on Climate 5.4.2 Influence on Marine Life 5.4.3 Influence on Trade 5.5 General of Influence Oceans on Climate 5.6 Case Study: Influence of Oceans on the Climate of the East African Coast 5.7 Summary 5.8 Terminal Questions 5.9 References 5.1 Introduction This lecture is the third and final part on the physical features of marine environments. The aim of the lecture is to discuss how ocean water movements and oceans in general influence the marine environment, particularly, the formation of coastal landforms and climate. The ocean is a significant influence on Earth's weather and climate. The ocean covers 70% of the global surface. This great reservoir continuously exchanges heat, moisture, and carbon with the atmosphere, driving our weather patterns and influencing the slow, subtle changes in our climate. In the next lecture we shall begin to address marine ecosystems. 3.2 Objectives By the end of this lecture, you should be able to: 1 Identify the geormorphic agents involved in the formation of coastal landforms and the schemes used to classify coasts. 2 Explain how ocean currents influence climate, marine life and economic activities. 3 Assess the influence of oceans on the general climate of a region. 4 Examine the influence of the Indian Ocean on the climate of the Eastern African coast and its associated islands. 5.3 Marine Coastal Landforms & Processes Although many factors work together to create the various coastal landscapes, ocean movement in the form of waves, tides and currents are the ones that leave outstanding impacts on ocean landscapes through erosion, 1 transport, and deposition. This is not to say that other factors not related to oceans do not affect the formation of coastal landscapes. Such factors include the nature of the landscape adjoining the coast, relative changes of sea and land, and special effects like coral, volcanoes, ice, and deltas. A factor whose influence in shaping coastlines has been increasingly growing over time is the influence of humanity’s engineering works. 5.3.1 Geomorphic Agents derived from Ocean Water Movements A coast or coastal zone is a dynamic region where land is carved and shaped by the action of waves, tides and currents whose combination results in a variety of gradational processes acting in the coastal zone. Among these three water movements waves are the most dominant force that shapes coastlines. As discussed in the previous lecture, wind blowing over the surface of the ocean forms waves and these waves transfer some of the wind's energy to the coastline. Except for the effects of tectonic uplift and sea level change, the dominant geomorphic process acting on coasts is erosion. Waves erode landscapes by hydraulic action, abrasion, solution, and attrition. Among these, abrasion is the most important. It is caused by the scraping or impact of sediment carried by water thrown against shore materials. Breakers are particularly effective at lifting larger rocks and hurling them against the shore. Hydraulic action caused by the direct impact of waves on the coast can be an effective geomorphic agent. Enormous pressures can build as water and air are compressed into rock fractures. Also important locally is solution especially where soluble rock is exposed along the shore. As waves approach the shore at an angle the water level in the surf zone increases. The rising water moves parallel to the shore as a longshore current. Beach drifting transports sand grains along the beach as waves strike the shore at an oblique angle. Sediment is carried landward when water rushes across the beach as swash. Sediment is carried back toward the ocean as backwash. The continual up rush and backwash carries sand in a zig-zag like movement along the shore. According to Ritter (2006) longshore transport and progradation (forward building) create a variety of depositional features like spits, baymouth bars and beaches. Drifting sand may be deposited inside breakwaters or against jetties or groins. 5.3.2 Schemes Used to Classify Coasts Coasts are often classified in many different ways. The most popular scheme is by categorizing coasts as being either submergent or emergent. There is, however, a problem with this scheme. It has been criticized for not adequately describing many coasts partly because many of them exhibit features of both submergence and emergence. Because of this problem, a classification scheme based on the most recent and predominant geologic agent forming the coast has become popular. Under this second scheme, there are essentially two major types of coasts: primary coasts and secondary coasts. Other classifications recognize categories, such as, erosional and depositional coasts and reefs. Below is a description of each of these six groups. I. Submergent Coasts Submergent coasts are those coasts which resulted from a rise in the relative sea level. ‘Submergence’ refers to the drowning of coastal land due to the rise in sea level. Young submergent coasts are usually irregular and have deep water offshore and many good harbors, either bays or estuaries. Good examples of young submergent coasts are much of the coast of New England in the United States and most of the Atlantic coast of Europe. 