adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Weathering is the process that breaks rocks down to smaller fragments and
alters minerals formed at higher temperature and pressure to those stable
under conditions present near the Earth's surface.
Geologists recognize two categories of weathering processes
Physical Weathering - disintegration of rocks and minerals by a
physical or mechanical process.
Chemical Weathering - chemical alteration or decomposition of rocks
Although we separate these processes, as we will see, both work together
to break down rocks and minerals to smaller fragments or to minerals more
stable near the Earth's surface.
Physical weathering takes place by a variety of processes. Among them are:
Development of Joints - Joints are regularly spaced fractures or
cracks in rocks that show no offset across the fracture (fractures
that show an offset are called faults).
Joints form as a result of expansion due to cooling or relief of
pressure as overlying rocks are removed by erosion.
Joints form free space in rock by which other agents of chemical or
physical weathering can enter.
Crystal Growth - As water percolates through fractures and pore
spaces it may contain ions that precipitate to form crystals. As these
crystals grow they may exert an outward force the can expand or weaken
Heat - Although daily heating and cooling of rocks do not seem to
have an effect, sudden exposure to high temperature, such as in a
forest or grass fire may cause expansion and eventual breakage of
rock. Campfire example.
Plant and Animal Activities -
Plant roots can extend into fractures and grow, causing expansion of
the fracture. Growth of plants can break rock - look at the sidewalks
of New Orleans for an example.
Animals burrowing or moving through cracks can break rock.
Frost Wedging - Upon freezing, there is an increase in the volume of
the water (that's why we use antifreeze in auto engines or why the
pipes break in New Orleans during the rare freeze). As the water
freezes it expands and exerts a force on its surroundings. Frost
wedging is more prevalent at high altitudes where there may be many
Since many rocks and minerals are formed under conditions present deep
within the Earth, when they arrive near the surface as a result of uplift
and erosion, they encounter conditions very different from those under
which they originally formed. Among the conditions present near the
Earth's surface that are different from those deep within the Earth are:
Lower Temperature (Near the surface T = 0-50oC)
Lower Pressure (Near the surface P = 1 - several hundred
Higher free water (there is lots of liquid water near the surface,
compared with deep in the Earth)
Higher free oxygen (although O is the most abundant element in the
crust, most of it is tied up bonded into silicate and oxide minerals,
at the surface there is much more free oxygen, particularly in the
Because of these different conditions, minerals in rocks react with their
new environment to produce new minerals that are stable under conditions
near the surface. Minerals that are stable under P, T, H2O, and O2
conditions near the surface are, in order of most stable to least stable:
Iron oxides, Aluminum oxides - such as hematite Fe2O3, and gibbsite
Note the minerals with *. These are igneous minerals that crystallize from
a liquid. Note the minerals that occur low on this list, are the minerals
that crystallize at high temperature from magma. The higher the
temperature of crystallization, the less stable are these minerals at the
low temperature found near the Earth's surface (see Bowen's reaction
The main agent responsible for chemical weathering reactions is water and
weak acids formed in water
An acid is solution that has abundant free H+ ions.
The most common weak acid that occurs in surface waters is carbonic
Carbonic acid is produced in rainwater by reaction of the water with
carbon dioxide (CO2) gas in the atmosphere.
Types of Chemical Weathering Reactions
Hydrolysis - H+ or OH- replaces an ion in the mineral.
Leaching - ions are removed by dissolution into water. In the
example above we say that the K+ ion was leached.
Oxidation - Since free oxygen (O2) is more common near the Earth's
surface, it may react with minerals to change the oxidation state of
an ion. This is more common in Fe (iron) bearing minerals, since Fe
can have several oxidation states, Fe, Fe+2, Fe+3. Deep in the Earth
the most common oxidation state of Fe is Fe+2.
Dehydration - removal of H2O or OH- ion from a mineral.
Complete Dissolution - all of the mineral is completely dissolved by
Weathering of Common Rocks
Clay Minerals + Hematite + Goethite
*Residual Minerals = Minerals stable at the Earth's surface and left in
the rock after weathering.
