Chapter 13: Oceans and Coastlines

Transcription

1 Chapter 13: Oceans and Coastlines 1. Our Changing Oceans 2. Ocean Basins 3. Ocean Waters 4. Oceanic Circulation 5. Tides 6. Wave Action 7. Shoreline Features 8. Shoreline Protection Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 Our Changing Oceans About 71% of Earth is covered with seawater. The Oceans were mostly in place by ~4 billion years ago. They are the final frontier for research on Earth.

3 Our Changing Oceans Elephant seal as researcher sensors glued to her back record information about temperature and salinity of the surface waters. This information cannot be gathered through satellites. Elephant seals migrate from California to Alaska and back, and dive as deep as 600 meters.

4 Our Changing Oceans Temperature vs. depth for NE Pacific ocean. The range of temperatures reflects different locations along the seal s journey. 4,000 marine animals from 20 species collect ocean data.

5 Our Changing Oceans Oceans are dynamic! Water is continually in motion. Oceanic and atmospheric circulation patterns move heat around and strongly influence climate. Coastlines are also dynamic, advancing and retreating depending on the balance of erosion and deposition. A storm at Nag s Head, NC

6 Our Changing Oceans How do oceans/coastlines change? Coastlines can advance or retreat Short term, the position of the coastline can change depending on daily tides and seasonal variations in stream flow Climate cycles measured over decades, centuries, or millennia can show rises and falls in sea level Tectonic cycles occurring over thousands to millions of years can revitalize coastlines through uplift Humans can influence oceans and coastlines as well, and be strongly affected by oceans and their weather (e.g. hurricanes) Malibu, Ca More than ¼ of the U.S. population lives along the Atlantic and Gulf coasts

7 Our Changing Oceans Self Reflection Survey Answer the following questions as a means of uncovering what you already know about oceans and coastlines: 1.How have you interacted with the world s oceans, either directly or indirectly?

8 Our Changing Oceans Self Reflection Survey Answer the following questions as a means of uncovering what you already know about oceans and coastlines: 2. Would you prefer to live along a coast or farther inland away from the ocean? What are the advantages and disadvantages of living along the coast?

9 Our Changing Oceans Self Reflection Survey Answer the following questions as a means of uncovering what you already know about oceans and coastlines: 3. Henry Boston identified three elemental sounds of nature: the sound of the ocean on a shore, the rain, and the wind in woods. Can you suggest three more?

10 Go back to the Table of Contents Go to the next section: Ocean Basins

11 Bathymetry (depth) of the Ocean Floor From what you learned about plate tectonics would you expect the depths to be the same throughout the world s oceans? The depth of the ocean (surface to floor) varies from zero meters (along the coast) to a maximum of nearly 11 km (7 miles) along the Mariana trench. Mt. Everest would sit in the trench with over 2,000 meters to spare! (More than 1500 people have stood atop Everest only 2 have visited the deepest region of the ocean floor). Average land elevation is less than 1 km, but average ocean depth is 3.8 km (2.3 miles). Volume of water in the oceans is nearly 10 times the volume of dry land that lies above sea level. If erosion leveled the continents, all the eroded material would fit in the ocean basins with room to spare!

12 Depth of the Ocean Floor The elevation of the ocean surface varies because the elevation of the ocean floor varies Bathymetry = the measurement of the depth to the ocean floor, and the mapping of its features Data from ships and submarines are combined with satellite data to reveal the topography of the ocean floor Ocean floor has mountains, valleys, and plains similar to those on land Masses of rock on the sea floor exert gravitational pull on the water causing it to pile up and form a mound on the ocean surface

13 Depth of the Ocean Floor Sea Level is assumed to be zero meters Sea Level changes are due to changes in the shape of the ocean basins, or long-term climate changes that trap water in ice caps or cause ice caps to melt. The sea surface has bumps and low points a satellite measures the difference in height between the bump over a volcano and the surrounding ocean. Radars on satellites are used to measure variations in gravity, revealing ocean floor topography.

14 Depth of the Ocean Floor The Four Major Depth Zones = Continental shelf, Abyssal plain, Oceanic ridge, Oceanic trenches Passive margin zones: Continental shelf, Continental slope, Continental rise, Abyssal plain Active margin zones: Continental shelf, Continental slope, Trench, Abyssal Plain

15 Depth of the Ocean Floor Zone 1 - The Continental Shelf The shallow ocean floor adjacent to the continent Submerged continental crust that slopes away from coast Maximum depth is a few hundred meters Wide when adjacent to passive margins, narrow when adjacent to active margins The width of the shelf decreases as sea level falls and increases as sea level rises

16 Depth of the Ocean Floor Checkpoint 13.1 On the following map, identify three active continental margins and three passive continental margins. (Don t worry about the x-y line)

17 Depth of the Ocean Floor During last N. Hemisphere glaciation when sea level dropped, the continental shelf off the coast of New Jersey was exposed. The Hudson River cut a deep, narrow canyon into the exposed shelf on its way to the lower sea level. The canyon was later submerged when sea level rose.

