Chapter 18: The Oceans And Their Margins Introduction: The World s Oceans Seawater covers 70.8 percent of Earth s surface, in three huge interconnected basins: The Pacific Ocean. The Atlantic Ocean. The Indian Ocean. The Oceans Characteristics The greatest ocean depth yet measured (11,035 m) lies in the Mariana Trench. The average depth of the oceans, is about 3.8 km. The present volume of seawater is about 1.35 billion cubic kilometers. More than half this volume resides in the Pacific Ocean.
Figure 18.1 Figure 18.2 Ocean Salinity (1) Salinity is the measure of the sea s saltiness, expressed in parts per mil ( = parts per thousand). The salinity of seawater normally ranges between 33 and 37. The principal elements that contribute to this salinity are sodium and chlorine.
Ocean Salinity (2) More than 99.9 percent of the ocean s salinity reflects the presence of only eight ions: Chloride. Sodium. Sulfate. Magnesium. Calcium. Potassium. Bicarbonate. Bromine. Ocean Salinity (3) Cations are released by chemical weathering processes on land. Each year streams carry 2.5 billion tons of dissolved substances to the sea. The principal anions found in seawater are believed to have come from the mantle. Chemical analyses of gases released during volcanic eruptions show that the most important volatiles are water vapor (steam), carbon dioxide (CO 2 ), and the chloride (CI 1- ) and sulfate (SO 4 2- ) anions. Ocean Salinity (4) Chloride and sulfate anions dissolve in atmospheric water vapor and return to Earth in precipitation, much of which falls directly into the ocean. Another source of ions is dust eroded from desert regions and blown out to sea.
Figure 18.3A Temperature And Heat Capacity of the Ocean (1) Global summer sea-surface temperature is displayed with isotherms that lie approximately parallel to the equator. The warmest waters during August (>28 0 C) occur in a discontinuous belt between about 30 0 N and 10 0 S latitude. In winter, the belt of warm water moves south until it is largely below the equator. Temperature And Heat Capacity of the Ocean (2) Waters become progressively cooler both north and south of this belt. Both the total range and the seasonal changes in ocean temperatures are much less than what we find on land.
Temperature And Heat Capacity of the Ocean (3) The range of temperature on land is 146 0 C. The highest recorded land temperature is 58 0 C (Libyan Desert). The lowest is 88 0 C (Vostok Station in central Antarctica). The range of temperature at the oceans surface is only 38 0 C. The highest recorded ocean temperature is 36 0 C (Persian Gulf). The coldest is 2 0 C (Polar Sea). Temperature And Heat Capacity of the Ocean (4) Coastal inhabitants benefit from the mild climate resulting from this natural ocean thermostat. In the interior of a continent, summer temperatures may exceed 40 0 C, whereas along the ocean margin they typically remain below 25 0 C. Figure 18.3B
Vertical Stratification (1) Temperature and other physical properties of seawater vary with depth. When fresh river water meets salty ocean water at a coast, the fresh water, being less dense, flows over the denser saltwater, resulting in stratified water bodies. Vertical Stratification (2) The oceans also are vertically stratified as a result of variation in the density of seawater. Seawater become denser as: Its temperature decreases. Its salinity increases. Gravity pulls dense water downward until it reaches a level where the surrounding water has the same density. These density-driven movements lead both to stratification of the oceans and to circulation in the deep ocean. Ocean Circulation Surface ocean currents are broad, slow drifts of surface water set in motion by the prevailing surface winds. A current of water is rarely more than 50 to 100 m deep. The direction taken by ocean currents is also influenced by the Coriolis effect.
Current Systems Low-latitude regions in the tradewind belts are dominated by the warm North and South Equatorial currents. Each major current is part of a large subcircular current system called a gyre. The Earth has five major ocean gyres. Two are in the Pacific Ocean. Two are in the Atlantic Ocean. One is in the Indian Ocean. Figure 18.4 Major Water Masses (1) Ocean waters also circulate on a large scale within the deep ocean, driven by differences in water density. The water of the oceans is organized into major water masses, each having a characteristic range of: Temperature. Salinity.
Major Water Masses (2) The water masses are stratified based on their relative densities. Cold water is denser than warm water; Salty water is denser than less salty water. Figure 18.5 The Global Ocean Conveyor System (1) Dense, cold, and/or salty surface waters that flow toward adjacent warmer, less-salty waters will sink until they reach the level of water masses of equal density. The resulting stratification of water masses is thus based on relative density.
