22. Shaping Earths Surface

8/22/2019 22. Shaping Earths Surface

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© 2012 Pearson Education, Inc.

Conceptual

PhysicalScience5th Edition

Chapter 22:

SHAPING EARTH’S

SURFACE





This lecture will help you

understand:• The Hydrologic Cycle

• Groundwater

•The Work of Groundwater

• Surface Water and Drainage Systems

• The Work of Surface Water

•Glaciers and Glaciation

• The Work of Glaciers

• The Work of Air




The Hydrologic Cycle

More than 97% of all

Earth’s water is in

the oceans.

Only 1 % of Earth’s

water is available to

us as water vapor,

groundwater, and

freshwater.




Earth’s waters are constantly circulating.

The driving forces are:

• Heat from the Sun

• Force of gravity





The hydrologic cycle is the set of processesthat controls the circulation of water onEarth.

Processes involved in the hydrologic cycle:• Evaporation

• Precipitation

• Infiltration

•

Runoff






Water that goes from the ocean back tothe ocean makes a complete loop in the

hydrologic cycle.

The journey is not always direct.

•

Water can flow as streams, rivers, andgroundwater

• Water can also be frozen in ice caps andglaciers


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Earth’s natural supply of fresh water comes from:

A. Ice caps.B. Artesian springs.

C. Glaciers.

D. Rain and snow.


CHECK YOUR NEIGHBOR



Earth’s natural supply of fresh water comes from:

A. Ice caps.B. Artesian springs.

C. Glaciers.

D. Rain and snow.

Explanation:

Rain and snow deliver the water that forms ice caps, aquifers,

glaciers, and streams.


CHECK YOUR ANSWER



Groundwater

Factors that influence storage andmovement of groundwater:

• Porosity : ratio of open space in soil, sediment,or rock to total volume of solids plus voids—the amount of open space underground.

• Greater porosity equals more potential tostore greater amounts of groundwater.

•

Particle size, shape, and sorting influenceporosity.—Soil with rounded particles of similar size has

higher porosity than soil with various sizes.



Groundwater

Permeability• Degree to which groundwater can flow

through a porous material—higher

permeability, greater potential for fluid flow.

• Sediment packing and connectedness of

pores influences permeability.

• Hydraulic conductivity—a measure of

permeability—tells us the degree to which thematerial can transmit water.



Groundwater The Water Table

Water beneath the ground exists asgroundwater and soil moisture.

•Groundwater occurs in the saturated zone

—

water has filled all pore spaces.

• Soil moisture is above the saturated zone inthe unsaturated zone—pores filled with water

and air.

The water table is the boundary betweenthese two zones.



Groundwater The Water Table

The depth of the water table varies withprecipitation and climate.

• Zero in marshes and swamps, hundreds of meters in some deserts.

• At perennial lakes and streams, the water table is above the land surface.

• The water table tends to rise and fall with thesurface topography.



Groundwater Aquifers and Springs

Aquifers are reservoirs of groundwater.

• Aquifers are a vital source of fresh water.

• Aquifers generally have high porosity and highpermeability.

• Aquifers underlie the land surface in manyareas.

•It is important to keep this vital source of freshwater clean and contaminant free.




Groundwater Aquifers and Springs

A perched water table occurs when

discontinuous, low-permeability layers

in an unconfined aquifer intercept

percolating water above the water

table.




Groundwater

Aquifers and SpringsGeologists can often use springs to locate

faults, because a spring can indicate

that there are cracks or breaks in therock.

G d t




A soil with rounded particles of similar size willhave a higher porosity than a soil with rounded

particles of various sizes, because

A. it will have a higher permeability.B. water flows more easily through rounded particles.

C. smaller sediment grains will fill the open pore spaces between

larger grains.

D. poorly sorted sediment will have more open pore spaces.

Groundwater

CHECK YOUR NEIGHBOR

G d t




A soil with rounded particles of similar size willhave a higher porosity than a soil with rounded

particles of various sizes, because

A. it will have a higher permeability.B. water flows more easily through rounded particles.

C. smaller sediment grains will fill the open pore spaces

between larger grains.

