15. Oceans and coastal processes

Oceans &
Coastlines
Introduction
Depth of the Ocean Floor
Salinity & Temperature of the Oceans
Oceanic Circulation
Coastlines
Wave Action
Shorelines & the Sediment Budget
Shoreline Protection
Summary
The oceans are the planet's last great living wilderness, man's only
remaining frontier on earth, and perhaps his last chance to prove himself a
rational species.
John L.Culliney
[The coastal zone is] rich in a variety of natural, commercial, recreational,
industrial, and aesthetic resources of immediate and potential value to the
present and future well-being of the nation.
Coastal Zone Management Act

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Introduction
• Over two-thirds of Earth’s surface is ocean.
• Global climate patterns are influenced by oceanic
circulation which in turn is controlled by the physical
characteristics of the oceans.
• The evolution of coastal landforms depends on the
interaction of wave action with processes in the continental
interior and human actions.
Over two-thirds (~71%) of the planet is covered in seawater.
The world’s oceans are unevenly distributed and lie mostly in
the Southern Hemisphere (88% seawater). The three major
oceans (Pacific, Atlantic, Indian Oceans) are connected
together along their southern margins by the Southern Ocean
that encircles Antarctica. Smaller, enclosed or partially
enclosed water bodies are termed seas or gulfs (Fig. 1). All of
these features are created by plate tectonic processes and
many continue to increase or decrease in size as plates diverge
or converge (see the Plate Tectonics chapter for more on plate
tectonics and the seafloor).
It is in the various small seas that we can observe the most
obvious negative consequences of human activity on the ocean
realm in the form of overfishing, pollution, eutrophication, and
habitat destruction. These same consequences are visited on the
margins of the major oceans.
Figure 1. The
distribution of the
world's oceans
and selected seas.
Nearly 90% of the
Southern
Hemisphere is
covered by
oceans.

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Today we appreciate the oceans for their direct and indirect
impacts on human activity. This chapter is divided into two
halves. The first part considers the physical characteristics of
the oceans and begins with the depth of the ocean floor. The
floor of the ocean is a few hundred meters below sea level
along the margins of the continents but averages over 3 km
depth between the major oceans and reaches a maximum depth
of 11 km in the western Pacific. The ocean floor has the
potential to be a great source of mineral wealth in the future
and mining companies are already staking claims to potentially
lucrative sections of subsea real estate.
Ocean currents are influenced not only by the extent and depth
of the oceans but also the salinity and temperature of ocean
waters. Ocean waters are thought to have originated from
meteorites and icy comets colliding with the early Earth. Both
temperature and salinity change with depth and latitude and
each is influenced by atmospheric weather patterns. Each
factor is examined in the section titled salinity and
temperature of the oceans.
The final section of the first half of the chapter examines how
depth, salinity, and temperature combine to generate
characteristic patterns of oceanic circulation. Surface currents
are controlled by dominant wind patterns that are in turn linked
to the rotation of the planet. Deepwater circulation patterns are
controlled by the density of ocean waters and the distribution
of landmasses. Both sets of currents redistribute the Earth's
heat budget and play a crucial role in controlling climate.
In the second half of the chapter we focus on how human
activity impacts the ocean margins and enclosed seas.
Coastlines represent the fragile strip of land that borders the
ocean. Developed coastal areas are threatened with potential
loss of life and billions of dollars in property damage as a result
of storm impacts and long-term erosion. For example
Hurricane Fran (Fig. 2) devastated parts of several eastern
Figure 2. Damage
from Hurricane
Fran along the
coast of North
Carolina. Note
change in position
of house indicated
by arrow. Images
courtesy of USGS
Recent Highlights-
Hazards.
Average ocean
area and depth:
Pacific Ocean
165,250,000 km2
4.28 km
Indian Ocean
73,440,000 km2
3.89 km
Atlantic Ocean
82,440,000 km2
3.33 km

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states and was especially destructive in North Carolina where
winds of over 100 mph generated more than $5 billion in
damages.
The evolution of landforms along a coastline depends upon the
interaction of wave action with the shore with processes in the
continental interior and human actions. The coastline is a
dynamic environment that advances or retreats depending upon
the balance between the supply of sediment and the material
removed by erosion. The section on shorelines and the
sediment budget examines this balance.
The National Park Service completed the tricky task of
relocating the historical Cape Hatteras lighthouse to a site
further inland in July 1999, to protect the light from erosion
that threatened to topple the structure. Twenty-six of the thirty
states bordering an ocean or Great Lake are presently
experiencing net loss of their shorelines. Well-intentioned
efforts at shoreline protection often resulted in the
construction of coastal structures such as jetties, groins, or
breakwaters that altered the natural movement of sediment
along the coastline and simply exacerbated existing erosion
problems. We finish by examining the interaction of shoreline
erosion and shoreline protection efforts along the southern
shore of Lake Erie.
Depth of the Ocean Floor
• The average depth of the ocean floor is nearly 4 km and the
maximum depth is a little over 11 km along the Mariana
Trench.
• Four principal depth zones can be identified in the oceans;
continental shelf (and rise), abyssal plain, oceanic ridge,
and oceanic trench.
• The oceanic ridge is a submarine mountain range that
occupies much of the floor of the Atlantic Ocean.
The depth of the ocean floor varies from sea level to a
maximum of over 11 km along the Mariana Trench in the
western Pacific Ocean. In contrast, the highest landform on the
continents is Mt. Everest, which is approximately 9 km (5.6

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miles) in elevation. In addition, the average elevation of the
land surface is less than a kilometer but the average depth of
the oceans is approximately 3.8 km (2.3 miles). We could
dump the continents in the ocean basins and still have plenty of
room to spare.
Recent analysis of originally classified satellite data at the
National Oceanographic and Atmospheric Administration's
(NOAA) National Geophysical Data Center (NGDC) has
allowed scientists to use slight variations in the elevation of the
ocean surface to determine the topography of the seafloor (Fig.
3).
Topography of the Ocean Floor
Beginning at the edge of the continents we can recognize four
principal depth zones in the oceans (Fig. 4). The first depth
level is the continental shelf, the shallow ocean floor (0-150
meters) immediately adjacent to continental land masses. The
shelf slopes gently toward the ocean from the coast with
maximum depths of a few hundred meters. The shelf may be a
relatively wide zone (hundreds of kilometers) adjacent to
passive margins (e.g., U.S. Atlantic Coast) or a narrow strip
Figure 3.
Characteristics of
the seafloor for the
eastern Pacific and
northwestern
Atlantic Ocean
basins. Map of
seafloor topography
from satellite
altimetry from
NOAA's National
Geophysical Data
Center.