Download Chapter 7 Review: Glaciers, deserts, landscape shaped by wind

Chapter 7 Review: Glaciers, deserts, landscape shaped by wind
Review: Chapter 7
Glaciers, Deserts,
Landscape shaped
by Wind
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind





As recently as 15,000 years ago,
up to 30 percent of earth’s land
was covered by an glacial ice.
Earth was covered by an ice age
A glacier is a thick ice mass that
moves slowly over the land
surface.
Glaciers originate on land in
places where more snow falls
each winter than melts in
summer.
The snowline is the lowest
elevation in an area that remains
covered in snow all year.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind



Glaciers are also important
agents of erosion. Like
rivers, they accumulate,
transport, and deposit
sediment
Unlike mountain streams,
mountain glaciers advance
only a few millimeters to
meters a day.
A valley glacier is a stream
of ice that flows between
steep rock walls from a
place near the top of a
mountain valley.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind



Ice sheets are enormous
ice masses that flow in all
directions from one or
more centers; much larger
than glaciers.
The only present day ice
sheets are those covering
Antarctica and Greenland
The Antarctic ice cap
holds 80% of the world’s
ice and nearly two thirds
of the world’s fresh water.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind




The movement of glaciers
is referred to as flow.
Glacial flow happens in two
ways:
 Plastic flow
 Basal slip
Plastic flow involves
movement within the ice
Basal slip is when the
entire ice mass actually
slips downhill due to
gravity. The layers closest
to the surface move faster
than the layers closer to
the top.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind



The uppermost zone that
does not have plasticity is
brittle. We call this zone
the zone of fracture.
The zone of fracture
experiences tension when
the glacier moves. This
tension results in deep
cracks called crevices.
Different glaciers move at
different rates. Glaciers
may move forward,
retreat, or stay in place.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind


In the zone of
accumulation; above the
snowline, the glacier gains
ice and forces movement
downslope.
The area of the glacier
below the snowline is
where the glacier melts,
losing ice and mass. We call
this area of glacial melting
the zone of wastage.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind


Glaciers lose ice when
large pieces break off
their front edge in a
process called calving.
Calving creates icebergs
when glaciers meet the
ocean.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind



The glacial budget is the
balance between the amount
of snow-ice accumulated at
the top of the glacier and the
amount of loss at the
glacier’s foot.
If more ice forms at the top
of the glacier than melts at
the bottom, than the glacier
advances.
If the ice melts faster at
the bottom than the glacier
accumulates at the top, than
the glacier retreats.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind

Glaciers erode the land in two ways:
 Plucking
 Abrasion


Plucking occurs when rocks are broken loose from
under the glacier. As the glacier flows, it loosens and
lifts the rocks and carries them with the ice flow,
plucking them from the Earth.
In abrasion, the load of rock and ice combined acts
like sandpaper as it slides over the surface. This
leaves telltale lines going in one direction showing the
direction of ice flow. These marks are called glacial
striations.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind

As with all agents of erosion; the rate of
erosion is controlled by several factors.
 rate of glacier’s movement
 thickness of the ice
 shape, hardness, and amount of rock fragments in
the ice at the glacier’s front edge
 the type of surface and hardness of the rock
beneath the glacier
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind






Glaciers are responsible for a variety of erosional landscape
features. Among these are
Glacial troughs: When a glacier moves through a mountain
valley, it widens, deepens, and straightens this valley. The once
V shaped valley becomes a U shaped glacial trough.
Hanging Valleys: Main glaciers cut deep U shaped valleys that
are deeper than the those carved by smaller side glaciers that
feed into the main glacier.
Cirque is a bowl shaped depression at the head of a glacial
valley that is surrounded on three sides by steep rock walls.
Aretes are snaking, sharp-edged ridges produced when cirques
form on either side of a divide
Horns are pyramid shaped peaks produced when several cirques
surround a mountain.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind


