Chapter 18: The Oceans and Their
Margins
J. Bruce H. Shyu
May 24, 2010
Introduction: The World’s Oceans

Seawater covers 70.8 percent of Earth’s surface, in
three huge interconnected basins:



The Pacific Ocean (太平洋).
The Atlantic Ocean (大西洋).
The Indian Ocean (印度洋).
Figure 18.1
The Oceans’ Characteristics



The greatest ocean depth yet measured (11,035 m)
lies in the Mariana Trench (馬里亞納海溝).
The average depth of the oceans, is about 3.8 km.
The present volume of seawater is about 1.35
billion cubic kilometers.

More than half this volume resides in the Pacific Ocean.
Figure 18.2
Ocean Salinity (1)



Salinity (鹽度) is the measure of the sea’s saltiness,
expressed in parts per mil (‰ = parts per
thousand).
The salinity of seawater normally ranges between
33 and 37‰.
The principal elements that contribute to this
salinity are sodium and chlorine.
Ocean Salinity (2)

More than 99.9 percent of the ocean’s salinity
reflects the presence of only eight ions:








Chloride.
Sodium.
Sulfate.
Magnesium.
Calcium.
Potassium.
Bicarbonate.
Bromine.
海水中的元素組成
鹽度 : 35gm/1000gm
9 種主要元素:
氯 (Cl)
鈉 (Na)
硫酸鹽 (SO4-2)
鎂 (Mg)
鈣 (Ca)
鉀 (K)
次碳酸鹽 (HCO3-)
溴 (Br)
鍶 (Sr)
Ocean Salinity (3)

Cations are released by chemical weathering processes
on land.


Each year streams carry 2.5 billion tons of dissolved
substances to the sea.
The principal anions found in seawater are believed to
have come from the mantle. Chemical analyses of gases
released during volcanic eruptions show that the most
important volatiles are water vapor (steam), carbon
dioxide (CO2), and the chloride (Cl-) and sulfate (SO42-)
anions.
Salinity of the oceans
Figure 18.3A
Temperature and Heat Capacity of the
Ocean (1)



Global summer sea-surface temperature is displayed
with isotherms (等溫度線) that lie approximately
parallel to the equator.
The warmest waters during August (>28°C) occur in
a discontinuous belt between about 30° N and 10° S
latitude.
In winter, the belt of warm water moves south until
it is largely below the equator.
Temperature of the oceans in August
Figure 18.3B
1994年5月時全球海洋表面平均溫度 (oF)
西
太
平
洋
表
面
海
水
年
平
均
溫
度
Temperature and Heat Capacity of the
Ocean (2)



Waters become progressively cooler both north and
south of this belt.
Since the water has a high heat capacity (熱容量),
both the total range and the seasonal changes in
ocean temperatures are much less than what we find
on land.
Coastal inhabitants benefit from the mild climate
resulting from this natural ocean thermostat.
Vertical Stratification (1)



Temperature and other physical properties of
seawater vary with depth.
When fresh river water meets salty ocean water at a
coast, the fresh water, being less dense, flows over
the denser saltwater, resulting in stratified water
bodies.
The oceans also are vertically stratified as a result of
variation in the density of seawater.
Vertical Stratification (2)

Seawater become denser as:




Its temperature decreases.
Its salinity increases.
Gravity pulls dense water downward until it
reaches a level where the surrounding water has
the same density.
These density-driven movements lead both to
stratification of the oceans and to circulation in the
deep ocean.
印
度
洋
海
水
溫
度
剖
面
海洋環流
表面洋流：風吹流
中層流與底層流：溫鹽環流
由行星風系所驅動
由海水的密度差所驅動
運行速度快
運行速度緩慢
兩者共同形成全球的海水輸送帶
Ocean Circulation



Surface ocean currents (表面洋流) are broad,
slow drifts of surface water set in motion by the
prevailing surface winds.
A current of water is rarely more than 50 to 100 m
deep.
The direction taken by ocean currents is also
influenced by the Coriolis effect (科氏力效應).
科氏力：由地球自轉速度，移動粒子的水平速度、所在
的緯度所決定，其方向與速度向量的方向成90度夾角
Current Systems


Each major current is part of a large subcircular
current system called a gyre (環流).
The Earth has five major ocean gyres.



