Download Surface Processes Sections 23.1-23.4

Chapter 23
Surface Processes
Physical Science II
Module 3, Part 3
Surface Processes - Introduction
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Within the context of geologic time, the Earth’s surface is
a dynamic area.
Nothing is permanent. As soon as a rock is exposed at
the Earth’s surface, weathering processes begin to destroy
it.
Due to various weathering processes, erosion, and gravity,
high places on Earth are constantly being worn down by
the transportation of sediment to low places.
Intro
Weathering
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Weathering – environmental agents cause the physical
disintegration and chemical decomposition of rocks and
minerals at or near the Earth’s surface
Weathering is generally a slow but powerful force that
will eventually breakdown even the most resistant
materials.
The rock/mineral types, moisture availability,
temperatures, and overall climate largely determine the
rate of weathering.
Section 23.1
Mechanical Weathering
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Mechanical (or physical) weathering – the breaking down
of rocks/minerals into smaller pieces (disintegration),
without changing its chemical composition
Frost wedging is a specific physical weathering process
caused by the expansion of freezing water in fractures
(joints) or cracks in rock outcrops.
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Water will increase in volume about 9% during the freezing
process. This force expands the fractures and loosens
fragments of the rock.
Section 23.1
Mechanical Weathering
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Permafrost is the permanent frozen state of soil profiles
in cold higher latitude areas.
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During the short summer season though, the topsoil may
thaw resulting in wet spongy conditions.
This annual freeze-thaw cycle contributes to the physical
weathering process.
Physical weathering is also promoted by the action of
burrowing animals and by the growing of plant roots that
grow into and expand rock crevices.
Section 23.1
Chemical Weathering
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Chemical weathering changes the chemical composition
of rocks/minerals principally through reactions with
water.
Water and temperature are the two most important
factors in chemical weathering.
Chemical weathering is most efficient in hot, moist,
tropical climates.
Limestone, a common rock at the surface, readily
undergoes chemical weathering.
Section 23.1
Chemical Weathering of Limestone
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Limestone is made up largely of the mineral calcite.
(CaCO3)
Normal rain water will react with CO2 in the atmosphere
to form a weak solution of carbonic acid. (H2CO3)
H2O + CO2  H2CO3
The more CO2 that water comes in contact with, the
more carbonic acid is formed.
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As water percolates through the soil, it make react with even
more carbon dioxide released by bacteria.
Section 23.1
Chemical Weathering of Limestone
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As carbonic acid (H2CO3) comes in contact with the
calcite (CaCO3) in limestone, they react.
H2CO3 + CaCO3  Ca(HCO3)2
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The product of this reaction, calcium bicarbonate
Ca(HCO3)2, readily dissolves in water and is carried away in
solution
Most chemical weathering in limestone occurs along
fractures where the most water flows.
Given enough time, underground caverns can form.
Section 23.1
Chemical Weathering
Effects of Temperature and Moisture
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Chemical weathering is most rapid in areas with high
temperatures and abundant moisture.
In very cold areas (polar regions) and very dry areas
(deserts) chemical weathering is relatively sluggish.
Section 23.1
Chemical Weathering
Chemical Composition of the Rocks
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Certain rocks and minerals are more resistant to
chemical weathering.
The effects of modern chemical weathering may be
readily observed by viewing cemetery headstones.
Marble is composed of the same mineral as limestone
(calcite CaCO3), and therefore readily reacts with slightly
acidic rainwater.
Granite is much more resistant to weathering.
Section 23.1
Erosion
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Erosion – the downslope movement of soil and rock due
to gravity and various geologic “agents”
The most common agents of erosion include gravity
(mass wasting), streams, glaciers, wind, and waves.
Section 23.2
Runoff
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When rainfall exceeds the amount of water that can be
absorbed by the ground, runoff of the excess water
occurs.
As this water flows over the surface of the ground it can
be particularly erosive in areas such as a freshly plowed
field without any vegetation to hold the soil in place.