2 Gradually, the young submergent coast that is often subjected to erosive attacks of the ocean and other agents becomes mature. This happens by, for instance, headlands being worn back to form cliffs. At the base of deposits, eroded material accumulate in the form of fringing beaches. Spits and bars also grow up from material that is carried by currents and deposited in deeper water. The shoreline is called mature when it is smooth, the headlands having been cut away and the bays either filled up or closed off by spits. II. Emergent Coasts Emergent coasts are those coasts that came about due to the emergence of land from the sea owing to a result of a fall or decline in sea level. Emergent shorelines usually have shallow water for some distance offshore. Such shorelines are found along the Atlantic coast of the south eastern USA and along part of the coast of Argentina, near the Río de la Plata. III. Primary Coasts Primary coasts are youthful coasts formed where the sea rests against a land mass whose topography was formed by terrestrial agents. These coasts include land erosion coasts, i.e., Maine in the US, volcanic coasts (Hawaii), deposition coasts (Nile Delta coast), and fault coasts (Red Sea). IV. Secondary Coasts Secondary coasts are formed mainly and most recently by marine agents, and may even be primary coasts that have been severely modified by wave action. These coasts include wave erosion coasts, marine deposition coasts, and coasts built by organisms such as reefs and mangrove coasts. V. Erosional Coasts Erosional coasts are associated with some of the most impressive scenery created by the effects of wave action. This grand scenery is associated with physical features, i.e., sea cliffs, sea caves, sea arcs, sea stacks, wave-cut benches and wave-built terraces. Below is a description of some of these features Sea cliff Wave erosion undercuts steep shorelines thereby creating coastal cliffs. A sea cliff is a vertical precipice (the steep sheer face of a cliff) created by waves crashing directly on a steeply inclined slope. Hydraulic action, abrasion, and chemical solution all work to cut a notch at the high water level near the base of the cliff. Constant undercutting and erosion causes the cliffs to retreat landward. 3  FIGURE 5.1: Features of an Erosional Coastline Sea caves, arcs and stacks Sea caves form along lines of weakness in cohesive but well-jointed bedrock. Sea caves are prominent headlands where wave refraction attacks the shore. A sea arch forms when sea caves merge from opposite sides of a headland. If the arch collapses, a pillar of rock remains behind as a sea stack. Wave-cut benches Seaward of the retreating cliffs, wave erosion forms a broad erosional platform called a wave-cut bench or a wave-cut platform. (d) Wave-built terraces After the constant grinding and battering, eroded material is transported to adjacent bays to become beaches or seaward coming to rest as a wave-built terrace. If tectonic forces raise the bench above the water level, a marine terrace forms. Some shorelines have several marine terraces created during various episodes of uplift. According to Chami (1994, p. 37) marine terraces on the coast of East Africa are better observed in the area between Malindi (to the north) and Dar es Salaam (to the south). These marine terraces differ in height in different places. VI. Depositional Coasts 4 Depositional coastal landforms include deltas, beaches, spits and bars, and barrier islands. Delta Coasts Delta coasts are those coasts formed by the deposition of sediment at the bottom of a river that enters an ocean. Deposition is caused by the rapid increase in water velocity as it enters the ocean. Sand and silt are deposited first. Meanwhile, the mixing of fresh and salt walter cuses clay particles to bind together. This forms larger particles that settle at the bottom. Beaches A beach is a deposit of loose sediment adjacent to a body of water. Most beaches constitute sand. A striking feature of beaches is the wide range of sediment size, which extends from boulders to fine silt on beaches worldwide. In places where wave action is high and where there are steeper slopes, larger particles are found. Fine particles and gentle slopes are characteristic of beaches exposed to low wave