1. Coasts

2. Key Concepts
The location of a coast depends primarily on global tectonic activity and the ocean’s water volume
The shape of a coast is a product of uplift and subsidence, the wearing down of land by erosion, and the redistribution of material by sediment transport and deposition

3. Key Concepts (cont’d.)
Coasts may be classified as erosional coasts (on which erosion dominates) or depositional coasts (on which deposition dominates)
Beaches change shape and volume as a function of wave energy and the balance of sediment input and removal
Human interference with coastal processes has generally accelerated the erosion of coasts near inhabited areas

4. 12.1 Coasts Are Shaped by Marine and Terrestrial Processes
Coast – zone affected by coastal processes
Shore – ocean meets land
Location – depends on global tectonic activity and volume of ocean water
Shape – depends on uplift, subsidence
Erosion, and redistribution of sediment

5. Coasts Are Shaped by Marine and Terrestrial Processes
Change of sea level
Three factors cause eustatic change (global sea level)
Amount of water in ocean
Volume of Earth’s ocean basins
Water temperature
Local changes in sea level
Tectonic and isostatic factors
Wind currents, seiches, storm surges, El Niño or La Niña, or other water in motion

6. Coasts Are Shaped by Marine and Terrestrial Processes
Figure 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 about
3 millimeters per year.
b. Projections of sea level through the year Seven research
groups (represented here by colored lines) have estimated future
sea level based on historical observations and climate models.
The most conservative of these predictions estimates a
Figure 12.2 S ea levels past and future. 20-centimeter (8-inch) rise.

7. 12.2 Erosional Processes Dominate Some Coasts
High-energy coast – rapid erosion
Low-energy coast – infrequent erosion
Complex features
Sea cliffs
Sea caves
Wave-cut platforms

8. Erosional Processes Dominate Some Coasts
Figure 12.4 The results of wave action on a coast.

9. Results of Wave Action On a Coast
Figure 12.5 Results of wave action on a coast.

10. Sea cliff Wave-cut Original land surface platform
Notch eroded by waves
Figure 12.5 Results of wave action on a coast.
Wave erosion of a sea cliff produces a shelflike, wave-cut platform visible at low tide. Remnants of the original cliff can protrude as sea stacks.
Stepped Art

11. Erosional Processes Dominate Some Coasts
Wave energy
Figure 12.6 Wave energy converges on headlands and diverges in the adjoining bays.
a. The concentrated forces shape the headland into platforms
and stacks. The accumulation of sediment from the headland
in the tranquil bays eventually forms beaches and straightens
the contours of the shore.
b. Waves approaching a shallow Caribbean reef refract around it.
Energy is clearly concentrated on the headland.

12. Land Erosion and Sea-Level Change
Figure 12.7 Drowned river valleys: submerging coasts that filled with water as the last ice age ended.
a. Delaware and Chesapeake bays on the East Coast of the United States.
b. Sydney Harbour, Australia, called by its discoverer “The finest harbour in the world.”

13. Volcanism and Tectonism
Figure 12.9 Volcanic alterations to coasts.
a. Lava flowing seaward from an eruption on the island of Hawai’i forms a
fresh coast exposed to erosion for the first time.
b. Two volcanic cones on the southeastern coast of the
Hawai’ian island of O’ahu. One of the volcanoes has
collapsed, and its crater has filled with seawater.

14. 12.3 Beaches Dominate Depositional Coasts
Beach – loose particles
Wave action, particle size, and permeability
Swash – carries particles onshore
Backwash – carries particles offshore
Figure b. The general relationship between grain size and beach slope.

15. A Typical Beach Profile
Figure A typical beach profile. The scale is exaggerated vertically to show detail.

16. Waves Transport Beach Sediment
Longshore drift and longshore current
Figure a. A longshore current moves sediment along the shoreline between
the surf zone and the upper limit of wave action.
b. Groins built at right angles to the shore at Cape May, New
Jersey, to slow the migration of sand. The groins interrupt
the flow of longshore currents, so sand is trapped on their
upcurrent sides. This view is toward the south; and south of
the groins, on the downcurrent sides, sand is eroded.

17. 12.4 Larger-Scale Features Accumulate on Depositional Coasts
Figure 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.

18. Barrier Islands and Deltas
Figure b. The heavy black lines south of Ocean City represent jetties constructed in the 1930s to protect the inlet. The jetties disrupt the
north-to-south longshore current. As a result, Assateague Island has been starved of sediment and has migrated about 500 meters (1,640 feet) westward.
Figure a. The bird’s-foot shape of the Mississippi Delta is seen clearly in this photograph. Lobed and bird’s-foot deltas form where deposition
overwhelms the processes of coastal erosion and sediment transportation. The sediment-laden water looks brown or tan in this photograph taken from low orbit.

19. 12.5 Biological Activity Forms and Modifies Coasts
Coral reefs
Fringing reefs
Barrier reefs
Atolls
Figure a. A fringing reef forms around an island in the tropics. The island sinks as the oceanic plate on which it rides moves away from a spreading
center. In this case, the island does not sink at a rate faster than coral organisms can build upward. The island eventually disappears beneath the
surface, but the coral remains at the surface as an atoll.

20. 12.6 Freshwater Meets the Ocean in Estuaries
Mixing fresh and salt water
Estuaries classified by origin
Characteristics influenced by water density and flow
Figure Types of estuaries in vertical cross sections. The salinity
values show the amount of mixing between freshwater and seawater in
the various types. Green color represents freshwater.

21. 12.7 The Characteristics of U.S. Coasts
Pacific
Actively rising
Recent tectonic activity
Atlantic
Passive margin
Gulf coast
Smaller wave size
Smaller tidal range

22. 12.8 Humans Interfere in Coastal Processes
Engineering to prevent/slow erosion
Figure Some measures taken to slow beach erosion.

23. Some Measures Taken to Slow Beach Erosion
Figure Some measures taken to slow beach erosion.

24. (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.
Figure Some measures taken to slow beach erosion.
Stepped Art
Figure p379

25. Chapter in Perspective
Location of coast depends on global tectonic activity and ocean water volume
Shape produced by many processes
Classified as erosional or depositional
Beaches are accumulations of sand
Wave action shapes beaches
Coral reefs and estuaries are biologically active
Humans interfere with coasts

26. Some Questions from Students
What are those little white pellets I find on the beach along the high-tide line? Surfers call them “nurdles.”
I’ve noticed that beaches composed of pebbles and cobbles tend to be fairly steep, but beaches made of fine sand are relatively flat. Why is that?
Those ubiquitous, insidious particles are the raw material for
molded plastic goods. They are transported from producers to
fabricators in containers loaded onto container ships. The pellets
escape if a container is mishandled, breaks open during a
storm, or is lost overboard. Virtually indestructible, “nurdles”
float with the winds and currents until they encounter a shore.
One researcher calculated that just 25 containers would carry
enough plastic pellets to spread 100,000 “nurdles” per mile
along all the seashores of the world!
You’ll learn more about the problem of plastic in the ocean
in this book’s last chapter.
A beach’s slope is determined by the energy necessary to move
the sand grains of which it is composed. Shallow water, smaller
waves, and coarse grains combine to form steep beach slopes,
but large waves can very easily move small particles and smooth
out the slope.