Shorelines and Oceans

Transcription

1 Shorelines and Oceans Ocean: a large body of water constituting a principal part of the hydrosphere Sea: a division of an ocean or a large body of salt water partially enclosed by land. Features of the world s oceans Processes of the oceans Ocean currents Tides Waves Seal level change Gravity flows Formally accepted as the fifth ocean by the International Hydrographic Organization in the Spring of 2000, the Southern Ocean extends from the shores of Antarctica to 60 degrees south latitude. Surface Area of the Planet (510,066,000 sq km) Ocean Sizes: Land Area on the Planet (148,647,000 sq km) 29.1% 1. Pacific (155,557,000 sq km) Ocean Area (335,258,000 sq km) 65.7% 2. Atlantic (76,762,000 sq km) Total Water Area (361,419,000 sq km) 70.9% 3. Indian (68,556,000 sq km) Type of Water (97% salt), (3% fresh) 4. Southern (20,327,000 sq km) 5. Arctic (14,056,000 sq km) Source: Features of the world s oceans 1

2 Continental shelves: from the shoreline to approximately 200 meters depth (on average). Most extensive on continental margins that are NOT at plate boundaries (termed passive margins). Gently dipping platform (0.1 degree slope, on average). 7.5% of total ocean area. Dominated by waves and storm-generated currents. Narrow along plate boundaries (termed active margins) Shelf Break: the seaward limit of the continental shelf. The slope into the ocean basin becomes markedly steeper. Average depth of about 200 m. Continental slope: relatively steep slope (4 degrees, on average) into the deep ocean. Submarine canyons locally cut the slope and extend up onto the shelf. 2

3 Cut by rivers when sea level was lower during the recent glaciation. Submarine fans are commonly present at the base of the canyons. Submarine fans are similar to deltas that form at the mouths of rivers. The Bengal Fan is 3,000 km long and 1,000 wide and is fed by the Ganges and Brahmaputa rivers. Continental rise: at the base of the continental slope; a gentle slope out into the deep ocean. Abyssal Plain: near horizontal floor of the ocean. The surface of the fan marked by channels that deliver sediment thousands of kilometres from the mouths of the rivers. Water depths range from 3 to 6 km below sea level. Relief as low as 3 metres over 1300 kilometres off the coast of South America. Hills locally protrude up through the sediment cover on the abyssal plain. Hills = relief on the underlying oceanic crust which has been blanketed by a cover of sediment. Pelagic ooze: the tiny shells of dead floating organisms mixed with fine mud derived from the land. The sediment cover thickens away from the Oceanic Ridge because the crust becomes older away from the ridge. 3

4 Newly formed crust at the ridge is young and has had little time to become deeply buried. The further from the axis the ridge, the older the crust and the thicker the sediment cover.increasing duration of time of deposition with distance from the ridge axis. Seamounts are large volcanic mountains, sometimes emerging as islands (e.g., Hawaiian Islands). The commonly occur in linear seamount chains because they form over hot spots in the upper mantle. As plate tectonics moves the volcanoes off the hotspots they become inactive. The Hawaiian Islands consist of eastern active volcanic islands and inactive volcanic islands to the northwest. 4

5 Further northwest of the islands are seamounts (underwater mountains that are submerged islands). Oceanic trench: deep, linear depressions on the sea floor (sites of plate subduction). The Tonga Trench, extends to 11 km below sea level, 7 km below the adjacent abyssal plain. 3-dimensional image of the Tonga Trench. Island arcs: replace the shelf break along many active margins. Very steep slopes from above sea level down to the floor of the trench. Backarc basins: marine bodies between the continental shoreline and an island arc. 5

6 Island arcs and their backarc basins border the northern and western Pacific Ocean. Processes in the world s oceans Any environment can be characterized by a series of processes and the response of sediment to those processes. Rivers: water flowing in one direction, bed load and suspended load transport, point bar migration, avulsion, etc. Deserts: blowing wind, sediment transport, abrasion, deposition, etc. Oceans: Currents Tides Waves Gravity flows Ocean currents Surface currents: governed by atmospheric circulation (i.e., surface winds). In polar regions cold water sinks as its density increases with cooling. This mass of sinking water causes deep ocean currents that flow along the sea floor. Vertical currents extend into the deep ocean. Some are due to displacement of water by surface currents. Where surface waters are blown away from a region by winds that water is replaced by rising water from the deep ocean. El Nino: a phenomena that is related to surface water circulation and vertical currents from the deep ocean. El Nino is Spanish for Christ child and refers to a warming of ocean surface waters off the coast of South America that sometimes occurs near Christmas. The frequency of El Nino varies from 2 to 7 years and the intensity of warming varies. The El Nino was the the most intense recorded over the 50 years for which data are available. 6

