Chapter 20: Coastal Processes and Terrain

Transcription

1 Chapter 20: Coastal Processes and Terrain McKnight s Physical Geography: A Landscape Appreciation, Tenth Edition, Hess and Terrain The Impact of Waves and Currents on the Landscape 2 The Impact of Waves and Currents on the Landscape Coastal processes affect a tiny portion of Earth s landscape Waves agents of erosion Currents agents of transportation and deposition Beaches mark transition between land and water and are highly variable Figure

2 Interface of three major components of Earth s environment Highly energetic due to constant motion of waters Importance of wind Ocean and lake formations similar except for: Tidal range smaller for lakes Water level change differences Reefs only in oceanic water Figure Waves transfer of energy through cyclical rising and falling of a substance Most are wind generated over oceans Wind stress generated waves, forced waves Swells 5 Waves of oscillation and translation Waves that move in a circular or oscillatory fashion with little forward movement, waves of oscillation Wave crests and troughs Wavelengths and wave heights Wave amplitudes Shallow water waves influenced by ocean floor, gain forward progress, called waves of translation; wave breaks Figure

3 Wave refraction Change in wave direction as they approach shore Uneven coastline and irregular water depth Waves bent due to uneven slowing of waves from irregular water depth Wave action focused on headlands, much gentler in adjacent bay areas Figure Wave erosion Consistent pounding of small waves results in erosion Large storms significantly enhance coastal erosion Air forced into cracks in coastal rocks when water moves inland; air released as water recedes and enhances erosion Chemical action of seawater Notches cut in the bases of cliffs Figure Tsunamis Waves triggered by disruptions in ocean floor When fault rupture on ocean floor generates tsunami, entire depth of ocean above rupture is displaced Inconspicuous in open ocean Figure 20-8 with long wavelengths and low heights Can travel over 400 mph Significant withdrawal of up to 40 meters before a significant surge of water 9

4 Tides Alterations of ocean level from gravitational pull of Sun and Moon Two high tides and two low tides per day Topographic effects generally small Significant agents of erosion only in narrow bays, around shallow seas, and in passages between islands Figure Changes in sea level and lake level Two primary causes of sea level changes Rising or sinking of landmass (tectonic change) Increase or decrease in amount of ocean water (eustatic sea-level change) Emergent versus submergence land characteristics Global warming and sea-level change Thermal expansion of water and melting of ice caps increasing water volume (eustatic) Sea level rise of up to 0.5 m by the end of the century 11 Ice push Bodies of water that freeze in winter, resulting in expansion and subsequent contraction Ice pushes onto land, significantly modifying land surface, similar to small glacial advance Most common in Arctic and Antarctic regions 12

5 Organic secretions Many aquatic animals form calcium carbonate shells Animals cluster together and form enormous masses of reefs, platforms, and atolls Stream outflow Streams important sources of sediment to oceans and lakes Figure Currents and coastal sediment transport Longshore currents Water moves parallel to shoreline ( along shore) Develop just offshore and set up by waves striking coast at an angle Wind direction reflected in longshore currents Figure Currents and coastal sediment transport (cont.) Beach drifting Short distance shifting of sand by breaking waves and retreating water Zigzag pattern of particle movement downwind and parallel to coast Affects of tides on debris movement Dune formation on coasts Figure

6 Coastal deposition Results when energy of moving water is diminished Maritime deposits more ephemeral than noncoastal deposits due to composition and lack of vegetative cover Sediment budget must be in balance to allow for deposit to persist Figure Depositional landforms Beaches Beaches relatively homogeneous Mark transition between land and ocean Backshore contains berms; foreshore regularly covered and uncovered by tides Offshore is zone that is permanently submerged Figure Spits At mouth of a bay, sediment moved into deeper water Deposit attached to land at one end and extends to open ocean in downcurrent direction is a spit Spits that extend across a bay, bay barriers or baymouth bars Tombolos: waves converge on each side and deposit sand so the bar connects Figure to land 18

7 Barrier islands Long, narrow sand bar built up in shallow offshore waters Oriented approximately parallel to shore Only rise a few meters above sea level, but some extend to great lengths Lagoon formation; mudflats Life cycle of a lagoon Figure Human modification of coastal sediment budgets Dams act as sediment traps, allowing less sediment to reach oceans and resulting in shrinking beaches Use of groins to help impede the downcurrent flow of sediment Jetties used to keep water moving and reduce sediment deposits in navigation channels Figure Shorelines of submergence Most oceanic coastline shows evidence of submergence at some time within last 15,000 years Ria shorelines Submergence results in drowning of previous river valleys, producing estuaries Coast with numerous estuaries is called a ria shoreline Figure

8 Shorelines of submergence (cont.) Fjorded coasts Extensive glaciation gouges out troughs by glaciers or ice sheets Troughs far below sea level, eventually fill with sea water Deep coastal indentations are called fjords Create extraordinarily irregular coastlines Figure Shorelines of emergence and erosion Shoreline features raised well above current sea level Wave-cut cliffs and platforms Constant pounding of waves at base of landforms cuts a notch at the high water level Broad erosional pattern called a wave-cut bench or wave-cut platform Most cut debris shifted just beyond wave-cut bench to wave-built terrace Figure Marine terraces Wave-cut platforms uplifted along tectonically rising coasts Several instances of marine terraces indicate several episodes of marine terrace formation Can be used to deduce history of the water levels in a region Figure

9 Coral coasts Most continents and islands fringed with coral reefs or another coralline structure Critical element is a group of anthozoan animals Calcium carbonate skeletons from coral polyps Have a blossomlike appearance similar to plants Have strict requirements for their survival Fringing reefs: those built right onto a volcano Barrier reefs: coral that appears to float around a volcano Atolls 25 Distribution of coral coasts worldwide Figure Summary Coastal regions make up a very small percentage of the Earth s landscape, but have unique structure and processes The coasts are the interface between three of the four primary spheres of the Earth The most energetic coastal processes are observed by wave motions Tsunamis are significant dangerous waves that result from underwater earthquakes, not from winds Tides play a small role in the sculpting of landforms of coasts 27

10 Summary Numerous processes are involved that modify the lake level and sea level of bodies of water Many other coastal processes, including ice push, organic secretions, and stream outflow, help structure coastal landforms Currents are primarily responsible for the transport of coastal sediment Coastal deposition takes place in areas where ocean water moves more slowly, and is typically ephemeral The most widespread coastal landform is called a beach 28 Summary Spits and barrier islands result from deposition of sedimentary material by the longshore currents Lagoons result when barrier islands cut off one small region of ocean water from the remaining ocean Humans have modified the structure of shorelines through damming and the building of groins and jetties Shorelines can be divided into two categories, emergence or submergence Coral coasts consist of organic material and typically surround volcanoes 29