18 Depth of the Ocean Floor Zone 2 - The Abyssal Plain Continental Slope and Rise are the transition to the abyssal plain - Rapid deepening of the ocean (continental slope) leads to a gentler slope (continental rise) that ends at the abyssal plain - The continental slope is marked by a rapid deepening of the ocean (couple thousand meters) - Continental rise is where sediments swept off the slope accumulate Abyssal Plain = deep ocean floor Over 4 km deep and are some of the flattest portions of Earth s surface Covered by layers of very fine sediment May be dotted by seamounts (underwater volcanoes)

19 Depth of the Ocean Floor Zone 3 - The Oceanic Ridge The oceanic ridge system is a submarine mountain chain that can be traced around the world! Ocean floor rises from the abyssal plain to the ridge 90% of Earth s volcanic activity happens at ocean ridges Doesn t heat the water much (rapidly dissipates) Depth is ~3 km above ridge crest Central valley beyond ridge crest region of submarine hot springs (hot smokers). They are home to some strange life! A white crab and tubeworm colony found near a hot smoker

20 Depth of the Ocean Floor Zone 4 - The Oceanic Trench Active continental margins, where two plates converge, form an oceanic trench near the subduction zone Narrow and deep deepest places on Earth! Mark the place where oceanic lithosphere descends into the mantle 7 to 11 km (4 to 7 miles) deep

21 Depth of the Ocean Floor Checkpoint 13.3 Note the X-Y line on the world map (below left). Which of the profile views (below right) most accurately models the bathymetry of the ocean floor along that line? Hint: Think about where there is an active margin vs. a passive margin.

22 Go back to the Table of Contents Go to the next section: Ocean Waters

23 Ocean Waters Where did our oceans come from? Early Earth was a hostile, hot mass of nearly molten rock Violent volcanic eruptions put gases, including water vapor, into the air As Earth cooled this water vapor condensed into liquid water The more the planet cooled, the more water could collect in hollows ( baby oceans that grew into our present oceans) Although the water in the oceans has been around for ~4 billion years, the present ocean basin configuration is the result of plate tectonics, and no ocean basin is older than about 200 million years old Even now, oceans and seas continue to grow or shrink as plates diverge or converge

24 Ocean Waters: Water Chemistry The oceans are salty because seawater contains dissolved salts and minerals Most of the dissolved solids in seawater is common salt (NaCl) Salinity = the measure of the concentration of salt in seawater More salt = higher density Q: What variables might influence what parts of the ocean (locations around the globe) are saltier than others?

25 Ocean Waters: Salinity Salinity is influenced by: -Temperature -Mixing caused by currents -Freshwater input from rain, streams, and melting ice Salinity is highest where temp is high and precipitation is low (evaporation leaves behind salts)

26 Ocean Waters Q: Why is salinity not highest at the equator? A: More precipitation occurs over equatorial regions, diluting the waters there and thereby reducing salinity. Q: Why might the salinity near the Hawaiian islands be only 0.2 % different than the salinity off the coast of Antarctica? A: Ocean currents are efficient mixers and even out some salinity differences in the oceans.

27 Ocean Waters Checkpoint 13.5 Examine the map of mean salinity for the Indian Ocean. Explain why salinity values are lower for the tropical Bay of Bengal (east of India) than for the cold waters of the Southern Ocean just north of Antarctica? Why do you think salinity is so high in the Red Sea (small red strip between Africa and Saudi Arabia)?

28 Ocean Waters: Salinity Salinity of the oceans also varies with depth Rapid decrease in salinity with depth in upper 500 meters Rapid change in salinity with depth = halocline Deeper waters not affected by surface processes that change salinity (evaporation and stream flow) N-S profile through Pacific Ocean (150 W)

29 Ocean Waters: Temperatures Temperature varies according to latitude Ocean temperatures are affected by: - Solar insolation - Ocean currents Temperatures are highest where solar energy is highest.

