Weathering & Soils
Weathering: The physical or chemical breaking down of rock material.
Most rocks form underground (at great depth and great temperature). The
conditions at the Earth’s surface are not favored by rocks. The
temperatures are too cold and the pressures to low for rocks to remain
stable.
I.
Physical weathering (mechanical weathering): breaks rocks into
smaller pieces.
a. As a rock physically weathers the total surface area exposed
increases – more of what was inside is exposed to the outside.
b. As the pieces get smaller their more likely to be eroded –
which will then increase the amount of physically weathering.
c. Physical weathering is dominant in cold wet climate zones.
d. Examples of physical weathering:
i. Frost Action / Frost Wedging: alternating freezing and
thawing of water breaks rocks apart.
ii. Root Wedging: Tree roots grow in the cracks between
rocks (think about sidewalks in front of large trees).
iii. Abrasion: Scraping or grinding.
1. Along stream bottoms as rocks roll along with the
current.
2. Wind blown sand blasts away and removes little
chips of larger rocks (pitted surface, ventifacts)
3. Talus – rock piles at the bottom of an avalanche or
landslide
iv. Exfoliation: peeling away of the outer layers of a rock
due to variations in temperature. Expansion in summer,
contraction in winter, over and over …
Resistance to weathering: Mineral composition and mineral hardness play an
important role in weathering. Quartz is a relatively hard mineral while
calcite is soft. Therefore a rock composed of quartz (granite) will be more
resistant to being weathered than a rock composed of calcite (limestone).
The shale unit is least resistant to weathering
II.
Chemical Weathering: Breaking down a rock at the molecular level.
The rock just doesn’t get broken into smaller pieces. Atomic bonds
are broken and new, weaker bonds are formed – A rusty nail is not
a strong as a new nail. Since the products of chemical weathering
are usually weaker, they are more susceptible to physical
weathering.
a. Chemical weathering is dominant in warm, humid climates.
b. Examples of Chemical Weathering:
i. Oxidation / Rust: 4Fe + 3O2  2Fe2O3
[Iron + Oxygen  Iron Oxide]
ii. Carbonation / Acid Rain: As rain falls through the
atmosphere CO2 gets dissolved in the drops. This makes
the rain drop acidic. Calcite is broken down by acids.
This is how caves, and sinkholes form.
iii. Hydrolysis: Water is the “universal solvent”, it dissolves
many minerals. Feldspars are no exception:
Feldspar + H20  clay and salts
Soils: The main product of weathering is soil. When rocks are broken down
into sediment and mixed with organic material (HUMUS) soil is formed.
Soils are developed slowly over many, many years. Depending on the
climate, a rich soil could take thousands of years to develop. In warm, humid
climates soils develop more rapidly. A developed soil has different layers or
horizons. As exposed bedrock breaks down, organic matter will begin to fill
into the spaces between the sediment. See the sequence in the diagram
below: