Transcript Soils and Weathering new

Weathering and Soils
and
Mass Movement
Weathering
Earth’s surface is constantly changing.
One of the main reasons is our planet’s
weather. Weathering breaks down rocks
and converts them into soil.
Weathering
1.
2.
There are two types of weathering:
Mechanical weathering
Chemical weathering
Types of Mechanical Weathering
Mechanical weathering occurs when
physical forces break apart rock.
*does not change the rocks chemical
composition.
Examples:
1. Frost wedging
2. Unloading
3. Biological Activity
4. Abrasion
Frost Wedging
The freezing and thawing of water in
cracks of rocks
- expansion of water splits rocks apart
- most common in the mountains of the
middle latitudes
- accumulation of material is called talus.
Sometimes geologists refer
to these piles as talus slopes
Unloading
When large masses of rock are exposed
through uplift and erosion
- Pressure on the underlying rock is
reduced – called unloading
- Unloading causes rock at the top to
expand more than rock at the bottom.
- Expansion causes large pieces to peel
off as sheets of rock this is called
exfoliation
Exfoliation
Biological Activity
Caused by:
- Plants - roots grow into the cracks of
rocks and force them apart.
- Animals - burrow and bring rock to the
surface – weathering happens faster
Abrasion
- Wind and water can cause abrasion
as rock fragments bounce off each
other
Chemical Weathering
Chemical weathering is defined as the
transformation of rock into new compounds.
- water the most important agent
Examples
1. Solution
2. Hydrolysis
3. Oxidation
Chemical Weathering
Solution
-the dissolving of solid rock material in
water. Water often contains substances such
as acids that dissolve other minerals.
Chemical Weathering
Oxidation
- Also known as rusting
- oxygen combines with iron to form
iron oxide
- greatly speeds up in the presence of
water and warm temperatures
Chemical Weathering
Hydrolysis
- a permanent chemical combination with
water
- the rock's minerals are chemically altered
by reacting with water and acid
- resulting in expanded and weakened
rock.
Rates of Weathering
Several factors effect the rates rocks
weather.
- Intensity (amount of acids) and
duration play pivotal roles in the rates of
weathering
1. Surface area – more area exposed the
greater the rate
2. Climate – the warmer and wetter an
area the greater the rate
3. Parent Material – different material
weather at different rates
Erosion
- Erosion involves removal and transport
- Rain, wind, and rivers are all agents of
erosion.
Soil
The natural resource soil is one of the most
important. Weathering creates layers of rock
and mineral fragments called regolith. Soil is
the part of the regolith that support plant life.
The Components of Soil
1.
2.
3.
4.
Soil is made of four things:
Broken rock and mineral matter – 45%
Organic matter (humus) – 5%
Water – 25%
Air – 25%
The percentages of each of these
components varies widely in soils.
Soil Texture
Soils are made of three components:
Sand - largest grain size
Silt - medium grain size
Clay - smallest grain size
Soil Texture Diagram-pg 134
Soil Texture
Texture strongly influences the soil’s ability
to support plant life
- Sandy soils – drain and dry out too
quickly
- Clay-rich soils – drain very slowly
- Loam soils – best for plant growth
The Soil Profile
The
composition of
soil varies with
depth. Color,
structure and
texture all
change with
depth. These
differentiated
layers are called
soil horizons.
Soil Horizons
A Horizon: The “A” horizon consists of organic
matter (decayed plant material) or humus.
Insects, fungus and microorganisms teem here.
The lower portion of the “A” horizon is a mix of
mineral and organic matter. Called topsoil.
B Horizon: Known as the subsoil, this layer
contains clay washed from above. It is the
lower limit for plant roots and burrowing
animals.
C Horizon: The “C” horizon consists of partially
weathered rock known as “saprolite”. This
horizon resembles the original or “parent” rock.
Soil Types
Pedalfers: The soils found around here
and in other temperate regions that get
significant rain. Their “B” horizons contain
iron- and aluminum-rich materials giving
us the red and orange clays so common in
the Carolinas and the rest of the East
Coast.
Soil Types
Pedocal: Are soil typical of dry climates
like those found n the western United
States. Chemical weathering is slower.
Because of this pedocals tend to have less
clay. Pedocals are rich in calcium based
minerals (calcite, limestone)
Soil Types
Laterite: These soils form in hot, wet tropical
areas. The large amount of precipitation carries
these soils deeply. The precipitation also
washes our much of the silicate material making
these soils orange. These soils are organically
poor. One major reason for rainforest
deforestation is the periodic need to replace the
land exhausted by farming.
Soil Erosion
Soil erosion is a natural part of the weathering
and transport process. However, excessive
erosion can damage or destroy a resource that
took 1000’s of years to develop.
Soil Erosion
Agents of Erosion
- Wind
- Water
- Gravity
- Ice (glaciers)
- Human activity - such as farming,
logging and construction remove natural
vegetation that slows and controls
erosion.
Rate of Soil Erosion

Rate depends on:
- Soil characteristics
- Climate
- Slope
- Vegetation
Water Erosion
Wind Erosion
Sediment Deposition
One problem caused by erosion is that of
sediment deposition. High rates of erosion
dump huge amounts of soil in rivers and lakes.
These sediments fill rivers, channels, deltas and
reservoirs and have to be removed by dredging.
Controlling Erosion
Though erosion cannot be stopped it can be controlled.
Windbreaks between fields reduce wind speed. Silt
fences control runoff from construction areas. “No till”
farming does not break the soil, but allows in to remain
consolidated. Terraced plowing follows the contours of
the farmland not allowing runoff to pick up too much
energy.
Mass Movement
The transport of
rock and soil
downhill due to
gravity is called
mass wasting.
Mass Movement
The causes (triggers) of mass wasting include:
Water: Water makes soils heavier and unconsolidates
individual soil particles.
Over-steepened slopes: Undercutting slopes removes
their structural support.
Removal of vegetation: Removing vegetation kills
roots that hold soils together.
Earthquakes: Earthquakes shake soils apart reducing
their structural integrity. The process is known as
“liquefaction”.
Types of Mass Wasting
Rockfalls: When rocks and rock fragments fall
through the air.
Slides: When blocks of material move along flat,
inclined surfaces.
Slumps: Slumps occur when large blocks of material
move downward.
slump
Types of Mass Wasting
Flows: Flows occur when material become saturated
with water and move down slope as a thick fluid.
Examples are mudflows and earth flows.
Creeps: Creeps are the slowest form of mass
wasting. Creeps occur when rock and soil move
forward centimeters per year. Creeps move due to
freezing and thawing cycles and cannot be directly
observed.