D. poorly sorted sediment will have more open pore spaces.

Groundwater

CHECK YOUR NEIGHBOR




Groundwater Groundwater Movement

The elevation of a water table above a

particular location—usually sea level—

is called the hydraulic head.




Groundwater Groundwater Movement

Darcy’s law:

Groundwater flow rate =hydraulic conductivity cross-sectional area hydraulic gradient




The Work of Groundwater

Flowing groundwater can alter andchange features at the surface:

• Land subsidence• Caves and caverns

• Sinkholes




The Work of Groundwater Land Subsidence

•

Extreme groundwater withdrawalby pumping from wells can resultin lowering of the land—landsubsidence.

•

Land subsidence is especiallyprevalent in areas underlain by

aquifers made of sandy sediments

and interbedded clays. The clays

leak water to the sand, then whenwater is pumped out, the clays

shrink and compact, causing

subsidence.




The Work of Groundwater Caverns and Caves

• The dissolving action of groundwater “eats

away” at rock—limestone in particular.

•

Rainwater chemically reacts with CO2 in the air and soil, producing carbonic acid. The acidified

water seeps into rock (especially limestone),

partially dissolving it.




The Work of Groundwater Caverns and Caves

Groundwater has carved out magnificent

caves and caverns (a cavern is a large

cave).




The Work of Groundwater Sinkholes

• Sinkholes are funnel-shaped cavities inthe ground that are open to the sky.

•Sinkholes are formed in a manner similar to caves.

• Sinkholes can also be formed fromconditions of drought and the over-

withdrawal of groundwater.




The Work of Groundwater Karst Regions

Karst regions are characterized by soft

rolling hills or sharp, rugged surfaces.

Karst regions are areas where sinkholes,

caves, and caverns define the land

surface.





What kind of formation is this?

A. Floodplain

B. Karst

C. Glacier

D. Artesian spring


CHECK YOUR NEIGHBOR





What kind of formation is this?

A. Floodplain

B. Karst

C. Glacier

D. Artesian spring


CHECK YOUR NEIGHBOR

S f W t




Surface Water

and Drainage Systems

Surface water includes streams, rivers,lakes, and reservoirs.

Infiltration of water is controlled by:• Intensity and duration of precipitation

• Prior wetness condition of the soil

•

Soil type• Slope of the land

• Nature of the vegetative cover




Surface Water


Running water shapes Earth’s surface

in two opposing ways:

• It carves out the landscape.

• It deposits sediments.

S f W t




Surface Water


Stream Flow Geometry

Factors that determine stream velocity:

• Gradient, or slope

• Discharge—volume of water moving past a

given point in a certain amount of time

• Channel geometry characteristics

— shape and size




Surface Water

and Drainage SystemsStream Flow Geometry

Average stream speed =

discharge / cross-sectional area

Stream speed is usually not constant along the

length of a stream. As the stream moves

downslope, the gradient decreases and thechannel widens. Discharge usually increases

as tributaries add water.

S f W t




Surface Water


Stream Flow GeometryStream speed varies within a channel. It is

slower along the stream bed (friction) and

faster near the surface.• In a straight channel, maximum flow speed

is mid-channel.

•In a curving channel,maximum flow speed

is on the outside of

each bend.

S f W t




Surface Water


Drainage Basins and NetworksThe land area that contributes water to a stream is called

the drainage basin.

Drainage basins are separated by drainage divides.

The largest drainage divides are continental divides.




The Work of Surface Water

Flowing surface water sculpts and shapes

Earth’s surface:

• Erosion—erosive sculpting action carves thelandscape

• Deposition—shapes the land as sediment is

deposited





Streamflow—two types of flow• Laminar flow—slow and gentle

• Turbulent flow—fast and rapid





Streams transport great amounts of sedimentfrom one location to another.

Laminar flows can lift and carry only the very

smallest and lightest particles.

A turbulent flow can move and carry a range of particle sizes—it moves particles downstream

mainly by lifting them into the flow or byrolling and sliding them along the channelbottom. The smaller, finer particles remainsuspended to make the water murky.