30 Ocean Waters: Temperatures Water has a high specific heat (amount of thermal energy required to raise the temperature of 1 gram of material by 1 C). -The temperature of a material with a high specific heat will not rise as rapidly as one with a low specific heat. -Water can absorb a lot of thermal energy without displaying much of a change in temperature. Why is it important that water has a high heat capacity? -Water can absorb, store, move, and release a lot of heat energy. -This is of major importance to global climate patterns. The density of water decreases with increasing temperature

31 Ocean Waters Checkpoint 13.7 The specific heat of the water in the oceans is about 4 times that of the rock and soil on the continents. In addition, water in the oceans moves, while rock and soil are effectively stationary. What are the implications of these observations for differences in maximum and minimum temperatures of the oceans and continents?

32 Ocean Waters Warm water is less dense than cold water Below 4 C this changes the density of really cold water decreases, especially when it goes from liquid to solid form Shallow layers of ocean water: Relatively warm, warmed by solar radiation Relative uniform temperature as water is mixed by currents Thermocline = The depth zone where temperature decreases most rapidly

33 Ocean Waters: Temperatures Temperature of the oceans also varies with depth Rapid decrease in temps with depth in upper 500 meters Rapid change in temperature with depth = thermocline N-S profile through Pacific Ocean (150 W)

34 Ocean Waters: Density The third factor that affects density - pressure Uniform increase in pressure with depth slightly increases density of the underlying water Salinity, temperature, and pressure combine to create density profile. Pycnocline = rapid increase in density from 200 1,000 meters depth. Density is uniform below the pycnocline. Ocean water - 3 main vertical density layers: surface (2%), middle (18%), and bottom (80%).

35 Go back to the Table of Contents Go to the next section: Oceanic Circulation

36 Oceanic Circulation Ocean water is in constant motion! Circular patterns (gyres) of ocean currents.

37 Oceanic Circulation: Currents Winds move ocean water Friction between wind and surface water Ocean currents follow prevailing wind direction except where the current encounters a barrier (e.g. landmass) Only about 10% of world s ocean water is moving in surface currents Narrow, high temperature Gulf Stream

38 Oceanic Circulation: Currents Circulation patterns in atmosphere generate gyres Clockwise in N Hemisphere, counterclockwise in S Hemisphere Water takes months to years to complete a gyre circuit Fast-flowing boundary currents at western extents of gyres redistribute warm tropical water toward the poles (e.g. Gulf Stream, Brazil) Eastern portions of gyres carry colder water from high latitudes toward equator (e.g. Canary, Benguela)

39 Oceanic Circulation Checkpoint 13.9 A shipment of rubber elephants falls overboard in the northern Pacific at location A on the map below. What path do the elephants follow? (Refer to figure 13.15) A. A-G-B-F-E-A B. A-E-C-G-A C. A-G-C-E-A D. A-E-F-B-G-A

40 Oceanic Circulation Coriolis Effect: Atmospheric and oceanic circulation patterns deflected to right in N Hemisphere and to left in S Hemisphere Earth rotates from west to east Objects near equator are moving faster than those near the poles (more distance to cover in a day s rotation) The planet beneath the circulating wind/water moves its position, leading to the deflection Imagine you are in Panama City, FL. At noon you fire a rocket directly north towards Columbus, OH. The rocket has a northward velocity, but also has a faster easterly velocity due to Earth rotating east. The rocket will land east of the city of Columbus the apparent deflection.

41 Oceanic Circulation Checkpoint How would the deflection of ocean currents be altered in the Northern Hemisphere if Earth rotated from east to west (instead of from west to east)? a) Currents stay the same, deflect right of their course b) Currents stay the same, deflect left of their course c) Currents switch directions, deflect right of their course d) Currents switch directions, deflect left of their course

42 Continents can affect ocean circulation patterns Closure of Isthmus of Panama influenced circulation patterns in Atlantic Oceanic Circulation Western currents forced N Strengthened gulf stream Warmer waters into N Atlantic Raised temperatures in Europe Winters milder in Europe and N. U.S.

43 Oceanic Circulation Antarctica used to be mostly free of ice About 34 million years ago ice growth was triggered Separation of S. America and Australia from Antarctica Before this occurred, warm tropical waters moved south and warmed Antarctica The separation of Antarctica and South America opened up the strong currents in the Southern Ocean Isolates Antarctica from moderating ocean currents

44 Gulf Stream Oceanic Circulation: Thermohaline Circulation Carries high-salinity, warm waters from central Atlantic to higher latitudes Water slowly cools as it travels N Cold, salty water sinks to the bottom of N Atlantic near Greenland and Iceland Sinking water is then carried southward along bottom of the Atlantic (NADW) Reaches Antarctica and is diverted eastward to the Indian and Pacific Deep current eventually comes up in N Indian and Pacific Oceans (upwelling) brings nutrients to surface waters The pattern of deep currents is termed thermohaline circulation (driven by both salinity and temperature)

45 Oceanic Circulation Checkpoint A fish tank is filled with water at room temperature. Cold water is added on one side of the tank and warm water is added on the other side. The water at each temperature is dyed a different color to show its movement. Predict what will happen when warm water and cold water are added to the tank simultaneously. Briefly describe your prediction and sketch it in the drawing of the tank below.

