The Work of Waves and Wind

The Work of Waves and Wind • The Work of Waves • Beaches • Types of Coastlines • Wind Action • Sand Dunes • Loess

The Work of Waves • Crests and troughs (high and low points) • Wave height is the vertical distance between trough and crest • Wave length is the horizontal distance from trough to trough, or from crest to crest • Thewave period is the time in seconds between successive crests or successive troughs that pass a fixed point

The Work of Waves • Wave height is related to wind speed, duration, and fetch • High winds over long distances and long durations can create very large waves • As waves approach the shore, wave length decreases and height increases until the wave becomes unstable and breaks

Waves • The energy of the breakers (crashing waves) causes erosion and transportation of shoreline materials. • Wave action is the most important agent in shaping shoreline landforms

Tidal Currents Tides rise (FLOOD) to produce a HIGH TIDE and fall (EBB) (LOW TIDE) because of the gravitational pull that the Sun and Moon exert on the earth’s surface (including the oceans)

The Work of Waves Waves erode weak materials rapidly to make marine scarps and resistant materials over a long period of time to make marine cliffs

The Work of Waves Marine cliff: the wave-cut notch, marks the line of most intense wave erosion

The waves find points of weakness in the bedrock and penetrate deeply to form crevices and sea caves

The Work of Waves • Where a more resistant rock mass projects seaward, it may be cut through to form a sea arch • After an arch collapses, a rock column, known as a stack, (eventually, the stack is toppled by wave action and is leveled)

San Diego’s Arch

Beaches • Beaches are thick wedge shaped accumulations of sand shaped by the swash and backwash of water along the shoreline • Breakers attacking the shore at an angle produce littoral drift, - a movement of sediment along the shoreline – as well as longshore drift - a sediment movement just offshore

Beaches • Littoral drift operates to shape shorelines - where a bay exists, the sand is carried out into open water as a long finger, or sandspit • As the sandspit grows, it forms a barrier, called a bar, across the mouth of the bay Figure 18.7, p. 601

San Diego’s Sandspit

Beaches • When sand arrives at a particular section of the beach more rapidly than it is carried away, the beach is widened and built oceanward - this change is called progradation (building out) • When sand leaves a section of beach more rapidly than it is brought in, the beach is narrowed and the shoreline moves landward - this change is called retrogradation (cutting back)

Types of Coastlines Two groups: One group of coastline types derives its qualities from submergence, the partial drowning of a coast by a rise of sea level or a sinking of the crust. Another group derives its qualities from emergence, the exposure of submarine landforms by a falling of sea level or a rising of the crust.

Types of Coastlines Figure 18.14, p. 608

Types of Coastlines • Coastlines created when new land is built out into the ocean: • Deltas • Volcano coasts • Coral Reef Coasts

Types of Coastlines A delta coast resultswhen a river empties into the ocean Figure 18.18, p. 610

Types of Coastlines coral reef coasts occur in warm oceans where corals build reefs at the land-sea margin (fringing reefs, barrier reefs and atolls) volcano coasts are found where fresh volcanic deposits of lava and ash reach the ocean

Wind Action Wind is selective: only the finest particles (clay and silt) are lifted and raised into the air sand grains are moved only when winds are at least moderately strong and usually travel within a meter or two of the ground (saltation) Figure 18.24, p. 615

Wind Action Wind performs two kinds of erosional work: Abrasion- wind drives sand and particles against an exposed rock or soil surface - causes the surface to be worn away by the impact of the particles Deflation- the removal of loose particles from the ground, acts on loose soil or sediment

Sand Dunes wind direction SLIPFACE BACKSLOPE crest angle of repose movement of sand

Sand Dunes Barchan dune is a crescent shaped heap of sand that moves across a flat, pebble-covered plain - the points of the crescent are directed downwind Figure 18.25, p. 615

Sand Dunes Parabolic dunes are bowed outward in the downwind direction – the opposite of barchan dunes Figure 18.27, p. 618

Sand Dunes Longitudinal dunes run parallel to the direction of the wind Figure 18.28, p. 619

LOESS A Loess is a deposit of wind-blown silt that may be as thick as 30 m in some regions of North America It forms highly productive but easily eroded soils Figure 18.30, p. 620

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The Work of Waves and Wind