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.

Read each slide then fill in each blank (underlined red

words) some slides may be information

only!

WeatheringThe 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/animationsweathering.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 sheets along

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 mineral such 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 feldsparPyroxeneAmphiboleBiotiteNa plagioclase feldsparPotassium feldsparMuscoviteQuartz

Most stable

Factors Affecting Weathering*Surface AreaTextureTemperatureHumidityTopographyTime

Factors Affecting WeatheringSurface 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.

Summary Types 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