Download 4.1 Surface Processes

SURFACE PROCESSES
WEATHERING

Rocks are continually recycled on the Earth’s
surface and in Earth’s interior, on a geological
time scale.

Rock is often exposed to an environment
different than the one in which it was formed.
EXAMPLE: GRANITE

Formation
 Forms
at great depth
 Under high temperature and
 Pressure

Exposure – if moved to the surface
 Colder
temperatures
 Less pressure
 Chemicals
 Physical contact

The granite becomes unstable in this new
environment under the new conditions.

The granite will change to attain an equilibrium
with its new conditions.

The change is called weathering, which
includes the physical breakdown and chemical
alteration of the parent rock.
WEATHERING (BREAKING DOWN)

Results in the decomposition of rock and its
conversion to gravel, sand, clay and/or
dissolved material.

There is little or no movement of the
weathering products as these accumulate at
the location of decomposition.
Breaking a rock apart creates
more surface area, which
results in faster weathering
Weathered rocks take
on a spheroid shape as
corners offer more
surface where more
mechanisms for change
can occur.
TYPES OF WEATHERING

Two methods
 Mechanical
 Chemical
or Physical weathering
weathering
MECHANICAL WEATHERING

This reduces solid rock to fragments or grains
but does not change the chemistry of the
original rock or the fragments.

Fragments are just smaller than the original
rock.
TYPES OF MECHANICAL OR PHYSICAL
WEATHERING
Pressure-release (exfoliation)
 Frost-wedging
 Salt pressure
 Organic activity
 Thermal expansion and contraction

PRESSURE-RELEASE (EXFOLIATION)
Many igneous and metamorphic rocks are formed
at great depth under very high pressure.
 Over time these rock may be uplifted to the Earth’s
surface which decreases the pressure on the rock.

The rocks expand upon the release of the pressure
but because the rock is cool and brittle it fractures
as it expands and layer break or peel off.
 This peeling is termed exfoliation.

FROST-WEDGING
This action occurs in areas where both freezing
and thawing can occur.
 When water freezes, it expands by nine
percent.
 If water (as a liquid) accumulates in cracks or
fractures of rocks, and then freezes, its
expansion pushes the rock apart.
 Upon melting of the ice, the weakened rock
may actually break into fragments.

ORGANIC ACTIVITY
If soil collects in a crack in a rock a seed may fall
there and sprout.
 The roots of the plant can work their way into the
crack, expand with growth, and may eventually
widen the crack.
 Burrowing animals, such as earthworms, can
break apart soft rocks such as gypsum or
limestone.

THERMAL EXPANSION AND CONTRACTION
Earth’s surface are continually exposed to
changes in temperature.
 Rocks expand when heated and contract when
cooled.
 During rapid temperature changes, this
expansion and contraction may fracture the
rock.

SALT PRESSURE
When groundwater containing
some salt, seeps into pores and
cracks in rocks is when this
takes place.
 If the water evaporates (as pure water), the salt
crystallizes.
 The growing crystals exert pressure on the rock.
 This pressure may be enough to loosen grains and
widen cracks in the rock . Very common along
coastlines where sea spray brings saltwater onto
rocks on land

CHEMICAL WEATHERING

Here the weathering involves a change in
chemistry when air and water chemically react
with the rock to alter its composition and
mineral content.

The final product of chemical weathering differ
both physically and chemically from the original
rock
METHODS OF CHEMICAL WEATHERING
Oxidation
 Acid dissolution
 Solution weathering
 hydrolysis

OXIDATION
This involves a chemical reaction with oxygen,
either atmospheric oxygen or oxygen dissolved
in water.
 Elements that react with oxygen include;

 Iron
- producing hematite or rust
 4𝐹𝑒
+ 3𝑂2 → 2𝐹𝑒2 𝑂3
 Copper
 Lead
 Zinc
With the oxidation of most of these elements in
nature sulphur is involved.
 When sulphur is released and reacts with water
it form sulphuric acid, a strong agent of
chemical weathering.

ACID DISSOLUTION

This is a chemical reaction that occurs between
rock and an acid.
 When
an acid mixes with water it breaks down
(dissociates) releasing hydrogen ions (H+). These ions
are very small and have a positive charge.
 When the hydrogen ions come in contact with a rock, the
hydrogen ions are substituted for ions in the minerals
that make up the rock. This changes the structure of the
mineral and causes it to break down.
The stronger the acid the more rapidly the rock
will break down. There are very few naturally
occurring strong acids on Earth.
 The most prevalent cause of acid dissolution is
from rainwater.
 Rainwater is slightly acidic. This is because
atmospheric carbon dioxide dissolves in
raindrops and react to form weak carbonic
acid. Acid Rain

SOLUTION WEATHERING
In this process it is similar to Acid Dissolution
except that in the chemical reaction the mineral is
dissolved, leaving no solids, and turned into a
liquid.
 The minerals are not changed in terms of the
chemical make up they merely change to a liquid
state.
 Rocks like limestone and gypsum, caves are
formed when groundwater dissolve the minerals.

HYDROLYSIS
Here the mineral of a rock bonds with the water.
 Water reacts with the mineral to form a different
mineral.
 This type of weathering is the most complex of all
chemical weathering processes.
 Still it produces the most common sediment on
Earth, clays.
 These clays, once deposited and lithified, become
shale.
