Weathering
Weathering
And
And
Erosion
Erosion
Weathering & Erosion:
Standard:
S6E5 Students will
investigate the scientific
view of how the earth's
surface is formed.
Elements:
• d. Describe processes that change rocks and
the surface of the earth.
• f. Explain the effects of physical processes
(plate tectonics, erosion, deposition, volcanic
eruption, gravity) on geological features
including oceans (composition, currents, and
tides).
Essential Question:
• What are the characteristics of weathering,
and how does weathering differ from erosion?
• KEY QUESTIONS:
– What is meant by weathering?
– How many different kinds of weathering
processes are there?
– How are weathering and erosion different?
– How are weathering and erosion related?
Weathering:
• The slow wearing away or
breaking down of objects
that are exposed to Earth’s
atmosphere and water.
Weathering breaks rock into smaller
and smaller pieces, such as sand,
silt, and clay. The terms sand, silt,
and clay are used to describe
specific sizes of sediments.
Sediment then changes gradually
into soil. The formation of soil
depends upon the amount of
weathering that occurs in a specific
place.
Two kinds of weathering:
Mechanical Weathering
Chemical Weathering
Both mechanical and chemical weathering work
together to shape Earth’s surface.
Mechanical Weathering:
• Objects are broken down into small pieces
but their chemical makeup doesn’t
change.
• Example: Wearing away a piece of wood
using sandpaper to smooth the wood. You
wear away the wood, but it’s still wood.
Exfoliation fractures in granite on
Stone Mountain, Georgia.
http://www.gly.uga.edu/railsback/FieldImages.html
Frost Action or ice wedging slowly breaks up this
sedimentary rock into unusual shapes.
• Root wedging in a
large boulder in
northern Georgia,
U.S.A
http://www.gly.uga.edu/railsback/Fi
eldImages.html
Ways rocks are
mechanically weathered:
• Animals: burrow by breaking apart
sediment and move it to the surface.
• Plants: roots can grow beneath a
sidewalk, cracking the concrete and
pushing it up.
• Ice wedging: water enters cracks in rocks
and freezes/expands. This pressure
breaks the rocks apart.
Chemical Weathering:
• Occurs when chemical reactions dissolve
the minerals in rocks or change them into
different minerals.
• Example:
– Leaving your bike outside and exposed to
elements such as rain, metal parts on the bike
will eventually rust. This is due to oxidation –
when metallic materials are exposed to
oxygen and water over prolonged periods
of time.
Ways rocks are
chemically weathered
• Natural Acids: water mixes with carbon
dioxide gas in the air or soil and forms a
weak acid called carbonic acid. (this is the
same weak acid that makes soft drinks
fizzy when shaken)
-- Carbonic acid reacts with minerals such
as calcite. The reaction causes the calcite
to dissolve. Carbonic acid has weathered
much limestone in caves.
Other methods of chemical
weathering:
• Naturally occurring acids come in contact with
other rocks. The mineral feldspar (found in
granite & some types of sandstone) is broken
down into a clay mineral called kaolinite. Clay
is an end product of weathering.
• Plant acids: some roots and decaying plants
give off acids that can dissolve minerals in rocks.
• Oxygen: (Oxidation) - occurs when metal is
exposed to oxygen & water over prolonged
periods of time and causes rust to occur.
1908
to
1969
Acid rain has eaten away this limestone statue.
Acid Rain
Climates effect on weathering
• Cold climates:
– Mechanical weathering rapidly breaks down rocks
through the process of ice wedging. Low
temperatures slow the chemical weathering
process.
• Warm Moist Climates
– Chemical weathering occurs more quickly in tropical
areas.
• Warm Dry Climates
– Lack of moisture slows down chemical weathering.
Weathering & Rock Type
• Rock type affects the rate of
weathering in a particular climate.
• Example:
–Wet climates: marble weathers
more rapidly than granite.
Weathering:
• affects rocks, caves, mountains, and
even buildings and streets.
• Is an important part of the rock cycle.
• Breaks down rocks.
• Produces sediment that can form
sedimentary rocks.
• Begins the process of forming soil from
rock and sediment.
Characteristics:
Definition:
Slow wearing away or
breaking down of
objects that are
exposed to Earth’s
atmosphere and
water.
Examples:
Rocks show signs of
decay:
pitting, breaking, flaking,
crumbling, swelling,
surface crusts and
discoloration.
