Weathering, Erosion, and Soil
What type of rock is
this, how can you tell?
Yosemite Valley, California
Mountains Carved by Glaciers
Grand Canyon, Arizona
Carved out by the
Colorado River
Bryce Canyon, Utah
Acidic Rainfall has worn away these rocks.
The harder rocks remain standing… but for how long?
Weathering
• Process by which rocks are broken down
due to exposure to processes occurring at
the Earth’s surface
• Caused by
– Water
– Waves
The 2 G’s and
– Wind
the 3 W’s
– Gravity
– Glaciers
2 Types of Weathering
• A. Mechanical Weathering also called
Physical weathering
– Rock is broken down into smaller pieces
of the same material
– (no change in composition)
• B. Chemical Weathering
– The breakdown or decomposition of rock that occurs
when minerals are changed into different substances
– (change in composition)
Mechanical/Physical Weathering
1. Frost (Ice) Wedging
– Process in which water freezes in the cracks
of rock and wedges (pushes) it apart
because water expands when it freezes.
– Occurs where there are frequent freezes and
thaws.
Explain what is
happening.
• Frost/Ice Wedging
can cause
Potholes to form
in pavement
Mechanical/Physical Weathering
2. Abrasion
– The wearing away of rock material by
grinding action
– Usually caused by sediment in Wind, Water,
and Glaciers
Wind abrasion- sandblasting effect on
stationary rocks as seen here in Arches National
Park
Hydraulic abrasion- water & sediments flowing over
boulders as seen here in Ohiopyle State Park, Pennsylvania
Notice the rounded river rocks
Mechanical/Physical Weathering
3. Plants and Animals
– Plant roots can split
rock
– Also known as: “Root pry”
or “Root action”
– Animals dig holes ,breaks
up rocks
Tree growing
out of rock,
Shenandoah
National Park
Tree roots
also break
up
sidewalks
Mechanical/Physical Weathering
Exfoliation- gradual peeling of layers
due to uplift and frost action, typical of
granite domes like those in Yosemite N.P.
Exfoliation of Igneous rock
Chemical Weathering
•
Chemical Weathering
– The breakdown or decomposition of rock
that occurs when minerals are changed into
different substances
– (change in composition)
• Involves Water
,water vapor, Acids,
and/or Oxygen
O2
Chemical Weathering
1. Hydrolysis
• Carbonic Acid in
water dissolves
Calcite. This
chemical
weathering can
hollow out
underground
Caverns
Limestone and Dolomite both
dissolve because they contain
Calcite
Acid rain (carbonic acid) weathering the details of
statues and tombstones Ex: Marble and Limestone
Chemical Weathering
2. Oxidation
•
Oxidation of minerals
with iron (magnetite,
pyrite) results in the
formation of rust or
iron oxide.
• This is why Mars
is the red planet
• Copper turns rocks
green
Rate of Weathering
• How fast a rock weathers depends
on 3 factors:
–Surface area
–Rock composition
–Climate
Rate of Weathering
• Surface area
– The greater the surface area, the faster
the weathering rate
– There are more surfaces to be weathered
Rate of Weathering
• Rock composition
– Some minerals are more
resistant than others
– For example, quartz is more
resistant (harder) than calcite
Quartz
Calcite
Rate of Weathering
• Climate
(long term pattern of moisture and temperature)
– Weathering rates are faster in warm,
climates
Desert vs. Rainforest
wet
Erosion
•
Erosion
– The removal and transport of
weathered materials by natural agents
such as
– Caused by
1.
2.
3.
4.
5.
Glaciers
Running water
Gravity
Wind
Waves
Glaciers
• Mass of compacted ice and
that moves under its own weight
snow
Glaciers
• Friction at the base of the glacier is reduced
by a thin film of melt water.
• Glaciers account for about 75% of the fresh
water on Earth
• Glaciers can leave behind large boulders
that are known as an erratic
Glaciers
• Glaciers can move lots of sediment that
can carve striations (grooves) into rocks
These Striations show how
glaciers moved
Glacial Deposits
• Drift or Till - all sediment that is deposited by
glacial activity
– Loess: fine powder, pulverized rock, that
blankets much of the northern mid-west
– Moraine: ridge or pile of boulders, gravel, sand, and silt
left at the end of a glacier
Esker: type of
outwash that
accumulates in the
channels and tunnels
of a glacier
Glacial Landforms
• Cirque
– Steep-walled
bowl-shaped
depression
• Horne: a pyramid
shaped peak where 3
or more cirques meet
Glacial Landforms
• Trough
–U-shaped valley, Glaciers carve U shaped
valleys
Glacial Landforms
• Hanging valley
• Arete
– Straight drop-off at the – Narrow flat-topped
ridge that forms between
end of a trough
two parallel troughs or
cirques
Glacial Landforms
• Moraine: Pile of mixed sediments that
have been carried and dropped by the
glacier
Glacial Landforms
• Eskers -Mounds of rock from melt tunnels
Streams and Rivers
• Velocity - How fast the water moves
• Gradient - The steepness, the slope that a river or
stream travels
• Discharge - The amount of water that moves past a
certain point in a river in a given amount of time
• If there is a steep gradient, high velocity, and
a large discharge, then Erosion will be
severe!