Two are in the Pacific Ocean.
Two are in the Atlantic Ocean.
One is in the Indian Ocean.
Figure 18.4
黑潮流徑
冬季
緯
速度比率
50 cm/s
度
夏季
緯
速度比率
50 cm/s
度
觀
測
：
用
衛
星
追
蹤
浮
球
細線條是1988~2006年間衛星追蹤約850個浮球隨流漂移的軌跡（又稱海流麵條
圖），箭矢是從軌跡計算出的海流平均速度。（台灣海洋大學胡健驊教授）
Major Water Masses


Ocean waters also circulate on a large scale within
the deep ocean, driven by differences in water
density.
The water of the oceans is organized into major
water masses, each having a characteristic range of:


Temperature.
Salinity.
大西洋的溫鹽環流
Figure 18.5
NADW 北大西洋深層水
AAIW 亞南極中層水
AABW 南極底層水
The Global Ocean Conveyor System (1)


Dense, cold, and/or salty surface waters that flow
toward adjacent warmer, less-salty waters will sink
until they reach the level of water masses of equal
density.
The resulting stratification of water masses is thus
based on relative density.
The Global Ocean Conveyor System (2)

The sinking dense water in the North Atlantic
propels a global thermohaline circulation system, so
called because it involves both the temperature
(thermo) and salinity (haline) characteristics of the
ocean waters.
The Global Ocean Conveyor System (3)

The Atlantic thermohaline circulation acts like a
great conveyor belt, transporting low-density
surface water northward and denser deep-ocean
water southward.

Heat lost to the atmosphere by this warm surface water,
together with heat from the warm Gulf Stream, maintains
a relatively mild climate in northwestern Europe.
Figure 18.6A
全球海水輸送帶
Figure 18.6B
Ocean Tides (1)

Tides (潮汐):



Twice-daily rise and fall of ocean waters.
Caused by the gravitational attraction between the Moon
(and, to lesser degree, the sun) and the Earth.
The Moon exerts a gravitational pull on the solid
Earth.
Tide-Raising Force (1)


A water particle in the ocean on the side facing the
Moon is attracted more strongly by the Moon’s
gravitation than it would be if it were at Earth’s
center, which lies at a greater distance.
This creates a bulge on the ocean surface due to the
excess inertial force (called the tide-raising force).
Figure 18.7
Tide-Raising Force (2)


At most places on the ocean margins, two high tides
and two low tides are observed each day as a coast
encounters both tidal bulges.
Twice during each lunar month, Earth is directly
aligned with the Sun and the Moon, whose
gravitational effects are thereby reinforced,
producing higher high tides and lower low tides.
Figure 18.8
Tide-Raising Force (3)





In the open sea tides are small (less than 1 m).
Along most coasts the tidal range commonly is less than 2 m.
In bays, straits, estuaries, and other narrow places along
coasts, tidal fluctuations are amplified and may reach 16 m
or more.
Associated tidal currents (潮汐水流) are often rapid and
may approach 25 km/h.
The incoming tide locally can create a wall of water a meter
or more high (called a tidal bore).
Ocean Waves (1)



Ocean waves receive their energy from winds
that blow across the water surface.
The water particles move in a loop-like, or
oscillating manner.
Because waveform is created by this loop-like
motion of water parcels, the diameters of the
loops at the water surface exactly equal wave
height (波高).
Figure 18.10
Ocean Waves (2)



Downward from the surface, a progressive loss of
energy occurs, resulting in a decrease in loop
diameter.
“L” is used to represent wavelength (波長), the
distance between successive wave crests or troughs.
At a depth equal to half the wavelength (L/2), the
diameters of the loops have become so small that
motion of the water is negligible.
Wave Base


