CHAPTER 13: OCEANS AND COASTLINES
SUMMARY
This chapter begins with a general discussion of the origin and distribution of oceans on
Earth. You are reminded that over two-thirds of the planet is covered by oceans that average
about 4 km deep. There are three major oceans (Indian, Pacific, and Atlantic) that are connected
along their southern margins by the Southern Ocean circling Antarctica. Physical features
common to most ocean basins are discussed in the context of plate tectonics. Oceans can be
described by four principal depth zones based on their relative depths: the continental margins
(~100s meters), to the near horizontal floor of the abyssal plain (4-5 km), rising to the crest of the
oceanic ridge (~3 km), and descending to the narrow depths of the oceanic trenches (7-11 km).
You are introduced to salinity as a percentage by volume (rather than ppt). Images of data
are used to lead you in “discovering” that salinity varies depending on temperature and the
mixing action of ocean currents. You will find that salinity is higher at low-mid latitudes (except
at equatorial areas) because high temperatures at these locations promote evaporation which
removes water but leaves the salt it contains behind. Salinity is lower at high latitudes because of
the lack of evaporation, high precipitation, and the influx of freshwater from melting ice sheets.
The mixing action of ocean currents ensures a consistent salinity range of 3.3 percent to 3.7
percent for much of the open ocean. Representative salinity curves are used to illustrate that
salinity increases with depth in the restricted northern ocean waters (Arctic Ocean), decreases
slightly with depth in the tropical open ocean, and are much more consistent at depth (2 km)
below the halocline.
The discussion on ocean circulation continues with a discussion of energy transfer in the
ocean. You will analyze data that shows how temperatures in the ocean vary both by latitude and
depth. You are reminded that solar radiation, the primary source of heating for Earth, is
distributed over a wider area and must penetrate a greater thickness of atmosphere at the poles,
thus reducing the amount of solar energy reaching the surface there. Consequently, ocean
temperatures are greater near the equator. The highest ocean temperatures (~27 ºC) are present
along the equator and temperatures decrease symmetrically to the north and south approaching 0
ºC at high latitudes. Temperature also declines steadily to a depth of approximately 1,000 meters
with deeper waters have a uniform temperature of 1 to 2 ºC.
The various observations are used to explain why water moves both along the surface of
the ocean and vertically. Winds drive surface currents called gyres. Because of the Coriolis
Effect, surface currents form a clockwise pattern in gyres of the Northern Hemisphere and a
counterclockwise pattern south of the equator. Although all currents contribute to global climate
patterns, the western boundary currents such as the Gulf Stream and Brazil currents have an
especially significant role as they transport warm tropical waters to higher latitudes.
Thermohaline circulation occurs in deeper ocean waters and is driven by density contrasts related
to differences in water temperature and salinity. Thermohaline circulation drives the global
energy conveyer belt that causes surface waters to sink in the northern Atlantic Ocean and sends
cold, deep currents through the world’s oceans before upwelling in the northern Pacific and
Indian Oceans.
The later half of the chapter deals with the coastline--a dynamic environment that is an
ever-changing place with large population centers. Tides are discussed in the context of why they
occur and how they affect coastal communities. The interplay between the force of gravity from
the moon and sun and their impact on tides is explained. The movement of water associated with
tides is linked to wave action and the impact of waves on coasts. The dynamics of wave
propagation, breaking waves, and ensuing near-coast currents are illustrated using examples.
These case studies lead to a discussion of the seasonal variations in stream flow and storm
activity that affect the volume of sediment supplied to the coast, the rate of erosion, and thus the
shoreline. Over longer time scales (1,000s of years), climate cycles that result in increasing or
decreasing sea levels affect the shoreline. Finally, tectonic cycles measured in hundreds to many
thousands of years may continually revitalize rugged coastlines by periodic uplifts. On very short
timescales (days to weeks), waves act to change shorelines. Material is eroded and re-deposited
as waves break along the shoreline. A longshore current is generated when waves strike the coast
at an angle. The current forms in the surf zone and transports sediment laterally along the
shoreline. Longshore currents can result in the formation of characteristic depositional landforms
parallel to the coastline.
