Chapters 7 and 8 Slide Lake 15.02.b1 Weathering – the breakdown of rock at or near the earth’s surface.

Chapters 7 and 8 Slide Lake

15.02.b1

Weathering – the breakdown of rock at or near the earth’s surface

Types of Weathering

• Physical – Mechanical weathering

• Chemical – Chemical weathering

A full, solid block has the least surface area.

The interior is safe from exposure.

A smashed piece has greatest surface area exposed. The interior can now be attacked.

Mechanical WeatheringAKA: physical weathering

Breaks a rock into smaller and smaller piecesOnly size changes occur in this processNo chemical composition change occurs in mechanical weatheringSame chemical composition as the parent rockResults in increased surface area

Types of Mechanical Weathering

• Frost Wedging

• Root Wedging

• Pressure Release

Let’s look at each one

-Repeated freezing and thawing of H2O in rocks

-Can form potholes in cold climate areas-Ice expands by over 9% in volume as it freezes

- Ice at -30oC is harder than granite

Frost wedging

Root Wedging• Plant action-

1.Tiny root hairs seek out small cracks and pits in rock.

2.Once the root hairs find a place they grow and expand.

3.The expansion causes great pressure and cracks the rock.

Pressure Release

1. Rocks formed deep in the Earth are made under high pressure.

2. When the pressure is released the rocks expand & crack.

3. May also be caused by alternate heating and cooling of rocks by weather conditions.

Chemical Weathering

• The breakdown of rocks by chemical means

• Depends on the stability of minerals

More Stable Less Stable

•Form at low Temps. • Form at high Temps.

• Quartz, K-Feldspar • Olivine

Two Agents of Chemical Weathering

• Oxygen causes oxidization (Rust)

• Carbon dioxide forms a weak acid

(carbonic acid), which helps dissolve

rock material.

Chemical Weathering of Statues, Bath, UK

The Result of Chemical Weathering

• New minerals are formed

• Dissolved particles are released

• The amount of chemical weathering is determined by the composition of the Parent rock

• Temperature increases increase chemical reactions

How Granite WeathersChemical weathering – the alteration of the internal

structure of minerals by chemical reactions

K Feldspar + carbonic acid + H2O Clay mineral + K bicarbonate + Silica

Rates of Weathering

Granite versus Marble

Factors influencing weathering:

-Mineral makeup

-Climate

Differential WeatheringRocks weather at different rates because of different

mineral makeup and exposure to the elements.

More resistant rock protrudes as ridges and pinnacles.

Bryce National Park, Utah

Where is the rock more resistant to weathering?

Where is the rock least resistant to weathering?