The depth L/2 is therefore referred to as the wave
base (浪基面或波底).
Landward of depth L/2, as the water depth
decreases, the orbits of the water parcels become
flatter until the movement of water at the seafloor
in the shallow water zone is limited to a back-andforth motion.
由於摩擦力隨
深度增加，水
分子運動的能
量隨水深而不
斷減弱，到水
深為波長的一
半時即消失。
波底 ＝
½波長水深
wave base =
depth of ½
wave length
Breaking Waves



When the wave reaches depth L/2, its base
encounters frictional resistance exerted by the
seafloor.
This causes the wave height to increase and the
wave length to decrease.
Eventually, the front becomes too steep to support
the advancing wave and the wave collapses, or
breaks.
Figure 18.11
波浪的形狀從深海
進入淺海所發生的
變化
波長和波速不斷減小，
波高不斷增加。最後
波浪愈變愈陡，圓周
狀轉動的速度 終於
超過波浪本身前進的
速度，波浪於是崩潰，
形成破浪(breaking
wave 或breaker)。
Surf




Such “broken water” is called surf.
The area between the line of breaking waves and
the shore is known as the surf zone (破浪帶).
Water piled against the shore returns seaward
partly in localized narrow channels as rip currents
(離岸流).
The geologic work of waves is mainly
accomplished by the direct action of surf.
Wave Refraction (1)


A wave approaching a coast generally does not
encounter the bottom simultaneously all along its
length.
As any segment of the wave touches the seafloor:




That part slows down.
The wave length begins to decrease.
The wave height increases.
This process is called wave refraction (波浪折屈).
Wave Refraction (2)

Wave refraction affects various sectors of a
coastline differently.



Waves converge on headlands, which are vigorously
eroded.
Refraction of waves approaching a bay will make them
diverge, diffusing their energy at the shore.
In the course of time, irregular coasts become smoother
and less indented.
Figure 18.13
波浪折屈 (wave refraction)
波峰線平行海濱線，波向線則垂直波峰線，波浪能量和侵蝕將
集中於岬角(headland)。波向線又稱能流線(energy-flow lines)。
Coastal Erosion and Sediment
Transport (1)

Erosion below sea level:



Ocean waves rarely erode to depths of more than 7 m.
The lower limit of wave motion is half the wavelength of
ocean waves.
Abrasion in the surf zone:


An important kind of erosion in the surf zone is the
wearing down of rock by wave-transported rock particles.
The activity is limited within a few meters, so the surf is
like an erosional knife edge or saw cutting horizontally
into the land.
Coastal Erosion and Sediment
Transport (2)

Erosion above sea level:


Waves pounding against a cliff compress the air trapped
in fissures.
Nearly all the energy expended by waves in coastal
erosion is confined to a zone that lies between 10 m
above and 10 m below mean sea level.
Sediment Transport by Waves and
Currents (1)

Longshore currents (沿岸流):



Longshore currents flow parallel to the shore.
The direction of longshore currents may change
seasonally.
The longshore current moves the sediment along the
coast.
Figure 18.14
Sediment Transport by Waves and
Currents (2)

Beach drift:


The swash (uprushing water) of each wave travels
obliquely up the beach before gravity pulls the water
back directly down the slope of the beach.
This zigzag movement of water carries sand and pebbles
first up, then down the beach slope in a process known as
beach drift.

