Chapter 5.1
What is Weathering?

Weathering can be
defined as any
chemical or
mechanical process
by which rocks
exposed to weather
undergo changes in
character or are
broken down into
smaller pieces.
Mechanical Weathering
Mechanical Weathering is a type of
weathering that breaks down rocks into
smaller pieces without changing the
chemical composition of the rock.
 For example, Granite can be broken
down into smaller pieces, but the
smaller pieces are still Granite.

Agents of Mechanical Weathering
Frost Wedging – When water freezes
in the cracks of rocks causing them to
expand and break the rock.
 The products of frost wedging are large
piles of rocks called talus.

Frost Wedging
Frost Wedging
Talus produced by Frost Wedging
Agents of Mechanical Weathering
Unloading and Exfoliation – When
large sections of rock buried underneath
the ground are exposed at the surface,
enormous amounts of pressure are
released. This release of pressure is
referred to as unloading.
 After unloading, the rocks begin to flake
off in sections like an onion. This is
referred to as exfoliation.

Unloading and Exfoliation
Unloading and Exfoliation
Giant Slabs of rock flake off
As a result of Unloading and
Exfoliation
Agents of Mechanical Weathering
Biologic Activity – Weathering that
occurs as a result of the actions of living
organisms like plants and animals.
 Plants often cause mechanical
weathering when their roots penetrate
down into the cracks of rocks.
 When the roots grow they expand.
 Animals burrowing into the ground can
also create cracks in the rocks.

Biologic Activity
Roots penetrating the cracks in rocks
Gophers and Earthworms Burrowing
Also contribute to Mechanical Weathering
Chemical Weathering


When rocks that
weather change their
composition into a
new type of rock, this
is referred to as
chemical weathering.
For example, if
limestone (calcite)
dissolves as a result
of acid rain, then it
becomes carbon
dioxide gas and
various ions.
Chemical Weathering
The three main types of chemical
weathering.
 Carbonation occurs when carbonic acid
(H2CO3) dissolves minerals in rocks.
 Hydrolysis occurs when hydrogen in
water dissolves minerals in rocks.
 Oxidation occurs when oxygen reacts
with minerals in rocks.

Chemical Weathering
Examples of each type include.
 Carbonation occurs when acid rain
(carbonic acid) dissolves limestone
bedrock to produce caves.
 Hydrolysis transforms feldspar (in
granite), to produce clay sediments.
 Oxidation transforms rocks with iron (like
basalt and gabbro) into hematite (rust).

Chemical Weathering
Oxidation of Mafic/Basaltic Rocks
To produce the mineral Hematite
Carbonic Acid dissolves limestone
To produce caverns
Weathering of
Granite to
Produce clay
sediments
Rate of Weathering
The most important factors that affect
the rate (time) of weathering are
 Surface Area - More cracks means
more surface area, more surface area
means more weathering.
 Composition – Certain minerals are
more resistant to weathering than
others. Quartz for example is more
resistant to weathering than calcite.

Rate of Weathering
How Weathering Affects the Surface Area and the Rate of Weathering
Rate of Weathering
Climate – Hot, humid climates have a
fast rate of weathering. Cold, dry
climates have a slow rate of weathering.
 Vegetation – Places with more
vegetation tend to have a higher rate of
weathering because certain plants help
produce acids that help speed up the
rate of weathering.

Rate of Weathering Varies
depending upon the Climate