Runoff usually only occurs over short distances before it
either soaks into the ground or empties into a stream.
Section 23.2
Streams
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Stream – any channeled flow of water occurring between
two well-defined banks
To a geologist the size of the channeled flow does not
matter.
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Both the Mississippi and a small mountain brook are
considered “streams.”
As a stream flows, other material is also transported.
This material is called the stream load.
Section 23.2
Stream Load
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The load of a stream varies from dissolved material to
fine particles to large rocks.
The stream load at any given location or time depends on
the streams gradient, stream volume, current velocity, and
channel material.
The stream’s load is usually divided into three
components according to how the material is carried by
the stream:
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Dissolved load, Suspended load, and Bed load.
Section 23.2
Stream Load Components
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Dissolved load – consists of minerals and elements
carried by the stream in solution
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Suspended load – typically consists of fine- to mediumgrained discrete particles suspended in the flowing water
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Up to 20% of the stream load may be this dissolved portion.
The suspended load changes dramatically depending of the
velocity of the flowing water.
Bed load – larger grains that roll or bounce along the
bottom of the channel
Section 23.2
Stream Base Level
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In the upstream portion of a stream, flowing water
erodes out a V-shaped stream valley.
As the stream eventually reaches the level of the standing
body of water that it flows into, the stream cannot erode
any further down.
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Most streams empty into the ocean and therefore sea level is
the lowest level to which a stream can erode.
Streams that empty into a lake have the lake level as their
lowest level of possible erosion.
The base level is the lower limit below which a given
stream cannot erode.
Section 23.2
Grand Canyon
of the
Yellowstone
River
V-shaped
Valley
Copyright © Bobby H. Bammel. All rights reserved.
Section 23.2
Stream – Downstream Portion
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In the downstream portion of a stream the level of the
flowing water begins to approach the stream’s ultimate
base level.
In this region the stream’s downward erosion slows and
most of the stream’s energy is expended laterally as it
erodes from side to side.
During flooding, the stream overflows its bank and
deposits sediments over a wide area.
Over time the stream develops a widened, flat valley floor
called a flood plain.
Section 23.2
Stream Floodplain
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New sediments that are received periodically within the
flood plain make these areas some of the most fertile
agricultural land in the world.
The rich soils also lead most flood plains to become
heavily populated.
Despite their usual quiet appearance, flood plains are still
prone to periodic flooding causing significant damage to
crops and structures.
Section 23.2
Stream Meanders
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Particularly in the downstream portions, the stream
twists and turns, making loop-like bends called meanders.
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Essentially, the stream will always take the path of least
resistance.
On the outside of each meander loop the water current
is slightly faster, causing erosion.
On the inside of each meander loop the water current is
slightly slower, causing deposition.
When an entire loop is cut off, an oxbow lake forms.
Section 23.2
Stream Delta
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Streams erode and transport enormous quantities of
materials.
Most of these sediments eventually end up in the ocean
where the stream ends.
The Mississippi river carries 500 million tons of sediment
into the Gulf of Mexico each year.
Most of these sediments accumulate at the mouth of the
stream as the stream velocity significantly diminishes.
These sediments form the stream’s delta.
Section 23.2
Glaciers
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Glacier – a large mass of ice that covers part of the Earth
year-round
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They consist of recrystallized snow and flow over the
surface of the land under the influence of gravity.
Although much of North America was covered with
glaciers over 10,000 years ago, there are still about 1100
glaciers within the lower 48 states.
Approximately 3% of Alaska is covered by glaciers.
Section 23.2
Glacier Formation
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A glacier will form in an area where, over a number of
consecutive years, more snow accumulates than melts.
When enough snow accumulates, it will compress and
recrystallize into solid ice.
Eventually the glacier will begin to “flow” downhill due to
its own weight.
Most icebergs in the oceans are large chunks that have
broken off the edges of the glacial ice sheets of Greenland
or Antarctica.