Weathering
Non-Examples:
Mechanical Weathering:
Animals, plants, & ice
wedging
Chemical Weathering:
Acids, oxidation.
Mining
Grinding
Carving
Amazing Caves !!!
• Formation of caves due to oxidation
• Carlsbad Cavern National Park
Words To Know:
• Mechanical Weathering: objects (rocks) are
broken down into small pieces but their
chemical makeup doesn’t change.
• Chemical Weathering: Chemical reactions
dissolve the minerals in rocks or change them
into different minerals.
• Ice wedging: where water enters cracks in
rocks & freezes, expands, and causes rocks to
crack.
• Oxidation: when metallic materials are
exposed to oxygen and water over prolonged
periods of time.
Erosion
There are many kinds of Erosion.
Here are a few:
• Wave Action
• Avalanche •Creep
• Running Water
•Glaciers
• Rock Fall
• Landslides
• Slump
• Mud Flow
EROSION:
• A natural process of moving
weathered rock and soil to another
place.
• Carves the Earth’s surface creating
canyons, gorges, and even
beaches.
• Changes the landscape by wearing
down mountains, filling in valleys,
and making rivers appear and
disappear.
Erosion:
• Usually a slow and gradual process that occurs
over thousands or millions of years.
• Human activity such as farming, construction
and mining can speed up erosion
• Particles carried by wind and moving water rub
against exposed objects. The rubbing wears
away objects or breaks them into small parts
Five agents of erosion:
• Gravity (constant pulling on all matter on
Earth.
• Running water (rivers and streams)
• Wind
• Glaciers
• Waves
This is a picture of a landslide.
A type of mass wasting.
In this picture the land has slumped
into the road below.
This is a diagram of creep. The hillside
slowly slips over many years from
temperature or water.
This is a picture of an avalanche. It
is similar to a landslide with ice
and snow instead of rock.
This is a picture of a glacier which carves
out a U-shaped valley where it flows
dragging rocks and boulders along the way.
Dangers of Erosion
• Soil particles moved during erosion are often
washed and blown into our waterways and can
cause the following problems:
– it decreases the water clarity (less light for plants to
photosynthesize)
– fine sediments can suffocate fish and other water life
– increased nutrients encourage undesirable plants and
animals.
Erosion and sedimentation control
in Georgia
• The US EPA (United States Environmental
Protection Agency) has listed soil erosion
and sedimentation as one of the biggest
threats to our nation's rivers, lakes, and
ponds. Due to our land disturbing
activities, the United States loses more
than 2 billion tons of topsoil each year to
erosion (USDA).
http://www.rivercenter.uga.edu/service/erosion.htm
Erosion is increased if land:
• has little vegetation on it
• is steep
• is on the bank of a river or lake
• is disturbed (farming, construction, etc)
• has erosion prone geology (for example mudstone or
pumice)
• is under pressure from high stock density or machinery
• is in an area of high and intensive rainfall.
Reducing the risk of Erosion
• planting trees on hills and stream banks
• fencing gullies and waterways to prevent stock access
• removing wild goats and rabbits (they eat the vegetation)
• keeping stock off steep pasture when it is wet
• planting cover crops when land is left fallow
• No-till farming methods
• ripping wheel tracks in cultivated land where runoff and erosion
along wheel tracks is a problem (Plant with the contour of the land)
• retiring unproductive land
• not cultivating steep land.
Erosion by Water:
• water in all its forms causes erosion
– Raindrops (especially in dry environments)
create splash erosion that moves tiny
particles of soil.
– Water collecting on the surface of the soil
collects as it moves towards tiny rivulets and
streams and creates sheet erosion.
– The faster water moves in streams the larger
objects it can pick up and transport.
Streams erode their banks in three
different ways
• 1) the hydraulic action of the water itself
moves the sediments
• 2) water acts to corrode sediments by
removing ions and dissolving them
• 3) particles in the water strike bedrock
and erode it.
In this picture the running water eroded the sides
of this canyon and created the V-shaped valleys.
Waves cause erosion by four
different methods:
• Breaking – as the breaking waves hit the
shoreline, their force knocks off pieces of
existing rocks.
• Forcing water into the cracks of the rocks
on the shoreline
• Abrasion- waves carry small rocks and
sand that scrape other rocks
• Chemical weathering (salt water breaks
down the rocks)
Wave action is when waves hit the rocks
and pieces of rock break off.
