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Chapter 20
Shaping Earth’s Surface
Weathering, Erosion, and
Transportation
• The Earth is constantly undergoing gradual changes.
• The materials the Earth is composed of are altered
by weathering and erosion, and then transported to
different areas on the Earth.
Weathering
• Importance of Weathering
– Crucial part of the rock cycle.
– Key in the formation of soils.
– Helps in the movement of rock material over the Earth’s
surface.
– Weathering breaks the rocks into fragments.
– Erosion picks up the material so it can be moved.
– Transportation moves materials.
• The movement of eroded materials by rivers, glaciers,
wind, or waves.
• As material is transported the weathering and erosion
process continues.
• This famous natural bridge is an example of a landform
created by the sculpturing power of weathering and erosion.
It is Rainbow Bridge in the Rainbow Bridge National
Monument, Utah.
• The piles of rocks and
rock fragments around
a mass of solid rock is
evidence that the solid
rock is slowly
crumbling away. This
solid rock that is
crumbling to rock
fragments is in the
Grand Canyon,
Arizona.
• Mechanical Weathering
– The physical breaking of rock material without any
change to their chemical composition.
– Exfoliation
• Spalling off layers of rock.
• Caused by reduced pressure on rocks as material is
removed from above.
– Frost wedging
• Caused as pores or cracks become filled with water
and then freeze and thaw.
• The process repeats; cracks and pores become larger.
• Eventually the rock will break off.
• (A)Frost wedging and (B) exfoliation are two examples of
mechanical weathering, or disintegration, of solid rock.
• Growing trees can break, separate, and move solid rock. (A)
Note how this tree has raised the sidewalk. (B) This tree is
surviving by growing roots into tiny joints and cracks,
which become larger as the tree grows.
• Spheroidal weathering of granite. The edges and corners of
an angular rock are attacked by weathering from more than
one side and retreat faster than flat rock faces. The result is
rounded granite boulders, which often shed partially
weathered minerals in onion-like layers.
• Chemical Weathering
– The alteration of materials by chemical reactions which
change the chemical composition of the material.
– Oxidation
• When oxygen reacts with minerals in rocks.
– Carbonation
• A reaction between carbonic acid and minerals in
rocks.
– Hydrolysis/Hydration
• A reaction between water and the minerals in rocks.
• Dissolves material.
• Water combines with the mineral to form a hydrate.
• Limestone caves develop when slightly acidic groundwater
dissolves limestone along joints and bedding planes,
carrying away rock components in solution. (A) Joints and
bedding planes in a limestone bluff. (B) This stream has
carried away less-resistant rock components, forming a cave
under the ledge.
Soils
• Soils
– Soil is a mixture of unconsolidated weathered Earth
materials and humus
• Humus
– Decayed organic matter.
• Bedrock
– Solid rock below the soil.
• Loam
– A soil that is a mixture of sand, silt, and clay.
– Sometimes referred to as topsoil.
Erosion and Transportation
• Mass Movement
– Criteria
• Material
– Bedrock that has been weathered and eroded.
– Debris that is carried away during the mass
movement.
• Rate
– The speed at which the movement occurs.
• Type
– Fall Slip Slump Slide Creep Flow
– Creep
• The very slow downhill movement of soil down a
steep slope.
– Fall
• The material moves in free fall down a cliff.
– Slip - materials moving together along a surface.
• Slump - Movement along a curved surface.
• Slide - Movement along a plane parallel to the surface.
– Flow – Mass movement of a liquid.
• The slow creep of soil is evidenced by the strange
growth pattern of these trees.
• Running Water
– Stream Channels
• Move materials in 3 ways.
– As dissolved material in a solution.
– As materials carried in suspension.
– As sand and larger materials rolled, bounced, and
slid along with a stream.
• Most of the erosion that a stream does is done by the
larger material that moves along with it on the stream
bed.
• Moving streams of water
carry away dissolved
materials and sediments
as they slowly erode the
land.
– Stream Erosion and Deposit Features
• Floodplain
– The wide, level floor of the valley adjacent to a
stream that has been built by the stream over time.
– This is the area that the stream will begin to move
into when moisture is high (flood).
– A young stream usually has a v shaped flood plain
whereas a mature stream has a flattened floodplain.
• Delta
– When the stream empties into an ocean or lake it
loses all of its sediment carrying ability.
– The sediments are deposited at the mouth of the
stream and form a layered deposit.
• A river usually stays in its channel, but during a flood it
spills over and onto the adjacent flat land called the
floodplain.
• Three stages in the aging
and development of a
stream valley, (A) youth,
(B) maturity, and (C) old
age.
• The waterfall and rapids
on the Yellowstone
River in Wyoming
indicate that the river is
actively down cutting.
Note the V-shaped
cross-profile and lack of
floodplain,
characteristics of a
young stream valley.
• (A)Delta of Nooksack River, Washington. Note the
sediment-laden water, and how the land is being built
outward by river sedimentation. (B) Cross section showing
how a small delta might form. Large deltas are more
complicated than this.
• Glaciers – A mass of ice on land that moves under its own
weight.
– Origin of Glaciers
• As snow melts and refreezes it is turned into ice.
• After years of repeated thawing of snow and
refreezing into ice, the weight above begins to pack
down the ice below.
• The increased pressure drives out air and reforms the
ice into a crystalline structure of interlocking ice
crystals with a very high mass.
• Alpine glaciers form in high elevations.
• Valley glaciers form and flow downhill through a
valley.
• Continental glaciers cover large areas of a continent.
• Valley glacier on Mount Logan, Yukon Territory.
– Glacial Erosion and Deposition
• A glacier erodes material by:
– Bulldozing
– Abrasion
– Plucking
• Material that is deposited by a glacier forms a
moraine.
• Plucking material produces a depression called a
cirque.
• As abrasion continues material becomes ground into
fine sediment called rock flour.
• (A)A stream-carved mountainside before glaciation.
(B) The same area after glaciation, with some of the
main features of mountain glaciation labeled.
• Wind
– Wind Erosion and Transportation
• Wind abrasion is the sandblasting process that occurs as
material is carried along with wind.
• Deflation is the picking up of loose materials from the
Earth’s surface.
– Wind Deposits
• Dunes
– A low mound or ridge of material sediments.
– Form where the wind that is carrying the sediment
encounters some obstruction.
• Loess
– Very fine dust or silt that is deposited over large areas.
– The source of the material is thought to be rock flour
from glacier action.
• Ventifact
formation
by
abrasion
from one
or several
directions
.
Development of Landscapes
• Rock Structure
– Type of rock
• igneous
• metamorphic
• Sedimentary
– If they have been disturbed by faulting or folding.
• Weathering and Erosion Process
– Controlled by climate and elevation.
• State of Development
– How effectively the landscape has been attacked by
erosion processes.
• This melting glacier (A) is the source for a stream
(B) that flows through a valley in the youth stage.