Section 23.2
Glacier Types
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Continental glaciers today cover most of Greenland and
Antarctica.
Cirque glaciers are much smaller and can be found in
mountain regions located in hollow depressions
protected from the Sun.
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Most the glaciers in the U.S. are cirque glaciers.
As the glacier flows it carves out an amphitheater-like
depression called a cirque.
If the ice melts, these glacial depressions (cirques) fill with
water and are called cirque lakes.
Section 23.2
Glacier Types
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Valley glaciers occur when entire valleys are covered with
glacial ice that flows down the valley.
Flow rates of glaciers depend on many factors and vary
from centimeters to meters per day.
Eventually at lower and warmer (lower) elevations the
glacial ice melts.
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If the rate of flow and the melting rate are equal, the glacier
appears to be stationary.
Section 23.2
Glacial Erosion
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As a glacier flows, it loosens and carries away eroded
rock material, much of which is eventually ground up.
Although much slower than streams, glaciers are not as
selective and are able to pick up huge boulders and gouge
deep troughs.
Deep U-shaped valleys are typical of glacial erosion.
Section 23.2
Boulder Left Behind as
the Glacier Retreated
Copyright © Bobby H. Bammel. All rights reserved.
Section 23.2
U-Shaped Valleys in the Grand Tetons
Copyright © Bobby H. Bammel. All rights reserved.
Section 23.2
Glacial Deposits
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Till – material that has been transported and deposited
by glacial ice
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Till deposits are generally not well sorted or layered.
The sides and end of a glacier may form ridges of till
called lateral and terminal moraines.
The end or terminal moraine marks the farthest point
that the glacier advanced.
Terminal moraines can be found as far south as Indiana,
Ohio, and Long Island, New York.
Section 23.2
Wind
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Although wind erosion is very slow, it can be a very
significant process in desert areas.
Deserts are defined by a lack of precipitation, but may be
any temperature range.
The episodic desert thunderstorm may cause tremendous
short-term erosion.
During most of the year the wind is the prime erosional
agent in the desert.
The “dust bowl” of the 1930’s acquired its name from
wind blown topsoil.
Section 23.2
Mass Wasting
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Mass wasting – a general term for the downslope
movement of rock and soil under the influence of gravity
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Gravity tends to pull the material down and friction tends to
keep the material stationary.
The presence of water in the material significantly affects
the forces of both gravity and friction.
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The addition of water adds weight to the material and also
reduces the friction between the particles.
The presence of water promotes slope failure.
Section 23.2
Mass Wasting - Classification
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Fast (catastrophic) mass wasting occurs in two basic
forms: landslides and mudflows.
Landslide – the rapid downslope movement of large
blocks of weathered materials
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If large volumes of rock are involved in the landslide it is
called a rockslide.
Mudflow – involves the rapid downslope movement of
soil that has become unstable due to the absorption of
large amounts of water
Section 23.2
Mass Wasting - Classification
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Slump – the relatively slow downslope movement of an
essentially unbroken block of overburden
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Creep – very slow, imperceptible particle-by-particle
movement of debris down a slope
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Slumps leave a curved depression on the slope marking the
detachment zone.
Creep is too slow to see the movement, but the results of
years of creep can be seen along slopes.
Abundant vegetation and its root structure help deter all
types of mass wasting.
Section 23.2
Bends in Tree Trunks Indicate Creep
Lassen Volcanic National Park, California
Copyright © Bobby H. Bammel. All rights reserved.
