Geology Cafe

Introduction to Geology

Chapter 14 - Ocean and Shoreline Processes

Despite being the 21st century, Earth's ocean basins remain mostly unexplored. Oceans cover more the 70% of the Earth's surface. Geologic investigations show that the oldest oceanic crust is only about 180 million years old, where rocks exposed in continents may be billions of years old (Figure 14-1). This chapter briefly reviews the origin of the world's ocean basin through Plate Tectonics Theory, then examines the physical environments ranging from the deep ocean to the shorelines along ocean margins. Factors including waves, currents, and tides influence erosion along coastlines. Stable and passive continental margins have very different kinds of coastal landscape features. The modern continental shelves around the world were exposed as coastal plains before the melting of the continental glaciers at the end of the Pleistocene Epoch that caused sea level to rise almost 400 feet (120 meters). Age of the seafloor crust
Fig. 14-1. Age of the seafloor

Plate Tectonics and the Origin of Ocean Basins (a general review)

Plate Tectonics Theory explains the evolution of ocean basins through geologic time (Figure 14-2, the theory is explained in detail in Chapter 5). Plates of crustal rocks "float" on the partially fluid upper mantle region called the asthenosphere. Because rocks in continental crust are less dense than rocks in oceanic crust, continents isostatically rise above the sea level of the global oceans (Figure 14-3). New ocean crust is formed along the spreading centers associated with the mid ocean ridges (such as the Mid-Atlantic Ridge, Figure 14-4). In the Late Permian Period (about 260 million years ago) all of the continents were assembled together in an ancient landmass called Pangaea (Figure 14-5). This ancient landmass gradually split apart, forming the Atlantic Ocean and rearranging the continents to where they occur today. As new ocean crust forms along ocean ridges, it is destroyed along subduction zones where some of the material sinks back into the mantle, but the lighter material becomes incorporated into new continental crust associated with volcanic arc chains (Figures 14-6). Figure 14-7 shows how the Pacific Plate has migrated over the Hawaiian Hotspot, creating the Emperor Seamount Chain, with the old Pacific Plate crust sinking into subduction zones associated with the volcanic arcs located along the western margin of the Pacific Basin region.
Seafloor spreading Isostacy and the density of crustal rocks Map of the Atlantic Basin
Fig. 14-2. Plate Tectonics Theory examines the processes occurring along plate boundaries including divergence, convergence and transform faulting. Fig. 14-3. Isostacy of crustal rocks compared with wood blocks floating on water. There would have to be more "light" crust under mountain ranges than under oceans. Fig. 14-4. Mid-Atlantic Ridge is a spreading center region. New ocean crust is forming along the mid-ocean ridge system that extends beneath parts of the world's oceans.
Pangaea Plate Tectonics Hawaiian hotspot
Fig. 14-5. Ancient super continent Pangaea formed in the late Paleozoic Era and split apart in the Mesozoic Era, forming the Atlantic Ocean Basin and other changes... Fig. 14-6. Generalized diagram showing Western North America's active margin located on boundary between the North American and Pacific Plates. Fig. 14-7. Hawaiian hotspot and Emperor Seamount Chain show how the Pacific Plate has moved over time, with subduction occurring along the ocean margins.

The Coriolis Effect on Atmospheric and Ocean Circulation Systems

Heat from "insolation" (INcoming SOLar radiATION) is the driving force behind the fluid motion of the atmosphere and the oceans. However, the patterns of motion are also influenced by the forces created by the rotation of the Earth on its axis. Because air and seawater have mass, they maintain momentum when moving from a location of high pressure to low pressure. However, because the earth is rotating, the rotation causes a right-turn deflection in the Northern Hemisphere and a left-turn deflection in the Southern Hemisphere (Figures 14-8 to 14-10).

The global atmospheric circulation system influences the movement of air masses in general "belts" that move air in rotating masses within zones around the planet. These relatively stationary wind belts impact the surface of the oceans, creating currents that circulate waters in the oceans, creating five large gyres (Figure 14-10) and lesser rotational currents in other regions (such as in Figure 14-11 for the Pacific Ocean).

