Download Soil Texture

What is weathering?
Rocks on the Earth’s
surface undergoing
changes in appearance
and composition
What are the 2 types of
Weathering?
1. Mechanicalphysically changing rock into
smaller pieces without changing its
composition
2. Chemicallybreakdown of rock by changing its
chemical composition
Types of Mechanical
Weathering
1. Exfoliation
(unloading)- process
where sheets of rock
peel or flake away
Example of exfoliation:
Types of Mechanical
Weathering
weathering
2. Ice wedge- occurs
when water seeps into
rocks and freezes
Ice
wedging
Types of Mechanical
Weathering
weathering
3. Biological activity- the
activity of organisms,
including plants, burrowing
animals, and humans that
weather away rock
Weathering by plants
Weathering by sandmartins
nesting in cliff
Types of Mechanical
Weathering
weathering
4. Abrasion- collision of
rock with one another
resulting in breaking
and wearing away.
• Abrasion takes place in many
environments:
– fast-moving streams
– beaches subject to storm waves
– desert environments with high winds
– beneath glaciers that are loaded
with fragments of rock.
Water-carried particles
eating away at the rock
Storm waves have eaten
away at the shoreline
The ocean waves have eroded
this beach leaving behind a
cliff as the soil was washed
away
Observe the effects of
mechanical
weathering.
Types of Chemical
Weathering
1. Hydrolysis:
Change in composition
of minerals when they
react with water
Feldspar combines with H2O to
form a common clay called kaolin
Types of Chemical
Weathering
2. Carbonation –
When some minerals come
in contact with carbonic
acid, they form a new
product
Stalactites caused by
carbonation
Limestone
is eaten
away by
acid and
deposits as
it drips
from ceiling
of cavern
Types of Chemical
Weathering
3. Oxidation –
When metallic elements
combine with oxygen
Oxidation =
rust!
Types of Chemical
Weathering
4. Acid
Precipitation
“acid rain”
Types of Chemical
Weathering
5. Plant acids – plants
secrete acids that
erode away the rock
Lichens and
mosses grow
on rocks and
secrete
weak acids
that dissolve
the surface
What 4 things
affect the
rate of
weathering?
1. Rock composition
• Igneous and
metamorphic rocks
don’t weather easily
• Sedimentary rocks do
2. Amount of exposure
• More exposure it receives,
faster it will weather
• Amount of time and
amount of surface area
exposed is also important
3. Climate
• Slow in very hot / very
cold climates
• Fairly rapid in warm,
humid climates
Cleopatra’s needle after only
one century in New York City
4. Topography
• Elevation or slope of
surface where rock is
located affects rate of
weathering
Regolith
• A layer of weathered
rock fragments
• covers much of the
Earth’s surface
Regolith
Soil
• part of the regolith that
supports plant growth
• Complex mixture of
minerals, water, gases,
and remains of plants and
animals
What is Soil Composition?
• has 4 major components:
–mineral matter
–organic matter (humas)
–water
–air
Composition of Soil:
What is Humus?
• Dark, organic material,
remains of animals and
plants
Humus:

•
What are the Soil
Textures?
Texture refers to the proportions
of different particle sizes.
- Sand (large size)
- Silt
- Clay (small size)
•
Loam (a mixture of all three sizes) is
best suited for plant life.
CLAY
SILT
SAND
Soil Texture
What are the most important
factors in soil formation?
(1) parent material
•source of the mineral
matter in soil
Residual soil—parent material is the
bedrock below the soil
• Transported soil—has been
carried from parent material and
deposited elsewhere
•
(2) Time
•
•
Important in all geologic processes
The longer a soil has been forming,
the thicker it becomes.
(3) Climate
• Greatest effect on soil formation
(4) Organisms
• Organisms influence the soil's
physical and chemical properties.
• Furnish organic matter to soil
(5) Slope
•Steep slopes often have poorly developed
soils.
• Direction the slope is facing influences soil
formation (affects soil temperature and
moisture)
Thinner on slope
because water erodes
soil & deposits it down
the slope
Tends to be
thicker here
(deposited)
What is a Soil profile?
• A cross-section in which the
layers of the soil and
bedrock can be seen
• Each layer is called a horizon.