Beach drift can reach a rate of more than 800 m/day.
Figure 18.15
Sediment Transport by Waves and
Currents (3)

Beach placers:


Gold, diamond, and several other heavy minerals have
been concentrated in beach sands by surf and longshore
currents.
Offshore transport and sorting:


Far from shore only fine grains can be moved.
Sediments grade seaward from sand into mud.
Figure 18.16
Coastal Deposits and Landforms


The shore profile (海岸剖面) is a vertical section
along a line perpendicular to the shore.
The three important features of the shore profile are:



Beaches (海灘).
Wave-cut cliffs (波蝕崖或海蝕崖).
Wave-cut benches (波蝕台地).
Beaches (1)

Beach is:



The sandy surface above the water along a shore.
A wave-washed sediment along a coast, including
sediment in the surf zone (sediment is continually in
motion).
Sediment of a beach may derived from:


Erosion of adjacent cliffs or cliffs elsewhere along the
coast.
Alluvium brought to the shore by rivers.
Beaches (2)

On low, open shores an exposed beach typically has
several distinct elements:



A rather gently sloping foreshore (前濱) (lowest tide to
the average high-tide level).
A berm (灘台) (bench formed of sediment deposited by
waves).
The backshore (後濱) (from the berm to the farthest
point reached by surf).
Figure 18.17
海灘標準平衡剖面及其各部名稱
何春蓀：普通地質學， p. 382
Rocky (Cliffed) Coasts

The usual elements of a cliffed coast due to erosion
are:




A wave-cut cliff, which may have a well-developed
wave-cut notch (海蝕凹壁) at its base.
A wave-cut bench, a platform cut across bedrock by surf.
A beach, the result of deposition.
Other erosional features associated with cliffed
coasts are sea caves (海蝕洞), sea arches (海蝕拱),
and stacks (海蝕柱).
Figure 18.18
A small sea notch in the western coast of Myanmar
A sea arch with sea cliffs in the coast of northern Chile
Factors Affecting the Shore Profile



Through erosion and the creation, transport, and
deposition of sediment, the form of a coast changes,
often slowly but sometimes very rapidly.
During storms, the increased energy in the surf
erodes the exposed part of a beach and makes it
narrower.
In calm weather, the exposed beach is likely to
receive more sediment than it loses and therefore
becomes wider.
Major Coastal Deposits and Landforms


Marine deltas (三角洲) are a typical constructional
coastal landform.
The extent of marine deltas is a compromise
between the rate at which a river delivers sediment
at its mouth and the ability of currents and waves to
erode sediment along the delta front.
Figure 18.21
Spits and Related Features

A spit (沙嘴) is an elongated ridge of sand or gravel that
projects from land and ends in open water.





It is merely a continuation of a beach.
It is built of sediment moved by longshore drift and dropped at
the mouth of a bay.
The free end curves landward in response to currents created by
refraction as waves enter the bay.
A spit-like ridge of sand or gravel that connects an island to the
mainland or to another island, called a tombolo (連島沙洲).
A ridge of sand or gravel may be built across the mouth of a
bay to form a bay barrier (灣口沙洲).
Figure 18.22
海岸地形及其侵蝕與沈積的現象
Beach Ridges and Barrier Islands



Beach ridges (灘脊) are low sandy bars parallel to
the coast. They are old berms.
A barrier islands (障蔽島或離岸沙洲) is a long
narrow sandy island lying offshore and parallel to a
coast.
An elongate bay lying inshore from a barrier island
or strip of land such as coral reef is called a lagoon
(潟湖).
The cross-section of a barrier island
Figure 18.24B
Organic Reefs and Atolls



A fringing reef (裙礁) is either attached to or
closely borders the adjacent land (no lagoon).
A barrier reef (堡礁) is separated from the land
by a lagoon that may be of considerable length and
width.
An atoll (環礁), a roughly circular coral reef
enclosing a shallow lagoon, is formed when a
tropical volcanic island with a fringing reef slowly
subsides.
Figure 18.26
Fringing Reef 裙礁
Barrier Reef 堡礁
Atoll 環礁
海洋火山隨著海洋地殼年齡老化而沈陷
中途島
東沙環礁
How Coasts Evolve (1)

The configuration of coasts depends largely on:




The structure and erodibility of coastal rocks.
The active geologic processes at work.
The length of time over which these processes have
operated.
The history of world sea-level fluctuations.
How Coasts Evolve (2)