46 Global Thermohaline Circulation

47 Normal Year Oceanic Circulation: ENSO El Niño and La Nina: The Earth system in action Pacific ocean waters heated Trade winds blow warm water west Cold upwelling occurs off coast of SA El Niño Year Western trade winds diminish Warm water remains in Pacific Heavy rains occur in SA Surface salinity decreases, reducing upwelling Droughts in western Pacific La Nina Year Cold conditions dominate Droughts in SA, western US Severe weather in western Pacific

48 Go back to the Table of Contents Go to the next section: Tides

49 Phases of the Moon Moon orbits Earth every 27.3 days New moon: Moon between Earth and sun Full moon: Earth is between Sun and moon

50 Tides Tides = changes in the sea surface height caused by the gravitational attraction of the moon (and a bit by the sun) a) Spring tides largest tidal bulges, highest tides b) Neap tides smallest tidal bulges, lowest tides Sun and moon exerting pull on the Earth in same direction. Occur during New Moon. Sun and moon exerting pull on the Earth in different directions.

51 Tides Checkpoint What would happen to spring tides if the moon were farther away from Earth? a)tides would be higher b)tides would be lower c) No change to spring tides

52 Tides Because the Earth rotates faster than the moon orbits, the location of the tidal bulge changes The moon is not always over the same spot on Earth Moon is essentially stationary while Earth rotates on its axis Imagine tidal bulges as stationary as Earth rotates below them A coastal site would rotate below two tidal bulges (high tides) on opposite sides of the Earth each day It would also pass through two minima (low tides) (Equal, but opposite, tidal bulges on the side of Earth away from Moon due to a balance of forces associated with gravitational attraction of moon, rotation of earth-moon system about a common center of mass called barycenter, and rotation of Earth on its axis)

53 Tides Depending on the position of the mood relative to Earth, and the latitude of a coastal site, the two daily tides may be very similar (semidiurnal) or varied (mixed). In panel b, notice that an equatorial coastal city would have a semidiurnal tide pattern, while at mid latitude the pattern would be mixed (very high on the right side of the image, and low on the upper left side).

54 Tides a) semidiurnal b) mixed c) diurnal

55 Tides Checkpoint Which tidal pattern is represented by the tide data for San Diego, California? a)semidiurnal b)diurnal c) Mixed

56 Tides Checkpoint Many planets have multiple moons. Discuss how the tides would be affected if Earth had two moons (A and B), each half the size of the current moon, in the following two scenarios: a) Assume the two moons followed the current orbit of the moon and were located on opposite sides of Earth (half an orbit apart; e.g., in the positions of the new moon and full moon). b) Assume the two moons followed the current orbit of the moon and were located one quarter of an orbit apart (e.g., in the positions of the new moon and the first quarter moon). Draw diagrams showing the locations of the moons relative to the Earth and the sun and illustrating how each scenario would change a typical semidiurnal pattern recorded on a tide gauge.

57 Go back to the Table of Contents Go to the next section: Wave Action

58 Wave Action: Open Ocean In the open ocean water simply bobs up and down. The wave shape (waveform) moves while the water particles follow a circular path and remain in place.

59 Wave Action: Open Ocean Wave size, speed, and direction are controlled by winds The waves we see in the ocean are the result of wind energy transferred to surface water Wave action affects only surface waters. Motion decreases downward to a depth equal to about ½ of the wavelength called the wave base. The deeper the wave base, the more volume of water involved in the wave.

60 Waves in the open oceans: What do you observe?

61 Wave Action Wind generated waves increase in size with increased wind speed Wind speed and distance over which wind blows determine the frictional force, and ultimately the wave height Large waves come from high velocity, steady winds blowing across a wide area with no obstructions Which ocean do you think has consistently taller waves the Atlantic or the Pacific? Why? Where do you think the largest waves (5-10 m) on Earth are found? Southern Ocean no continents to interrupt the distance over which winds blow.

