HCDE-LPI Workshop onSedimentary Processes

Good Morning!!

WeatherinWeathering…g…Let’s break it Let’s break it

down…down…

Weathering• Chemical and physical alteration of rock

and minerals when exposed to air, moisture, and organic matter– Chemical weathering - minerals in a

rock are chemically altered or dissolved

– Physical weathering - solid rock fragmented by mechanical breaking

University of Georgia

Examples?

What Controls the Rate of Weathering?

• Parent Material (minerals and structure)• Climate (rainfall and temperature)• Biology• Slope• Presence / Absence of Soil • Exposure Time

Chemical Weathering• Chemical weathering - minerals in a rock

are chemically altered or dissolved • Minerals weather at different rates

because of differences in chemical stability; the resistance of a mineral to change

Which Minerals are More Stable?

Less?

Our Hero Bowen & His Reaction Series

Skinner and Porter, 1992

Physical Weathering• Physical weathering - solid rock

fragmented by mechanical breaking• Factors affecting:

– Slope – Zones of weakness - joints– Organisms - flora (and fauna)– Freeze/thaw (frost wedging)– Crystal growth – Heat / Cold

Slope• Things move downhill …. Why?• Gravity aids in the breakdown and transport of material• Slope starts it all!

Mass Movements (Mass Wasting)

• Mass movement is the down slope movement of earth materials under the influence of gravity

www.physicalgeography.net

Rock Mass Movements• Fall - fast; rocks break and fall

www.physicalgeography.net

www.geology.com

Rock Mass Movements

• Slide - (land slide) moderate; rocks slide as a unit

Source: USGS

Unconsolidated Mass Movements

• Creep - very slow flow downhill; top moves faster than base

Source: North Dakota State University

Unconsolidated Mass Movement

• Slump - moderate; slide of material as a unit along a lower surface (spoon-shaped)

Why Do We Care?• Human Impacts of mass wasting …

• Year Location Type Fatalities• 1916 It/Aust Landslide 10,000• 1920 China Earthquake triggered landslide 200,000• 1945 Japan Flood triggered landslide 1,200• 1949 USSR Earthquake triggered landslide 12-20,000• 1954 Austria Landslide 200• 1962 Peru Landslide 4,000-5,000• 1963 Italy Landslide 2,000• 1970 Peru Earthquake related debris avalanche 70,000• 1985 Columb Mudflow related to volcanic eruption 23,000• 1987 Ecuador Earthquake related landslide 1,000

• In a typical year in the United States, landslides cause over $1.5 billion in damages and 25 to 50 deaths

http://www.tulane.edu/~sanelson/geol204/masswastproc.htm

Joints

http://www.geo.duke.edu/geo41/st045.gif

http://www.geo.duke.edu/geo41/wea017.gif

Chemical Weathering Along Joints

….. Where the Water Goes …..

http://www.geo.duke.edu/geo41/wea017.gif

Spheroidal Surfaces

http://www.geo.duke.edu/geo41/wea003.gif

Roots and Critters

Crystal Growth

http://www.geo.duke.edu/geo41/wea001.gif

http://ww

w.usd.edu/esci/figures/150651.JPG

Frost Wedging

In what environments will chemical weathering be

greatest?

Physical?