7 Normal circulation in the equatorial Pacific is referred to as "Walker circulation" Winds over the surface of the Pacific are generally from east to west. Moist, warm air blows against the western Pacific lands where it rises upwards and cooler, dryer air descends from the upper atmosphere along the coast of South America. Most precipitation is along the western side of the Pacific Ocean. The winds also drive surface waters of the ocean towards the west. Along the western Pacific sea level is, on average, 0.5 m higher than along the eastern Pacific. To replace the surface waters along the eastern Pacific cold, deep water rises towards the surface. These cold waters are rich in nutrients and support a rich population of sea life. The TAO Array is an array of sensor s descending from floating buoys that extend across the equatorial Pacific Ocean. The sensors measure water temperature to a depth of 500 metres. The surface waters along the western Pacific are several degrees warmer than on the eastern Pacific. 7

8 During an El Nino year anomalous Walker Circulation develops: two counter-rotating cells of atmospheric circulation. Cool dry air descends along the west and eastern sides of the Pacific and warm moist air rises in the central Pacific. Surface winds are much lighter than during normal conditions. The westward flux of warm surface waters does not take place and warm waters extend across the Pacific. Most precipitation falls on the mid-pacific and it is much dryer on the eastern and western sides of the ocean. Upwelling of cold, nutrient-rich waters ceases, greatly depleting fish stocks. Average surface water temperatures rise significantly (5 to 5 degrees C) along the eastern Pacific. The anomalous Walker circulation sets up a pattern of zones of high and low pressure that effect weather around the world. These, in turn, cause many regions to be wetter, warmer, colder and/or dryer than normal. 8

9 Map showing the difference in average temperatures (degrees C) during the months of December, January and February of an El Nino Year compared to the long term average. Map showing the difference in average precipitation (cm of water) during the months of December, January and February of an El Nino Year compared to the long term average. 1. Australia: drought and bush fires. 2. Indonesia, Philippines: crops fail, starvation follows. 3. India, Sri Lanka: fresh water shortages. 4. Tahiti: 6 tropical cyclones. 5. South America: fish industry devastated. 6. Coral reefs die across the Pacific Ocean. 7. Colorado River basin: flooding, mudslides. 8. Gulf states: downpours cause death & property damage. 9. Peru, Ecuador: floods, landslides. 10. Southern Africa: drought, disease, malnutrition. Tides For current surface water temperatures in the Pacific go go: Influences all of the world s marine coastlines. Due to the gravitational interaction between the Earth, Moon and Sun. Without the moon the oceans would form a sphere, of uniform thickness, about the Earth. 9

10 Forces acting on this sphere include: 1. Earth s gravity (pulling the ocean towards Earth) 2. Centrifugal force due to Earth rotation (pushing it away from Earth). Force 1 > Force 2 so that the oceans (and us) remain on the planet. The Moon exerts a strong gravitational force on the Earth. The gravitational force is strongest on the side of the Earth facing the Moon, weakest on the opposite side. The Earth s centrifugal force is equal to the average lunar gravitational force. On the side of the Earth facing the Moon its gravitational force pulls the ocean surface outwards. On the opposite side of the Earth the centrifugal force is greater than the gravitational force. Water is deepest beneath the bulges and becomes shallower away from the bulges. As the Earth rotates about its axis, points on its surface move in and out of the bulges on the ocean surface. The centrifugal force pushes the ocean surface away from the Earth. Result: two bulges of the ocean surface on opposite sides of the Earth Therefore, with respect to the Earth s surface, the height of the ocean surface moves up and down twice over the period of one complete rotation (one 24 hour day). 10

