Weathering and Erosion

Weathering, Erosion and Soil

Weathering is the process in which materials
on or near Earth’s surface break down and
change.

Two types of weathering

○ Mechanical Weathering
○ Chemical Weathering

What is Weathering?

What causes weathering?

● Rain
● Wind
● Sleet
● Snow

● Temperature
● Gravity
● Plants
● Animals

Mechanical Weathering (Physical
weathering)

Is a type of weathering in which rocks and
minerals break down into smaller pieces

● Temperature and pressure play

major role in Mechanical
Weathering

Abrasion (Physical battering)

● Abrasion occurs when rocks scrape or crash

against each other and overtime become
smoother and more rounded

● Rocks wear down due to water, gravity, and

even wind

Abrasion

1. Water- rocks bang and crash against each other in

waves or streams. Water can wear down rock.

2. Gravity- Rocks can fall on each other causing them

to crack, scrape, or break into smaller pieces

3. Wind- Sand blown by wind can wear down rock like

sandpaper

Water carries
rocks, smoothing
and breaking them
down

Running water can
smooth rock
surfaces

Rocks falling causes
them to break up
into smaller rocks

Gravity plays a
role in
abrasion

Wind
Abrasion

Wind blown
sand and
particles can
wear down rock
over time.

How does temperature play a role in
mechanical weathering?

Frost Wedging

The freeze-thaw cycles of water in the cracks

of rocks

-Water finds its way into cracks of rocks and freezes and
thaws over and over. When water freezes, it EXPANDS
putting pressure on the rock. Overtime this pressure
causes the cracks to become bigger eventually causing the
rock to break.

Frost Wedging

Frost Wedging

Exfoliation (onion skin weathering)

The process by which outer rock layers are

stripped away

-Rocks are exposed to the sun and heat up during
daylight causing them to expand. At night the
temperature cools down and the rock contracts.
Over time this cycle continues to repeat itself
causing the rock to flake and peel apart.

Exfoliation (onion skin weathering)

Exfoliation (onion skin weathering)

Half Dome at Yellowstone
National Park

Sugarloaf Mountain- Rio De Janeiro,
Brazil

How does pressure play a role in
mechanical weathering?

Root Wedging/Biological Weathering

Process by which plants and their roots wedge
into cracks of rocks splitting them apart

Root Wedging/Biological

Weathering

What climate is most likely to
experience physical weathering and
why?

● Climates with wet weather and cool temperatures

are more likely to experience physical weathering.
Water and cool temperatures allow for abrasion,
frost wedging, exfoliation and plants to grow.

Multiple Choice

How does freezing water cause the weathering of rocks?  The freezing water—

• keeps the rocks in place

• makes the rocks last longer

• expands cracks and breaks rocks

• causes rocks to fall in landslides

Multiple Choice

The process that breaks down rocks and other materials on Earth's surface is called

• weathering

• erosion

• soil conservation

• decomposition

Multiple Choice

How does wind abrasion work?

• Sediment is blown off a surface from wind.

• Wind-blown sediment strikes a surface.

• Wind blows large chunks of rock off a surface.

• Sediment falls off of a surface on its own.

Multiple Choice

What is mechanical weathering?

• Process through which rocks are physically broken into smaller pieces.

• Rocks are broken down through chemical changes.

• Rocks are worn away from acid in plant roots.

• Rocks broken down by machinery.

Multiple Choice

The diagrams  shows a natural process that weathers rock.
 Which statement best explains why this process results in weathering?

• frozen water acts as a solute

• water expands when it freezes

• the mass of water increases when it freezes

• frozen water dissolves most types of rock

Multiple Choice

Which of the following is an example of mechanical weathering?

• Acidic groundwater dissolving limestone

• frost wedging

• Paper being burned

Multiple Choice

Which agents  break down earth materials?

• wind only

• earthquakes and gravity

• ice and 007

• wind, water, and ice

Multiple Choice

Which type of climate will have faster chemical weathering?

• Hot and Wet

• Cold and Dry

• Hot and Cold

• Cold and Wet

Multiple Choice

When wind blows over the land, it picks up the smallest particles of sediment.

• True

• False

Multiple Choice

Which of the following is NOT a type of physical weathering?

• burrowing plants

• ice wedging

• plant roots

• acid rain

Multiple Choice

What is mechanical weathering?

• Process through which rocks are physically broken into smaller pieces.

• Rocks are broken down through chemical changes.

• Rocks are worn away from acid in plant roots.

• Rocks broken down by machinery.

Multiple Choice

How do plants weather rocks?

• during photosynthesis

• when roots grow into cracks of rocks and expand the rock

• when the plant moves water from the roots to the leaves

Multiple Choice

These rocks were made smooth by wind blowing smaller rock particles across their surface. This is an example of:

• ice wedging

• chemical weathering

• acid rain

• abrasion

Multiple Choice

How might a weathered mountain appear different than an unweathered mountain?

• Weathered mountain would be taller.

• Weathered mountain would be rounder and less jagged.

• Weathered mountain would be more jagged and less rounded.

• Weathered mountain would have no trees.

Chemical Weathering

Is the process by which rocks and minerals undergo
changes in their composition.