Glaciers transport huge loads of debris as they slowly advance
across the land. When glaciers begin to melt, they deposit
these large amounts of sediments and rocks they carry.
The term glacial drift is used to include all sediments of glacial
origin. There are two types of glacial drift:
 Till is material deposited directly by the glacier, deposited as the
glacier melts and drops it’s load of rock debris. Because everything
is dropped at once, till is usually an unsorted mixture of rocks made
up of all sizes.
 Stratified drift is sediment laid down by glacial melting water.
Stratified drift contains particles that are sorted by size and
weight of the debris. Some deposits of debris come from streams
coming directly from the glacier. Stratified drift often consists of
sand and gravel
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind

Glaciers are responsible for a variety of depositional features
including:
 Lateral moraines are ridges that form along the sides of glacial valleys
when the glaciers melt and leave the material it has gathered.
 End Moraines form when glaciers stay stationary for long periods of
time. Within the glacier, the ice still flows. This flow still carries rock
debris to the foot of the glacier like a conveyer belt. Here the debris
builds up and produces End Moraines.
 Ground moraines form when glaciers begin to recede. The glacier front
continues to deliver debris with the ice movement but instead of
creating a ridge (as in the end moraine) the retreating foot of the
glacier deposits the debris as a rock strewn plain.
 Terminal Moraines: Glaciers can periodically retreat and than find
balance again and remain stationary for a long time period. A glacier will
form an end moraine when stationary than create ground moraines in it’s
periods of retreat. This pattern can repeat many times before the
glacier completely melts.
 The farthest end moraine created in this pattern of stopping and
retreating is called the terminal moraine.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind

Glaciers are also responsible for :
 Kettles: Ponds and small lakes called kettles. Kettles form
when blocks of stagnant ice become buried in drift and
eventually melt.
 Drumlins are long elongated hills composed of glacial till. The steep
side of the hill faces the direction the glacier came from and the
gentle slope side the direction the ice moved toward.
 Eskers are snakelike ridges composed of sand and gravel that were
deposited by streams once flowing in tunnels beneath glaciers.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind



The ice sheets greatly affected the drainage patterns of major
rivers in North America. Before the glaciers:
 The Missouri River flowed north toward Hudson Bay
 The Mississippi flowed through central Illinois
 The Great lakes did not exist
The sheets of glacial ice also triggered changes in the climates
of North America beyond their edges. Regions that are arid
today became cooler and wetter.
Desert landscapes also reveal the affects of running water and
wind. These combine in many ways to provide a wide variety of
desert landscapes.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind



Deserts are very different than landscape is humid areas of
the world. Humid areas generally have rounded hills and curving
slopes. By contrast, deserts generally have angular rocks, sheer
canyon walls, and surfaces covered in sand and small pebbles.
In humid regions, well developed soils support a continuous layer
of plant growth on top. In these areas, slopes and rock edges
are rounded from chemical weathering.
In contrast, in a desert much of the debris of rock is from
mechanical weathering. The minerals that make up the rock
debris are unchanged chemically. Because of the dryness of the
climate, the rock debris do not break down into rich soil as they
do in humid climates.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind




Chemical weathering is not entirely absent from a desert however. Over
long spans of time, clays and thin soils do form. Many of the iron-rich
silicate minerals oxidize producing rich rust colors in the landscape.
In humid or temperate climates, streams that run year round are a
normal fixture. In contrast, in deserts many streams are “ephemeral”
lasting only a short time. They run usually only after a rare heavy rain
washes over the landscape. We call these ephemeral streams.
These ephemeral streams are especially dangerous because of flash
floods that occur. During heavy rains, waters falls so fast that the
ground can not absorb it. The lack of vegetation allows the water to run
quickly off the land to fill these dry creek beds.
Because arid regions lack permanent streams, they have interior
drainage. This means these temporary streams do not flow out to the
ocean but end within the desert itself. In the United States the Dry
Basin and Range is an example of this. This area includes southern
Oregon, all of Nevada, western Utah, southeastern California, southern
Arizona, and southern New Mexico.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind



When the occasional runoff of water from rainstorms does happen; it is
heavily loaded with sediment from the large amount of erosion that
happens in a short span of time. Emerging from the mouths of the
canyons, the runoff spreads over the slopes at the base of the
surrounding mountains and loses it’s velocity and force. When this occurs
the water deposits all of it’s sediment suddenly, dropping it within a
short distance of space. Most of load is dumped within a short distance
of leaving the canyon mouth. The result is a cone of debris known as an
alluvial fan at the mouth of the canyon.
On rare occasions of abundant rainfall or snowmelt in the mountains,
streams may flow across the alluvial fans to the center of the basin,
converting the shallow basin floor into a shallow “playa lake”. Playa lakes
last only a few days or weeks before evaporation and infiltration dry
them up. The dry flat lake bed that remains is called a playa. Most areas
of the country have stream and river systems that drain the land and lead
water back to the ocean.
In desert areas, streams dry up long before water reaches the ocean.
Water quickly disappears between evaporation and infiltration into the
soil.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind



Some permanent streams and even rivers manage to cross arid areas.
The Colorado River, Arkansas River, and the famous Nile River all
begin in mountains with abundant rain. These rivers are large enough
and with enough flow that they lose little in their crossing of the arid
desert landscape.
The Nile River, for example, leaves the lakes and mountains of central
Africa and crosses over 3000 kilometers of the famed Sahara desert
without a single tributary joining it’s flow.
The point to remember about water in a desert environment, is that
although it is rare, it is a major force of change through erosion.
Most desert erosion results from running water.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind



Wind can still be an important force however and contributes to
shaping the landscape. In deserts, the soil is dry and dusty and there
are few plants with roots to hold the soil together.
Strong desert winds also pick up, move and deposit sediment in great
quantities. These are known as dust storms. In the mid 30s,
overfarming and drought destroyed much of the midwest. With all
the vegetation removed to grow crops; the area became a Dust Bowl
when the strong winds picked up the soil and blew it in huge dust
storms.
Winds erode in the desert through two ways
 Deflation is the lifting and removal of loose particles of clay and silt.
Courser sand particles roll or skip along the surface in a process called
saltation. These larger sand particles make up the “bed load” much as they
do in a stream, rolling along the bottom.
 Abrasion is when wind-blown sand cuts and polishes exposed rock surfaces.
Blowing sand can grind away at boulders and smaller rocks, sometimes
sandblasting them into odd shapes.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind



Loess is windblown silt that blankets the landscape. Dust storms pick
up this material, transport it and deposit it. The thickest and most
extensive deposits of loess are in China.
Like running water, wind releases it’s load of sediment when it’s
velocity falls and the energy available for transport diminishes. Sand
begins to accumulate whenever an obstruction crosses it’s path and
the wind diminishes.
Unlike deposits of loess, which forms blanket-like layers over broad
areas, winds commonly deposit sand in mounds or ridges called dunes.
Dunes can occur whenever an obstruction, such as a rock or small
plant, causes the wind’s velocity to slow and particles drop to the
ground. Once the sand starts to mound it acts as it’s own windbreak
and more and more sand collects.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind


Dunes are often steeper on the sheltered side and more gently
inclined on the side facing the wind. Wind blows sand up the gentler
windward side. Once the sand blows over the crest of the dune, the
wind slows and the sand drops down. The sheltered side of the dune
gradually becomes steeper and the sand eventually slides down the
side of the slope.
As sand is deposited on the sheltered side of the dune, it forms
layers inclined in the direction the wind blows. These sloping layers
are called cross-beds. When the dunes are eventually buried under
sediment and become sedimentary rock, the cross beds remain as a
record of their origin.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind

Sand Dunes are not random. They occur in a variety of consistent shapes. There
are six major types of sand dunes
 Barchan Dunes: Solitary sand dunes shaped like crescents are called





barchan dunes.
Transverse dunes: If prevailing winds are steady, sand is plentiful, and
vegetation is sparse, dunes form in a series of long ridges. These are
called transverse dunes because the ridges are perpendicular to the
direction of the wind.
Barchanoid Dunes: A common dune form that is between a barchan and
transverse dune is the barchanoid dune. These scalloped rows of sand
form at right angles to the wind.
Longitudinal dunes are long ridges of sand that form parallel to the
prevailing wind. These dunes occur where sand supplies are moderate and
the prevailing wind direction varies slightly.
Parabolic Dunes: Parabolic dunes look like backwards barchans. Their tips
point into the wind instead of away from it.
Star Dunes: Star dunes are isolated hills of sand mostly found in parts of
the Sahara and Arabian deserts. Their bases resemble stars and they
usually have three or four sharp ridges that meet in the middle.