adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Cover about 71% of the surface of the Earth.
The greatest ocean depth of 11,035 m occurs in the Mariana Trench
Have an average depth of 3,800 m.
Have a present volume of about 1.35 billion cubic kilometers, but
the volume fluctuates with the growth and melting of glacial ice.
Salinity, a measure of amount of dissolved ions in the oceans,
ranges between 33 and 37 parts per thousand.
The dissolved ions have been concentrated in seawater as a result of
chemical weathering (Na, Ca, Mg, S, K, Br, and HCO3 ) and degassing of
the mantle by volcanic activity (Cl
Seawater would contain higher concentrations of dissolved ions if
some were not removed by chemical precipitation, plants and animals,
and absorption onto clay minerals.
Salinity varies in the oceans (see figure 13.3 in your text)
Surface waters evaporate, rain and stream water is added, and ice
forms or thaws.
Salinity is higher in mid-latitude oceans
Salinity is higher in restricted areas of the oceans like the
Mediterranean and Red Seas (up to 41 parts per thousand).
Salinity is lower near the equator because precipitation is higher.
Salinity is low near the mouths of major rivers because of input of
The temperature of surface seawater varies with latitude, from near 0o C
near the poles to 29oC near the equator. But restricted areas can have
temperatures up to 37oC.
Properties of seawater also vary with depth.
The density and salinity of seawater increase with depth.
Temperature decreases with depth.
Surface Ocean currents are result of drift of the upper 50 to 100 m of
the ocean due to drag by wind. Thus, surface ocean currents generally
follow the same patterns as atmospheric circulation with the exception
that atmospheric currents continue over the land surface while ocean
currents are deflected by the land. The surface currents
Circulation is clockwise in the northern hemisphere and
counterclockwise in the southern hemisphere.
In each hemisphere cooler waters from higher latitudes circulate
toward the equator where they are warmed and circulate back toward
In addition to surface circulation, seawater also circulates vertically
as a result of changes in density controlled by changing salinity and
temperature (see figures 13.5, 13.6a, and 13.6b in your text).
Such circulation, because it controlled by both temperature differences
and differences in salinity of the water, is called thermohaline
Tides are due to the gravitational attraction of moon and to a lesser
extent, the sun on the Earth. Because the moon is closer to the Earth
than the sun, it has a larger effect and causes the Earth to bulge
toward the moon. At the same time, a bulge occurs on the opposite side
of the Earth due to inertial forces.
These bulges remain stationary while Earth rotates. The tidal
bulges result in a rhythmic rise and fall of ocean surface, which is not
noticeable to someone on a boat at sea, but is magnified along the
coasts. Usually there are two high tides and two low tides each day, and
thus a variation in sea level as the tidal bulge passes through each
point on the Earth's surface. Along most coasts the range is about 2 m,
but in narrow inlets tidal currents can be strong and fast and cause
variations in sea level up to 16 m
Because the Sun also exerts a gravitational attraction on the Earth,
there are also monthly tidal cycles that are controlled by the relative
position of the sun and moon. The highest high tides occur when the Sun
and the moon are on the same side of the Earth (new moon) or on opposite
sides of the Earth (full moon). The lowest high tides occur when the Sun
and the moon are not opposed relative to the Earth (quarter moons).
These highest high tides become important to coastal areas during
hurricane season and you always hear dire predications of what might
happen if the storm surge created by the hurricane arrives at the same
time as the highest high tides.
Waves are generated by winds that blow over the surface of oceans. In a
wave, water travels in loops. But since surface is the area affected,
the diameter of the loops decreases with depth. The Diameters of loops
at the surface is equal to wave height (h)
Wavelength (L) = distance to complete one cycle
Wave Period (P) = time required to complete on cycle.
Wave Velocity (V) = wavelength/wave period (L/P).
Motion of waves is only effective at moving water to depth equal to one
half of the Wavelength (L/2). Water deeper than L/2 does not move. Thus,
waves cannot erode the bottom or move sediment in water deeper than L/2.
