Earth Science--Chapter 10 wk 10
Waters of the ocean are constantly in motion.
winds generate surface current, the moon and sun produce tides, and
density differences create deep-ocean circulation.
I. Surface Currents - huge patterns covering whole oceans
- the Gulf Stream is a portion of a huge, slowly moving, circular whorl, or
gyre (Fig. 10.2)
A. Ocean Circulation Patterns
- energy is passed from moving air to the water through
friction.
- drag exerted by winds blowing steadily across the ocean
causes the surface layer of water to move
-winds are the primary driving force of surface currents.
- a clear relationship exists between the oceanic circulation
and the general atmospheric circulation.
Coriolis effect (Fig. 10.2) must be considered…learn more about
mechanics in Ch 13…for now REMEMBER…
- currents are deflected to the right in the Northern
Hemisphere and
- to the left in the Southern Hemisphere viewed from above.
-the direction of surface currents does not coincide
precisely with the wind direction
-Equatorial Currents are north and south of the equator,
westward moving currents
-Because of the Coriolis effect, these currents are
deflected poleward to form clockwise gyres in the Northern
Hemisphere and counter clockwise gyres in the Southern
Hemisphere.
-found in each major ocean basin (Fig. 10.2)
Eth Sci wk 10 p.2
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Example – Gulf Stream….
In the North Atlantic, it continues northeastward beyond the Grand
Banks, widens, slows until it becomes known as the North Atlantic Drift
-As North Atlantic Drift approaches Western Europe, it
splits, Canary current moves south
this eventually completes the loop, back into North Equatorial
Current
-Large central area inside the gyre has no well-defined currents
(Sargasso Sea)

In S. Atlantic, surface circulation is much same as N.
Atlantic, just diff names (Brazilian at west, Benguelan on east), and
everything circles to left

N. and S. Pacific same deal as N. & S. Atlantic
-Antarctica, cold surface waters circulate continuously,
moving in response to prevailing westerly winds --West
Wind Drift
B. Ocean Currents and Upwelling
-In addition to producing surface currents, winds also
cause vertical water movements.
- known as Upwelling, the rising of cold water from deeper layers to
replace warmer surface water that moves away
-most characteristic along the eastern shores of the
oceans (Fig. 10.2)
-Upwelling occurs in these areas when winds blow
toward the equator and parallel to the coast.
eath Sci wk 10 p.3
How does upwelling happen??
-the surface water movement is deflected away from the shore.
As the surface layer moves away from the coast, it is replaced by water
that upwells from below the surface
-flow from depths of 50 to 300 meters (165 to 1000 feet)
-creates a characteristic zone of lower temperature near
the shore.
-chilling surprise = dip in Pacific
-Coastal upwelling also brings to the ocean surface
greater concentrations of dissolved nutrients. Nutrient
enriched waters promote the growth of plankton which
supports extensive populations of fish
C. The Importance of Ocean Currents Fig 10.3
-Important effect on climate
-moderating effect of poleward moving warm ocean currents is well known
-Great Britain and much of northwestern Europe
warmer during the winter than one would expect
for their latitudes.
-warm ocean currents whose effects are felt most in the
middle latitudes in winter, like Gulf Stream heating England
the influence of cold currents is most pronounced in the tropics or during
summer months in the middle latitudes. Cold currents, such as
the Benguela Current moderate the tropical heat.
-the aridity along these coasts is intensified because the lower air is
chilled . When this occurs the air becomes very stable and resists the
upward movement
Earth Sci wk 10 p.4
Heat Transfer a key job….
-Ocean currents also play a major role in maintaining Earth’s heat
balance.
--transferring heat from the tropics to the polar regions
-Ocean water movement accounts for about a quarter of
this heat transport, winds transport the remaining three
quarters.
II. Deep-Ocean Circulation
-Deep water movements occur as well as surface
-deep ocean circulation is governed by gravity and driven by
density differences (vs main driver of surf currents…..).
-Two factors--temperature and salinity
most significant in creating a dense mass of water
-Thermohaline circulation (thermo and haline…remember?)
-after sinking from the surface of the ocean, waters will
not reappear at the surface for an average of 500-2000
years.