62 As a wave approaches shore and shallower water it is slowed by friction, its length decreases, and it becomes taller and steeper. Wave Action Wave eventually collapses due to over-steepening (breaker). Water actually moves forward here.

63 Wave Action Checkpoint At which location on the following diagram would the waves begin to break farthest from the beach?

64 Wave Action a. Path of Hurricane Katrina is a station that recorded wind speed and wave height. b. Average wind speed for 10 minute intervals. c. Significant wave height. Notice correspondence between highest waves and fastest wind speeds.

65 Wave Action Rip Currents Narrow currents of water flowing through gaps in sandbars lying just offshore. Rip currents are caused by variations in the surf zone such as sandbars and channels. Do you see a location in the picture at right that might be dangerous if you were swimming there? Do you think you could see it from the beach? Rip currents cause ~100 deaths in the U.S. each year If you get caught in one let it sweep you out past the structure that is causing it. Once past it, swim parallel to the beach and then back toward shore.

66 Wave Action Irregularities in the shoreline or changes in seafloor can change shape and direction of the waves Can cause bending of the waves toward the shore (refraction)

67 Wave Action: Turning waves into energy Ocean waves are actually energy moving through the oceans If that energy could be harnessed, it would be clean and renewable What is the best location to build an ocean wave-driven power generation facility? What problems might you face?

68 Go back to the Table of Contents Go to the next section: Shoreline Features

69 Shoreline Features Shorelines are constantly changing as materials are eroded, transported and deposited through a process known as the sediment budget.

70 Shoreline Features What do waves do to coastlines? Cause erosion (wearing away headlands and filling in bays straightens out coastline) Transport material Deposit sand and other materials Twelve homes in Pacifica, CA were condemned when the cliff retreated 33 feet.

71 Shoreline Features Erosion rates of the coastlines along the Atlantic shore and Gulf coast are 3.3 ft per year on average Erosion is worst on loose, unconsolidated sediments, and can be accelerated by surges caused by storms

72 Shoreline Features Shorelines can also be experiencing deposition Shoreline grows in width with deposition of sediment Head on currents carry sediment onto and off the beach, and may deposit sand in sand bars off shore during storms Longshore currents transport sediment parallel to the beach in the surf zone Sand was moved left to right during a storm.

73 Shoreline Features Spit sand bar partially blocking a landform Baymouth Bar sand bar that completely blocks a channel The bay at Puget Sound, Washington. This narrow spit may become a baymouth bar.

74 Streams and Coastal Systems Checkpoint Place the terms/phrases in the correct location on the Venn diagram. 1. Erosion creates underwater channels. 2. Source: continental interior 3. Source: offshore sandbar 4. Sand deposited in bars 5. Erosion more pronounced in winter 6. Occur at my range of elevation 7. Occur at sea level 8. Erosion by wave action 9. Similar erosion rates 10. Longshore current 11. Mix of grain sizes 12. Uniform grain sizes

75 Go back to the Table of Contents Go to the next section: Shoreline Protection

76 Shoreline Protection Natural Features that protect coastal residents of Florida from erosion: -Tall dunes behind beaches protect against large storms -Wide, stable beaches absorb wave energy -Exposed offshore sand bars absorb the force of breaking waves These features are not found at all beaches. Humans can erect artificial barriers to help slow erosion, but these features may speed up erosion in other coastal locations.

77 Shoreline Protection The sediment budget = the balance between material deposited on the shore and material eroded from the shore. Humans can influence the sediment budget, and coastline features, by their actions. Damming on major rivers can result in sediment starvation because sediment that would have been deposited along the shoreline is trapped upstream. Humans can also build structures to try to combat dangerous erosion processes Seawall - Rock wall built to try and slow erosion of a cliff north of Monterey, Ca.

78 Shoreline Protection Groins wall-like structures built perpendicular to the shoreline as barriers to longshore currents Causes deposition on upcurrent side, but erosion on downcurrent side

79 Shoreline Protection Breakwaters barriers built offshore to protect part of the shoreline Slow the waves and allow the beach to grow behind them Unprotected parts of the shoreline often erode more quickly.

80 Shoreline Protection Checkpoint Compare and contrast seawalls and breakwaters.

81 Shoreline Protection Checkpoint Examine figure and explain why the shoreline erosion/deposition process at the site of Cape Hatteras required the lighthouse to be moved.

82 Oceans and Coastlines Concept Map Complete the concept map to evaluate your understanding of the interactions between the earth system, oceans and coastlines. Label as many interactions as you can using information from this chapter.

83 The End Go back to the Table of Contents