The Work of Surface Water Erosion and Transport of Sediment

Chemical Weathering: Stream water carries

dissolved substances that chemically weather and

erode rock.

Hydraulic Action: Erosion

and movement of great

quantities of sediment

and rock. Abrasion: Abrasion occurs

when sediments and particles

scour a channel.

Th W k f S f W t




The Work of Surface Water Erosion and Transport of Sediment

Stream erosion:

• Loosely consolidated particles are lifted by

abrasion and dissolution.

Stronger currents lift particles more effectively:

• Stronger currents have “higher ” energy

• Lift and transport more and bigger particles• Turbulent versus laminar flow




The Work of Surface Water Stream Valley and Floodplains

Stream channels in high

mountain areas cut into

underlying rock.Fast-moving rapids and

beautiful waterfalls are

characteristic of V-shaped mountain

stream valleys.





Stream Valleys and FloodplainsStream speed plays a role in erosion and

deposition.

In a meandering stream channel, maximum stream speed

is on the outside bend of the channel—the cut bank—a

place of erosion.

On the inside bend, stream

speed slows to create a

point bar —a place of

deposition.





Stream Valleys and FloodplainsMeandering streams create a wide belt of

almost flat land: a floodplain.

When a flood occurs, sediment is deposited inthe floodplain. Large, coarse sedimentcreates natural levees.





Deltas: The End of the Line for a River A delta is where a flowing stream meets a

standing body of water. The flow slows down

and the stream dumps sediment.The result is a fan-shaped deposit of new land.





This picture shows an example of what

geologic feature?

A. Floodplain

B. Delta

C. Turbulent flowD. Rapids

CHECK YOUR NEIGHBOR





This picture shows an example of what

geologic feature?

A. Floodplain

B. Delta

C. Turbulent flow

D. Rapids

CHECK YOUR ANSWER




Glaciers and Glaciation

Glaciers are powerful agents of erosion. Aglacier is like a plow as it scrapes andplucks up rock and sediment.

Glaciers are also powerful agents of deposition. A glacier is like a sled as itcarries its heavy load to distant places.




Glaciers and GlaciationGlacier Formation and Movement

A glacier is an accumulation of snow and ice

thick enough to move under its own weight.

• Two types of glaciers:

— Alpine

—Continental





Glacier Formation and Movement• When a glacier ’s ice mass becomes about 50

meters thick, the pressure of the overlyingmaterial causes the base of the ice to move

plastically —

the entire mass shifts.• Also, meltwater at the base of the glacier

creates basal sliding.





Glacier Formation and MovementThe mass of a glacier changes over time. As

snow falls, accumulation makes the glacier

grow. As ice melts, sublimates, or breaks off,

ablation occurs.




The Work of GlaciersGlacial Erosion and Erosional Landforms

Alpine glaciers develop in mountainous areas,generally confined to individual valleys.—Cascades, Rockies, Andes, Himalayas

—Erosional landforms: cirque, arête, horn, hangingvalley,U-shaped valley




The Work of GlaciersGlacial Sedimentation

and Depositional LandformsWhen glacial ice melts, it drops a poorly sorted,

heterogeneous load of boulders, pebbles,

sand, and clay.

A wide range of particle sizes is the hallmark

that differentiates glacial sediment from the

much-better-sorted

material depositedby streams and

winds.




The Work of Air

Wind blows everywhere, but its impact onsculpting the land is minor.

Impact is greatest where:

• Strong winds blow frequently

• Vegetation is sparse or absent

—Plant roots keep particles together

—Plants deflect wind and shelter particles

• Surface particles are small—Small particles are more easily lifted and

transported

Surface Processes




There are many erosive agents that sculpt Earth’s surface.Overall, the erosive agent that does the most work is

A. wind.

B. groundwater.

C. running water.

D. glaciers.

CHECK YOUR NEIGHBOR

Surface Processes



There are many erosive agents that sculpt Earth’s surface.Overall, the erosive agent that does the most work is

A. wind.

B. groundwater.

C. running water.

D. glaciers.

CHECK YOUR NEIGHBOR

22. Shaping Earths Surface

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