Ocean circulation is also influenced by seawater temperature and density. Cold and salty water (concentrated by surface evaporation) sinks. Elsewhere seawater rises where it is displaced by colder and saltier water (Figure 14-12). Warm water in the tropics flows in currents (like the Gulf Stream, Figure 14-13) to polar regions where it cools and the formation of sea ice concentrates the salt in seawater, increasing its density so that it sinks.
Coriolis Effect World wind zones Gyres in the global ocean circulation system
Fig. 14-8. The coriolis effect is cause by the rotation of the Earth on its axis. This rotation causes air masses moving from high to low pressure to deflect. Fig. 14-9. Global wind circulation patterns impact regional climates and drive the large current systems in the global ocean circulation system. Fig. 14-10. Five large gyres circulate surface waters in the global oceans. These rotating gyres are influenced by the patterns of atmospheric winds and the coriolis effect.
Pacific Circulation System Ocean Circulation System Gulf Stream
Fig. 14-11. The Pacific Gyre and subordinate currents in the northern Pacific Ocean basin. Fig. 14-12. Cold and salty ocean water is dense and sinks, warm water stays at the surface. Fig. 14-13. The Gulf Stream is the world's largest ocean current (revealed here by water temperature patterns)

The Coriolis Effect Influences Superstorms

Large rotating storms are called hurricanes (near North America), typhoons (near Southeast Asia) and cyclones (in the Indian Ocean). All are the same, caused by warm moist winds being drawn to the center of low pressure near the center of the storm (called the "eye" in well developed storms). North of the equator the coriolis effect causes low-atmospheric pressure to rotate counterclockwise, but south of the equator they rotate in a clockwise direction. The lower the air pressure in the eye of the storm, the greater the wind speed and rotation. Note on the map in Figure 14-13 that there are no hurricanes along the equator or near the poles. These are regions where the coriolis effect is not a significant force in deflecting storm winds to cause rotation.
Storm paths of Hurricanes, Typhoons, and Cyclones Atlantic hurricane Hurricane Katrina
Fig. 14-13. World map showing historic paths of hurricanes, typhoons, and cyclones. The large storms are the same (different names for different regions); storm rotation is influenced by the Coriolis Effect. Fig. 14-14. Superstorms like Hurricane Andrew (1992) can erode and redeposit vast quantities of sediments, both offshore and onshore. Hurricanes and other storms rotate counterclockwise in the Northern Hemisphere. Fig. 14-15. The eye of a hurricane is the center of low pressure. Hurricane Katrina (2005) shown here, was the most costly and destructive hurricane disaster in US history, killing more than 1,800 people.

The Continental Margin: Shelf, Slope, and Rise

Bordering continental landmasses are submarine geographic regions located between the shoreline and deep ocean (Figures 14-16 to 14-19).

A continental shelf is a submerged nearshore border of a continent that slopes gradually and extends to a point of steeper descent to the ocean bottom. During the peak of the last ice age, the world's continental shelves were mostly exposed coastal plain environments.

A continental slope is the slope between the outer edge of the continental shelf and the deep ocean floor. The continental slope is cut by submarine canyons in many locations.

A shelf break is a general linear trend that marks the boundary between the relatively flat continental shelf and the dropoff into deeper water on the continental slope. The shelf break generally follows the ancient shorelines that existed at the peak of the continental glaciation periods of the ice age when sealevel was as much at 400 feet (120 meters) lower that present sealevel.

A continental rise is a wide, gentle incline from a deep ocean plain (abyssal plain) to a continental slope. A continental rise consists mainly of silts, mud, and sand, deposited by turbidity flows, and can extend for several hundreds of miles away from continental margins. Although it usually has a smooth surface, it is sometimes crosscut by submarine canyons extending seaward of continental slope regions. The continental rise is generally absent in regions where deep-sea trenches exist where subduction zones are active.

abyssal plain—an underwater plain on the deep ocean floor, usually found at depths between 3000 and 6000 meters that extends from the continental rise (continental lithogenic sediments accumulate along continental margins) to the distant deep ocean basin where continental-derived sediment deposition is not significant.
Depositional Environments Ocean margin features
Fig. 14-16. Depositional environments. Most sediments eventually end up being deposited offshore. Fig. 14-17. Submarine landscape features associated with a continental margin to ocean basin.
Monterey Bay Monterey Canyon and Big Sir region
Fig. 14-18. California central coast topography and bathymetry showing shelf, slope and rise (offshore) Fig. 14-19. Monterey Canyon and other features carved by gravity-driven turbidity currents (discussed below).
Turbidity Currents and Development of Submarine Canyons and Fans

A turbidity flows is a turbid, dense current of sediments in suspension moving along downslope and along the bottom of a ocean or lake. In the ocean, turbidity currents can be massive episodic events. They typically form and flow down through a submarine canyon (carved by previous turbidity flows) and accumulate near the base of the continental slope on deep-sea fans. Turbidity flows produces deposits showing graded bedding (Figure 14-20). Slowing turbid currents drop their coarser fractions first (gravel and sand) and the finer silt and clay fractions settle out last.