The Global Ocean Conveyor System (2) The sinking dense water in the North Atlantic propels a global thermohaline circulation system, so called because it involves both the temperature (thermo) and salinity (haline) characteristics of the ocean waters. The Global Ocean Conveyor System (3) The Atlantic thermohaline circulation acts like a great conveyor belt, transporting low-density surface water northward and denser deep-ocean water southward. Heat lost to the atmosphere by this warm surface water, together with heat from the warm Gulf Stream, maintains a relatively mild climate in northwestern Europe. Figure 18.6A
Figure 18.6B Ocean Tides (1) Tides: Twice-daily rise and fall of ocean waters. Caused by the gravitational attraction between the Moon (and, to lesser degree, the sun) and the Earth. The Moon exerts a gravitational pull on the solid Earth. Ocean Tides (2) A water particle in the ocean on the side facing the Moon is attracted more strongly by the Moon s gravitation than it would be if it were at Earth s center, which lies at a greater distance. This creates a bulge on the ocean surface due to the excess inertial force (called the tide-raising force).
Ocean Tides (3) On the opposite side of Earth, the inertial force exceeds the Moon s gravitational attraction, and the tide-raising force is directed away from Earth. These unbalanced forces generate opposing tidal bulges. Figure 18.7 Ocean Tides (4) At most places on the ocean margins, two high tides and two low tides are observed each day as a coast encounters both tidal bulges. Twice during each lunar month, Earth is directly aligned with the Sun and the Moon, whose gravitational effects are thereby reinforced, producing higher high tides and lower low tides.
Figure 18.8 Ocean Tides (5) At position halfway between these extremes, the gravitational pull of the Sun partially cancels that of the Moon, thus reducing the tidal range. In the open sea tides are small (less than 1 m). Along most coasts the tidal range commonly is no more than 2 m. Ocean Tides (6) In bays, straits, estuaries, and other narrow places along coasts, tidal fluctuations are amplified and may reach 16 m or more. Associated currents are often rapid and may approach 25 km/h. The incoming tide locally can create a wall of water a meter or more high (called a tidal bore).
Tidal Power Energy obtained from the tides is renewable energy. One important difference between hydroelectric power from rivers and that from tidal power is that rivers flow continuously whereas tides can be exploited only twice a day. Ocean Waves (1) Ocean waves receive their energy from winds that blow across the water surface. The size of a wave depends on how fast, how far, and how low the wind blows. Ocean Waves (2) Because waveform is created by a loop-like motion of water parcels, the diameters of the loops at the water surface exactly equal wave height. Downward from the surface, a progressive loss of energy occurs, resulting in a decrease in loop diameter.
Ocean Waves (3) L is used to represent wavelength, the distance between successive wave crests or troughs. At a depth equal to half the wavelength (L/2), the diameters of the loops have become so small that motion of the water is negligible. Ocean Waves (4) The depth L/2 is referred to as wave base. Landward of depth L/2, as the water depth decreases, the orbits of the water parcels become flatter until the movement of water at the seafloor in the shallow water zone is limited to a back-and-forth motion. Figure 18.10
Ocean Waves (5) When the wave reaches depth L/2, its base encounters frictional resistance exerted by the seafloor. This causes the wave height to increase and the wave length to decrease. Eventually, the front becomes too steep to support the advancing wave and the wave collapses, or breaks. Ocean Waves (6) Such broken water is called surf; The geologic work of waves is mainly accomplished by the direct action of surf. Figure 18.11
Wave Refraction (1) A wave approaching a coast generally does not encounter the bottom simultaneously all along its length. As any segment of the wave touches the seafloor: That part slows down. The wave length begins to decrease. The wave height increases. Wave Refraction (2) This process is called wave refraction. Wave refraction affects various sectors of a coastline differently. Waves converge on headlands, which are vigorously eroded. Refraction of waves approaching a bay will make them diverge, diffusing their energy at the shore. In the course of time, irregular coasts become smoother and less indented. Figure 18.13
Coastal Erosion And Sediment Transport (1) Erosion by waves. Erosion below sea level: Ocean waves rarely erode to depths of more than 7 m. The lower limit of wave motion is half the wavelength of ocean waves, which is the lower limit of erosion of the ocean floor by waves. Coastal Erosion And Sediment Transport (2) Abrasion in the surf zone: An important kind of erosion in the surf zone is the wearing down of rock by wave-transported rock particles, The surf is like an erosional knife edge or saw cutting horizontally into the land. Erosion above sea level: Waves pounding against a cliff compress the air trapped in fissures. Nearly all the energy expended by waves in coastal erosion is confined to a zone that lies between 10 m above and 10 m below mean sea level. Coastal Erosion And Sediment Transport (3) Sediment transport by waves and currents. Longshore currents: Longshore currents flow parallel to the shore. The direction of longshore currents may change seasonally. The longshore current moves the sediment along the coast.
