Download WeatheringSoil Formationand Erosion

Weathering, Soil Formation,
and Erosion
Weathering
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Breaking down of rocks and other materials on
the Earth’s surface
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it is a slow, continuous process
effects aren’t easily observed
Two Types:
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Mechanical: rocks are broken into smaller pieces but
their chemical makeup does not change
Chemical: chemical makeup of rocks is changed as
rocks are broken down
Mechanical Weathering
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What causes Mechanical Weathering?
Temperature:
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Frost wedging:
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Water seeps into small cracks, freezes and expands, which
enlarges cracks. (cracks in sidewalk, potholes in road)
Organic Activity: (caused by living things)
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Rocks expand/contract (cycle).
Causes exfoliation (flaking)
Plant roots can loosen rock, make cracks larger (grass in
sidewalk)called root-pry
Abrasion: wearing away of rocks by particles carried by
wind, water, etc.
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Rough edges of particles ‘scrape’ off parts of rocks. Rocks in a
riverbed are scraped by moving objects in the waterthey
become smooth
Chemical Weathering
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What causes Chemical Weathering?
Water
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Dissolves minerals that hold rocks together (hydrolysis)
Forms acids when mixed with gases in atmosphere (Acid
precipitation)
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Acid precipitation has a ph of less than 5.6
Combines with mineral to make new mineral (combine w/
feldspar to make clay)
Oxidation
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Oxygen chemically reacts with something else. (Iron + Oxygen
= rust) Inner rock will be diff. color than outer rock.
Chemical Weathering
Chemical Weathering, cont.
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Carbonation
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Sulfuric Acid
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Carbon dioxide and water form a weak acid. It dissolves certain
rocks (limestone) but not strong enough to harm plants/animals
caused by pollutants-they dissolve in rain to form acid rain. It
corrodes (wears away) rocks, metals, etc. quickly.
What do you think it does to monuments and buildings?
Plant Acids
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Plants produce weak acids that dissolve certain minerals
(mosses) and break rocks into smaller pieces.
What affects the rate of
Weathering?
Rock composition (different rocks weather
differently)
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Stable rock resists chemical weathering
Climate
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Ex: limestone OK in warm/dry climate; when wetweak acids weather
Time exposed on surface
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Old unexposed rocks-no big changes
New exposed rocks-weather quickly
Climate and Chemical Weathering
What affects the rate of
Weathering?
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Surface area
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When rock is in small pieces, more surface
area is available for weathering
Topography, other variables
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Materials on slopes are more likely to move
due to gravity.
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This exposes underlying rock, providing more
opportunities for weathering.
Erosion
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Erosion
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Process by which weathered rock and soil particles
are moved form one place to another
Carries away products of weathering
Deposition
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Process by which sediments are laid down in new
locations
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Final stage in the erosion process
Erosion moves materials; deposition builds new
landforms
Agents of Erosion
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Gravity
Wind
Running water
Glaciers
Waves
These are all forces that move materials
from place to place
Gravity
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Pulls rocks and soil down slopes
Agent of mass movements
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Landslides
Mudflows
Avalanches
Wind
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Major agent of erosion in hot, dry climate or
places with little vegetation
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When wind erodes soil to depth that water is present,
shrubs/grasses can growcalled oasis (happens in
desert)
Wind barriers (windbreaks) are used to reduce
effects of wind erosion
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Trees, plants planted perpendicular to wind’s direction
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Reduces soil erosion, can trap blowing snow, protect crops,
etc.
Running Water
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Water has more power than wind to move
particles (exceptions are hurricane and tornado
winds)
When water moves faster, erosion is greater
Erosion by running water in small channels on
side of slope is rill erosion
When channels become deep it evolves into
gully erosion
Gully Erosion vs. Rill Erosion
Glaciers
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Produce large-scale, dramatic effects
Have capacity to carry HUGE rocks, piles
of debris over great distances
Scratch/grind the surface
Can polish others
Famous for their deposition
Plants, Animals, Humans
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Materials get moved from place to place
Digging tunnels underground (animals)
Excavation
Planting gardens, etc.
Building roads, buildings, etc.