A deep-sea fan is a fan- or delta-shaped sedimentary deposit found along the base of the continental slopes, commonly at the mouth of submarine canyons. Deep sea fans form from sediments carried by turbidity flows (density currents) that pour into the deep ocean basin from the continental shelf and slope regions and then gradually settle to form graded beds of sediment on the sea floor. Deep-sea fans can extend for many tens to hundreds of miles away from the base of the continental slope and an coalesce into a broad, gently sloping region called a continental rise.

Graywacke is a fine-to-coarse-grained sedimentary rock consisting of a mix of angular fragments of quartz, feldspar, and mafic minerals set in a muddy base (commonly called a "dirty sandstone or mudstone" because of its mixed size fractions). Graywacke is the general term applied to sediments deposited by turbidity flows, and they commonly show graded bedding. Graywacke is common in the Coast Ranges of California and other active continental margin regions. It is exposed on land where tectonic forces push up rocks that originally formed in the deep ocean (examples in Figures 14-21 to 14-23). "Turbidites" (deposits associated with turbidity flows) commonly appear as interbedded layers of sandstone and shale. Conglomerate typically occurs in thicker beds and were originally deposited as gravel and mud on ancient submarine fans closer to the mouths of submarine canyons or in channels carved into the seabed.
Turbidity current and deposits Gazos Creek
Fig. 14-20. Turbidity flows are essentially underwater landslides or density-driven currents. Sediments laden with sediment are heavier than clear seawater. Fig. 14-21. Seas stacks composed of submarine channel deposits (mostly conglomerate) exposed at Gazos Creek State Beach, California
Turbidites exposed on Loma Prieta Turbidites at Bean Hollow State Beach
Fig. 14-22. Cretaceous age turbidites exposed on Loma Prieta Peak, Santa Cruz Mountain, California Fig. 14-23. Cretaceous age turbidites (turbidity current deposits) at Bean Hollow State Beach, California

"Passive" vs. "Active" Continental Margins

In North America, the Pacific Coast is an "active continental margin" whereas the Atlantic Coast is a "passive continental margin,"

An active continental margin is a coastal region that is characterized by mountain-building activity including earthquakes, volcanic activity, and tectonic motion resulting from movement of tectonic plates. Active continental margins typically consists of sediments from the sea floor are scraped from the oceanic plate and plastered onto the edge of the continent. Active continental margins are also associated with subduction zones, often include a deep offshore trench, and occur in places where leading edge of a continent is overrunning oceanic lithosphere. The Pacific Coast is an active margin that is characterized by narrow beach, steep cliffs, rugged coastlines with headlands and sea stacks (see features discussed below).

Passive continental margins occur where the transition between oceanic and continental crust which is not an active plate boundary. Examples of passive margins are the Atlantic and Gulf coastal regions which represent setting where thick accumulations of sedimentary materials have buried ancient rifted continental boundaries formed by the opening of the Atlantic Ocean basin. The Atlantic Coast of the United States is characterized by wide beaches, barrier islands, broad coastal plains (see features discussed below).

Emergent and Submergent Coasts: In some regions around the world, tectonic forces are pushing rocks up along coastal regions, mostly in regions associated with active continental margins. There areas are called emergent coasts and display features including sea cliffs and marine terraces (see below). Where sea level is rising faster than land is rising, or where coastal areas are sinking, it is called a submergent coast. Submergent coasts are associated with passive continental margins with wide coastal plains and continental shelves. Estuaries are associated with submergent coastlines formed when sea level rises and floods existing river valleys. Active margins can have both emergent and submergent coastlines in close proximity to each other.

active and passive margins North Carolina Outer Banks satellite view
Fig. 14-24. Active and passive margins of North America. The East Coast and North Slope are now passive margin regions located within the greater North American Plate. Fig. 14-25. Passive margin: North Carolina's Outer Banks region showing coastal plain, rivers, tidal estuaries, lagoon, barrier islands, and shallow Atlantic continental shelf
Satellite view of San Francisco and Monterey Bay region Satellite view of the Central Coast region
Fig. 14-26. Active margin: San Francisco Bay and Monterey Bay region has actively rising coastal range mountains and sinking coastal basins Fig. 14-27. Active margin: the Big Sur, Santa Lucia Range, Salinas Valley, Gavilan Range, and southern Diablo Range of central coastal California
Features associated with emergent coastlines on active continental margins

Emergent coastlines typically have sea cliffs carved by wave and current action along the shoreline. The geometry of a coastline is largely a reflection of how some rocks along a coastline are more resistant to erosion.

