Physical Science 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.

Physical Weathering

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

Repeated freezing and melting of ice in pore space or fractures.
Water expands when it freezes.
Forces fractures to widen.
Can result in the formation of a talus slope, or talus cone.

Salt-Crystal Growth

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

Most rocks form under pressure - confining pressure.
As pressure is released, rock will expand.
Outer layers break free in sheets - exfoliation.
Exfoliation example 1
Exfoliation example 2

Thermal Expansion

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?

Root Wedging

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 Weathering

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
blast furnaces


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)

Ion Exchange

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:

Mass Wasting

Down slope movement of materials under the influence of gravity
Type of mass wasting is controlled by:


Very slow movement: mm/yr
Creep Example 1
Creep Example 2


Can be rapid or slow
Slump Example 1
Slump Example 2


Can be rapid or slow
Landslide Example 1
Landslide Example 2
Landslide Example 3

Mudflows / Debris Flows

Mudflows / Debris Flows:
Moderately rapid movement
Debris Flow Example 1
Debris Flow Example 2
Mudflow Example 3

Rock Slides / Rock Falls

Rock Slides / Rock Falls:
Rapid to very rapid movement: (m/s)
Rock fall equation: Vf = -(2dg)0.5
Rock Fall Example 1
Rock Fall Example 2
Rock Slide Example 3

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