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Chapter 12 Coasts. Look For The Following Key Ideas In Chapter 12.
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Look For The Following Key Ideas In Chapter 12 • Coasts are temporary structures, often subject to rapid change. The shape of a coast is a product of many processes: uplift and subsidence, the wearing down of land by erosion, and the redistribution of material by sediment transport and deposition. • The location of a coast depends primarily on global tectonic activity and the volume of water in the ocean. • Change in sea level greatly influences coastal processes. For most of Earth's recent history, sea level has been lower than we find it today. • Erosional coasts are new coasts in which the dominant processes are those removing coastal material. Depositional coasts are those coasts that are steady or growing because of their rate of sediment accumulation or the action of living organisms. • Depositional coasts often support beaches, accumulations of loose particles. Beaches change shape and volume as a function of wave energy and the balance of sediment input and removal.
Key Ideas Continued… • Depositional coasts, especially along subsiding continental margins, often exhibit characteristic large-scale features. • Deltas form on broad continental shelves where rivers deposit sediment, tidal range is low, and wave and current action generally mild. • Coasts can be extensively modified by the actions of living organisms. • Estuaries can form at river mouths where fresh water mixes with seawater. They are among the most complex and biologically productive coasts. • Human interference in coastal processes rarely increases the long-term stability of a coast.
Coasts Are Shaped by Marine and Terrestrial Processes Sea levels past and future. (a) Sea level rose rapidly at the end of the last ice age as glaciers and ice caps melted and water returned to the ocean. The rate of rise has slowed over the past 4,000 years and is now believed to be between 1.0 and 2.4 millimeters per year. (b) Projections of sea level through the year 2100. Seven research groups (represented here by colored lines) have estimated future sea level based on historical observations and climate models. Even the most conservative of these predictions estimates a 20-centimeter (8-inch) rise.
Coasts Are Shaped by Marine and Terrestrial Processes (top-left) The southeastern coast of the United States looked much different 18,000 years ago, during the last ice age. Because of lower sea level, the position of the gently sloping southeastern coast has been as much as 200 kilometers (125 miles) seaward from the present shoreline, leaving much of the continental shelf exposed. (bottom-left) In the distant future, if the ocean were to expand and the polar ice caps were to melt because of global warming, sea level could rise perhaps 60 meters (200 feet), driving the coast inland as much as 250 kilometers (160 miles)
Erosional Coasts Often Have Complex Features Features of an erosional coast at low tide. Wave erosion of a sea cliff produces a shelflike, wave-cut platform visible at low tide.
Erosional Coasts Often Have Complex Features Wave energy converges on headlands and diverges in the adjoining bays. The accumulation of sediment derived from the headland in the tranquil bays eventually smoothes the contours of the shore. • Marine erosion is usually most rapid on high-energy coasts, areas frequently battered by large waves. • Low-energy coasts are only infrequently attacked by large waves.
Beaches Often Have Distinct Profiles A beach is a zone of loose particles that covers a shore. What are the features of a beach? • Berm • berm crest • Backshore • Foreshore • beach scarp • longshore trough • longshore bars
Waves Transport Sediment on Beaches A longshore current moves sediment along the shoreline between the surf zone and the upper limit of wave action
Sand Input and Outflow Are Balanced in Coastal Cells The general features of coastal cells, in which sand is introduced by rivers, transported southward by the longshore drift, and trapped within the nearshore heads of submarine canyons. Sections of coast in which sand input and sand output are balanced are referred to as coastal cells.
Larger-Scale Features Accumulate on Depositional Coasts A composite diagram of the large-scale features of an imaginary depositional coast. Not all these features would be found in such close proximity on a real coast. • A sand spit forms where the longshore current slows as it clears a headland and approaches a quiet bay. • A bay mouth bar forms when a sand spit closes off a bay by attaching to a headland adjacent to the bay. • Depositional coasts can also develop narrow, exposed sandbars that are parallel to but separated from land - known as barrier islands. • A long, shallow body of seawater isolated from the ocean is known as a lagoon.
Estuaries Are Classified by Their Origins An estuary is a body of water in which fresh river water mixes with ocean water. Estuaries can be classified by their origin: • Drowned river mouths • Fjords • Bar-built • Tectonic (a) Drowned river mouths: the mouths of the James, York, and Susquehanna rivers; Chesapeake Bay; Sydney Harbour, Australia. (b) Fjords: New Zealand’s Milford South; the Strait of Juan de Fuca in Washington state. (c) Bar-built: Albernarle and Pamlico sounds in North Carolina (d) Tectonic: San Francisco Bay; Tomales Bay
Estuary Characteristics Are Influenced by Water Density and Flow Types of estuaries in vertical cross sections. The salinity values show the amount of mixing between fresh water and seawater in the various types. (a) Salt wedge estuary. (b) Well-mixed estuary. (c) Partially mixed estuary. (d) Fjord estuary.
Characteristics of U.S. Coasts The Pacific Coast - An actively rising margin where indications of recent tectonic activity can be observed. The Atlantic Coast - A passive margin on the trailing position of the North American plate. The Gulf Coast - Smaller wave size and a smaller tidal range characterize the Gulf Coast.
Humans Have Interfered in Coastal Processes What are some ways that humans try to influence coastal processes? • Groins • Seawalls • Importing sand (left) A few of the many types of measures taken to slow and prevent beach erosion. Some of these methods are actually counterproductive. In many cases these methods help serve as a reminder that shorelines and beaches are constantly changing, and are not under human control.
(a) Groin Groins are structures that extend from the beach into the water. They help counter erosion by trapping sand from the current. Groins accumulate sand on their updrift side, but erosion is worse on the downdrift side, which is deprived of sand. Current (b) Seawall Seawalls protect property temporarily, but they also increase beach erosion by deflecting wave energy onto the sand in front of and beside them. High waves can wash over seawalls and destroy them and property. (c) Importing sand Importing sand to a beach is considered the best response to erosion. The new sand often is dredged from offshore, can cost tens of millions of dollars, and can disturb aquatic biodiversity. Because it is often finer than beach sand, dredged sand erodes more quickly. Stepped Art Fig. 11-31, p. 267
Chapter 12 Summary The location of a coast depends primarily on global tectonic activity and the ocean’s water volume, while the shape of a coast is a product of many processes: uplift and subsidence, the wearing-down of land by erosion, and the redistribution of material by sediment transport and deposition. Coasts are classified as erosional coasts (on which erosion dominates) or depositional coasts (on which deposition dominates). Depositional coasts often support beaches, accumulations of loose particles. Generally, the finer the particles on the beach, the flatter is its slope. Beaches change shape and volume as a function of wave energy and the balance of sediment input and removal. Coral reefs and estuaries are among the most complex and biologically productive coasts. Human interference with coastal processes has generally accelerated the erosion of coasts near inhabited areas.