Headlands
are rocky shorelines that have resisted wave erosion more than surrounding areas, forming points or small peninsulas that jut seaward. Small sandy beaches typically occur in bays between headlands (Figure 14-28).

Sea stacks
are large rocky outcrops that have resisted wave erosion and stand offshore as the beach and sea cliff continues to erode landward (Figure 14-29).

A wave-cut bench is a flat bench-like platform of rock that form by wave erosion at the base of a an actively eroding sea cliff on an emergent coastline (Figure 14-30).

A sea cave is an underground passage or enclosed overhang carved into a sea cliff carved by focused wave action.

A sea arch is a natural rock arch caved by wave action (Figure 14-31).

Marine terraces are elevated step-like benches formed by the combined effects of long-term wave erosion during the rise and fall of sea level on an emergent coastline
Point Reyes Headlands sea stacks at Olympia National Park
Fig. 14-28. Headlands and bays at Point Reyes National Seashore. Fig. 14-29. Sea stacks along the coast at Olympic National Park, WA
Wilder Ranch State Park, wavecut bench and sea cave sea arch at Natural Bridges State Park, Santa Cruz
Fig. 14-30. Wave-cut benches and a sea cave at Wilder Ranch State Park, Santa Cruz, CA Fig. 14-31. A sea arch at Natural Bridges State Park, Santa Cruz, CA
Elevated marine terraces on the California coastline.

California preserves much evidence of geologic, geographic, and climatic changes caused by ice ages. During the last ice age, alpine glaciers and ice caps covered upland regions in the Sierra Nevada Range and Cascades volcanoes, but lower elevations were ice free (Figure 14-32). The formation of continental glaciers in North America and Europe caused sea level to fall almost 400 feet, causing the shoreline to migrate seaward as much as 10 to 70 miles westward of the current coastline in some locations. This rise and fall of sea level happened with each glaciation cycle (of which there were many through the ice ages of the Pleistocene Epoch). In places where the California coastline is slowly rising, each of the major glaciation cycles is preserved as a step-like bench, called a marine terrace. The formation of marine terraces is illustrated in Figure 14-33. Examples in northern and southern California re illustrated in Figures 14-34 and 14-35.
California glaciation
Fig. 14-32. California at the peak of the last ice age. Glaciers covered the higher mountains, lakes filled inland valleys, and a coastal plain was extended offshore.
Formation of marine terraces
Fig. 14-33. Formation of marine terraces. This example shows the formation of two terraces. At least seven major terrace levels are preserved in some areas along the California coast.
Davenport marine terraces
Fig. 14-34. Marine terraces at Davenport, California
Marine terraces on San Clement Island, CaliforniaFig. 14-35. Step-like marine terraces on San Clement Island located offshore in southern California

Biogenous Sediments in the Marine Environment

Biogenous sediments are sediments mostly composed of the remains of organisms (including skeletal remains of both microplankton (plants and animals), plant remains (wood, roots, and leaves) and remains of larger animals including shells of invertebrates, such as shells, coral fragments, and vertebrate teeth and bone, and excrement left behind by any type of organism. Biogenous sediments may be partly mixed with lithogenic sediments (continental-derived sediments) in coastal regions, particularly where streams and rivers contribute sediments. In many passive margin regions in tropical regions, carbonate sediments form and accumulate forming deposits both offshore and along coastlines (see below).

Coral Reefs

A "reef
" is a general name for a ridge of jagged rock, coral, or sand just above or below the surface of the sea. A coral reef is one that is made of skeletal material composed of coral, coralline algae, and other carbonate skeletal material. Figure 14-36 illustrates the variety of settings and features associated with carbonate depositional environments.