Types of coasts:



Most of the Pacific coast of North America is steep and
rocky.
The Atlantic and Gulf coasts traverse a broad coastal
plain that slopes gently seaward and are festooned with
barrier islands.
Where rocks of different erodibilities are exposed
along a coast, marine erosion is strongly controlled
by rock type and structure.
Coastlines of
Croatia, which
are highly
controlled by
the trends of
local structures
Figure 18.27
Geographic Influences on Coastal
Processes



Coasts lying at latitudes between about 45° and 60°
are subjected to higher-than-average storm waves
generated by strong westerly winds.
Subtropical east-facing coasts are subjected to
infrequent but often disastrous hurricanes or
typhoons.
Sea ice is an effective agent of coastal erosion in the
polar regions.
Changing Sea Level

Sea level fluctuates:


Daily as a result of tidal forces.
Over much longer time scales as a result of:



Changes in the volume of water in the oceans as continental
glaciers advance and retreat.
The motions of lithospheric plates that cause the volume of the
ocean basins to change.
Sea level fluctuations, on geologic time scales,
contribute importantly to the evolution of the
world’s coasts.
Submergence: Relative Rise of Sea
Level


Nearly all coasts have experienced submergence, a
rise of sea level that accompanies the most recent
deglaciation.
Most large estuaries, for example, are former river
valleys that were drowned by the recent sea-level
rise.
Figure 18.28
Emergence: Relative Fall of Sea Level



Many marine beaches, spits, and barriers exist from
Virginia to Florida.
The highest reaches an altitude of more than 30 m.
These features indicate that the past sea level was
higher.
Sea-Level Cycles and Relative
Movements of Land and Sea




Many coastal and offshore features date to times when
relative sea level was either higher or lower than now.
The major rises and falls of sea level are global
movements.
By contrast, uplift and subsidence of the land, which
cause emergence or submergence along a coast, involve
only parts of landmasses.
Movements of land and sea level may occur
simultaneously, in either the same or opposite directions.
Sea level fluctuation history of the past 140000 yrs
Figure 18.29
Coastal Hazards (1)





Storms cause infrequent bursts of rapid erosion.
A strong earthquake, landslide, or volcanic eruption
can generate a potentially dangerous tsunami (海
嘯) .
Tsunamis can travel at a rate as high as 950 km/h.
They have long wavelength up to 200 km.
They can pile up rapidly to heights of 30 m.
Tsunami travel times
Figure 18.31
Coastal Hazards (2)



Cliffed shorelines are susceptible to frequent
landslides as erosion eats away the base of a
seacliff.
Sometimes landslides on cliffed shorelines give rise
to giant waves that are even more destructive than
the slides themselves.
Very large tsunamis have also been produced by
massive coastal landslides (Lituya Bay, Alaska, in
1958).
Ocean Circulation and the Carbon
Cycle




Photosynthesizing marine organisms exchange
dissolved CO2 for dissolved O2 in surface waters.
A wide variety of organisms draw bicarbonate
anions out of seawater to form calcium carbonate
shells.
Calcium carbonate accumulates on the seafloor if it
is shallower than about 4 kilometers.
Cold O2-rich water sinks into the deep ocean from
the surface waters.
Sediments at the Beginning and the
End of an Ocean’s Life Cycle (1)



Unusual depositional conditions are common when
an ocean basin initially opens, and in its last stages
of closure.
If evaporation dominates the regional climate,
salinity increases in small semi-isolated ocean
basins.
Evaporite deposits can form if the connection to the
world’s oceans is broken by tectonic activity or by a
drop in sea level.
Sediments at the Beginning and the
End of an Ocean’s Life Cycle (2)


Geologists have estimated that the Mediterranean
would evaporate completely in only 1000 years if
the Straits of Gibraltar were blocked.
Thick salt deposits beneath the Mediterranean
seafloor tell us that it dried out as many as 40 times
between 5 and 7 million years ago.