Section 23.2
Water
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Water is essential for life and is required to maintain our
body function
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About 55% to 60% by mass of the human body is water
Early civilizations flourished where water was abundant,
generally in river valleys
Modern societies are also keenly concerned about the
distribution of fresh water
The global water supply is constantly in flux, being
redistributed throughout the Earth
Section 23.3
The Hydrologic Cycle
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Hydrologic Cycle – the constant movement of water
from large reservoirs (oceans and lakes), to the
atmosphere (evaporation), onto the land (precipitation),
and eventually back to the sea (runoff)
Much of this process we can readily see as clouds, rain,
snow, lakes, and flowing rivers
A significant portion of precipitation soaks into the soil
and bedrock and becomes part of the very slowly moving
groundwater system
Section 23.3
The Hydrologic Cycle
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The Earth’s 1.25 x
1018 m3 of water
is constantly
moving
Section 23.3
Water
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Approximately 70% of the Earth is covered with water
97.2% is ocean water, about 2.15% is frozen in icecaps and
glaciers, and only 0.65% is found in lakes, streams,
groundwater, and the atmosphere
Groundwater alone serves as the source of half of our
drinking water and nearly half of the water used for
agriculture and industry
Section 23.3
Groundwater
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The study and proper management of groundwater
resources has become one of the key environmental
issues facing the world
At the present time human usage of groundwater is
increasing each year and already exceeds the natural rate
of recharge
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In most areas groundwater reserves took thousands of years
to accumulate but can be depleted in only a few years
In many areas, groundwater is also polluted by human,
industrial, and agricultural wastes
Section 23.3
Porosity
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The movement of groundwater is largely dependent upon
physical characteristics of the soils and bedrock
Porosity – the percentage of the soil, bedrock, or
sediment’s volume that is composed of pores
It is the porosity of the underground reservoir that
determines the material’s capacity to store water
Another important physical property of the soil and
rocks is permeability
Section 23.3
Permeability
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Permeability – a measure of the material’s ability to
transmit fluids
The permeability of a substance is a function of the size
of the pores, the abundance of the pores, and how well
the pores are connected.
Loosely packed sand/gravel has large pores, permitting
easy fluid movement.
Muds and clays have small microscopic pores, resulting in
very poor permeability.
14 m/y is the average rate of groundwater flow.
Section 23.3
Groundwater
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Due to gravity, some of the water at the Earth’s surface
slowly percolates down through the unsaturated zone of
aeration until it reaches the zone of saturation.
The water table marks the boundary between these two
zones.
Within the zone of aeration air partially or completely
fills the pores.
In the zone of saturation water completely fills the pore
space.
Section 23.3
Groundwater Reservoir
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If the zone of saturation exhibits sufficient porosity and
permeability it forms an underground reservoir of water
that may be tapped by drilling wells to depths below the
water table.
Lakes, streams, and springs are found where the water
table intersects the surface.
Water table levels display seasonal variations and may dip
below shallow wells during a dry spell.
Section 23.3
Aquifer
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Aquifer – a body of permeable rock that both stores and
hold groundwater
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Sand, gravel, and porous sedimentary rocks typically make
good aquifers.
Most aquifers require pumping by wells. These are called
water-table wells.
Section 23.3
Aquifer
Zone of Aeration and Zone of Saturation
Section 23.3
Artesian Wells
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In some situations groundwater will flow to the surface
under its own pressure, called an artesian well.
Some aquifers are sandwiched between a pair of sloping
and impermeable rock layers.
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This is called a confined aquifer.
If the higher end of this aquifer layer is exposed at the
surface and receives water, wells penetrating the
downslope aquifer may flow under their own pressure.
Section 23.3
Confined Aquifer
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As long as the top of the well is below the water table,
the well will flow under its own pressure.
Water table
Section 23.3
Groundwater Recharge
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Groundwater supplies are a limited resource.
Water tables will drop during dry seasons and will also
drop if the rate of extraction through water wells is too
great.
In order to maintain a stationary water table, water must
be replaced into the aquifer at the same rate that moves
out of the aquifer.
Recharge is the rate of water replacement into an aquifer.
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Precipitation and/or flowing streams at the surface generally
lead to recharge.
Section 23.3
Groundwater Depletion
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Many aquifers are being depleted due to high extraction
rates.
The Ogallala aquifer of the central U.S. is a good example
of recent aquifer depletion.
This aquifer received most of its water over 10,000 years
ago during the wetter climate of the last ice age.
