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Introduction to Geology |
Chapter 12 - Ocean and Shoreline Processes |
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1. Review the concept of seafloor spreading, hotspots, and the development and breakup of Pang ea. 2. Portray a continental margin, showing the shelf, slope, and rise; illustrate the features of deep-ocean basins. 3. Illustrate ocean circulation systems. 4. Compare and contrast the Atlantic and Pacific coasts, and distinguish between emergent and submergent coasts. 5. Describe the development of turbidity currents in submarine canyons. 6. Describe the features of coral reefs, atolls, and seafloor sediments. 7. Review the characteristics of waves, and distinguish between waves of oscillation and waves of translation. 8. Explain the tides and how they are created. 9. Illustrate the common types of shoreline features. 10. Define wave refraction and longshore transport. 11. Explain how waves contribute to erosion and illustrate shoreline erosion problems that affect humans. |
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| Keywords and Essential Concepts | ||||||||||||||||||||
| 1. Review the concept of seafloor spreading, hotspots, and the development and breakup of Pang ea. The oldest oceanic crust on the Earth today is about 180 million years old, where rocks exposed in continents may be billions of years old.
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2. Portray a continental margin, showing the shelf, slope, and rise; illustrate the features of deep-ocean basins. continental shelf—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. continental slope—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. shelf break—a general lines 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 shoreline that existed at the peak of the last ice age when sealevel was as much at 400 feet (120 meters) lower that present sealevel. continental rise—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. abyssal plain—an underwater plain on the deep ocean floor, usually found at depths between 3000 and 6000 meters.
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3. Describe Ocean Circulation Systems.
Ocean circulation is also influenced by seawater temperature and density. Cold and salty water (concentrated by surface evaporation) sinks. Water rises where it is displaced by colder and saltier water. |
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| 4. Describe the development of turbidity currents in submarine canyons. deep-sea fan—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 flow (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. turbidity flows—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 flow through a submarine canyon (carved by previous turbidity flows), produces deep-sea fans, and produces deposits showing graded bedding. graywacke—a sedimentary rock consisting of a mix of angular fragments of quartz, feldspar, and mafic minerals set in a muddy base (a "dirty sandstone or mudstone, common in the Coast Ranges of California and active continental margin regions.
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| 5. Compare and contrast the Atlantic and Pacific coasts, and distinguish between emergent and submergent coasts. The Atlantic coast of the United States is characterized by wide beaches, barrier islands, broad coastal plains: The Pacific coast is characterized by narrow beach, steep cliffs, rugged coastlines with headlands and sea stacks. *Estuaries are associated with submergent coastlines formed when sea level rises. active continental margin—a continental margin 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 typically associated with subduction zones, often include a deep offshore trench, and occur in places where leading edge of a continent is overrunning oceanic lithosphere. passive continental margin—a passive margin is the transition between oceanic and continental crust which is not an active plate margin. 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.
Features associated with emergent coastlines on active continental margins headlands—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. sea stacks—large rocky outcrops that have resisted wave erosion that stand offshore as the beach and sea cliff continues to erode landward. wave-cut bench—flat benches of rock that form by wave erosion at the base of a an actively eroding sea cliff on an emergent coastline. sea cave—an underground passage carved into a sea cliff carved by focused wave action. sea arch—a natural rock arch caved by wave action.
marine terraces—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 (see diagram below).
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| 6. Describe the features of coral reefs, atolls, and seafloor sediments. reef—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. * Over time lime sediments 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. Examples include the Bahamas, South Florida, and the Yucatan Peninsula. The world's largest is the reef tracts and tidal shoals associated with the Australian Great Barrier Reef.
Seamounts, atolls, guyots, and deep sea sediments atoll—A ring-shaped reef, island, or chain of islands formed of coral, typical on a foundation of an extinct volcano in the ocean. guyot—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 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. carbonate compensation depth (CCD)—the depth in the ocean in which cold water temperature and pressure causes carbonate materials (particularly calcareous plankton) 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.
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| 7. Explain the 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. Tidal forces can affect of land and 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 tides. * During a full moon, the gravitational forces of the Sun and Moon are maximized, producing very large ranges of tidal highs and lows (spring tides). * During the quarter moon phase, the gravitational forces of the Sun and Moon are at their minimum, producing very small ranges of tidal highs and lows (neap tides). spring tide—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. neap tide—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.
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| 8. Illustrate the common types of shoreline features. tidal flat—a nearly flat coastal area (at or near sea level) that is alternately covered and exposed by the tides, and consisting of unconsolidated sediments. estuary—the mouth of a river or stream where the tide-driven flow allows the mixing of freshwater and ocean saltwater. beach—an accumulation of mostly sand (and some gravel) along a shoreline where wave action winnows away finer sediment. wrackline—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. barrier island—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. lagoon—a saltwater-filled bay or estuary located between a barrier island and the mainland. headlands—rocky sections of coastline that have resisted wave and storm erosion and stick out into the ocean. Small beaches form in bays between rocky headlands.
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| 9. Review the characteristics of waves, and distinguish between waves of oscillation and waves of translation. * Ocean waves are created by wind blowing over water. * Ocean wave intensity reflects characteristics of wind speed, wind duration, and fetch (the distance the wind has traveled over open water). *A water molecule in the open ocean wave illustrates movement in a circle. *The period of a wave is the time interval between passing crests (completing one cycle), measured as wave crest pass a stationary point. *Wave period is inverse to the wave frequency (wave cycles per second). *The greater the period, typically the higher the wave crests as it approaches the shore. *Ocean waves typically "break" where the water depth is about one half of its wavelength. wave of oscillation—a wave in the open ocean where movement in the water below a passing wave is in a vertical circular motion. wave of translation—a tumbling wave that continues onshore after it crests when entering a shallow coastal setting. ocean swell—a 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.
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| 10. Define wave refraction and longshore transport. longshore drift—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. 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 approach a beach at an oblique angle, when the pass an island, or when they pass a point or other structure at the mouth of a harbor. rip current—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.
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| 11. Explain how waves contribute to erosion and illustrate shoreline erosion problems that affect humans. * Prevailing wind and wave swell patterns and storm events affect shoreline erosion and deposition, affecting shoreline geometry over time. * Manmade structures to control wave and storm damage (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 day what may take 100 years or more of "normal storms."
Coastal erosion problems in the New York City region
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Quiz Questions