Figure 18.14 Coastal Erosion And Sediment Transport (4) Beach drift: The swash (uprushing water) of each wave travels obliquely up the beach before gravity pulls the water back directly down the slope of the beach. This zigzag movement of water carries sand and pebbles first up, then down the beach slope in a process known as beach drift. Beach drift can reach a rate of more than 800 m/day. Figure 18.15
Coastal Erosion And Sediment Transport (5) Beach placers: Gold, diamond, and several other heavy minerals have been concentrated in beach sands by surf and longshore currents (Namibia, Alaska). Ilmenite, a primary source of titanium, is highly concentrated along several beaches in India. Magnetite-rich sands occur in Oregon, California, Brazil, and New Zealand. Chrome-rich sands are mined in Japan. Coastal Erosion And Sediment Transport (6) Offshore transport and sorting: Far from shore only fine grains can be moved. Sediments grade seaward from sand into mud. Figure 18.16
Coastal Deposits And Landforms Waves dash against firm rock, erode it, and move the eroded rock particles. The three important features of the shore profile are: Beaches. Wave-cut cliffs. Wave-cut benches. Beaches (1) Beach is: The sandy surface above the water along a shore. A wave-washed sediment along a coast, including sediment in the surf zone (sediment is continually in motion). Sediment of a beach may derived from: Erosion of adjacent cliffs or cliffs elsewhere along the coast. Alluvium brought to the shore by rivers. Beaches (2) On low, open shores an exposed beach typically has several distinct elements: A rather gently sloping foreshore (lowest tide to the average high-tide level). A berm (bench formed of sediment deposited by waves). The backshore (from the berm to the farthest point reached by surf).
Figure 18.17 Rocky (Cliffed) Coasts The usual elements of a cliffed coast due to erosion are: A wave-cut cliff, which may have a welldeveloped notch at its base. A wave-cut bench, a platform cut across bedrock by surf. A beach, the result of deposition. Other erosional features associated with cliffed coasts are sea caves, sea arches, and stacks. Figure 18.18
Factors Affecting The Shore Profile (1) Through erosion and the creation, transport, and deposition of sediment, the form of a coast changes, often slowly but sometimes very rapidly. During storms, the increased energy in the surf erodes the exposed part of a beach and makes it narrower. Factors Affecting The Shore Profile (2) In calm weather, the exposed beach is likely to receive more sediment than it loses and therefore becomes wider. Storminess may be seasonal, resulting in seasonal changes in beach profiles. Winter storm surf tends to carry away fine sediment, and the remaining coarse fraction assumes a steep profile. Major Coastal Deposits And Landforms Marine deltas are a compromise between the rate at which a river delivers sediment at its mouth and the ability of currents and waves to erode sediment along the delta front.
Figure 18.21 Major Coastal Deposits And Landforms (2) A spit is an elongated ridge of sand or gravel that projects from land and ends in open water. It is merely a continuation of a beach. It is built of sediment moved by longshore drift and dropped at the mouth of a bay. Major Coastal Deposits And Landforms (3) The free end curves landward in response to currents created by refraction as waves enter the bay. A spit-like ridge of sand or gravel that connects an island to the mainland or to another island, called a tombolo. A ridge of sand or gravel may be built across the mouth of a bay to form a bay barrier.
Figure 18.22 Major Coastal Deposits And Landforms (4) Beach ridges are low sandy bars parallel to the coast. A barrier islands is a long narrow sandy island lying offshore and parallel to a coast. An elongate bay lying inshore from a barrier island or strip of land such as coral reef is called a lagoon. Figure 18.24B
Major Coastal Deposits And Landforms (5) Organic reefs and atolls: A fringing reef is either attached to or closely borders the adjacent land (no lagoon). A barrier reef is separated from the land by a lagoon that may be of considerable length and width. Great Barrier Reef off Queensland, Australia. An atoll, a roughly circular coral reef enclosing a shallow lagoon, is formed when a tropical volcanic island with a fringing reef slowly subsides. Figure 18.26 How Coasts Evolve (1) The configuration of coasts depends largely on: The structure and erodibility of coastal rocks. The active geologic processes at work. The length of time over which these processes have operated. The history of world sea-level fluctuations.
