Earth Science 5.1 Weathering
Weathering
Key Concepts we will cover today. . .

Students will learn. . . .
 What is mechanical weathering
 What is chemical weathering
 What factors affect the rate of weathering
Weathering

Earth’ surfaces are always
changing. Internal forces gradually
raise some parts of the surface
through mountain building and
volcanic activity.

At the same time, external
processes continually break down
rock and move the sediments to
lower locations.

Weathering is the breaking down of
rocks at or near Earth’s surface.
Mechanical Weathering

Mechanical weathering occurs
when physical forces break
rocks into smaller and smaller
pieces without changing the
rock’s mineral composition.

Each piece has the same
characteristics of the original
rock, it is just broken into a
smaller pieces so that the
surface area is increased.
Mechanical Weathering

In nature, three physical processes
are especially important causes of
mechanical weathering:
 Frost wedging
 Unloading
 Biological activity
Frost Wedging

When water freezes, it expands by about 9%,
exerting great outward force.

In nature, water works it’s way into every
crack in a rock. When the water freezes and
expands, it enlarges the crack in the rock.

After many repeated cycles of this freezingthawing, the rock will eventually split in two
halves.

This process is called frost wedging.

Frost wedging is most common in
mountainous regions in the middle latitudes.
Uploading

Large masses of igneous rock may be exposed by the uplift of the
rock and erosion over time.

Reduced pressure on the rock, called unloading, causes the outer
rock layers to expand more than the rock layers beneath.

This causes the top rock layers to peel off like the layers of an onion
in a process called exfoliation
Uploading

Exfoliation is
especially common
in rock masses
made of granite.

It often produces
large, dome-shaped
rock formations.
Biological Activity

The activities of organisms, including
plants, burrowing animals, and
humans, can cause mechanical
weathering.
 Plant roots can grow into cracks in rocks
and apply pressure over time, splitting
rocks apart.
 Burrowing animals cam move rocks to the
surface where weathering occurs more
rapidly.
 Decaying organisms can produce
compounds called acids that cause
chemical weathering.
Chemical Weathering

Chemical weathering is the
transformation of rock into one or more
new compounds.

In mechanical weathering, the rock
breaks into smaller pieces but is
compositionally unchanged

In chemical weathering, the rock
undergoes a transformation in it’s
composition as a result of the process.
Water

Water is the most important agent of chemical
weathering.

Water promotes chemical weathering by absorbing
gases from the atmosphere and the ground. These
dissolved substances than chemically react with
various minerals.
Water

Oxygen dissolved in water
reacts easily with certain
minerals, forming oxides.

Iron rich minerals get a
yellow-reddish coating of
iron oxide when they react,
forming rust.

Iron oxide is the rust that
forms when objects with
iron are exposed to water.
Water

Water also absorbs carbon
dioxide when rain falls through
the atmosphere. Water that
seeps through the ground also
picks up carbon dioxide from
decaying plant material.

The carbon dioxide dissolved in
water forms carbonic acid. This
is the weak acid in carbonated
soft drinks. Carbonic acid reacts
with many common minerals.
Water

Water in the atmosphere also absorbs
sulfur oxides and nitrogen oxides.

These oxides are produced by the burning
of coal and petroleum.

Through a series of chemical reactions in
the atmosphere, these pollutants are
converted into acid rain.

Acid rain precipitation accelerates the
chemical weathering of stone monuments
and structures considerably.
Chemical Weathering of Granite

Chemical weathering affects rocks in different
ways depending on their compositional
makeup. Lets look at granite as one example.

When granite is exposed to water containing
carbonic acid, the feldspar is converted mostly
to clay materials.

As the feldspar is converted to clay, the quartz
grains are released from the granite.

The quartz grains are deposited near the shore
where they become the main component of
beaches and sand dunes.
Chemical Weathering of Silicate Materials

When silicate materials undergo
chemical weathering, the sodium,
calcium, potassium, and magnesium
they contain dissolve and are carried
away by groundwater.
Spheroidal Weathering

Chemical weathering can change the
shape of rock as well as it’s chemical
composition.

When water enters along the joints in
a rock, it weathers the corners and
edges most rapidly. The corners and
edges break off and the rock becomes
more rounded.

The rock takes on a spherical shape
and this process is called spheroidal
weathering.
Rate of Weathering

Mechanical weathering
affects the rate of chemical
weathering.

By breaking rock into smaller
pieces, mechanical
weathering accelerates
chemical weathering by
increasing the surface area of
rock exposed to the chemical
components.
Rate of Weathering

Two other factors affect
the rate of weathering:
 Rock
Characteristics
 Climate
Rate of Weathering

Rock Characteristics:

Physical characteristics of
rock, such as cracks, are
important because they
influence the ability of water
to penetrate the rock.

However, the rock’s mineral
composition makes the most
difference.
Rate of Weathering : Characteristics

Gravestones made of
granite are relatively
resistant to weathering.

In contrast, marble
gravestones undergo much
more rapid chemical
weathering
Rate of Weathering: Climate

Climatic factors, especially temperature
and moisture, have a strong effect on the
rate of weathering.

These factors control the freeze-thaw
cycle which affect the amount of frost
wedging.

Temperature and moisture affect the
growth of plants which also contribute to
breaking rock down so chemical
weathering can infiltrate cracks and
decay rocks further.
Differential Weathering

Different parts of rock
masses often weather at
different rates. This process,
called differential
weathering, has several
causes.

Differences in mineral
composition are one cause.

More resistant rocks
protrude as pinnacles or
high peaks.
Concept Review. . . .

Mechanical weathering occurs when physical force breaks
rocks into smaller and smaller pieces without changing the
rock’s mineral appearance.

In nature, three physical processes are especially important
causes of mechanical weathering: frost wedging, unloading,
and biological activity.

Chemical weathering is the transformation of rock into one or
more new compounds.

Two factors that affect the rate of weathering are rock
characteristics and climate.