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Physical Science Notes

Weathering, Soil and Mass Wasting

Earth's external processes

1. Weathering - the disintegration and decomposition of material at or near the surface
2. Mass wasting - the transfer of rock material downslope under the influence of gravity
3. Erosion - the incorporation and transportation of material by a mobile agent, usually water, wind, or ice

Weathering:

Mechanical: physical breaking up of rocks.
Chemical: changes in chemical composition

Mechanical Weathering

Breaking into smaller pieces increases surface area. More surface exposed increases the rate of chemical weathering.

Types of Mechanical Weathering

• Frost Wedging → Talus Slopes
• Expansion from Unloading → Exfoliation Dome
• Biological Activity
• Salt Cracking
• Thermal Expansion and Contraction

Chemical Weathering

Water contains dissolved oxygen and carbon dioxide. Some important chemical reactions are:
reaction with oxygen to form oxides
reaction with weak acids to dissolve minerals into solution

Feldspar weathers to form clay and quartz weathers to form sand.

Rates of Weathering

Mechanical weather accelerates chemical weathering by exposing more surface area.

Chemical weather occurs more rapidly in wet warm conditions

Marble (calcite) readily dissolves in weakly acidic solutions

Soil

Soil - an interface in the Earth system, a combination of mineral matter, water and air - that portion of the regolith that supports the growth of plants

Soil texture and structure

Texture - refers to the proportions of different particle sizes: sand (large), silt (medium) and clay (small)

Loam - a balanced mixture of sand, silt and clay - best soil for growing plants

Soil structure - platy, prismatic, blocky or spheroidal - the way soil particles clump

Controls of soil formation

There are two main types of soil: residual and transported. Residual soils are made from the bedrock beneath them. Transported soils are made from material that has been transported from another place and deposited.

Soil takes time to form. It is a natural resource. If you dig it up and then put it back right away - it is NOT the same.

Soil formation is influenced by

• parent material
• time
• climate
• plants and animals
• the angle and orientation of the land where the soil is formed (topagraphy)

A soil profile is a vertical look that the soil - looking at each horizontal layer - and identifying these layers: O - organic matter, A - organic and mineral matter, E - not much organic matter, B - zone of accumulation, C - partly altered parent material

solum - true soil - layers O, A, E and B

top soil - layers O, A and E only.

Mass Wasting

mass wasting is the downslope movement of rock, regolith, and soil under the direct influence of gravity

triggers:

• saturation of material with water
• oversteepening of slopes
• removal of anchoring vegetation
• earthquakes

types:

• slump: rapid, movement along a curved surface
• rockslide: rapid, avalanche
• debris (mud) flow: rapid, confined to channels, dry areas that receive heavy rain
• earth flow: rapid, water saturates soil (liquefaction is specific to earthquakes due to ground vibration)
• creep: slow, fences and poles tilt
• solifluction: slow, areas underlain by permafrost

A search on You Tube is a worthwhile exercise to find videos of mass wasting

some videos...

• Debris Flow, Clear Creek County, Colorado

• Landslide, La Jolla, California - CNN

• Debris Flow, Devore San Bernardino California

• Landslide, Aowanda, Taiwan

more links to videos...

• Mudslide, La Conchita, California
• Laguna Mudslide, Laguna Beach, California
• Mud Flow, Arolla Glacier, Alps

try is yourself, go to you tube and enter search terms like: mudslide, avalanche, landslide