Natural recharge of this aquifer cannot keep pace with
over 150,000 wells that are constantly extracting water
for agricultural use.
Section 23.3
Groundwater Subsidence
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Water within the zone of saturation helps support the
overlying weight of the strata.
As the zone of saturation shrinks, the overlying strata
begins to compact and the ground surface subsides.
Ground subsidence can lead to shifting in building
foundations, pipeline ruptures, and cracks in roads.
In low-lying coastal areas subsidence can lead to flooding.
Section 23.3
Groundwater Contamination
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In most coastal areas, the groundwater is separated by
density.
Fresh water is less dense than salt water and therefore
tends to stay on top of the salt water.
Excessive withdrawal of the fresh groundwater can result
in salt water being pulled up into the well bore.
If the denser salt water is pulled up, the water well will be
ruined and the level of the water table will change.
Section 23.3
Coastal Salt Water Contamination
Excessive pumped can pull salt water into the well
Section 23.3
Coastal Salt Water Contamination
Excessive pumping pulls salt water into the well
Section 23.3
Groundwater Over Usage
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Numerous problems arise when aquifers have too much
water extracted or the extraction rate is too rapid.
Water tables are lowered, ground subsidence occurs, and
in coastal areas salt water contamination is possible.
The proper management of our groundwater reservoirs
is critical.
Section 23.3
Groundwater Contamination
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Groundwater contamination also occurs when other
substances percolate down into the zone of saturation.
Some examples include industrial and household
chemicals, fertilizers, pesticides, sewage, herbicides,
detergents, refined petroleum products, and other
Water quality may be adversely affected, depending upon
the pollutant and the concentrations present in the
groundwater.
Section 23.3
Water Treatment
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Municipal water supplies are generally treated with
chlorine to kill bacteria and viruses.
However, chlorination does not remove chemical
pollutants.
Chemical or mineral pollutants in the groundwater must
be removed by some filtration or chemical process.
Section 23.3
Hard Water
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Hard water is water that contains high concentrations of
Ca and Mg salts.
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Fe salts may also contribute to hard water.
Although fairly high amounts of Ca/Mg in water is not
harmful it can cause several problems.
Hard water leads to the formation of scale and eventual
clogging inside pipes.
Hard water also inhibits detergent action, resulting in
yellowing, bathtub rings, and less lathering action.
Section 23.3
The Ocean
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Coastal regions of the world are dominantly shaped by
the influence of surface waves.
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Shoreline zones range from rocky cliffs to broad, low, sandy
beaches.
The five major oceans of the world include the Pacific,
Atlantic, Indian, Antarctic, and Arctic.
Oceans cover approximately 70% of the Earth’s surface
and have an average depth of around 4 km.
The Marianas Trench in the western Pacific is the deepest
portion of the ocean at 11 km.
Section 23.4
Ocean Water Motion
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There are basically three types of seawater movement.
These include; waves, currents, and tides.
Ocean waves are formed by the interaction of
atmospheric winds and the surface of the water.
Waves of varying sizes continually lap the world’s
shoreline.
Section 23.4
Shoreline Wave Motion
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Although the wave form continually propagates through
the water, the water “particles” themselves are only
moving in small, generally circular orbits.
Since the water is actually not moving much, debris will
simply bob up and down as wave forms continually pass
under it.
As the wave form enters shallow water along the shore,
the underwater circular motion of the wave is interfered
with, resulting in surf.
Section 23.4
Surface Wave Approaching Shore
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As the wave approaches the shore, the underwater
circular motion becomes elliptical, and finally the crest
falls forward.
Elliptical motion
Section 23.4
Long-Shore Currents
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Along most shorelines, offshore debris will steadily move
parallel to the shoreline.
This type of movement indicates the presence of a longshore current.
This type of current results from waves breaking at an
angle to the shoreline.
The component of motion parallel to the shoreline
creates a current in that direction.