This depth is called wave base. In the Pacific Ocean, wavelengths up to
600 m have been observed, thus water deeper than 300m will not feel
passage of wave. But outer parts of continental shelves average 200 m
depth, so considerable erosion can take place out to the edge of the
continental shelf with such long wavelength waves.
When waves approach shore, the water depth decreases and the wave will
start feeling bottom. Because of friction, the wave velocity (= L/P)
decreases, but its period (P) remains the same Thus, the wavelength (L)
will decrease. Furthermore, as the wave "feels the bottom", the circular
loops of water motion change to elliptical shapes, as loops are deformed
by the bottom. As the wavelength (L) shortens, the wave height (h)
increases. Eventually the steep front portion of wave cannot support the
water as the rear part moves over, and the wave breaks. This results in
turbulent water of the surf, where incoming waves meet back flowing
Rip currents form where water is channeled back into the ocean.
Wave Erosion- Rigorous erosion of sea floor takes place in surf
zone, i.e. between shoreline and breakers. Waves break at depths
between 1 and 1.5 times wave height. Thus for 6m tall waves,
rigorous erosion of sea floor can take place in up to 9 m of water.
Waves can also erode by abrasion and flinging rock particles against
one another or against rocks along the coastline.
Wave refraction- Waves generally do not approach shoreline
parallel to shore. Instead some parts of waves feel the bottom
before other parts, resulting in wave refraction or bending.
Wave energy can thus be concentrated on headlands, to form cliffs.
Headlands erode faster than bays because the wave energy gets
concentrated at headlands
Coastal Erosion and Sediment Transport
- Since most waves arrive at the shoreline at an angle even after
refraction. Such waves have a velocity oriented in the direction
perpendicular to the wave crests (Vw), but this velocity can be resolved
into a component perpendicular to the shore (Vp) and a component
parallel to the shore (VL). The component parallel to the shore
can move sediment and is called the longshore current.
- is due to waves approaching at angles to beach, but retreating
perpendicular to the shore line. This results in the swash of the
incoming wave moving the sand up the beach in a direction perpendicular
to the incoming wave crests and the backwash moving the sand down the
beach perpendicular to the shoreline. Thus, with successive waves, the
sand will move along a zigzag path along the beach.
Offshore Transport and Sorting
Particles picked up by wave motion move downslope, but the deeper the
water, the less energy is involved in wave motion, so smaller and
smaller particles are moved farther off shore. This results in size
sorting of sediment, with grain size decreasing away from coast.
Shaping of Coasts
Coast represents the boundary between sea and land. When waves hit the
coast, they can erode by breaking up rocks into finer particles and
abrading other rocks by flinging rocks, sand and water against them.
Over time, the effects can be large. Sediment is moved and redeposited
to increase the size of continental shelves. The effects on the land
surface can be seen by examining the shore profile.
Beaches occur where sand is deposited along the shoreline. A beach can
be divided into a foreshore zone, which is equivalent to the swash zone,
and backshore zone, which is commonly separated from the foreshore by a
distinct ridge, called a berm. Behind the backshore may be a zone of
cliffs, marshes, or sand dunes.
Rocky Coasts - Where wave action has not had time to lower the coastline
to sea level, a rocky coast may occur. Because of the resistance to
erosion, a wave cut bench and wave cut cliff develops. If subsequent
uplift of the wave-cut bench occurs, it may be preserved above sea level
as a marine terrace.
The cliff may retreat by undercutting and resulting mass-wasting
processes. In areas where differential erosion takes place, the
undercutting may initially produces sea caves. If sea caves from
opposite sides of a rocky headland meet, then a sea arch may form.
Eventual weakening of the sea arch may result in its collapse to form a
Coastal Deposits and
Coastlines represent a balance between wave energy and sediment supply.
If wave energy and sediment supply are constant, then a steady state is
reached. If anyone of these factors change, then shoreline will adjust.