Factors that affect the rate of
erosion at the shoreline include:
• Size of the waves
• Force of the waves
• Type of the rock
As erosion of the shore continues
several features may result:
Sea Cliffs, Terraces, Sea Stacks, and
Sea Caves
Where does beach sand come
from?
• Streams and rivers deposit sediment from
mountains into the ocean water.
• Waves erode rocks at the shoreline.
The color and texture of the sand depends
on the type of rock that was eroded.
Erosion by Wind:
• The most active agent of erosion with sand is
wind. As the wind blows it picks up small
particles of sand/sediment and blasts large rocks
with the abrasive particles, cutting and shaping
the rock. The ability for wind to erode larger
rocks is controlled by:
–
–
–
–
Size of particles
Speed of wind
Length of time wind blows
Resistance of rocks
Wind Erosion
• occurs almost always in
deserts
• Major cause of Dust Bowl of
the 1930’s
• physically removes the lighter,
less dense soil particles such
as organic matter, clays, and
silts
• removes the most fertile part
of the soil and lowers soil
productivity
• Wind erosion in the United
States is most widespread
on agricultural land in the
Great Plains states
• It is worse in arid and
semiarid regions
• Some soil from damaged
land enters suspension and
becomes part of the
atmospheric dust load. Dust
obscures visibility and
pollutes the air and water,
causes automobile
accidents, fouls machinery,
and imperils animal and
human health.
Wind erosion reduces the land's
ability to produce crops.
These rocks have been blasted by wind and
sand causing them to become rounded.
WIND ABRASION
Deposition of Sediment:
• Deposition is the process by which
sediments (small particles of rock) are laid
down in new locations.
• Deposition builds new landforms. Usually
water is responsible for deposition but
landslides can be caused by earthquakes
and volcanoes.
The Grand Canyon
Channels
• A channel is a body of water that connects two larger
bodies of water (like the English Channel). A channel
is also a part of a river or harbor that is deep enough
to let ships sail through.
Created by
slowing of
flowing water &
depositing of
sediment.
Satellite view of the English Channel
http://en.wikipedia.org/wiki/English_Channel
Definition:
Characteristics:
Affected by wind, water, ice,
slope, gravity, waves, sediment
size, and human activity.
Natural moving of material
from one place to another.
In the United States almost 2 billion
tons of Earth’s surface has been
moved from one place to another.
Logging
Mining
Farming
Construction
EROSION
Examples:
Grand Canyon
Non-Examples:
Adding Plants to an
area.
Weathering
Dust Bowl
Channels
Logging:
• If a forest is “clear-cut” all of the trees in an
area are cut down. Roots that had
previously held soil in place die and decay.
• Rain falls on the bare soil and washes
away much of the topsoil, robbing the area
of nutrients and minerals needed by plants
to grow.
Mining
• Land is stripped away.
• Large pits are cut into the land.
• Both strip mining and open-pit mining
remove plants from an area. Again, plants
are needed to “anchor” the soil so that it is
not easily eroded.
Farming:
Farmers tend to plow away topsoil with each
crop.
Droughts can strike farmland. When plants
die due to a drought, the soil becomes
exposed to forces of nature (water and
wind) and is carried away.
Construction:
• Building of roads, buildings, and
communities removes natural barriers
(plants) that help prevent erosion of soil.
• Erosion speeds up and land becomes
barren and ugly.
Important Words:
1. Runoff: Water that does not soak into the
ground or evaporate but flows across Earth’s
surface.
2. Abrasion: occurs when windblown sediments
strike rocks and sediment, polishing and pitting
their surfaces.
3. Deposition: dropping of sediments that occurs
when an agent of erosion (such as gravity, a
glacier, wind, or water) loses its energy and can
no longer carry its load.
4. Mass movement: any type of erosion that
happens as gravity moves materials down
slope. (see next slide)
5. Slump: when a mass of material slips down
along a curved surface.
6. Creep: occurs when sediments slowly shift
their positions downhill. Common in areas of
freezing and thawing.
7. Mudflows: thick pasty mixture (made when
heavy rains add water with loose sediments) is
pulled downward by gravity.
8. Channels: groove created when water moving
down the same path slows and deposits
sediment.
Mass Movements
• Landslides, mudslides, slump and creep
creep
landslide clip.mpeg
Ticket-Out-The-Door
Monday 11-26-07
• Explain how mechanical
weathering and chemical
weathering are different
from each other.