• In fully developed residual
soil, there are three horizons
A horizon
(topsoil)
• mixture of organic matter
and small rock particles
B horizon
(subsoil)
•contains minerals and clay
C horizon
(regolith)
•partially
weathered
bedrock
Bedrock
• Solid, unweathered
rock that lies beneath
regolith
Bedrock
What role does Climate play
in soil formation?
• Climate is most
important factor
influencing soil
formation
What is a Temperate climate?
• Temperatures range from
cool to warm
• Rain fall is not excessive
• Two types of soil are
found dependent on
amount of rainfall:
What are the soil types of
temperate climates?
1. Pedalfer – soil formed if an area
receives 65 cm or more of rain per
year (clay, quartz, iron)
-found in eastern United States
2. Pedocal – soil formed if rainfall is
less then 65 cm a year (calcium
carbonate)
-found in western United States
Pedalfer
Pedocal
What is the soil formation
in tropic areas?
• forms laterites (thick and
infertile soil)
*rain always is washing
away the A horizon but
constant rotting
vegetation covers the B
Horizon
Laterite
What is soil formation of a
desert?
• Forms soils from
mechanical weathering
• Soil is thin and mostly
of regolith
What is erosion?
• erosion is the process by
which the products of
weathering are transported
• agents of erosion are
gravity, wind, glaciers,
water, ocean waves,
currents streams,
groundwater
What is Soil Erosion?
1. occurs world wide and is
normally a slow process
2. ordinarily, new residual soil
forms about as fast as the
existing soil erodes
3. However, unwise use of the
land and unusual climatic
conditions can upset this
natural balance
How is erosion accelerated?
1. unwise farming and ranching
methods increase soil erosion
• one example is clearing of
trees,small plants, and animal
overgrazing
• another example is furrows
plowed in land
When soil is washed away by rainfall,
furrows become larger forming gullies
(gravity and water at work)
•Sheet erosion is another type of
soil erosion that strips away
parallel layers of topsoil
•may occur when continuous
rainfall evenly washes away the
topsoil
•can also be caused by wind during
unusually dry periods
Severe soil erosion from wind in the 1930's
was a result of drought coupled with unsound
farming practices.
The photo on the right though, was taken at
10:00 AM, March 13, 1991 in downtown Topeka,
Kansas, looking into the sun. There was so much
soil in the air it darkened the sky. So the
problem is not just something of the past.
constant erosion reduces
the fertility of the soil by
removing the A horizon,
which is rich in humus
(unable to grow crops)
What is soil conservation?
• Methods of protecting the soil by reducing
soil erosion and loss of soil nutrients.
1. contour plowing: soil is
plowed in circular bands
that follow the shape of
the land (prevents water
from flowing directly down
slope)
2. strip cropping: crops are
planted in alternate bands
•Contour cropping of corn, alfalfa and wheat.
Contour farming (planting crops along the slope of
the land) and contour strip cropping (alternating
different crops in the same field) reduces soil
erosion. Traps soil on the field.
3. terracing: construction of
step-like ridges that follow the
contours of a sloped field
During heavy rain storms terraces catch water and
lead it slowly off the field. Slowing down the speed
of flowing water reduces soil erosion.
4. crop rotation: farmers plant
one type of crop one year and a
different type of crop the next
year
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With the development site cleared of all vegetation,
area is now prone to soil losses in excess of 70 tons
per acre.
Gravity and Erosion
• Gravity, through its downward
pull, causes rock fragments to
move down inclines
• mass movement is the movement
of fragments down a slope (rapid
or slow)
rockfall: the fall of rock
from a steep cliff
•is the most rapid
•can be from tiny fragments
to large boulders
Figure 21 Rockslide The scar on the side of this
mountain in northwestern Wyoming was made by an
enormous rockslide that happened more than
75years ago.The debris in the slide formed a dam 70
m high across the Gros Ventre River.
landslide: sudden movement
of masses of loose rock and
soil down the slope of a hill
Slumgullion landslide
Hinsdale County, Colorado
700 years old and still moving
mudflow:
the rapid
movement of
a large mass
of mud
slump: downhill movement of a
large block of soil under the
influence of gravity
A slump is a type of slope failure which involves
the rotational movement of soil or rock
creep: extremely slow downhill
movement of weathered rock
material
erosion of mountains:
weathering and erosion wear
down rugged peaks to rounded
peaks and gentle slopes
Rocky Mountains
Appalachian Mountains
Examine a landscape
formed by erosion.