basic driving mech:
water at surface is made colder & more salty and becomes more
dense.
Denser water then sinks and displaces lighter water
light water moves back toward the zone where the denser water
formed
 Dense waters flow away from their source near the poles and
are replaced by warmer waters from lower latitudes.
Earth Sci wk 10 p.5
-Arctic and Antarctic waters represent the two major regions where dense
water masses are created.
Temps in Antarctic are cold enough to form sea ice
salt is not frozen into the ice, the remaining water becomes saltier.
This is the densest water in all the oceans.
sinks to bottom of sea floor where it moves through the ocean basins in
sluggish currents.
III. Tides
-Tides are daily changes in the elevation of the ocean surface
-Newton applied law of gravitation to them
-atmosphere and the ocean both are fluids, deformed by this
force
-ocean tides result from gravitational attraction exerted upon
Earth by the moon
A. Causes of TIdes
-moons gravitational force can cause the water to bulge on the side
of Earth nearest the moon. Fig 10.6
equally large tidal bulge is produced on the side of Earth directly opposite
the moon (like stretching a rubber band).
-the tidal bulges remain in place while Earth rotates “through” them --Imagine self on seashore for 24 hours …Earth rotates you through
alternating areas of deeper and shallower water as you pass through the
bulges.
Earth Sci wk 10 p.6
-tidal bulges migrate as the moon revolves around Earth every 29 days.
Shift about 50 minutes later each day
B. Spring and Neap Tides
- sun also influences the tides
-effect is considerably less than that of the moon
-half that of the moon
- Spring tides: new and full moons - near these times, sun and
moon are aligned and their forces are added together,
combined gravity causes higher tidal bulges (high tide) and lower troughs
(low tides)
largest daily tidal ranges
- Neap tides: first and third quarters of the moon, sun and moon act on
Earth at right angles,
each partially offsets the influence of the other.
daily tidal range is less.
C. Tidal Currents
- Tidal current is term used to describe the horizontal flow of water
accompanying the rise and fall of the tide.
-Tidal currents that advance into the coastal zone as the tide
are called flood currents (a true “tidal wave”)
rises
Earth Sci wk 10 p.7
As the tide falls seaward moving water generates ebb currents (“ebb
tide”….remember the Righteous Bros??).
Periods of little or no current, called slack water, separate flood and ebb.
Areas affected are called tidal flats.
-Tidal currents that accompany a high tide may
miles per hour
IV.
attain a speed of 12
Waves
-shoreline is constantly being modified by waves (causing cliffs
of poorly consolidated glacial sediment to retreat at nearly 1
meter per year)
-Wind-generated waves provide most of the energy that
shapes and modfies shorelines
-the shore is where a practically irresistible force meets an
almost immovable object.
A. Characteristics of Waves
-waves derive their energy and motion from the wind
-if wind exceeds 3 km per hour, more stable waves form
progress with the wind (Fig. 10.8)
Parts of Waves ---learn them…
-Crests- tops of waves
-Troughs
-Wave height- vertical distance between trough and crest
-Wave length-horizontal distance between successive wave crests
-wave period - time interval between the passage of
successive crests at a stationary point
Earth Sci wk 10 p.8
-height, length and period depend to 3 factors:
1-wind speed,
2-length of time the wind has blown,
3-fetch, the distance that the wind has traveled across open water
-As the quantity of energy transferred from the wind to
the water increases, the height and steepness of the
waves increase as well
-When the wind stops or changes direction, or the waves
leave the storm area where they were created, they
continue on without relation to local winds.
B. Types of Waves
-the motion of the wave is different from the motion of the
water particles within it Fig 10.8
-wave form or shape that moves forward, not the water
itself
-Waves of energy simply move through the nearly
stationary medium of the water.
-When water is part of the wave create, it moves in the
same direction as the advancing wave form
-When it is in the trough, the water actually moves
backward
- these are waves of oscillation - waves in the open sea
energy transfer downward as well as lateral…
-beneath the surface, the circular motion rapidly
depth equal to about one-half the
wavelength, the movement of
water particles becomes negligible. (Fig. 10.8) KEY POINT
earth Sci wk 10 p.9
-When a wave approaches the shore, the water
becomes shallower and influences wave behavior.
The wave begins to “Feel bottom” at a water depth equal to about one half
its wavelength (Fig. 10.10)
-As the speed and length of the wave diminish, the energy must go
somewhere,
the only way to go is up, so the wave steadily grows higher.
Finally a critical point is reached when the steep wave front collapses or
breaks.
- this is wave of translation - in which the water advances up the shore
-“surf” - turbulent water created by breaking waves
-“swash” - the turbulent sheet of water from collapsing
breakers.
When it’s energy has been expended the water flows back down the
beach toward the surf zone as Backwash
V. Wave Erosion
- just as streams do most of their work during floods, so do waves perform
most of their work during storms.
Waves awesome in their violence.
Each breaking wave has thousands of tons of water against the land,
earth Sci wk 10 p. 10
The pressures exerted by Atlantic Ocean waves are more than 2000
pounds per square foot.
Wave impact opens cracks, crevices, etc with lots of force from
compressed air forced ahead of wave Fig 10.11
Wave erosion by abrasion too - the sawing and grinding action of the
water loaded with rock fragments
- especially intense in surf zone
VI. Wave Refraction
-wave refraction: bending of waves
-plays part in shoreline processes,
strongly influences where sediment transport and deposition
will take place.