You will review case studies that illustrate the sediment budget and how that affects
coastal communities. You will find that the sediment budget is the balance between material
added to the shore by deposition and the material removed by erosion. Coastal erosion occurs
where erosion by wave action is not balanced by local deposition of the eroded material and the
supply of sediment from streams. The construction of dams will reduce the volume of sediment
reaching the coast. Drought conditions may also reduce stream flow and thus diminish sediment
transported to the shore by streams. Coastal development may result in the construction of
structures designed to reduce erosion (e.g., breakwaters, seawalls) or to control the local
depositional patterns to prevent infilling of navigation channels (jetties). Seawalls, groins, and
breakwaters act to prevent erosion and/or encourage deposition. However, structures built to
protect coastlines may prevent erosion of part of the shoreline but can result in accelerated
erosion elsewhere. Artificial beach nourishment occurs when sand is dredged and pumped onto
the beach from offshore but has a limited life span unless the processes that caused the original
erosion are stopped.
13.2 Ocean Basins.
Some features of the ocean floor include the continental shelf, slope and rise, the abyssal
plain, midoceanic ridge, seamonts and trenches. A cross sectional view of the ocean floor shows
these topographic elements (see Figure 13.5 on page 347).
Checkpoint 13.3: Note the line X-Y on the world map used for Checkpoint 13.1 (page
347). Which of the profile views most accurately models the bathymetry of the ocean floor
along that line?
a) Profile a
b) Profile b
c) Profile c
Hint: please refer to Figure 4.20 page 93 for help with this checkpoint. When viewing this
figure, make sure to understand what the symbols mean and what type of natural features
are represented by these symbols (for example, what is found at a convergent boundary
that involves oceanic crust?). Also, make sure to understand where the continents are
located (they are outlined in black) versus the seafloor.
13.3: Ocean Waters.
Salinity is the measure of the concentration of salt in seawater (see Table 13.1 for elements
found in seawater; notice that Cl and Na comprise the highest percentage of dissolved material in
seawater, and they combine together to form NaCl...salt).
Increasing the amount of salt in seawater increases the density of the seawater! Notice in Figure
13.8 on page 351 where the highest (and lowest) salinty values are in the world's oceans. The
big picture shows that the highest salinity values are found around 25 degrees N and 25 degrees
S of the equator (this is where temperatures are high, evaporation is high and rainfall is low).
Read and study this map; you should understand why salinity values are not highest at the
equator after reviewing this section, and why salinity values are low at the polar regions.
Salinity values change rapidly in the upper 500 meters. This rapid change in salinity with depth
is known as a halocline.
Notice in Figure 13.9 that temperatures of the world's oceans are highest at the equator
(extending to 25-30 degrees N and S of the equator) and lowest at the poles. Because water has a
high specific heat, it can absorb alot of energy without a big change in temperature and ocean
currents can carry this energy around the world (and help moderate temperatures at higher
latitudes). In other words, the oceans can absorb, store and release heat energy which plays a
crucial role in controlling global climate patterns.
Warm water is less dense than cold water! (so, water density increases by adding salt and
lowering the temperature). This creates layers in the oceans because warm water floats on top of
cold water (and cold water sinks beneath warmer water). Notice in figure 13.11 (a crosssectional view across the Pacific Ocean) that temperatures decrease rapidly in the upper 1000
meters and this is known as the thermocline.
Take note that the halocline and thermocline are located near the surface of the oceans where the
water is most affected by the Earth's surface processes such as evaporation, precipitation, mixing
by winds and river input.
Salinity, temperature and pressure are factors that affect the density of seawater. Figure 13.12
shows that the density of seawater increases most rapidly from about 200-1000 meters depth
called the pycnocline. This separates the ocean into three main vertical density layers which
plays an important role in controlling how heat is transferred between the world's oceans
(discussed in section 13.4).
13.4: Oceanic Circulation.