Weathering Rinds, Exfoliation, and Spheroidal Weathering
When rock weathers, it usually does so by working inward from a surface
that is exposed to the weathering process. This may result in:
Weathering Rinds - a rock may show an outer weathered zone and an
inner unweathered zone in the initial stages of weathering. The outer
zone is known as a weathering rind. As weathering continues the
thickness of the weathering rind increases, and thus can sometimes be
used as an indicator of the amount of time the rock has been exposed
to the weathering process.
Exfoliation - Concentrated shells of weathering may form on the
outside of a rock and may become separated from the rock. These thin
shells of weathered rock are separated by stresses that result from
changes in volume of the minerals that occur as a result of the
formation of new minerals.
Spheroidal Weathering - If joints and fractures in rock beneath the
surface form a 3-dimensional network, the rock will be broken into
cube like pieces separated by the fractures. Water can penetrate more
easily along these fractures, and each of the cube-like pieces will
begin to weather inward. The rate of weathering will be greatest along
the corners of each cube, followed by the edges, and finally the faces
of the cubes. As a result the cube will weather into a spherical
shape, with unweathered rock in the center and weathered rock toward
the outside. Such progression of weathering is referred to as
Bedding planes, joints, and fractures, all provide pathways for the
entry of water. A rock with lots of these features will weather more
rapidly than a massive rock containing no bedding planes, joints, or
If there are large contrasts in the susceptibility to weathering
within a large body of rock, the more susceptible parts of the rock
will weather faster than the more resistant portions of the rock. This
will result in differential weathering.
Slope - On steep slopes weathering products may be quickly washed
away by rains. On gentle slopes the weathering products accumulate. On
gentle slopes water may stay in contact with rock for longer periods
of time, and thus result in higher weathering rates.
Climate- High amounts of water and higher temperatures generally
cause chemical reactions to run faster. Thus warm humid climates
generally have more highly weathered rock, and rates of weathering are
higher than in cold dry climates. Example: limestones in a dry desert
climate are very resistant to weathering, but limestones in a tropical
climate weather very rapidly.
Animals- burrowing organisms like rodents, earthworms, & ants,
bring material to the surface were it can be exposed to the agents of
Time - since a rate is how fast something occurs in a given amount
of time, time is a crucial factor in weathering. Depending on the
factors above, rates of weathering can vary between rapid and
extremely slow, thus the time it takes for weathering to occur and the
volume of rock affected in a given time will depend on slope, climate,
Soils are an important natural resource. They represent the interface
between the lithosphere and the biosphere - as soils provide nutrients for
consist of weathered rock plus organic material that comes from
decaying plants and animals. The same factors that control weathering
control soil formation with the exception, that soils also requires the
input of organic material as some form of Carbon.
When a soil develops on a rock, a soil profile develops as shown below.
These different layers are not the same as beds formed by sedimentation,
instead each of the horizons forms and grows in place by weathering and
the addition of organic material from decaying plants and plant roots.
Caliche - Calcium Carbonate (Calcite) that forms in arid soils in
the K-horizon by chemical precipitation of calcite. The Ca and
Carbonate ions are dissolved from the upper soil horizons and
precipitated at the K-horizon. In arid climates the amount of water
passing through the soil horizons is not enough to completely dissolve
this caliche, and as result the thickness of the layer may increase
Laterites - In humid tropical climates intense weathering involving
leaching occurs, leaving behind a soil rich in Fe and Al oxides, and
giving the soil a deep red color. This extremely leached soil is
called a laterite.
Paleosols - If a soil is buried rapidly, for example by a volcanic
eruption, the soil may be preserved in the geologic
as an ancient soil called a paleosol.
In most climates it takes between 80 and 400 years to form about one
centimeter of topsoil (an organic and nutrient rich soil suitable for
agriculture). Thus soil that is eroded by poor farming practices is
essentially lost and cannot be replaced in a reasonable amount of time.
This could become a critical factor in controlling world population.