Weathering & Erosion
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What is weathering?
Weathering is a set of physical,
chemical and biological
processes that change the
physical and chemical properties
of rocks and soil at or near the
earth's surface.
More about weathering

Definition – the breakdown of rock to
form sediment [very small pieces of
rock]
Weathering happens to rocks that are
NOT MOVING
 Weathering is part of the Rock Cycle

There are three types of
weathering
Mechanical [sometimes called physical]
 Chemical
 Biological [can be mechanical or
chemical]

Let’s investigate
mechanical weathering first.
In mechanical weathering, a rock is
broken down into smaller pieces
without changing its mineral
composition.
Mechanical weathering breaks rocks
down into smaller pieces.
Types of mechanical weathering include
frost wedging, exfoliation, and thermal
expansion.
There are several different processes
that cause mechanical weathering.
Enchanted rock
is a huge
example of
exfoliation
mechanical
weathering
close to home.
Observe the effects of
mechanical weathering
Click on the arches and follow the
instructions to observe mechanical
weathering:
Observe photographs of the
different types of mechanical
weathering.
Watch the movie to find out
more about mechanical
weathering:
Review mechanical weathering:
thermal expansion –
heat and cold
causing the rocks to expand and contract
exfoliation –
peeling of layers; In
weathering, exfoliation describes sheets of rock
peeling off a bare rock face.
ice wedging –
water freezes and thaws
causing rocks to break loose
root pry –
tree roots causing roots to break
apart
Now, let’s look at
chemical weathering.
Chemical weathering breaks rocks down
chemically adding or removing chemical
elements, and changes them into other
materials.
Chemical weathering consists of chemical
reactions, most of which involve water.
In chemical weathering, a rock
is broken down by chemical
reactions that change its
mineral composition and
physical and chemical
properties
Chemical weathering happens
when the minerals that make up
a rock are changed, leading to
the disintegration of the rock
Chemical weathering happens
quickly in warm, moist
environments because water is
needed for the chemical
reactions.
The warm weather speeds up the
reactions.
Not all minerals are prone to chemical
weathering. The rock granite, has
very different levels of resistance to
chemical weathering than marble.
Chemical Weathering also occurs
when minerals are dissolved below
the surface, resulting in the bedrock
being removed.
Review chemical and mechanical
weathering. Take a quiz:
Biological weathering involves
processes that can be either
chemical or physical in nature.
Biological weathering can be
considered special types of
mechanical or chemical
weathering.
Biological weathering – What is it?
Some biological weathering
processes are:



1. Rocks can break because of animal
burrowing.
2. Tree roots grow into cracks and widen
them, which helps physical weathering.
3. Bacteria, lichens and other organisms
secrete acidic solutions, which helps
chemical weathering.
Lichens on rocks
cause biological
weathering by
secreting acids.
Is this physical or
chemical
weathering?
Plants growing in rocks break the
rocks apart. Chemical or physical?
Large
trees
break
apart rock
Burrowing animals break
apart rock. Chemical or
physical?
Now let’s move on to
EROSION
What is erosion?
Erosion is defined as the removal and
movement of earth materials by
natural agents.
Some of these agents include glaciers,
wind, water, earthquakes, volcanoes,
tornadoes, hurricanes, mud flows,
and avalanches.
How are erosion & weathering
different?
Weathering involves two
processes
[mechanical, chemical]
that often work together to break
down rocks. Both processes
occur in place. No movement is
involved in weathering.
As soon as a rock particle (loosened
by one of the two weathering
processes) moves, we call it
erosion or mass wasting.
Mass wasting is simply movement
down slope due to gravity.
Rock falls, slumps, and debris flows
are all examples of mass wasting.
We call it erosion if the rock particle
is moved by some flowing agent
such as air, water or ice.
Click to see examples of rock falls
and landslides. Scroll through the
pictures.
In a nutshell: if a particle is
loosened, chemically or
mechanically, but stays
put, we call it weathering.
Once the particle starts moving,
we call it erosion.
Water is the most important
erosional agent and erodes
most often as running water in
streams or rivers.
Water in all its forms is erosional.
Raindrops create splash erosion
that moves tiny particles of soil.
Water collecting on the surface of the
soil collects as it moves towards tiny
streams and creates sheet erosion.
Observe river erosion causing
waterfalls
Erosion by wind is known as
aeolian erosion (named after
Aeolus, the Greek god of winds)
and usually occurs in deserts.
Aeolian erosion of sand in the
desert is partially responsible for
the formation of sand dunes.
Painted Desert National Monument
The erosive power of moving ice is
actually a greater than the
power of water.
However since water is much
more common, it is responsible
for a greater amount of erosion
on the earth's surface.
Glaciers cause erosion two ways they pluck and abrade. Plucking
takes place by water entering cracks
under the glacier, freezing, and
breaking off pieces of rock that are
then moved by the glacier. Abrasion
cuts into the rock under the glacier,
scooping rock up like a bulldozer
and smoothing and polishing the
rock surface.
Click on arches
to see more on
glaciers
Pic.1 A boulder nearly dislodged due to plucking,
Beartooth Plateau, Montana. © William Locke
Athabasca Glacier - Jasper, Canada
Waves in oceans and other large
bodies of water cause coastal
erosion.
The power of ocean waves is
awesome; large storm waves can
produce 2000 pounds of pressure
per square foot. The pure energy
of waves along with the chemical
content of the water is what
erodes the rock of the coastline.
Watch wave erosion
Observe landscapes formed by
erosion
Observe sea cliffs formed by
erosion
Observe sea caves formed by
erosion
Wind and Water Erosion
Wind and water work together to erode
huge rock layers, forming magnificent
landforms. Read how these well-know
landforms were formed.
Let’s look at some erosion
close to home.
Remember the flood of 2002?
Clicking on the picture will take you
to a website with several
photographs of the erosion that
took place at Canyon Lake during
the flood.
DEPOSITION
As erosion occurs, the sediment is
moved and then deposited at a new
location. The size of the particles
determines how far it will travel.
Click on the movie to see how sizes
of sediment are deposited.
Deposition

The faster the stream, the larger size
sediment can move. As the stream slows
down, large sediment is deposited and
only small sediment can still move.
Deposition in Rivers

As water flows in a river, erosion occurs in
some areas, deposition occurs in others.
Draw a picture of Oxbow Lake and show
where erosion and deposition occur.
Review weathering & erosion
vocabulary with online flashcards.
http://www.studystack.com/javastudysta/frames.jsp
Note: Some of the terms on the cards
have not been covered during this
lesson. Read them anyway; it never
hurts to expand your vocabulary!
The End!