11 Water depth has two maximums per day (one for each bulge) and two minimum s per day. Moon tides, alone, would produce a uniform, symmetrical oscillation in the water surface over time. This rising and falling of the water is responsible for tides. The Sun also exerts a gravitational attraction on the Earth that influences the ocean surface. Sun has 27,000,000 times the mass of moon. Sun is 390 times more distant from Earth. Sun s gravitational attraction on Earth is 46% that of the moon. The Sun s gravity creates another pair of bulges that either add or subtract from the lunar bulges, depending on the position of the Moon with respect to the sun. New Moon: Sun and Moon are in line and their bulges merge to form larger bulges. First Quarter: Sun and Moon align at right angles and the resulting bulges are smaller. 11

12 Full Moon: Sun and Moon are in line and their bulges merge to form a larger bulge. Third Quarter: Sun and Moon align at right angles and the resulting bulges are smaller. Due to this interaction between bulges, the tides vary over the lunar month. New Moon and Full Moon: highest tides (spring tides). First and third quarters: lowest tides (neap tides). Along the shoreline tides flow inland as the tidal wave rises (termed flood tide). The tidal range is the difference in the water level from high to low tide. As the tidal wave falls the waters flow seaward (ebb tide). Tidal range varies with the geometry of the coastline and the location of the coast on the Earth s surface. 12

13 Open coasts: generally 1.5 to 2 m tidal range. Narrowing basins the tidal range increases (e.g., Bay of Fundy, up to 16 meter tidal range). The incoming tide at the Bay of Fundy is constricted due to the narrowing of the basin inland, forcing the tidal waters into an increasingly smaller area so that the water depth increases. Widening basins the tidal range decreases (e.g., Gulf of Mexico, tidal range of less than 1 meter). Where are the world s highest tides? Burntcoat Head, Cobequid Bay, Bay of Fundy Nova Scotia: >16 m. Leaf River Estuary, Ungava Bay, Quebec: 18 m Tidal ranges over the daily cycle are not commonly the same. When a the tidal wave (bulge) reaches a landmass it reflects a wave back that will interfere with the next bulge as the Earth rotates through it. The interference of these waves causes considerable variation in tidal ranges within a daily cycle 13

14 Semidiurnal tides: two tidal cycles per day which are very similar in range. Diurnal tides: one tidal cycle per day. The inrush and outflow of the tides creates currents that reverse in direction every 6 hours for semidiurnal tides and every 12 hours for diurnal tides. These currents are important in moving sediment in tidally influenced environments. The energy of the tides is also harnessed to produce electricity at some locations by directing the tidal currents through turbines. The Annapolis River (Bay of Fundy) has had a tidal power station since Estimated potential electricity for the Bay of Fundy: 6,000 megawatts (Peak Ontario consumption = 22,000MW) Estimated tidal electricity world-wide: 3,000,000,000 megawatts Only 2% of this could be extracted so that the resource is: 60,000,000 megawatts Some negative implications include: Loss of coastal wetlands, home to a variety of organisms, including sea birds. Changes in sedimentation patterns (channels may become blocked by sediment). A tidal power plant in the Minas Basin (Bay of Fundy) might raise tides along the coast of Cape Cod. Tides and Earth Rotation? The Earth spins due to forces that acted near the time that it formed. The relative movement between the Earth and the Oceans under the gravitational influence of the Sun and the Moon causes friction. This friction acts to very gradually slow the rate of rotation of the Earth. Waves Waves on the surface of the oceans are an important process for erosion, sediment transport and deposition. A wave propagates across the water surface its passage involves the rising and falling of the surface. Most waves are sinusoidal in form and are characterized by their: Over the last few centuries the rate of slowing is seconds per century. When complex life first developed on the Earth (about 570 million years ago) the day was 20.6 hours long. 14

15 Length, the horizontal distance from crest to crest. Waves of a wide range of scale are present in the world oceans. Height, the vertical distance from trough to crest. Celerity, the speed at which the wave travels. Waves are commonly characterized by their wave period, the time (usually in seconds) that it takes for one wavelength to pass a point on the water surface. Wave period = wave length/celerity Wind-generated gravity waves: waves the are produced by winds blowing over the water surface. Storm waves: generated by particularly high winds during storms. Tsunamis: the waves that are generated by underwater landslides, earthquakes and volcanic explosions. Tides Wind-generated waves are the most important process for erosion, sediment transport and deposition along many of the world s shorelines. Waves can move sediment on the bottom out to the edge of the continental shelf. Wind speed controls the size and energy of the waves. With increasing wind speed: Wave length increases Wave height increases Fluid motion under surface waves With the passage of a wave the water surface rises and falls. Fluid beneath the wave follows a circular path called a wave orbital. Celerity (and wave period) increase. Wave orbital diameter depends on the height and length of the waves and the depth below the water surface. 15