Over time lime sediments produced by biological activity will accumulate in and around coral reefs and in warm, shallow marine water settings. Wave action and currents will erode and redistribute lime sediments offshore where it may accumulate, building carbonate platforms (becoming regions underlain by limestone). Examples of carbonate platform regions include the Bahamas, South Florida, and the Yucatan Peninsula (Figure 14-38).

The world's largest reef system is the reef tracts, islands, and tidal shoals associated with the Great Barrier Reef located along the east coast of Australia (Figure 14-39). The Great Barrier Reef is composed of over 2,900 individual carbonate reefs and about 900 islands stretching for over 1400 miles (2,300 km) along the northeast coast of Australia and encompassing about 133,000 square miles (344,400 km2). It is the largest feature of biological origin on Earth. Similar reef tracts have formed throughout geologic history in other locations around the world.
Carbonate depositional environments South Florida satellite view
Fig. 14-36. Carbonate depositional environments include coral reefs, keys, shoals, tidal flats, bays, and other coastal and offshore features. Fig. 14-37. South Florida is part of a growing carbonate platform with the Keys consisting of an ancient and modern forming a barrier reef complex.
Gulf of Mexico
Fig. 14-38. Carbonate platforms surround much of the Gulf of Mexico. They include continental shelf regions around the Yucatan Peninsula, South Florida, and islands of the Caribbean where Biogenous sediments form and accumulate.
Great Barrier Reef
Fig. 14-39. Great Barrier Reef - The world's largest organic deposit. The growth of the great reef tract has kept pace with the global rise in sea level since the end of the Wisconsin ian ice age.

Seamounts, Islands, Atolls, and Guyots

A seamount is any isolated mountain-sized feature that rises above the seafloor. A seamount may be a large tectonic block that separated from a large continental landmass or may be an ancient or even active submarine volcano. A submarine mountain that is partly exposed above the ocean surface is called an island. For instance, the Hawaiian Islands are part of the Emperor Seamount Chain (see Figure 14-7). The South Pacific region is a region with numerous seamounts, of which many are islands, atolls, or guyots (Figure 14-40).

An atoll is a ring-shaped reef, island, or chain of islands formed of coral, typical on a foundation of an extinct volcano in the ocean. The limestone ring forms along the margins of the volcano. Over time, the volcano either erodes away or sinks below the surface, but the limestone rim continues to grow and expand over time. A guyot is a submarine mountain (seamount) with a flat top. Most guyots are ancient submarine volcanoes that have been beveled by wave action before sinking into ocean depth and may lack the fringing limestone reefs associated with atolls.

Siliceous Deep-Sea Sediments

Siliceous microfossils
may dominate sediments deposited in deep ocean basin environments—especially in cold water below the CCD [carbonate compensation depth] and in regions far offshore, away from land and nearshore sediment sources. Deep-sea seafloor sediment consisting mostly of the shells and skeletal remains of small (microscopic) planktonic organisms is called ooze.

The carbonate compensation depth (CCD) is the depth in the ocean in which cold water temperature and pressure causes carbonate materials (particularly calcareous plankton remains) to dissolve faster than they can accumulate. Sediments in deep-sea settings are mostly void of fine-grained carbonate material, however, siliceous plankton skeletal remains can accumulate as siliceous ooze (the host material that may become chert). Ancient sea ooze that has undergone dewatering, compaction and cementation (lithification) becomes a sedimentary rock called chert.
South Pacific region atolls, guyots, seamounts
Fig. 14-40. The South Pacific region has numerous islands, seamounts, atolls, and guyots. Fig. 14-41. Atolls, guyots, and ocean basin volcanoes
Teteria Atoll Ribbon chert in the Marin Headlands
Fig. 14-42. A satellite view of an atoll displaying a fringing carbonate reef platform (with islands) surrounding an eroding central volcanic peak. Most atolls of the world are located in tropical regions of the South Pacific and Indian Oceans. Fig. 14-43. Ribbon chert exposed in the Marin Headlands (near the Golden Gate Bridge, CA) formed from deep-sea siliceous ooze deposited on an ancient abyssal plain setting far offshore during the Jurassic Period.

What are tides and how they are created?

Tides are cause by the gravitation pull of extraterrestrial objects, the sun and moon being the most significant tidal forces on planet Earth (Figure 14-44). Tidal forces can affect crustal rocks and especially water (oceans and great lakes). Water will flow in the direction of gravitational pull. However, because the earth is rotating, this gravitational pull is constantly changing causing daily tide cycles.