For example, winter storms may increase wave energy, if sediment supply
is constant, fine grained beach sand may be carried offshore resulting
in pebble beaches or cobble beaches. Due to input of sediment from
rivers, marine deltas may form, due to beach and longshore drift such
features as spits, bay barriers, and tombolos may form.
Depositional Features along coasts.
Deltas -- Deltas form where sediment supply is greater than
ability of waves to remove sediment. An example is the Mississippi
River Delta, which is composed of several lobes that were deposited
within the last several thousand years. Erosion of the older delta
lobes has taken place due to subsidence, sea level rise, and lack of
new sediment being supplied to the delta because of the human-made
Spits - elongated deposits of sand or gravel that projects from
the land into open water. Spits usually form at the mouth of a bay
due to long shore current and beach drift. Generally they curve
inward towards the bay due to refraction of the waves around the
mouth of the bay.
Bay Barriers - if a spit extends across a bay, it is called a bay
barrier. Exchange of water between the bay and the ocean is
accomplished through the groundwater system.
Tombolos - a spit that connects the mainland to an offshore island
is called a tombolo.
Barrier Islands - A barrier island is a long narrow ridge of sand
just offshore running parallel to the coast. Separating the island
and coast is a narrow channel of water called a lagoon. Most barrier
islands were built during after the last glaciation as a result of
sea level rise. Barrier islands are constantly changing. They grow
parallel to the coast by beach drift and longshore drift, and they
are eroded by storm surges that often cut them into smaller islands.
Reefs and Atolls - Reefs consist of colonies of organisms, like
corals, which secrete calcium carbonate. Since these organisms can
only live in warm waters and need sunlight to survive, reefs only
form in shallow tropical seas. In the deeper oceans reefs can build
up on the margins of volcanic islands, but only do so after the
volcanoes have become extinct. After the volcanism ceases, the
volcanic island begins to erode and also begins to subside, due to
the weight of newly added material. As the island subsides, the
reefs continue to grow upward. Eventually, the original volcanic
island subsides and is eroded below sea level. But, the reefs trap
sediment and a circular or annular island, called an atoll.
The shape of coast is controlled mainly by tectonic forces and
meteorological conditions. Examples:
The southern coast of Australiais a rugged coast with many sea cliffs
and raised wave cut benches (marine terraces). This is what is called an
emergent coastline and in this case is due to a recent episode of uplift
of the land relative to the sea by tectonic forces.
The rest of eastern coast, on the other hand, is a submerged coast, and
shows submerged valleys, barrier islands, and gentle shorelines, all due
to rise of sea level since last glaciation age (during glacial ages,
seawater is tied up in ice, and sea level is lower; when the ice melts
sea level rises).
Storms - great storms such as hurricanes or other winter storms
can cause erosion of the coastline at much higher rate than normal.
During such storms beaches can erode rapidly and heavy wave action
can cause rapid undercutting and mass-wasting events of cliffs along
Tsunamis - a tsunami is a giant sea wave generated by an
earthquake. Such waves travel at speeds up to 950 km/hr, have
wavelengths up to 200 m, and upon approaching a shallow coastline,
can have wave heights up to 30 m. Such waves have great potential to
wipe out cities located along coastlines.
Landslides - on coasts with cliffs, the main erosive force of the
waves is concentrated at the base of the cliffs. As the waves
undercut the cliffs, they may become unstable and mass-wasting
processes like landslides will result. Massive landslides can also
Protection from Shoreline Erosion
Seacliffs, since they are susceptible to landslides due to undercutting,
and barrier islands and beaches, since they are made of unconsolidated
sand and gravel, are difficult to protect from the action of the waves.
Human construction can attempt to prevent erosion, but human engineering
cannot always protect against abnormal conditions. Humans construct such
things as sea walls, breakwaters, and groins in an attempt to slow
coastal erosion, but sometimes other problems are caused by these
engineering feats. For example, construction of groin (a wall built
perpendicular to the shoreline) can trap sand and prevent beach drift
and longshore drift from supplying sand to areas down current along the
coastline. These down current areas may then erode, causing other