A shipment of rubber elephants falls overboard in the northern Pacific at location A on the map
(below). What path do the elephants subsequently follow?
a) A-G-B-F-E-A
c) A-G-C-E-A
b) A-E-C-G-A
d) A-E-F-B-G-A
The Coriolis Effect: Because the Earth spins from west to east on its geographic axis and objects
at different latitudes on Earth's surface move at different speeds (fastest at the equator, slowest at
the poles), atmospheric and oceanic circulation patterns are deflected to the right of their course
in the Northern Hemisphere (and to the left in the Southern Hemisphere). The wind or current
does not actually switch direction, but the spinning planet beneath it changes position.
To picture this, pretend you are at a merry-go-round and standing at the middle of the merry-goround with a basketball (this would be analogous to standing at the north pole) and a friend of
yours is standing at the very edge of the merry-go-round (analogous to standing at the equator).
Now spin the merry-go-round counterclockwise (looking down on the north pole from space the
Earth spins counterclockwise) and throw the basketball along a straight path to your friend.
Because your friend is spinning faster than you are, the ball will seem to deflect to the right of
your friend (from your point of view). This is the coriolis effect; an ocean current traveling from
the equator toward the north pole is also traveling eastward at a faster speed than the ground
beneath it (as the current moves north), so it has an apparent deflection to the right. If this is
confusing, read more about this on page 356.
New section added to second edition on El Nino/La Nina...to be discussed in class.
Thermohaline Circulation: Deep currents (see pages 360-361, deep currents are shown in blue)
transport water far below the ocean surface and are driven by differences in water density
(dependent on contrasts in both temperature and salinity). Remember, density increases as water
cools and salinity increases. This pattern of deep currents is termed thermohaline circulation and
moves energy from the tropics to the poles which helps moderate Earth's climate by
redistributing heat around the various oceans. A key step in the global conveyor belt occurs with
sinking water in the North Atlantic. These deep currents move along the bottom of the oceans
and return to the surface at the Indian and Pacific Oceans.
13.5: Tides
Tides are changes in the sea-surface height caused by the gravitational attraction of the moon and
the sun. To be discussed in class! By the way, a tsunami is NOT a tidal wave (tsunami not
caused by gravitational pull of moon and sun, but by events such as earthquakes, landslides,
volcanic eruptions and meteor impacts).
13.6: Wave Action and Coastal Processes.
Wave size can be measured in terms of wave height and wave length (see Figure 13.24). Wave
height is measured as the distance between the lowest part of the wave (trough) and highest point
of the wave (crest). Wave motion only affects surface waters, so wave motion decreases
with depth. Wave action decreases downward to a depth equal to about one-half of the
wavelength, and this is called the wave base. Wavelength is measured from wave crest to wave
crest and can vary from a few centimeters up to 600 meters.
What factors control the actual size of a wave? The wind speed and the distance over the ocean
that wind blows (called the fetch). Water basically follows a circular path in the open ocean and
remains in place (as the waveform moves through it; keep in mind there are currents that we
discussed earlier where water does move forward in the open oceans). As a deep ocean wave
approaches shallow regions near the shore, water in contact with the seafloor is slowed by
friction, the length of the wave decreases and the wave becomes steeper and taller. Here the
water actually does move forward as the wave collapses and breaks forward in the surf zone (it
washes up the shore and flows back to the sea). The point where waves feel the bottom of the
ocean (as discussed above) and break forward is usually where the depth of the ocean bottom is
less than one-half the wavelength (recall this is called the wave base).
Checkpoint 13.17: At which location on the following diagram would the waves begin to break
farthest from the beach?
Make sure to read about rip currents on page 367 in preparation for Checkpoint 13.20.
LEARNING OBJECTIVES
1. Students will explain concepts related to oceans and coastlines.
2. Students will identify and describe bathymetric features of the sea floor.
3. Students will describe differences in the salinity of ocean waters and account for these
differences.
4. Students will describe differences in the temperature of ocean waters and account for these
differences.
5. Students will explain the relationship of the salinity, temperature, and density of seawater.
6. Students will define and describe the Coriolis Effect.
7. Students will predict why salinity values vary from one geographic location to another.
8. Students will explain why tides occur and describe the variety of factors that determine high
and low tides.
9. Students will discuss the features of waves and their effects on the shoreline.
10. Students will compare and contrast sediment transport and sediment budget.
11. Students will describe examples of prevention and adjustment strategies for managing
coastal erosion.