action. Most beaches in the midlatitude undergo a cycle of erosion and deposition following the seasonal changes in wave action. During the winter, midlatitude storms are more vigorous producing more wave action and erosion. Hence, beaches tend to narrow during the winter. Wave action subsides during the summer as storms weaken somewhat favoring deposition over erosion and producing broader beaches. Spits and Bars One of the most common coastal landforms is the sand spit. It can be defined as a linear accumulation of sediment that is attached to land at one end. Sand carried parallel to the shore by longshore drift may eventually extend across a bay or between headlands especially where water is relatively calm. Spits are typically elongate, narrow features built to several meters by wind and waves. Spits often form when wave energy decreases as a result of wave refraction in a bay. When a coastline turns abruptly, wave energy is dissipated by divergence of wave trajectories, causing sediment to accumulate as the water loses its ability to transport. Spits can extend across the mouth of a bay, but wave action is usually strong enough to wash sand out to sea or be deposited in the embayment. They may curve into the bay or stretch across connecting to the other side as a baymouth bar. When the bay is closed off by a bar it becomes a lagoon. Wave energy also dissipates in the lee of large sea stacks or islets. Wave refraction sweeps sediment behind the obstruction from two directions, depositing it as a slender finger called a tombolo. 5  FIGURE 5.2: Features of a Depositional Coastline Barrier Islands Barrier islands are coastlines paralleled by offshore narrow strips of sand dunes, salt marshes and beaches. A good example of barrier islands is found in the US where barrier island complexes stretch along the southeastern coast of North America from Long Island, New York to the Gulf coast of Texas. Many authorities argue that barrier islands originated as offshore bars built by waves breaking on a shallow shore. When waves begin to feel the tug of the ocean floor, they push sand toward shore as Many authorities argue that barrier islands originated as offshore bars built by waves breaking on a shallow shore. When waves begin to feel the tug of the ocean floor, they push sand toward shore as they break. The return undertow sweeps sand back to settle on the developing bar. These offshore bars were later exposed when the continent rebounded after ice age glaciers melted. Reefs Reefs emanate from coral, which is a simple marine animal with a small cylindrical sac-like body called a polyp and an exoskeleton of calcium carbonate. As old colonies of coral die, new ones form on top. Thus, corals and calcareous algae grow on top of one another and form a three-dimensional framework that is modified in various ways by other organisms and inorganic processes. This ultimately leads to the formation of limestone (corals and coral reefs will be discussed in greater detail in Lecture 7). 6 Reefs appear in two forms, coral reefs and mounds. A coral reef is a marine ridge or reef comprising coral and other organic matter consolidated into limestone. Reefs are held up by a macroscopic skeletal framework while mounts are not. There are three kinds of coral reefs. These are fringing reefs, barrier reefs and atolls: Fringing reefs are platforms of coral attached to land. They tend to be wider where wave action is prominent and where the water is well aerated. Barrier reefs form offshore with lagoon in between. Many form along slowing subsiding islands, growing at a rate that keeps them near sea level. Others form on continental shelves. Atolls are circular reefs enclosing a lagoon formed from the subsidence and disappearance of a volcanic island cone. The most famous barrier reef is the Great Barrier Reef of Australia which is over 2025 km long and 16 - 145 km wide. 5.4 Effects of Ocean Currents Ocean currents have three main impacts. These are influence on climate, influence on marine life, and influence on trade as elaborated in the following three sub-sections 5.4.1 Influence on Climate As gathered from our earlier discussion in Lecture 4, currents move from warm temperature areas to colder temperature areas and vice versa. As they move from one place to another they partly attempt to modify the temperature. The temperature of a mass of water affects the temperature of the air above it. Therefore, the ocean current that moves from the equatorial region to the colder latitudes raises the temperature of the air in the areas into which it moves. Good examples of this phenomenon can be drawn from Europe and North America. In the first instance is the warm Gulf Stream (North Atlantic Drift upon crossing the