Phases of the moon and tides: The gravitational pull of the moon is slightly stronger than the sun. However, sometimes the gravitational forces of the sun and moon join together to make higher tides.

* During full moon or new moon phases, the gravitational forces of the Sun and Moon are maximized, producing very large ranges of tidal highs and lows called spring tides. A spring tide is the exceptionally high and low tides that occur at the time of the new moon or the full moon when the sun, moon, and earth are approximately aligned.

* During the quarter moon phases, the gravitational forces of the Sun and Moon are at their minimum, producing very small ranges of tidal highs and lows (neap tides). A neap tide is the lowest level of high tide; a tide that occurs when the difference between high and low tide is least. Neap tide comes twice a month, in the first and third quarters of the moon.

Tidal ranges vary considerably around the world and are influenced by factors including shoreline and continental shelf geometries, latitude, size of the body of water, and other factors.
spring and neap tides Tides of the Bay of Fundy
Fig. 14-44. Spring and neap tides are related to the orientation of the Earth, Moon, and Sun (note polar orientation in this view). Fig. 14-45. Tides at the Bay of Fundy, Maine and Canada, are the largest in the world with spring tide ranges more than 50 feet!
Tides at Mont St.-Michel Del Mar Dog Beach sea cliffs and beach.
Fig. 14-46. Tides and tidal flats at Mont Saint-Michel, France, a region with a high tidal range. Fig. 14-47. High tide allows waves to erode the base of sea cliffs such as here at the Del Mar Dog Beach, CA.

Common shoreline features

Figure 14-48 illustrates common shoreline features associated with beaches and barrier islands.

A beach is an accumulation of mostly sand (and some gravel) along a shoreline where wave action winnows away finer sediment. Beaches occur in the intertidal zone (the zone between highest and lowest tides). Above the high tide line the upper part of the beach is mostly impacted by wind (forming dunes) and storm surges.

A wrackline is an accumulation of shell material and debris that typically marks the location of the last high tide cycle on a beach or after a storm.

A barrier island is a long and typically narrow island, running parallel to the mainland, composed of sandy sediments, built up by the action of waves and currents. Barrier islands serve to protect the mainland coast from erosion by surf and tidal surges. Examples include the Outer Banks in North Carolina and Padre Island in Texas. Barrier islands are most common on submergent coastlines associated with low-relief regions such as is present along the Atlantic Coast and Gulf Coast of the eastern United States.

A tidal flat is a nearly flat coastal area (at or near sea level) that is alternately covered and exposed by the tides, and consisting of unconsolidated sediments.

An estuary is the mouth of a river or stream where the tide-driven flow allows the mixing of freshwater and ocean saltwater. A lagoon is a saltwater-filled bay or estuary located between a barrier island and the mainland.

Coastal Depositional Environments Coastal Dunes at Point Reyes Peninsula
Fig. 14-48. Coastal environments extend from offshore to inland estuaries and bays. Fig. 14-49. Beach and coastal dunes at Point Reyes National Seashore, California
Moss Landing beach Elkhorn Slough
Fig. 14-50. Beach, dunes and harbor jetty, Moss Landing, California Fig. 14-51. Tidal marshes and estuary, Elkhorn Slough, California

Characteristics of Waves

Ocean waves are created by wind blowing over water. The distance between two wave crests or two wave troughs is called the wavelength. The height of a wave is a measure of wave amplitude (Figure 14-52). The period of a wave is the time interval between passing wave crests (completing one cycle) and are measured as wave crests pass a stationary point (such as a buoy or pole on a pier). The greater the period, typically the higher the wave breaks as it approaches the shore.

* Ocean wave intensity reflects characteristics of wind speed, wind duration, and fetch (the distance the wind has traveled over open water). Wind energy is gradually transferred to the waves forming on a body of water, causing waves to absorb energy and grow in amplitude and period over distance and time (Figure 14-53).

Why do waves break?

In a wave passing through the open ocean a water molecule in the water will move in a circular motion parallel to the direction the wave is moving. However, as a wave approaches the shore, its internal circular motion begins to impact the seabed causing the wave to drag along the bottom and slow down, shortening the wavelength (and wave period), but increasing the wave amplitude. Ocean waves typically "break" where the water depth is about one half of its wavelength or when the slope of the wave approaches a steepness ratio of 1 to 7 (feet or meters) (Figure 14-54).