• Rivers have energy to move lots of
sediment, and even large boulders.
River
stages
affect:
Velocity,
Gradient,
&
discharge
which
cause
Erosion
Seven Bends of the Shenandoah
River meanders
Bends in the river, shows how mature a river is.
Is the Shenandoah and old river or a young river?
Erosion-Mass Movement
• The downward transportation of weathered
materials by gravity
Massive Landslide
Erosion-Mass Movement
•
•
•
•
Creep
Slump
Mudflow/Earthflow
Landslide/Rockslide
Erosion-Mass Movement:
A large mass of sediment drops down!
Erosion-Mass Movement
• Creep
– Very slow movement of earth material.
– Caused by repeated freezing and thawing.
Sediments are
loosened by
expanding frost and
contracting thaw.
Leaning fence posts
and telephone poles
are a sign of Creep.
Fence posts leaning due to creep
Erosion-Mass Movement
• Slump
– Slow downward movement of a large
mass of soil
Occurs when
underlying sediments
are weakened by
heavy rains
Characterized by a
curved scar in the
land surface.
Slump
can
cause
road
hazards
Erosion-Mass Movement
• Mudflow/Earthflow
– Thick pastes of sediments that travel downhill at
great speeds
Usually occur in
dry areas that get
a large quantity of
rain all at once
Leaves a cone
shaped deposit
Mudflow in the Blue Ridge
Mooreman’s Gap near Charlottesville
Erosion-Mass Movement
• Landslide/Rockslide
– Fast movement of large blocks of rock
Occurs in very
steep vertical cliffs
Accelerated by
ice wedging
At the bottom of an old rock slide in N.C. 2011
Wave Erosion
• The crashing of waves on a shoreline
combined with storms continually shape the
beach. This is a shoreline with erosion.
Wind and Waves
• Wind moves
sand dunes at
the beach and in
the deserts.
Wind and Waves
• Wind can carve out landscapes
as it carries sediment
Wind and Waves
• Many storms combine water
and wind to cause significant
erosion on landforms.
• Strong winds are capable of
moving large amounts of
sediment
Deposition: The process of
dropping or depositing soil
and sediments.
Soil (dirt)
• Soil includes loose weathered rock,
and organic material in which plant
roots can grow
Soil Composition
Use this pie chart to answer the questions in your notes.
Soil Composition
What do the arrows mean?
Where soil forms
• Residual soil
– The parent rock is the bedrock beneath the
soil (the soil has not moved)
• Transported soil
– Soil forms from parent material left by
winds, rivers, glaciers, or soil that was
moved from its original location.
This
soil has been transported by erosion
and deposition
Factors that affect Soil Composition
• Parent material: What bedrock is it from?
• Time: How long has it had to decompose?
• Plants and animals: How have roots and
animals helped break up the rocks? Were
there acid producing mosses that help break
down the rocks (chemical weathering)?
• Topography: What is the shape of the
landscape?
• Climate: Is it warm & wet? Were there
glaciers present? Was there acid rain?
Soil Profile
• Cross
section of
soil layers
revealing all
soil horizons
Soil Horizon
• A soil layer with physical and chemical
properties that are different from
adjacent layers
• O Horizon= organic material (humus)
• A Horizon = topsoil
• B Horizon = subsoil
• C Horizon = partially weathered
parent material
Idealized Soil Profile
Humus:
Dark organic
matter in soil
that is rich in
nutrients.
Leaching: The removal of nutrients or toxins in
soil as water passes through the layers.
Soil Texture
• The size
of soil
particles
Particle size ranges for sand,
silt and clay
Type of Mineral
Particle
Sand
Size Range
2.0 - 0.06 millimeters Large/
Coarse
Silt
0.06 - 0.002
millimeters
Clay
less than 0.002
millimeters
Medium
Small/
Fine
erosion
EROSION
Weathering
Deposition
Sedimentation
to “settle”