Atlantic), which flows northwards to West European coast. This current helps in keeping the coast of Great Britain and Norway free of ice in winter. The effect of the ocean current becomes clearer if you compare the winter-condition of the British Isles with that of the North East Coast of Canada situated on the same latitudes. Since the North East Coast of Canada comes under the influence of cold Labrador Currents, it remains ice bound during the winter time. As we noted when discussing the Atlantic currents, without the warm Gulf Stream, England and other places in Europe would be as cold as Canada. As seen from earlier discussions, when cold and warm currents meet they produce mist and fog. For example, during the discussion of currents in the Atlantic Ocean, we noted that near New Foundland, the warm Gulf Stream meets the cold Labrador Current and produces fog. They also create conditions for storms. Hurricanes in New foundland and Typhoons in Japan are perhaps the result of the meeting of warm and cold currents. 5.4.2 Influence on Marine Life The influence of ocean currents on marine life, especially fish, is profound. Temperature has a great influence on marine life. It determines the type of flora and fauna. The areas where warm and cold currents meet are among the most important fishing grounds of the world. This is mainly a result of upwelling. As we saw in the earlier discussions, 7 rich fishing grounds include Newfoundland where the cold Labrador current meets the warm Gulf stream, and in South Africa where the cold Benguela current meets the warm Aghulhas current. When two currents, a warm current and a cold current meet, there develops a phenomenon referred to as upwelling. Upwelling is the factor which determines the abundance of marine fish. Upwelling Upwelling is an oceanographic phenomenon involving wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. Upwelling occurs in five different forms. The upwelling that concerns us is coastal upwelling. This kind of upwelling is the one most closely related to human activities because it supports some of the most productive fisheries in the world, for instance, small pelagic fish like sardines and anchovies. Deep waters are rich in nutrients including nitrate and phosphate, which result from the decomposition of sinking organic matter like dead plankton from surface waters. When these nutrients are brought to the surface, they are used by phytoplankton, together with dissolved carbon dioxide and light energy from the sun, to produce organic compounds, through the process of photosynthesis. It follows, therefore, that upwelling regions experience very high levels of primary production as to other areas of the ocean. High primary production propagates the food chain. This is due to the fact that phytoplanktons are at the base of the oceanic food chain. Examples of regions of upwelling, and which form rich hunting grounds include the coasts of Peru, Chile, Arabian Sea, western South Africa, eastern New Zealand, southeastern Brazil and the California coast. The food chain follows the following course: Phytoplankton → Zooplankton → Predatory zooplankton → Filter feeders → Predatory fish Because this is a food chain, it means that every species is a key species within the upwelling zone. How does upwelling arise? Upwelling originates from the Coriolis Effect. In this process, wind-driven currents tend to be driven to the right of the winds in the Northern Hemisphere and to the left of the winds in the Southern Hemisphere. For example, in the northern Hemisphere, when winds blow either equatorward along an eastern ocean boundary or poleward along a western ocean boundary, surface waters are driven away from the coasts and replaced by denser waters from below. 5.4.3 Influence on Trade Ocean currents influence the trade. The ports and harbors of higher latitudes which are affected by warm currents are ice free and open for trade all the year round. For example, the ports of North Western Europe remain open throughout the year while port of Quebec in Canada gets frozen in winters. 