Download Weathering Process

Weathering Process
Rocks are classed as “dead”, hard and chemically
inactive.
Soils are considered to be “alive”, soft, porous and
chemically active.
They are inter-linked through bio–geo-chemical
cycles.
Weathering Process
Rocks are formed in the Earth’s crust under high
pressures and temperatures. Over time they are
exposed at the surface through a process known as
uplift.
Air, water and acidic gases all contribute towards the
weathering process.
The weathering of rocks involves 4 chemical
processes and 1 physical.
Weathering Process
Once exposed to the
elements and there has
been a release of
pressure on the rocks,
cracks / fissures
develop and physical
weathering can take
place – this increased
by the following:
Cracks are increased by;
Ice expansion
Wetting & drying out
Heating & cooling
Flora, roots etc..
Weathering Process
Chemical weathering
can also take place.
In doing so it forms
other minerals, different
character clays / clay
minerals or soluble
components may
recombine with other
compounds = new
minerals
Chemical weathering
can be divided into 4
types;
Simple dissolution
Simple hydrolysis /
complex hydrolysis
Carbonation
Reduction / oxidation
reactions
Weathering Process
Simple dissolution
Rocks / minerals
decompose by water or
reactions with naturally
occurring acids.
NaCl(s)
Na+(aq) + Cl-(aq)
Weathering Process
Simple hydrolysis
Hydrolysis – more
extensive reaction
occurs with water
molecule, breaks the
O-H bonds
Hydrolytic weathering
involves breaking the
metal-oxygen bond.
Weathering Process
Complex hydrolysis
This is a more important as it produces clays / soils.
It breaks down 3D silicates into 2D silicates.
Example:
Feldspar – KAlSi3O8 is a 3D silicate, that is weathered
to a clay by changing its composition and removing
some of the metals, forming a 2D silicate.
Further weathering, by loss of silica, results in the
formation of oxides.
Weathering Process
Carbonation involves the reaction of water and / or
carbon dioxide with materials in the Earth’s crust
CO2 + H2O ↔ (H2CO3) ↔ H+ + HCO3H+ + CaCO3 ↔ Ca
2+
+ HCO3-
The presence of the acidic H2O / CO2 weathers out
certain metals from rocks
Weathering Process
Oxidation /
reduction
This is an important
chemical process which
occurs where cations /
anions have >1
oxidation state
Example: Fe
(II)