A wave of oscillation is a wave in the open ocean where movement in the water below a passing wave is in a vertical circular motion (in open, deep water).

A wave of translation is a tumbling wave that continues onshore after it crests and breaks when entering a shallow coastal setting. Breakers then turn into a wave of translation and is called "surf." When the wave runs up on the beach and then retreats it is called "swash."

Ocean swell refers to series of ocean surface waves that were not generated by the local wind. Ocean swell waves often have a long wavelength. Swell can develop on lakes and bays, but their size varies with the size of the water body and wave intensity. Swells are generated by storms over the open ocean, but many ocean swells originate in the oceans around Antarctica where there is high winds with nearly infinite duration and fetch (Figure 14-55).
Wavelength Fetch
Fig. 14-52. Wavelength and amplitude of wave cycles. Fig. 14-53. Waves energy depends on wind speed, wind duration, and fetch.
Waves Origin of ocean swells in the southern oceans
Fig. 14-54. Waves of oscillation, breakers, and waves of transition moving onto the beach. Fig. 14-55. Most ocean swells originate in the southern oceans where strong winds combine with unlimited fetch.
New York Bight Wave diffraction
Fig. 14-56. Effects of winds and wave swells on longshore currents in the NYC region Fig. 14-57. Wave diffraction around offshore obstruction on waves nearshore

Wave Refraction and Longshore Transport

When waves approach a beach, they slow down. If the waves approach a beach at an angle, the slowdown near the beach will cause the line of a wave crest to "bend" in a process called wave refraction. Some of the energy from the waves approaching the beach at an angle create currents that move parallel to the beach (moving downwind). Longshore drift is the process by which sediments (sand and gravel) move along a beach shoreline, caused by currents created by waves approaching the shore at an oblique angle (example in Figure 14-56).

Wave diffraction refers to various phenomena which occur when a wave encounters an obstacle or change in geometry of the seabed. For example waves are diffracted when they when they pass an island, or when they pass a point or other structure, such as a jetty at the mouth of a harbor (Figure 14-57).

Rip current, also commonly referred to simply as a rip (or by the misnomer rip tide), is a strong channel of water flowing seaward from near the shore, typically through the surf line. Rip currents tend to form when wave swells approach directly onto the beach, causing water to bunch up and then spill seaward in locations along the beach (Figure 14-58).
Riptides Longshore drift
Fig. 14-56. Longshore drift and the impact of groin construction. Groins are constructed to stop beach erosion, but they are not completely effective. A groin can trap sand moving by longshore drift on one side, but on the down-current side, the shut off of a sand supply results in beach erosion.
Fig. 14-59. Formation of rip current. Rip currents are common when wave approach in a line parallel to the beach. The bigger the waves, the stronger the rip currents. Rip current are revealed by frothy bubbles streaming offshore.

How do waves contribute to coastal erosion?

Prevailing wind and wave swell patterns and storm events affect shoreline erosion and deposition, changing shoreline geometry over time. Manmade structures designed to control wave and storm damage include seawalls, groins, and jetties. These features affect wave energy dispersion and longshore currents, modifying shoreline geometries. They must be designed for long-term stability or they may fail.

Great coastal storms can severely impact coastal communities and change shoreline geometries. Great storm can move as much sediment in a couple days what may take 100 years or more of "normal storms."Coastal erosion is a HUGE problem for humans intent on living along the shore. Coastal erosion is inevitable and unstoppable in the long term. Coastal environments are locations where people should probably be building parks and nature preserves, not megacities! All coastal communities are at some risk of potential disaster. The question isn't "if..." but the answers are in "how, where, and when!"

Manmade structures, such as groins, jetties, and seawalls, are designed to control erosion, but their construction often creates other problems. Groins and jetties trap sand moving along a beach with longshore drift (Figure 14-56). Whereas they may trap sand in one location, their construction shuts off the sand supply for beach areas down current. This scenario was well illustrated by a disaster involving the community Westhampton Beach on Long Island, New York where a wealthy community decided to have a groin field built to protect coastal homes. The groins constructed by the US Army Corp of Engineers successfully stopped the erosion, but down current of the groin field, it was the recipe for disaster which took place when a typical "nor'easter" storm hit in the winter of 1982 (Figure 14-57). The storm surge and waves eroded the beach, cutting a new inlet across the barrier island and hundreds of homes were either destroyed or heavily damaged. In the end, it was the tax payers in New York who had to pay many millions of dollars to clear up the damage for a relatively small population of coastal dwellers. This is a story has been generally repeated many times, but in many different locations.

