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Coastlines-Lecture Notes
Our present coastlines-that thin strip of real estate that forms the border between the ocean and land-reflect the interplay of geology, the dynamic forces of the ocean, and various biological and surficial processes (e.g., weathering). To understand the variety of coastlines, we will first look at the processes that affect the coast and then model the various coastlines. Erosive processes The oceans erode the various coastlines through the battering of waves, and sediment in the water rubbing against the land. This energy is concentrated in the zone between the tides where waves break. Coastlines with narrow tidal ranges erode more quickly than those with broader ranges. Because of refraction, the strongest oceanic waves are bent toward headlands so those receive the brunt of activity. In addition to waves eroding the coasts, rivers also grind through the coastline on their way to the ocean. Also, there are a host of oceanic organisms that chew away rocks for either nutrients or to create homes and various chemical processes act to break down materials. The effect of these various erosive processes is relative to the strength of the material making up the coastlines. Well-lithified rocks, such as granite, erode more slowly than poorly-lithified rocks such as mudstone. Also, rock layers that slant toward the ocean tend to fall apart more quickly than layers that tilt away from the ocean. No where are these effects more readily observed than in our own backyard. The coastline of Humboldt County is made up of lots of different materials of various strengths stuck together like a cheap casserole, called a "tectonic melange." The hard parts stick out as "sea stacks" or "knockers" (e.g., Trinidad Head), and the weak layers are eroded back and commonly landslide. Smaller weak zones can be eroded out to form sea caves. The continual onslaught of waves leads to the erosion of a broad platform near wave base. This flat shelf ramps slightly offshore and is overlain by complex sand waves. During very low tides, this wave-cut shelf is exposed. If tectonic processes raise the land near the ocean (see below), these broad platforms stick up above sea level, forming flat "terraces." Building processes Tho' the ocean taketh away, it also giveth back. As the waves grind away the land, the new sediment is moved by the waves and nearshore currents to form new beaches in areas of less strong erosion (beaches are areas of coastline where sediment builds up). An important process in moving sediments is the longshore current. Waves tend to strike the coast at an angle, forming a wedge of water. As the water escapes this wedge, it is pushed parallel to the coast. This effect changes daily due to the fluctuating direction of the waves, but the average direction for the West Coast is from the north to the south, due to the strength of northern storms. The longshore current is responsible for moving sediment from its input source (such as at a river) to the channel where the sand is ultimately moved offshore (such as at a submarine canyon). This "pathway" is called a "littoral cell." Rivers are a major source for bringing new sediment into the system. Under ideal conditions, a wedge of sand builds up where the river spills into the ocean. These triangular-shaped wedges are called "deltas." The shape and life span of a delta is a function of the relative forces of sediment input, tides, waves, and the width of the continental shelf. Although Humboldt County rivers carry a lot of sediment, the narrow shelf and strong waves and currents act against the formation of deltas. In addition to rivers, new land can be added to coastlines through other geologic processes. The most important of these are volcanic eruptions and glaciers. As glaciers move across the land, they bulldoze the landscape piling up the debris at the head and sides of the glacier as it melts. These piles are called "morraines." Although not conspicuous now, in the past, this process led to construction of some the islands off of the northeastern U.S. (e.g., Long Island, Martha's Vineyard). Another way coastlines buildup is through the accumulation of organic material. Perhaps the most well-known process is the development of large, rigid buildups of corals and other invertebrates forming reefs. Large-scale reef buildup is confined to the tropics where the warmer water temperatures enhance the precipitation of calcium carbonate and the lack of nutrients "keep out" competitor algae. More on this when we discuss marine life. Other common biological buildups include large tracts of sturdy vegetation that can handle the salty water, such as mangrove swamps. Role of Sea Level Changes and Tectonics in forming Coastlines The main effect of rising and lowering of sea level is the movement of the position of the coast. As sea level rises, the coastline is moved toward the land. Geologists refer to this movement as a "transgression." Conversely, if the sea level drops, the coastline moves out toward the sea, or a "regression." The effect of transgressions is most readily seen as the sea floods river or glacial