Ch 10 – WeatheringObjectives

1.Contrast chemical and mechanical weathering.

2.List and describe the types of mechanical weathering.

3.List and describe the types of chemical weathering.

4.List the products resulting from the chemical

weathering of Igneous rocks.

5.List and discuss the factors that influence the type

and rate of rock weathering.

2

There are two primary types of weathering:

Chemical and Physical

1. Physical Weathering – the breakdown of rocks and minerals

into smaller pieces without a change in chemical composition.

Root/Plant Action

2. Chemical Weathering – the breakdown of rocks and minerals into smaller

pieces by chemical action. The rocks breaks down at the same time as it

changes chemical composition. The end result is different from the original

rock.

Example. Oxidation – oxygen combines with the elements in the rock and it

reacts. This the scientific name for rust.

Read each slide then

fill in each blank

(underlined red

words) some slides

may be information

only!

Weathering

The breakdown of the materials on Earth’s

crust into smaller pieces.

Water causes weathering

What evidence of

weathering do you see

in this picture?

Wind causes weathering

What evidence of

weathering do you see

in this picture?

Why wasn’t this mass

of land weathered

away?

Ice causes weathering

Describe how ice

causes weathering?

Weathering

• Breakdown of rock

due to surface

processes

• 3 types of weathering

– Physical (Mechanical)

– Chemical

– Biological

http://www.geography.ndo.co.uk/animation

sweathering.htm#

Physical Weathering (Mechanical)

1. Abrasion – Means that the surface of the rock is weathered due to the action of wind, water & gravity

2. Frost wedging – Freezing–Thawing water expands when it freezes

3. Exfoliation or unloading –

– rock breaks off into leaves or sheetsalong joints which parallel the ground surface;

– caused by expansion of rock due to uplift and erosion; removal of pressure of deep burial

Physical Weathering

(Mechanical)4. Thermal expansion -

– repeated daily heating and cooling of rock;

– heat causes expansion; cooling causes contraction.

– different minerals expand and contract at different rates causing stresses along mineral boundaries.

5. Plant Roots – break rocks apart as they grow

6. Animals – animals burrowing through the soil-examples ants, molds, coyotes, worms, mice

What type of physical weathering?

What type of physical

weathering?

What type of physical

weathering?

What type of physical

weathering?

What type of physical weathering?

What t ype of physical weathering?

What type of physical weathering?

What type of physical weathering?

Chemical WeatheringRock reacts with water, gases and solutions

(may be acidic); will add or remove

elements from minerals.

1. Dissolution (or solution)

- also includes leaching

2. Oxidation

3. Hydrolysis

4. Biological Action/Acid in living thing

Dissolution• Dissolution occurs when rocks and/or

minerals are dissolved by water. The dissolved material is transported

away leaving a space in the rock. One consequence of this process is the formation of caves in limestone areas.

• Several common minerals dissolve in water

– halite

– calcite

• Limestone and marble contain calcite and are soluble in acidic water

• Marble tombstones and carvings are particularly susceptible to chemical weathering by dissolution.

Oxidation• Oxygen, the second most

common element in the air we breathe, reacts with iron in minerals to form iron oxide minerals, e.g. hematite (rust). As many minerals contain iron, it is not unusual to see red-colored rocks

• Oxygen combines with iron-bearing silicate minerals causing "rusting"

• Iron oxides are red, orange, or brown in color

Acid Precipitation

• 3. Acid Precipitation-Rain,

sleet, or snow, that contains a

high concentration of acids is

called acid precipitation.

Precipitation is naturally acidic.

However, acid precipitation

contains more acid than normal

precipitation.

Acid Rain

3. Acid Precipitation-Rain, sleet, or snow, that

contains a high concentration of acids is called acid

precipitation. Precipitation is naturally acidic. However,

acid precipitation contains more acid than normal

precipitation.

Acid in groundwater

Caves and caverns typically form in limestone

• speleothems - cave formations; made of calcite

• form a rock called travertine

– stalactites - from ceiling

– stalagmites - on ground

Karst topography forms on limestone

terrain and is characterized by:

• caves/caverns,

• sinkholes,

• disappearing streams,

• springs

More Dissolution

Acid Leaching

Hydrolysis – affected by H20• Hydrolysis occurs when minerals react

with water to form other products. Feldspar, the most common mineral in rocks on the earth's surface, reacts with water to form a secondary mineralsuch as kaolinite (a type of clay) and additional ions that are dissolved in water. The weaker clay is readily worn away by physical weathering.

• Feldspar alters to clay

– Feldspars = stable at high temperatures and pressures

– Clays are stable under conditions at the Earth's surface

• Quartz turns to sand

Biological Action

• Biological weathering is the actual molecular

breakdown of minerals. There are things called

lichens (combinations of fungi and algae)

which live on rocks. Lichens slowly eat away

at the surface of rocks.

• Lichens, fungi, and other micro-organisms

• Chemically and physically change rock

Mineral Content

Least stableOlivine

Ca plagioclase feldspar

Pyroxene

Amphibole

Biotite

Na plagioclase feldspar

Potassium feldspar

Muscovite

Quartz

Most stable

Factors Affecting Weathering*Surface Area

Texture

Temperature

Humidity

Topography

Time

Factors Affecting Weathering

Surface Area – Most

important

• Smaller particles,

more surface area

faster• Examples

– Crushed ice cools

faster

– Granulated sugar

dissolves faster

– More exposed rocks

weather more

Temperature and Water

• The rate of chemical weathering is

faster in warm, humid climates

Weathering and Elevation

• Rocks at higher elevations, as on a

mountain, are exposed to more wind,

rain, and ice than the rocks at lower

elevations are. This increase in wind, rain,

and ice at higher elevations causes the

peaks of mountains to weather faster.

Gravity

• Gravity affects weathering, too. The

steepness of mountain slopes

increases the effects of mechanical and

chemical weathering. Steep slopes cause

rainwater to quickly run off the sides of

mountains.

SummaryTypes of Weathering

• Mechanical or Physical - the breakdown of rock material into smaller and

smaller pieces with no change in the chemical composition of the

weathered material.

• Chemical - the breakdown of rocks by chemical agents. Obviously the chief

chemical agent is water which carries dissociated carbonic acid.

• Mechanical Weathering Expansion and Contraction - the thermal heating

and cooling of rocks causing expansion and contraction.

• Frost Action - Water freezes at night and expands because the solid

occupies greater volume. Action wedges the rocks apart. Requires

adequate supply of moisture; moisture must be able to enter rock or soil;

and temperature must move back and forth over freezing point.

• Exfoliation - process in which curved plates of rock are stripped from a

larger rock mass. Example Half Dome. Exact mechanism uncertain but

probably due to unloading.

• Other types - Cracking of rocks by plant roots and burrowing animals.

• Chemical Weathering Factors which effect the rate of chemical weathering

are: Particle size - Smaller the particle size the greater the surface area

and hence the more rapid the weathering

• Climate