Ch 5 – Weathering & ErosionObjectives

1.Briefly contrast weathering and erosion.

2.Contrast chemical and mechanical weathering.

3.List and describe the types of mechanical weathering.

4.List and describe the types of chemical weathering.

5.List the products resulting from the chemical weathering of Igneous rocks.

6.List and discuss the factors that influence the type and rate of rock weathering.

The Rock Cycle

Last Unit – Ch 3 &4

This Unit –

Ch 5 & 6

What kinds of rock can be melted? Can be weathered & eroded? Heated & Pressurized?

Weathering • Breakdown of rock

due to surface processes

• 2 types of weathering– Physical

(Mechanical)– Chemical

Physical Weathering1. Abrasion - wind & water 2. Frost wedging - 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

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 grow6. Salt Crystal Growth – evaporation, crystals grow

between rocks

Type of physical weathering?

Type of physical weathering?

Type of physical weathering?

Type of physical weathering?

Type of physical weathering?

Type of physical weathering?

Type of physical weathering?

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

5. Spheroidal

Acid Rain

Dissolution

• 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.

More Dissolution

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

Oxidation• Oxygen combines with iron-

bearing silicate minerals causing "rusting"

• biotite • Iron oxides are red, orange, or

brown in color

Hydrolysis – affected by H20

• 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

• Lichens, fungi, and other micro-organisms

• Chemically and physically change rock

Spheroidal Weathering

• chemical weathering of jointed rocks.

• weather to form spherical shapes

Mineral Content

Least stableOlivine Ca plagioclase feldsparPyroxeneAmphiboleBiotiteNa plagioclase feldsparPotassium feldsparMuscoviteQuartz

Most stable

Factors Affecting Weathering*Surface Area

Texture

Temperature

Humidity

Plant growth

Topography

Time

Factors Affecting WeatheringSurface Area –

Most important

• Smaller particles, more surface area

• Examples– Crushed ice cools

faster

– Granulated sugar dissolves faster

– More exposed rocks weather more

Factors Affecting Weathering• Type of material – both are from 1780’s;

one is slate, the other is marble. Which is which? Why is there a difference?

Erosion• Movement / transportation

– Wind, water, glacier

• Mast Wasting - Movement of large amounts of material downhill under gravity – Creep – Mudflows – Slump – Rockfalls – Landfalls

– Avalanches

What happens to Granite? Granite contains Na Plagioclase feldspar, K feldspar, Quartz

– Lesser amounts of biotite, amphibole, or muscovite

Weathering

1. The feldspars undergo hydrolysis to form ________ and Na and K ions The Na and K ions removed by leaching.

2. The biotite & amphibole will undergo hydrolysis to form clay, and oxidation to form _________.

3. The quartz (and muscovite, if present) will remain as residual minerals because they are very resistant to weathering. They get smaller to make ________.

Weathered rock fragments are one of the constituents of soil – our next topic.