How Coasts Evolve (2) Types of coasts: Most of the Pacific coast of North America is steep and rocky. The Atlantic and Gulf coasts traverse a broad coastal plain that slopes gently seaward and are festooned with barrier islands. The result is an embayed, rocky, coastline that shows the effects of both: Differential glacial erosion. Drowning of the land by the most recent sea-level rise. How Coasts Evolve (3) Where rocks of different erodibilities are exposed along a coast, marine erosion is strongly controlled by rock type and structure. Coasts of Norway, Ireland, and Croatia. Geographic Influences on Coastal Processes Coasts lying at latitudes between about 45 and 60 0 are subjected to higher-than-average storm waves generated by strong westerly winds. Subtropical east-facing coasts are subjected to infrequent but often disastrous hurricanes (called typhoons west of the 180th meridian). Sea ice is an effective agent of coastal erosion in the polar regions.
Changing Sea Level Sea level fluctuates: Daily as a result of tidal forces. Over much longer time scales as a result of: Changes in the volume of water in the oceans as continental glaciers wax and wane. The motions of lithospheric plates that cause the volume of the ocean basins to change. Sea level fluctuations, on geologic time scales, contribute importantly to the evolution of the world s coasts. Figure 18.27 Submergence: Relative Rise of Sea Level Nearly all coasts have experienced submergence, a rise of water level that accompanies the most recent deglaciation. Most larges estuaries, for example, are former river valleys that were drowned by the recent sea-level rise.
Figure 18.28 Emergence: Relative Fall of Sea Level Many marine beaches, spits, and barriers exist from Virginia to Florida. The highest reaches an altitude of more than 30 m. These landforms are related to a combination of broad up-arching of the crust, as well as submergence. Sea-Level Cycles (1) Many coastal and off-shore features date to times when relative sea level was either higher or lower than now. The major rises and falls of sea level are global movements. By contrast, uplift and subsidence of the land, which cause emergence or submergence along a coast, involve only parts of landmasses.
Figure 18.29 Sea-Level Cycles (2) Movements of land and sea level may occur simultaneously, in either the same or opposite directions. Unraveling the history of sea-level fluctuations along a coast can be difficult and challenging. Coastal Hazards: Storms Storms cause infrequent bursts of rapid erosion. Atlantic hurricanes can be exceptionally devastating.
Coastal Hazards: Tsunamis A strong earthquake, landslide, or volcanic eruption can generate a potentially dangerous tsunami (a seismic sea wave). It can travel at a rate as high as 950 km/h. It has long wavelength up to 200 km. It can pile up rapidly to heights of 30 m. Figure 18.31 Figure 18.32
Coastal Hazards: Landslides Cliffed shorelines are susceptible to frequent landsliding as erosion eats away at the base of a seacliff. Sometimes landslides on cliffed shorelines give rise to giant waves that are even more destructive than the slides themselves. Very large waves have also been produced by massive coastal landslides during earthquakes. Protection Against Shoreline Erosion (1) Seacliffs can be protected by: An armor consisting of tightly packed boulders so large that they can withstand the onslaught of storm waves. A strong seawall built parallel to the shore. Protection Against Shoreline Erosion (2) Protection of beaches: A breakwater is an offshore barrier designed to protect a beach or boat anchorages from incoming waves. A groin is a low wall built out into the water at a right angle to the shoreline. Another way of protecting an eroding beach is to haul in sand and pile it on the beach at the updrift end.
Effects of Human Interference Dams trap the sand and gravel carried by the streams, thus preventing the sediment from reaching the sea. Large resort developments may interfere with the steady state that had existed among the supply of sediment to the coast, longshore current and beach drift, and deposition of sediment on beaches. Ocean Circulation And The Carbon Cycle (1) Photosynthesizing marine organisms exchange dissolved CO 2 for dissolved O 2 in surface waters. A wide variety of organisms draw bicarbonate anions out of seawater to form calcium carbonate shells. Calcium carbonate accumulates on the seafloor if it is shallower than about 4 kilometers. In greater depths, the calcite tends to dissolve. Ocean Circulation And The Carbon Cycle (2) Cold O 2 -rich water sinks into the deep ocean from the surface waters of the North Atlantic and offshore Antarctica. Unusual depositional conditions are common when an ocean basin initially opens, and in its last stages of closure.
Ocean Circulation And The Carbon Cycle (3) If evaporation dominates the regional climate, salinity increases in small semi-isolated ocean basins. Evaporite deposits can form if the connection to the world s oceans is broken by tectonic activity or by a drop in sea level. Ocean Circulation And The Carbon Cycle (4) Geologists have estimated that the Mediterranean would evaporate completely in only 1000 years if the Straits of Gibraltar were blocked. Thick salt deposits beneath the Mediterranean seafloor tell us that it dried out as many as 40 times between 5 and 7 million years ago.