16 Orbitals diameter diminishes with increasing depth beneath the surface. At a depth of ½ of the wavelength the orbitals are very small and fluid motion is negligible. Transitional waves when water depth is <½ of a wavelength but >1/20 of a wavelength. Under transitional waves the orbitals become flatter as they approach the bottom. Deep water waves: when water depth is > ½ of a wavelength of the surface waves. At the bottom the orbitals are flat and the motion of the water is back and forth (oscillating motion). Orbital velocities are greatest under the crests and troughs but in opposite directions. The motion of sediment on the bed is similarly, back and forth. The larger height, length and period the more powerful the oscillating currents. As waves approach a shoreline the water shallows and they change from deepwater to transitional waves. When surf reaches the beach it rushes up the beach face as swash and then runs back down the slope as backwash. As water shallows the waves steepen and finally break to form surf which surges towards the shoreline. Swash and backwash moves sediment up and down the beach face. 16

17 When waves approach at an angle to a shoreline their crests bend to become more parallel to the shoreline (termed wave refraction). The more nearshore portion of a wave will propagate more slowly than the more offshore portion due to the interaction with the bed. The deeper portion of a wave will catch up to the shallower portion, causing the wave crests to become more parallel to the shoreline. As a result, when waves break they are nearly parallel to the shoreline. With wave approach water is driven towards the shore. This causes currents that flow along the shore, termed longshore currents. Longshore currents will transport sediment along the shoreline. Where there is a sharp bend in the shoreline the currents will maintain a path that diverges from the shoreline. These currents can build beaches that extend out into the body of water. Such bodies of sand are termed spits and can form very long sandy beaches for considerable distance offshore. 17

18 When longshore currents transport sediment across the mouth of a bay, a spit may extend across the mouth. When the spit isolates the bay entirely it is called a baymouth bar. Longshore transport of sediment can cause problems for landowners and shipping in many areas. Groins are walls that extend perpendicular to the shoreline and act to trap sediment moving along shore. Jetties are walls that extend parallel to channels that enter a water body; they inhibit longshore drift from blocking the channel. 18

19 Rip currents are flows that move offshore at regular intervals along a beach. Shoreline geomorphology The profile of a shoreline includes: The submerged shoreface. Shoaling zone: where waves begin to interact with the bottom. Breaker zone: where waves steepen and collapse (break). Surf Zone: where the water surges towards the land. 19

20 The foreshore, where swash and backwash move up and down the beachface. The backshore. The berm or ridge is a linear mound at the top of the beachface. The runnel is a linear trough on the landward side of the berm. Windblown dunes are common inland from the beach (due to the abundance of sand). During storms the entire shoreline is modified by the powerful waves. Sand is eroded from the foreshore and backshore and transported onto the shoreface. When fair weather returns the shoreface sands move onshore, replenishing the beach. Along the west coast of North America most major storms are in the winter. Winter storms erode the beaches annually. 20

21 Deep Ocean Processes Fine-grained sediment (mud) is deposited by settling from the water column dominates the deep ocean. Sediment includes mud derived from the land and the microscopic shells of floating organisms. Sediment gravity flows: mixtures of water and sediment that flow downslopes due to gravity. On land mudflows and avalanches are a type of sediment gravity flow. Turbidity currents are made up of suspended sand and mud mixed in turbulent waters. Turbidity currents can cut channels, build levees and construct extensive submarine fans. Some erosion of submarine canyons may be due to turbidity currents. The currents can flow for many hundreds of kilometres, down the continental slope and out onto the abyssal plain. An important mechanism for transporting relatively coarse sediment (sand) into the deep ocean. The first and largest recorded turbidity current occurred in 1929 off the coast of Newfoundland. Most large submarine fans are located where rivers deliver large quantities of sediment to the oceans (e.g., the Indus and Ganges fans of the Indian Ocean). An earthquake caused a massive slump on the continental slope. The earthquake caused the largest loss of life of any historical earthquake in Canada: 28 people died due to the tsunami that was caused by the submarine landslide. 21