Section 23.4
Tides
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Tides – the periodic rise and fall of the ocean surface
caused by the gravitational pull of the Moon and to a
lesser extent the Sun
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Review Section 17.2 for a detailed explanation.
In some areas of the world the daily tidal range may be as
much as 12 meters. In other areas the daily range is less
than 1 meter.
Waves, long-shore currents, and tides are all important
agents of erosion along coastlines.
Section 23.4
Coastal Erosion
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A number of different features form along irregular
coastlines.
The portions of the shoreline that jut out the most are
subjected to more intense wave energy and therefore
more erosion.
When the coastal area is elevated wave action can attack
only the base of the slope below the high-tide level.
Wave-cut cliffs, sea stacks, sea arches, and sea caves may
all form.
Section 23.4
Wave Erosion Along
an Elevated Coastline
Section 23.4
The Green Bridge of Wales
Copyright © Bobby H. Bammel. All rights reserved.
Section 23.4
Coastal Deposition
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Most shoreline sediments come from either shoreline
erosion or from stream transport.
These sediments are distributed along the coast by waves,
long-shore currents, and tidal action.
Sediment deposition occurs along calmer stretches of the
coast and results in several characteristic forms.
The more common depositional features include pocket
beaches, barrier islands, lagoons, and spits.
Section 23.4
Features of Coastal Deposition
Section 23.4
Coastal Deposition
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Pocket beaches form in low-energy, protected zones
between coastal headlands.
Barrier islands are long narrow deposits that form parallel
to the mainland.
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Both North Carolina and Texas have a number of barrier
islands separated from their mainland.
Lagoons form between the mainland and barrier island.
Spits are elongate extensions of the shorline.
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Cape Cod, MA is a classic example of a spit.
Section 23.4
Seafloor Topography
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Before modern technology revealed a detailed view of
the ocean floor, most scientists thought that the floors of
the oceans were relatively smooth and sediment-covered.
We now know that the topography of the seafloor is as
irregular as are the surfaces above sea level.
Hidden beneath the deep ocean water lie submarine
mountain chains, isolated volcanic mountains, trenches,
and extensive flat areas.
Section 23.4
Seafloor Topography
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Submarine mountain chains form along divergent plate
boundaries and are called mid-ocean ridges.
Volcanic mountains are known as seamounts and form
over isolated hot spots.
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The Hawaiian Islands are examples.
Some submarine seamounts have unusually flat tops and
are called guyots.
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Guyots were once volcanic islands that had their tops
eroded off by wave action.
Section 23.4
Seafloor Topography
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Seafloor trenches are another significant feature found
under the oceans.
These trenches are generally long, narrow, and mark the
locations of submarine subduction zones.
Continental erosion supplies huge quantities of sediments
onto the seafloor.
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These sediments have the tendency of covering and/or
masking some of the irregular seafloor features.
Huge flat areas called abyssal plains form from this
sediment influx.
Section 23.4
Continental Shelves
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Most of the oceans are underlain by oceanic crust and
contain very deep columns of water
Continental crust, however, extends and underlies some
areas near the continents.
Portions of the ocean underlain by continental crust are
called continental shelves.
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These zones are much shallower than the oceanic basins.
The widths of the continental shelf areas vary from
almost nothing to over 1200 km.
Section 23.4
Continental Shelves - Economics
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Continental shelves are of particular economic interest to
their bordering countries for a number of reasons.
Continental shelves provide most of the major
commercial fishing zones throughout the world.
In addition, continental shelves are locations for significant
offshore oil deposits and serve as areas for future
potential oil reserves.
Section 23.4
Continental Shelf
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The continental slope actually marks the true edge of the
continental landmasses.
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Sediments erode off the continental shelf and accumulate
near the base.
Section 23.4
Continental Slope
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Beyond the continental shelf the seafloor slopes more
steeply downward to the floor of the ocean basin.
From a ‘plate tectonics’ view, the continental slope marks
the transition from continental crust (less dense) to
oceanic crust (more dense).
Section 23.4