valleys, forming "drowned valleys" or "fjords," respectively. Although not prominent on our coast, this is a common feature on the East Coast of the United States. There are two classes of sea level movement: localized or eustatic. Localized sea level changes only affect a limited area of coastline and can be caused by tectonic movements or rapid influx (or removal) of sediment. A good example of this occurred near Petrolia during the 1992 earthquake when the coastline raised several meters, moving the shoreline farther out to sea (localized regression). Eustatic changes refer to the removal or addition of water to the entire global ocean system, thus lowering or raising sea levels world wide. Eustatic changes are brought on by the removal and locking up of ocean water in glaciers, or by major plate tectonic movements causing changes in the volume of the ocean basins. One example of the latter is that during time of rapid sea-floor spreading, the spreading centers become warmer and rise, thus lowering the volume of the oceans. Tectonics also play a role in forming the coastlines by reshaping the landscape along faults. In California, land is generally rising up along faults or folds thus forming a landscape of uplifted shorelines (terraces) that resembles a staircase (think of the flat areas above sea level in Eureka, Arcata, and McKinleyville. In fact, CR is built on an uplifted terrace). The opposite can also occur when faulting or folding causes the land to move down, and become flooded. This explains the location of Humboldt Bay and Big Lagoon, among other features. Another effect of faulting is in enhancing or lowering the erosion potential. Generally speaking, faults are more easily eroded than the landscape between them, so embayments can form along faultlines. Tomales Bay is a good example of this; Elk Head is a local, albeit smaller, example. Rarely, the faults are a zone of increased strength and form resistant "walls" sticking out of the coastlines. Age of the Coastlines Taken together, all of these processes act in concert to eventually smooth out the coastlines. Land masses that project out into the ocean concentrate the wave energy and erode faster. Areas of less wave activity accumulate sand. As long as the tectonic movements or sea level rise is essentially nil, the coastlines will become flat, ramps of land that project into the sea with broad beaches. Of course, this will never truly happen, but the idea helps us to classify coasts based on age-either young or old. Tectonically active Humboldt County is made up of young coasts (also called primary coasts) and the quiescent Gulf Coast, for example, is made up of old coasts (secondary coasts). With all this said, we can look at various coastal features with hopes of recognizing where the coastline is in its development. We will first look at typical beaches, and then briefly summarize the main geographical features of coastlines. Beaches Beaches are among the most dynamic environments on the land and are constantly in a state of change. A typical profile, or vertical cut, through a beach would show a linear hill at about the high tide level. This is the "beach berm." Landward of the beach berm (the backshore area) is a low area in which waters that spill over the berm during storms can be channeled away. A second berm may be found landward of this channel and is built up of storm-strewn flotsam and jetsam. In front of the berm (the foreshore area) is a ramp down to the low tide level. Offshore, sand bars are developed parallel to the coast through the action of the longshore currents. Features of Coastlines The "river of sand" pushed along by longshore currents will be deposited in areas where the current is slowed down. These leads to the formation of several distinctive sand bodies: Sand spits-commonly
observed linear features that grow downcurrent of the longshore current.
Sand spits are anchored to coast off of natural or man-made projections,
such as sea stacks or jetties. Usually, the youngest end of the spit
is wide and/or hook-shaped. Lagoons are closed or partially bays protected from the coast, but containing salty water. The barrier can be made from sand (such as our lagoons or the numerous lagoons developed behind barrier islands), reefs (common in the Pacific), rock (such as volcanic flows or glacial morraines), or other materials. Estuaries are important, biologically diverse zones where fresh water and oceanic water mix. The organisms that make estuaries their home have to be able to handle large fluctuations in salinity. However, if they can overcome this, they live in relative safety. Most estuaries develop at either the mouths of drowned rivers or flooded glacially carved valleys (fjords); others develop in lagoons or bays that have significant fresh water runoff, or in tectonically controlled embayments. As mentioned, Humboldt Bay is an example of the latter. We can classify estuaries based on how the fresh and salt waters mix and circulate. In one form, the incoming fresh water does not mix with the more dense, oceanic water. Instead, it "pushes" the oceanic water back, and flows above it. This is called a "salt water wedge." If