5.5 General of Influence Oceans on Climate Why are coastal regions often much warmer than interior regions? The most important role played by oceans is that of modifying the climate. This happens such that coastal areas are often warmer than interior regions as explained below: 8 Firstly, oceans act as stores of heat – Because the ocean stores a large quantity of heat it is often referred to as "the saving bank for the solar energy, receiving deposits in season of excessive insolation and paying them back in seasons of want". The extensive deep waters of oceans gain as well as loose heat more slowly than the land when both are subjected to the same amount of insolation. The contrast in the temperature of the ocean and land explains the difference in the temperature of coastal and interior region. This is especially significant in those coastal areas that would otherwise experience extremely severe winters. Secondly, oceans are sources of precipitation – The oceans supply water vapor to the atmosphere and thus are the basic source of all precipitation on earth. They are also the vital source of fresh water on earth. Thirdly, oceans act as regulators of temperatures – on the earth’s surface ocean currents are important regulators of temperature. They help in exchange of heat between low and high latitudes and are essential in sustaining the global energy balance. On the local scale, the warm ocean currents bring a moderating influence to coasts in higher latitudes; cool currents reduce the heat of tropical deserts along narrow coastal belts. Finally, oceans constitute origins of centers of high pressure – The influence of oceans on climate becomes clearer if we consider the distribution of pressure and prevailing wind system over the sea surface. The surface of the ocean possesses six or more permanent centers of high pressure. These high pressure areas create the planetary wind system over the earth. These planetary winds determine the amount of rainfall and its distribution over the earth's surface. The westerlies provide rainfall on the West European Coast after collecting moisture from the warm North Atlantic Drift while the Monsoons are the major source of rainfall in eastern Africa. 5.6 Case Study: Influence of Oceans on the Climate of the East African Coast Climatic conditions have played an important role in the development of the East African coast from the ancient past to its present status. The climate of the coastal area of East Africa is in turn greatly influenced by the Indian Ocean. Monsoon winds blowing over the Indian Oceans have played a great role in not only moulding the climate but also enabling socioeconomic developments on the East African coast (Chami 1994, p. 36). Most of the rain received on the coast is attributed to these winds. The NE monsoons blow between November and February resulting in rainfall to the coast south of the Equator. From May to August, the SW monsoons blow, bringing rainfall north of the Equator. The reversal of direction of the monsoons results in two annual rainfall seasons on the northern Tanzania and southern Kenyan coasts. These seasons are experienced on the islands of Pemba and Zanzibar. Thus, the winds have made the coastal area to receive a somewhat evenly distributed rainfall to support agriculture, especially the perennial crops like bananas, coconuts and other fruits as well as the forests of the region. According to Pearce and Smith (1984), the average annual rainfall received on Kenya’s coastal region is over 40 inches (1000m) except in the north where it is rather less. The wettest season is in April and May as the inter-tropical rain belt moves north. The second rainy season is experienced in October and November. The Tanzanian coastal regions, including the large offshore islands of Pemba and Zanzibar have a heavier and more reliable rainfall than most of the areas inland. The average annual rainfall is almost everywhere above 1000mm on the coast to 1500mm in the wetter places. Thus, significance of the Indian Ocean to the climate is that it has enabled the availability of rainfall. The coastal region is rather exceptional in that it gets some rainfall in all months with the main rains falling between March and May. 9 Another influence of the ocean on the climate of the coast is temperature. In general, temperatures remain quite higher throughout the year as does humidity; but the weather is not oppressive because of the regular and strong offshore winds in day time and the greater number of sunshine hours averaging 7-8 hours a day in all months. The whole coast of East Africa has a mean temperature of about 26 to 29º C, but this diminishes steadily southwards. In the early historical times, temperatures on the East African coast also played a very significant role in the development of the region. Favorable temperature conditions attracted people from inland areas and from across the ocean to settle on the coast and develop the Swahili civilization. In addition to the above, the warm temperatures of the coastal area have contributed to the warming up of the ocean waters, which favors the growth of marine life like polyps that are responsible for coral reefs, whose value we have already discussed. Another important climatic factor influenced by the ocean and which has affected socio-economic developments of the East African coastal area is the Monsoon winds. In early historical