22 100 km 3 of sediment was involved in the current. The turbidity current flowed for hundreds of kilometres, snapping underwater telephone and telegraph cables along the way. The current flowed at speeds up to 95 km/hr and covered an area of 100,000 km 2. Sea Level Change Sea level changes over a wide range of time scales. Over many millions of years the largest fluctuations in sea level are due to changing spreading rates of the oceanic ridge. During the Cretaceous Period (144 to 66.4 million years ago) spreading rates were very rapid. Much of western Canada was covered by an inland sea. The oceanic ridge had a very large volume and sea level was hundreds of metres higher than today. In addition to the larger ridge, global temperature was the highest of the past 600 million years. During the period of highest sea level temperatures were too warm to maintain the ice caps that we have today. Continental glaciation causes a lowering of sea level because large volumes of water remain on land as ice. 18,000 years ago, during the last major glaciation, sea level was 140 metres lower than today. Their melting contributed to the higher sea levels. Fluctuations in temperature over the past several million years led to the growth and decay of ice caps and resulted in large fluctuations in sea level. 22

23 Much of the continental shelves were exposed with shorelines 100 to 200 kilometres from their current position. If the current ice sheets melted entirely much of the coastal lands will be inundated by the oceans. Modern rates of sea level change are known from long term records. At present sea level is rising by 2 mm per year. Global warming is expected to cause an increase in the rate of sea level rise: 18 cm above present by 2030 (5.6 mm/yr, average) 44 cm above present by 2070 (6.1 cm per year average) Disintegration of Modern Ice Sheets Animated Earth with changing sea level. ( Ice sheets are the most extensive glaciers (>50,000 km 2 ). Flow outward, from the centre of thickest ice. A continental ice sheet Ocean x Ice sheet Ice y At the shores of the land mass the ice sheet may flow onto the ocean to form an extensive, floating ice shelf. x Ice Shelf y Greenland: 80% of total land mass is covered by an ice sheet. Average thickness 1.5 km, 3 km locally. 23

24 Antarctica: Ice sheet covers 13.5 million square kilometres. Larsen Ice Shelf: a relatively small ice shelf that is breaking up. Exceeds 4 km in thickness. Bounded by extensive Ice Shelves; e.g., Ross Ice Shelf: total area of 500,000 km 2 Regional temperature has increased by 2.5 degrees C over the past 50 years. In February 1995 a major storm initiated the breakup of Larsen A Ice Shelf. The largest iceberg was 70 km long and 25 km wide. Larsen Ice Shelf continues to break up. Hundreds of icebergs 1 2 km in size were liberated by the breakup. 24

25 March 2003 Larsen B disintegrated. Over the year following the disintegration of Larsen B the glaciers that fed the shelf increased their flow rates from 1.7 m/day to 3.1 m/day (a 250% increase). Implications: Suggests that global warming may lead to breakup of other shelves. Such breakup leads to an increase in the rate of flow of glaciers into the Ocean. This, in turn, will increase the rate at which sea level rises with global warming. There is evidence that just such an event took place in the geologic past, when average global temperature was a few degrees warmer, raising sea level by more than 6 metres. In March, 2003 Iceberg B-15 broke off the Ross Ice Shelf. One of the largest icebergs ever seen, it was 300 km long and 40 km wide (11,000 km 2 ). The Ward Hunt Ice Shelf on the coast of Ellsmere Island (the Arctic s largest ice shelf) broke up in 2003 due to climatic warming. The Point: With global warming the ice shelves are breaking up. This is expected to increase the rate at which glacier ice flows into the world s oceans. This will lead to an increase in the rate of sea level rise. 25

26 18,000 years ago Images are from William Haxby at the Lamont-Doherty Earth Observatory Melting of the Western Antarctic Ice Sheet Melting of the Eastern and Western Antarctic Ice Sheet 18,000 years ago Melting of the Western Antarctic Ice Sheet 26

27 Melting of the Eastern and Western Antarctic Ice Sheet 18,000 years ago Melting of the Western Antarctic Ice Sheet Melting of the Eastern and Western Antarctic Ice Sheet 18,000 years ago Melting of the Western Antarctic Ice Sheet 27

28 Melting of the Eastern and Western Antarctic Ice Sheet 28