The movement of the coastline is unstoppable. It is well illustrated along the Atlantic shore of Long Island. For example, coastal erosion is causing the shoreline to migrate landward, removing sediments from the increasingly narrow barrier island, but that sand is moving and accumulating in other locations. Since the construction of a lighthouse at the western end of Fire Island in 1825, the island has grown nearly 4 miles longer. built from sediments contributed by erosion from the eastern end of the barrier island (Figure 14-58). The sediments are accumulating at the entrance to Moriches Inlet, which is making dredging at the harbor entrance an unending expense. Eventually, the Fire Island barrier island will no longer exist as the landward erosion of the coastline continues. A similar situation exists farther west on Long Island in an area called the Rockaways. The end of the barrier island has grown nearly 2 miles since 1866 (Figure 14-59). The westward migration of the end of the barrier was temporarily halted with the construction of a jetty to protect the harbor entrance to Jamaica Bay, but the supply of sand filling in the harbor entrance continues unabated, and will require dredging to keep it open. As an aside, the community of Breezy Point constructed near the jetty was nearly completely destroyed by fire during Superstorm Sandy in 2013.

Billions have been spent trying to cope with the coastal erosion problems in California. A big problem that affecst the California coastline is landsliding. In many areas, the coastline is extremely steep, and wave erosion is constantly gnawing away the at the base of the seacliffs along the coast. In many places, the rocks exposed along the shoreline consist of soft sedimentary deposits which easily erode. This make the development of prime real estate of oceanfront property a costly venture for everyone. Figure 14-61 illustrates the expensive efforts to shore up eroding sea cliffs in Encinitas, California. Seawall construction only delays the inevitable erosional retreat of the seacliffs and visually impacts the view of the natural environment. Figure 14-62 shows the jetty constructed around the Oceanside Harbor and to Fort Pendleton. Although the jetty protects the harbor, it impacts the natural longshore drift of sand to the beaches of Oceanside south of the harbor.

Another big problem in California has been the construction of dams along rivers that drain to the coast. Dams trap sediments that the rivers would otherwise contribute to coastal beaches. When the beach washes away, the seacliffs are exposed to erosion, undercutting hillsides. The result can be catastrophic landsliding (Figures 14-63 and 14-64). Coastlines are not ideal locations for construction!
Westhampton Beach Disaster, 1982 Formation of Jones Beach
Fig. 14-57. Construction of a poorly designed groin field led to the Westhampton Beach disaster of 1982. The groins shut off the natural sand supply to a coastal community next door. Fig. 14-58. Jones Beach and Robert Moses State Park, New York has grown by nearly 4 miles by sand accumulation by longshore drift. Erosion is making the island longer and narrower.
Rockaway Beach and Jamaica Bay Coastal dynamics of Sandy Hook, New Jersey
Fig. 14-59. Beach wracklines and historic changes to the coastal landscape of Rockaway Beach caused by longshore drift. Rockaway beach, located Queens, New York (seaward of New York Harbor), has grown nearly 2 miles since 1866, largely influences by construction of groins and a jetty. Fig. 14-60. Sandy Hook is a sand spit on the southern New Jersey side to the entrance to New York Harbor. The spit has changed its geometry continuously through historic times, and is now preserved as a national park. Many attempts have been made to control coastal erosion on Sandy Hook.
Sea wall in Encinitas, California Oceanside Harbor, CA
Fig. 14-61. Attempts to halt sea cliff erosion to protect coastal properties in Encinitas, California. Sea cliff erosion naturally provides a sand to the beach. Fig. 14-62. A jetty constructed around the entrance to Oceanside Harbor impacts longshore drift, requiring occasional dredging operations.
Thornton Beach cliffs, CA Daly City coastal landslide
Fig. 14-63. In 1905, a railroad line was constructed along the coast south of San Francisco, it was destroyed by coastal landsliding in the 1906 earthquake. Today very little evidence of the railroad line is visible along the coast. Fig. 14-64. A massive coastal landslide took out many homes along the sea cliffs in Daly City, a coastal community near San Francisco. As coast erosion proceeds the entire mountain face is subject to landslides.

Chapter 14 Quiz Questions


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3/16/2015