times, during the early centuries AD, the monsoon winds contributed to the flourishing of commercial activities on the coast. These winds are also called ‘Trade Winds’ because of their role in transoceanic trade. The Trade Winds were used especially by Arab traders of the 1st millennium AD to propel boats and ships carrying traders from the Orient to and from East Africa through the waters of the Indian Ocean. They also made possible the development of local links between the different trading communities along the East African coastal settlements. In order to maintain their monopoly in the Trans-Oceanic trade, the Muslim traders who knew the working of these winds kept it secrete from rival traders for a long time (Datoo 1975, Horton 1984, Casson 1989, p. 11). 5.7 Summary Ocean water movements, especially waves and currents, shape the landscape of the coasts through the geomorphic processes of erosion, transportation and deposition. Their activity results in the formation a wide variety of coastal landforms including sea cliffs, sea caves, arcs and stacks, wave-cut benches, wave-built terraces, deltas, beaches, spits and bars, barrier islands, and reefs. There are three types of reefs – fringing reefs, barrier reefs and atolls. Coasts are often classified in many different ways. The most popular scheme is by categorizing coasts as being either submergent or emergent. There is, however, a problem with this scheme. It has been criticized for not adequately describing many coasts partly because many of them exhibit features of both submergence and emergence. Because of this problem, a classification scheme based on the most recent and predominant geologic agent forming the coast has become popular. Under this second scheme, there are essentially two major types of coasts: primary coasts and secondary coasts. Other classifications recognize categories such as erosional and depositional coasts and reefs. Below is a description of each of these groups. Currents have a direct influence on climate. Cold currents cause drought off the coasts where they flow. However, they cause warm conditions off the coasts they flow, hence not only helping to regulate temperature but by attracting precipitation. Currents also enable the flourishing of rich ecosystems and are also useful for trade as they enable marine transportation. 10  Oceans are in general very valuable to humans through their influence to the climate. Not only are they the sources of high pressure cells where wind systems originate, they also help to store heat, are regulators of temperatures, and are also sources of precipitation. Finally, the Indian Ocean has for a long time been the single most important factor that has shaped the climate on the East African coast. This coast receives rain throughout most of the year resulting from the moisture laden winds from the Indian Ocean. Together with warm temperatures and high humidity, this has attracted human settlement on this coast since the ancient times and. It has also enabled the East African coast to witness tremendous socioeconomic developments over time. Question 1 5.8 Terminal Questions Discuss how ocean water movements and oceans in general influence the formation of coastal landforms and climate. Support your answer with clear examples. Question 2 (a) What geormorphic agents are involved in the formation of coastal landforms? (b) Describe the main features of erosional coasts and depositional coasts. Question 3 (a) How do ocean currents influence climate, marine life and economic activities? (b) Assess the influence of oceans on the general climate of a region. (c) Explain how the Indian Ocean shaped the climate on the East African coast and human activities? 5.9 References Alongi, D.M. (2020) Coastal Ecosystem Processes. 2nd edn. Boca Raton: CRC Press. Cleveland, C.J. (ed.) (2007) Encyclopedia of Earth. Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). Davis, R.A. Jr & FitzGerald, D.M. (2020) Beaches and coasts. 2nd edn. Hoboken, NJ: John Wiley and Sons. Davidson-Arnott, R., Bauer, B. & Houser, C. (2019) Introduction to Coastal Processes and Geomorphology. 2nd edn. Cambridge, UK & New York, NY, USA: Cambridge University Press. Livingston, R.J. (ed.) (2012) Ecological Processes in Coastal and Marine Systems. New York, NY, USA: Springer US. Mann, K.H. & Lazier, J.R.N. (2013) Dynamics of Marine Ecosystems. 3rd edn. Oxford, UK: Blackwell Publishing. Marshak, S. (2012) Portrait of a Planet. 4th edn. London, UK: W.W. Norton and Company. 11  Pinet, P.R. (2019) Invitation to Oceanography. 8th edn. Boston, MA, USA: Jones and Bartlett Publishers. Thomson, W. (2018) Tides and the Ocean Water's Movement around the World, from Waves to Whirlpools. Philadelphia, PN, USA: Running Press. 12