Physical Geology 101
Weathering and Mass Wasting
Weathering and Erosion
Weathering - the destructive processes by which materials at or near the Earth's surface are changed in color, texture, composition, firmness or form, with little or no transportation of the altered material.
Erosion - the mechanical or chemical destruction of the land and removal of material by moving water, ice or wind.
Produces finer particles by fracturing rock.
Forces applied must exceed rock strength.
Impact and Abrasion
Moving material impacts or abrades a surface.
Causes fracturing or flaking of small particles.
Frost Action - Ice Wedging
Three minerals are involved in this process: Halite (salt), Calcite, Gypsum.
As water evaporates, minerals are left behind to grow between grains in the rock.
Can result in cavities or alcoves forming in vertical faces.
Exfoliation - Unloading
Rock: low thermal conductivity
Heat on a rock surface that does not travel inward very rapidly
Rock exterior will expand, causing exterior to fracture: Spalling.
Sources of heat?
Plant, especially trees, sink root systems into existing joints and fractures.
Root growth forces fracture to expand.
• Root Wedging example 1
• Root Wedging example 2
• Root Wedging example 3
• Root Wedging example 4
Chemical alteration of rocks and minerals.
Chemistry changes → Minerals change → Crystal structure changes
This can weaken the rock structure.
Weaker rocks means physical weathering is easier.
Decomposition reaction involving water and a silicate mineral.
Alteration by H+ ions or OH- ions
Example: Orthoclase + H+ → Muscovite + Quartz + K++
Reaction involving water and silicate mineral
Alteration by H2O
Water causes clay minerals to expand. (physical weathering)
Grus is formed from granite
Hydration reactions also produce clay minerals. (chemical weathering)
Orthoclase + H2O + HCl → Illite + Quartz + K+ + Cl-
Element combines with oxygen ion, O-2
Combustion and rust
– 2 Fe+2 + 3 O-2 → Fe2O3 (hematite)
– reddish colored rocks often have hematite.
Reduction – mineral loses an oxygen
Fe2O3 + C → 2Fe + CO2 + O-2
Acids (sulfuric, nitric, hydrochloric) dissolve rock and minerals, especially carbonate minerals.
CO2 + H2O → H2CO3 Carbonic Acid
Water and carbon dioxide are readily available:
– atmosphere, surface water, and groundwater
Carbonic acid is a weak acid
– can dissolve large quantities of calcite, limestone, marble
Dissolution: beginning of cave formation
Dissolved calcite is deposited in:
– oceans as limestone
– fractures in rock
– caves as stalactites / stalagmites (travertine)
Substitution of ions in solution for ions in minerals
Water contains ions: H+, OH-, Ca+, Na+, K+, Cl-
Occurs in many minerals – most common in clay minerals
Exchange dependant on:
Down slope movement of materials under the influence of gravity
Type of mass wasting is controlled by:
Mudflows / Debris Flows
Rock Slides / Rock Falls
Mass Wasting - Avalanche
Very rapid movement
Rock slides and avalanches are similar but avalanches are larger and involve more material
Rock vs. Snow Avalanches
• Avalanche Example 1