Mass Movements
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Downslope movement of loose sediments
and weathered rock caused by gravity
A form of erosion
Only occur on slopes
Also Known As Mass Wasting
Factors that affect Mass
Movements
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Weight of material
Resistance to sliding/flowing
Triggers such as earthquakes
Amount of friction between material and slope
Amount of erosion that has taken place at the
bottom of the slope
Mass movement occurs when the forces pulling
material down a slope are more than the
resistance of the materials
Types of Mass Movements
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Creep
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Slow, steady downhill flow of loose, weathered
materials such as soil
Can be just a few centimeters per year
Flow
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Weathered materials can “flow” like a liquid
Mudflows
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Swift moving mixture of mud and water
Triggered by EQ, common in volcanic areas, sloped areas
with short periods of intense rain (CA)
Types of Mass Movements
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Slides
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Rapid, downslope movement that occurs
when a block of loose soil, rock, debris
separates from the bedrock underneath
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Landslide
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Little internal mixing of materials
Very fast movement, some up to 200km/hr (124 mph)
Rockslide
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When rock sheets move downhil
Often caused by EQ
Rockslide on Westbound I-90
Landslide: El Salvador 2000
(Earthquake)
Types of Mass Movements
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Slumps
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Happens when material in a landslide rotates while it
slides downhill
Common after rain, leave a crescent scar on the land
Happen in areas with thick soil and medium-steep
slope
Avalanche
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Landslide with snow
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Early in winter, warm ground melts snow (it refreezes, turns
to ice). Vibrations from EQ, etc can trigger snow on top of
this ice sliding down the mountain
Slumps
Types of Mass Movements
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Rock Falls
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Occur at high elevations, steep road cuts, and
rocky shorelines
Cliffs become weathered, eroded, and rocks
fall into a cone-shaped pile
Not likely in humid areas (water promotes
plant growth, which lessens likelihood of rock
falls)
Wind Erosion
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Occurs in places with little precipitation
Suspension
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Saltation
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Particles bounce around
Deflation
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Particles are in the air for a long period of time
Lowering of land surface due to removing surface
particles
Abrasion
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Rocks shaped by abrasion are called ventifacts
Wind Deposition
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Wind carries particles like sand through the air
When something blocks their movement, the
sand gets deposited
Over time, the pile of sand becomes a dune
There are different types of dunes
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Dunes are classified by shape
Human activity in coastal regions has disrupted
dune formation/damaged sand dunes
Loess
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Fine, lightweight particles that have been
carried and deposited by the wind
Glaciers
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A moving mass of ice
Form at Earth’s poles and in high elevations
(mountainous areas)
Valley Glaciers
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Form in valleys of mountainous areas
Occurs when ice is too heavy, flows down the mountain like
liquid
Continental Glaciers
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Cover a continent-sized area
Thickest at center
Callled Ice Sheets
Glacial Erosion
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Most powerful agent of erosion because of
size, weight, density
Glaciers carve U-shaped valleys, put
scratches/grooves on rocks
Glacial Deposition
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Moraines
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Ridges of glacial till
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Glaciers can melt and the ice recedes/water flows into the valley
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Elongated landform caused by glacial movement
Esker
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The location where the sediments get deposited is the Outwash plain
Drumlin
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Till is the mix of debris carried by the glacier
Winding ridges of layered sediment deposited by streams under a
glacier-esker
Glacial Lake
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Occurs when block of ice breaks away from a glacier, melts in a
depression. Precipitation, surface water combine with the water to form
a glacial lake
Soil Formation
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Soil is an important natural resource
It is found almost everywhere on Earth
Exactly what is it?
Soil Formation
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Soil
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Loose covering of broken rock and humus on
the earth’s surface
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Humus is decaying organic matter (leaves, etc)
Soil formation takes a very long time
Soil Composition
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Soil forms in layers during the process of its
development.
The parent rock is the solid bedrock from which
weathered pieces of rock first break off.
The smallest pieces of weathered rock, along with
living and dead organisms, remain in the very top
layer.
Rainwater seeps through this top layer of
materials, dissolves soluble minerals, and carries
them into the lower layers of the soil.
Soil Composition
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Residual soil is soil located above its
parent bedrock.
Transported soil is soil that has been moved to
a location away from its parent bedrock by agents
of erosion, such as running water, wind, and
glaciers.
The parent bedrock determines what kinds of
minerals a soil contains.
The parent rock and climatic conditions of an area
determine the length of time it takes for
soil to form.
Soil Profiles
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What is a soil profile?
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A vertical sequence of soil layers
A soil horizon is a distinct layer, or
within a soil profile.
There are three major soil horizons:
A, B, and C.
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zone,
A, B,
Horizon A contains high concentrations of organic matter and
humus.
Horizon B contains subsoils that are enriched with
clay minerals.
Horizon C, below horizon B and directly above solid bedrock,
contains weathered parent material.
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The O horizon is organic material, that may or may not be present.
Topography
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The topography of a region affects the
thickness of developing soil.
Soils on slopes tend to be thin, coarse,
and infertile.
Soils formed in lower areas, such as in
valleys, are thick and fertile.
Soil Types
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Polar Soils
 Form at high latitudes and
high elevations
 Located in Greenland,
Canada, Antarctica
 No distinct horizons, very
shallow soil
 Just below the soil the
ground is permanently
frozen-known as permafrost
Permafrost in Denali
Soil Types
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Temperate Soils
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Lots of variation
Support forests, grasslands, prairies
Amount of rainfall determines what grows in
this type of soil
Grasslands-lots of humus-soil is rich and fertile
 Forests-soil is less deep/less fertile, containing
clays and iron oxides (Eastern US)
 Prairies-dry soil-lots of grasses and bushes
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Soil Types
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Desert Soils
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Very little precipitation
High levels of salts
Limited vegetation
Little or no organic matter
Very thin topsoil (A horizon)
Light colored and coarse soil
Soil Types
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Tropical Soils
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High temperatures, heavy rainfall
Soil is intensely weathered, infertile
Source of ores (iron), but not great for
growing plants
Soil Textures
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Particles of soil are classified according
to size as being clay, silt, or sand, with
clay being the smallest and sand being
the largest.
The relative proportions of these
particle sizes determine a soil’s texture.
The texture of a soil affects its capacity
to retain moisture and therefore its
ability to support plant growth.
Soil Textures
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To determine the
texture of a soil
sample, find its
percent for sand,
silt, and clay.
The texture of the
soil will be where all
three lines intersect.
Soil Fertility
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A measure of how well a soil can support
plant growth
Factors involved include:
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Availability of nutrients/minerals
Precipitation
Topography
Acidity
Number of microorganisms present
Soil Color
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Factors that determine soil color include:
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Climate
Soil’s composition
Topsoil-usually dark (rich in humus)
 Red/yellow soils-caused by iron minerals oxidizing
 Yellow soils are often poorly drained, associated
with environmental problems
 Gray/bluish soils-poorly drained, constantly wet,
lacking in oxygen
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