Regardless of angle of attack, waves try to line up parallel to shore
- mechanism …wave slows down in shallow water, wave front
refracts toward shore
shore
Waves

refraction focuses energy on headlands, disperses it in bays Fig 10.12
Earth sci wk 10 p. 11
 in bay with lowered wave energy, sediment can fall out of suspension
and accumulate in the bay….so you see “pocket beaches” between
headlands
VII Sand movement alongshore Fig 10.13


though refracted, waves still hit shore at angle.
2 components of wave travel
- perpendicular to shore
- parallel to shore (longshore current)


sand grains zig-zag up and down shoreface
longshore current moves suspended sediment along the shore
- quantities are large: NJ – 750,000 tons/yr
Calif – 1,500,000 tons/yr
Beach often called “river of sand”
VIII Shoreline Features
Fig 10.17
Vary from one shore to another
-
Wave cut cliffs..erosion at base, notch develops, rx fall in
Wave cut platform left behind
Arches (Fig 10.15) form if 2 sea caves join
Stack (one side of arch) left after top part of arch collapses
Where sand involved:
- Spit: thin sand bar pulled by longshore drift into a bay
Example Sandy Hook NJ (Fig 10.16)
Baymouth bar: sand bar completely sealing off mouth of a bay
Tombolo: sand spit connects mainland to island or island to island
(Fig 10.17)
- Barrier Islands
- espec prominent on Atlantic and Gulf Coasts
- usu 15-30 km long (10-20 mi)
- lagoons behind them are quiet water, very fine grained seds
-
Earth Sci wk 10 p.12
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B.Islands may form several ways
- spits severed from mainland
- sand heaped by breakers
- former sand dune ridges from last ice age and lower sea leve
shoreline evolution
generally evolve from irregular to smoother over
time…embayments fill while headlands retreat
shore
from:
shore
to:
waves
waves
IX. Shoreline Erosion Problems



lots of human activity complicates situation….very desirable place to
be, but very unstable
storms can radically change the beach
lots $ spent to attempt erosion control, beach protection Fig 10.18
important factors in how a coast reponds:
- proximity to sed input
- tectonism
- topography, lithology
- winds/weather
- coastline configuration
some artificial ingineering structures:
- groins - shore-perpendicular strcs designed to trap sand Fig 10.18
- can have big problem with down-drift erosion because wave activity
still hits shore but no sand blanket to absorb energy
Earth Sci wk 10 p.13
-
seawalls – shore-parallel structure designed to protect property
from breaking waves..
loss of beach in front of seawall because wave energy reflected off
wall, tends to scour and erode sediment.
One strategy increasing employed..beach nourishment
Sand placed on beach, often mined from offshore
Problem with this sol’n is that it is temporary, because processes
that removed original sand to begin with will also remove the nourishment
Nourishment not cheap..
Another alternative (#3) is to abandon coastal development
by relocating buildings, etc
X. Emergent/submergent coastlines
Some people classify coastline by either emergent or submergent
1. emergent – land rising relative to sea level
for example, California
2. submergent – land subsiding relative to sea level
for example, Chesapeake & Delaware Bays- drowned river mouths known
as estuaries