Chapter 12
Weathering & Erosion
Breakup of rock
transport of weathered material
by wind, water or ice
Types of Weathering
A. Mechanical
-breaking rock into smaller pieces (no change in rock composition)
B. Chemical
-process by which rocks are decomposed, dissolved or loosened
(rock composition changes –new minerals are formed)
Mechanical Weathering – 4 types
1. Ice & Water
-Water seeps into cracks in the rock. Over time, repeated freezing & thawing, or wetting &
drying, expands the crack, breaking the rock apart. Also called “Ice Wedging” or “Frost
Wedging.”
*Heating during the day and cooling at night has the same effect.
2. Abrasion
-Wind-blown sand or moving water carry rocks. The moving rocks grind and scrape against
each other and is worn away.
-The longer a rock is abraded by water, the rounder and smoother it is
3. Plants & Animals
-“Root Action” - plant roots grow through rocks and split/ break them.
-Animals dig holes in the soil which allow air & water to reach the bedrock which speeds up the
weathering process.
4. Exfoliation
-Caused by the loss of rock & soil over bedrock. Rocks weather by peeling off in sheets rather
than eroding grain by grain.
Chemical Weathering – 3 types
1. Hydrolysis
 When rocks react with water. Specifically, when silicate minerals (like feldspar)
react with water to form clay.
2. Acids
 Acid forms from decaying plants like moss.
Dissolution:
 CO2 or Carbon Dioxide from the air mixes with water to form carbonic acid.
 It dissolves calcite completely to form underground caves in limestone bedrock.
“Karst Topography”
 Pollutants like Sulfur Dioxide and Nitrogen compounds mix with water in the
atmosphere producing acid rain. (Sulfuric or Nitric acid)
3. Oxidation
 When iron-bearing minerals in rocks react with oxygen to form reddish or yellow
“rust” or iron-oxide.
Examples of iron minerals: magnetite & pyrite
The Rate of Weathering – depends on…
1. Surface Exposure
 Weathering takes place on the surface.

The more surface exposed, the faster it weathers!
2. Mineral Composition
Some minerals are more resistant to weathering than others.

Quartz is resistant to weathering = slow

Calcite is susceptible to weathering = fast

A quartz granite would take longer to weather than a calcite limestone.
3. Climate
Depends on temperature & the amount of rainfall
 Cold-Wet/Dry and Hot-Dry climates favor mechanical weathering. (occurs
slow)

Hot-Wet climates favor chemical weathering. (occurs fast)
Chapter 12 – Types of Erosion
Differential Erosion
Because each rock has a different
mineral composition, surface area or
climate, they can erode at different
rates.
Resistant rock will form ridges while
less resistant rock that weathered quickly form valleys.
Mass Movements (or Mass Erosion)
-The downward transport of weathered material down slope. “Landslides”
- Trees, grass and plants protect the bedrock from weathering! Roots hold
soil in place.
FAST Movements
1. Mud Flow – rapid movement of water that contains large amounts of suspended clay and silt.

Mudflows that accompany volcanic eruptions are called Lahars.
2. Rock Fall (rock slide) gravity pulls down mostly dry rock

Talus – pile of rocks that collect at the base of a slope
3. Earth Flow – mass of weathered material that has been saturated with water flows downhill.

The movement is slower and less fluid than a mudflow.
SLOW Movements
1. Creep (slowest type)
- slow imperceptible movement of soil down-slope
2. Slump
- when blocks of land tilt & slide down along a curved surface
Chapter 12 - Soil
Weathering has helped form a priceless resource… Earth’s life supporting
SOIL – weathered rock and organic material in which plants with roots can
grow. -- [Regolith is term used to describe any loose material covering the solid bedrock including soil]
a mature
soil profile
A/O - topsoil - organic layer, made mostly of leaf matter
& humus.
B - subsoil; red or brown color from iron oxides; made of
clay & sand. “leached zone” - minerals are washed down
from above.
C - slightly weathered bedrock
Bedrock – solid rock beneath soil
Soil Processes & Formation
According to the image at the right, what is
the most important factor affecting soil
development? Climate!
How can we protect our soil?



Plant windbreaks of trees around fields
Terracing prevents water washing soil
away
Strip cropping - alternating rows of tall &
short plants to prevent wind erosion