there is a little mixing along the front between the two water masses, we classify the esturary as "partially mixed." In a "well mixed estuary," there is a gradation between the two water masses. In some tropical areas, the incoming ocean water gets stuck in the back of the estuary and begins to evaporate, increasing the salinity. This is a "reverse estuary." The estuarine mixing pattern is a function of river output, tidal strength, and shape of the basin. If there is a submerged wall at the the base of the estuary (common in fjords), stagnant or unmixed water can be trapped at the base. Finally, keep in mind that most estuaries are large enough for the Coriolis Effect to offset circulation. Key Points: 1. Coastlines are being eroded by the battering of waves, grinding by moving sediment in the ocean and rivers, destruction by organisms, and chemical processes. 2. Headlands receive more physical battering than pocket beaches and coves. 3. Coastlines with narrow tidal ranges erode more quickly than those with broader ranges. 4. The amount of erosion is relative to the strength of the material making up the coastlines. 5. The waves erode a broad platform near wave base that ramps slightly offshore and is overlain by sand waves. 6. Sediment, either from erosion at the coasts or introduced from rivers, is moved by the waves and the longshore currents to form new beaches in areas of less strong erosion. 7. The longshore current forms because the waves strike the coast at an angle forcing the water to be pushed out as a wedge. Although it fluctuates, the average longshore current for our coast is from the north to the south. 8. A "littoral cell" refers to the path of movement of sediment from its source, along the coast, to deposition offshore. 9. Triangular-shaped deltas develop at the mouths of rivers. 10. The shape and life span of a delta is a function of the relative forces of sediment input, tides, waves, and the width of the continental shelf. 11. New land is also added to coastlines through volcanic eruptions, glacial deposits (morraines), and organic buildups (e.g., reefs, swamps). 12. Transgressions are when sea level rises, and the coastline is moved toward the land. Regressions are the opposite. 13. During a transgression, former river or glacial valleys can become flooded. Flooded glacial valleys are called (fjords). 14. Sea level changes can be localized or global (eustatic). Locallized changes are caused by tectonic movements or rapid influx (or removal) of sediment. Eustatic changes are driven by water removal and storage in glaciers, or by major plate tectonic movements. 15. A landscape of uplifted shorelines (terraces) that resembles a staircase is common where local tectonics raise the coastline. Flooded bays or valleys occur when the local tectonics drop the coastline. 16. Typically, embayments form along easily-eroded faults. 17. Primary coasts are young coasts; secondary coasts are old coasts that have had much erosion and deposition. 18. Beaches are among the most dynamic environments on the land and are constantly in a state of change. 19. From ocean to land, a typical beach is composed of offshore sand bars parallel to the coast, a flat ramp (foreshore), a high beach berm, a low area behind the berm (backshore), and a secondary storm berm. 20. Sand spits grow downcurrent of the longshore current and are anchored to coast. The youngest end of the spit is wide and/or hook-shaped. 21. Tombolos are sand spits that grow from the shore to an offshore land mass. 22. Bay mouth bars are spits that grow across and block the entrance to a bay. 23. Barrier Islands are long sand bars that develop offshore, but parallel to the coast. 24. Lagoons are closed or partially bays protected from the coast, but containing salty water. 25. Estuaries are important, biologically diverse zones where fresh water and oceanic water mix. 26. Estuaries develop in river mouths, fjords, or in tectonically controlled embayments. 27. Estuaries are classified on how the fresh and salt waters mix and circulate. 28. Salt water wedges form when the incoming fresh water does not mix with the salt water. Well-mixed estuaries show a clear gradation between the fresh and oceanic waters. Partially-mixed estuaries are a combination of salt water wedges and well-mixed estuaries. Reverse estuaries occur where the oceanic water evaporates and raises the salinity near the shore. 29. The estuarine mixing pattern is a function of river output, tidal strength, shape of the basin, and the Coriolis Effect. 30. Submerged
walls at the the base of the estuary (common in fjords) can lead to
stagnant bottom waters. http://www.geosci.unc.edu/classes/Geo11_2/Images/Coastlines/ Lots of photos that were gathered for a similar course at the University of North Carolina. http://www.epa.gov/docs/nep/coastlines/ Site for an online newsletter about environmental issues relating to estuaries and other coastlines. http://www.epa.gov/OWOW/oceans/ More governmental sites for coastline information. Site for NOAA's Coastal Ocean Program. http://www-staff.lboro.ac.uk/~cvcfs/simulations/estuarinedynamics/menu.html Good site for figures on estuary circulation. |