●

Water, Oxygen, Carbon Dioxide, and Acid Precipitation

play major role in Chemical Weathering

●

When rocks interact with these agents, it can cause

rocks to dissolve and some new minerals to form

Examples of Chemical Weathering

Iron reacts with Oxygen to
form rust color in rocks that
have iron (Hematite)

Acid precipitation can dissolve
calcium carbonate and form
caves

How does WATER play a role in Chemical
Weathering?

● Water can dissolve many kinds of

minerals and rocks

● Serves as a medium in which reactions

can occur

● Water can react directly with minerals in

a reaction

HYDROLYSIS

Break down of rock and minerals
caused by its reaction with water

A-B + H2O A-OH + B-H+
Reactants Water Product A Product B

Water reacts with Feldspar (big molecules) and
breaks it down into Kaolinite (smaller molecules)

How does OXYGEN play a role in chemical
weathering?

Oxidation

The chemical reaction of oxygen with
other substances

● Earth’s Atmosphere is 21 percent oxygen

How does OXYGEN play a role in chemical
weathering?

Iron in rocks and minerals combines with
oxygen in the water and air to form
minerals with the oxidized form of iron.

● Creates compounds called OXIDES
● Hematite- a common mineral that contains the

oxidized form of iron

How does CARBON DIOXIDE play a role in
Chemical Weathering?

● Carbon dioxide (CO₂) reacts with water (H₂0) to form a

weak acid called carbonic acid.

● The natural pH of rainwater is slightly acidic (pH5.6)

● Decaying matter, soil and respiration have high levels of

CO₂

● Water can seep into the ground and combine with CO₂ in

the soil creating carbonic acid. This acid can dissolve

limestone and form caves

Acid Precipitation

● Is caused by sulfur dioxide, carbon dioxide and

nitrogen oxide

● These oxides are released into atmosphere by human

activities, such as burning fossil fuels

● These oxides combine with Oxygen and Water in

atmosphere to form strong acids

● Acid precipitation is anything under pH of 5.6

Effects of Acid Precipitation

● Acid precipitation is harmful to many organisms

● Destructive to many human made structures

● Can do much damage to forests and make forest more

vulnerable to disease and insect damage

Effects of Acid Precipitation

Acid rain can
seep into the
soil and
dissolve
nutrients, such
as magnesium
and calcium,
that trees
need to grow
and be healthy

Effects of Acid Precipitation

Cleopatra’s
Needle in
Egypt did not
undergo
weathering,
but then it was
moved to NYC,
chemical
weathering
dissolved the
symbols

What Affects the Rate of Weathering?

The rate of weathering is very, very slow. It can
take 2000 years just to weather 1 centimeter of
limestone.

Four factors affect the rate of weathering:
● Climate
● Rock type and composition
● Surface Area
● Topography

Climate

● Climate is the major influencer for the rate of

weathering

● Climate includes all precipitation, temperature, and

evaporation

● Chemical weathering is faster in regions with warm

temperatures, abundance rainfall, and lush vegetation,
like regions near the Equator

● Physical weathering is faster in cool environments

where water can undergo freeze-thaw cycles (frost
wedging)

World map of chemical weathering

Rock Type and Composition

● Different rock types and compositions weather at

different rates.

● Some rocks are considered more resistant to

weathering

● Example: Calcite rocks like limestone weathers faster

than quartz rocks like granite, which weather at a
slower rate.

Surface Area

● Weathering occurs on the surface of rocks. The more

surface that is exposed, the greater the effect of
weathering.

● Weathering creates more surface area
● Weathering has more effect on multiple, smaller rocks

as opposed to one big rock.

● Mechanical weathering can increase the rate of

chemical weathering

24cm²
48cm²

Multiple Choice

What are the two factors that affect the rate at which weathering occurs?

• climate and mass of rock

• climate and size of rock

• type of rock and size of rock

• type of rock and climate

Topography

● Rocks on level ground tend to stay in place longer than

rocks on slopes

● Gravity helps move rocks down sloped areas. This can

expose new layer of rock to weathering due to erosion

Erosion

● The removal of weathered rock and soil from its

original location

● Remember: weathering does not involve the movement

of anything. It is just making things smaller or
changing their composition.

● How does erosion occur?

Agents of Erosion

● Running water
● Glaciers
● Wind
● Living Things
● Gravity
●

Water

● Water is the most powerful agent of erosion
● Running water can produce dramatic changes in

landscape

● Streams can reshape entire landscapes and have the

greatest impact when there is a large volume of water
that is moving very fast.

● Water flowing down slopes will cut downward and carve

steep valleys

Mississippi River

The Mississippi
river can carry
550 million
metric tons of
sediment into
the Gulf of
Mexico each
year!

Destructive Results of Erosion

● Water can carry away fertile soil, which is bad for

agriculture

● Running water can cut small channels into the side of a

slope forming a Rill. Overtime, Rills can become deeper
and wider and turn into Gullies.

Rills- form when water
forms small depressions
as water runs downhill

Gully- when rills
become deeper and
wider

Erosion by Water

● Remember, water is the most powerful agent of

erosion and is responsible for dramatically changing
the landscape.

● Let’s look at some examples of how water changes the

landscape…

Streams carry sediment to coastal areas
forming Deltas