Download Weathering

Weathering
Weathering is the physical breakdown and
chemical alteration of rocks and minerals.
•Weathering happens on or near the surface.
Weathering is caused by
•Atmospheric gases
•Water
•Acids
•Living organisms
There are two types of weathering, mechanical and
chemical
Why is Weathering important?
It produces the raw products for both
Sedimentary rocks and soils
It also products some important mineral resources
like aluminum ores
What is erosion?
Erosion is the removal of weathered material from the
weathering source area.
Mechanical Weathering
This is the physical breaking of bedrock into smaller
pieces.
Types of Mechanical weathering include
•Frost wedging
•Salt crystal growth
•Pressure release
•Thermal expansion & contraction
•Activities of organisms
Frost wedging:
This occurs in regions where temperatures fluctuate
above and below freezing
The material produced by frost wedging accumulates as
talus at the base of mountains forming a talus slope.
Pressure release:
This common in intrusive rocks that were formed deep under
ground. E.g. granite batholiths.
When this rock is exposed to the surface by uplift and erosion
the rock expands and sheet joints form parallel to the rock
surface. This is also called exfoliation.
Exfoliation domes form as the slabs of rock slip or slide off the
bedrock. E.g. Yosemite
Thermal expansion and contraction:
A common feature in desert environments where
temperatures can vary by as much as 30°C (86 °F)
Rock is a poor conductor of heat so the outside expands
more than the interior.
But this is not an important agent of mechanical
weathering.
Chemical Weathering
This is the decomposition of the rocks by chemical alteration
of the bedrock.
Water (H2O)
Acids (H2SO4, HNO3 , H2CO3)
atmospheric gases (carbon dioxide and oxygen)
and organism are important in chemical weathering.
Water:
Some minerals will dissolve in water like the mineral halite
(NaCl)
H2O + NaCl
 Na+ +
Cl-
This minerals are transported as dissolved salts in water.
Living organisms:
Many minerals will not dissolve in water, but they will
in acid.
Lichen is a fungi and algae living together.
Lichen covered rocks weather faster than rocks not
covered by lichen. Why is this?
During the process of photosynthesis, lichen removes
CO2 from the atmosphere.
This gas then reacts with water to form an acid.
The chemical reaction:
Water + carbon dioxide
H2O
CO2

Carbonic acid
H2CO3
Carbonic acid  hydrogen ion + bicarbonate ion
H2CO3
H+
Rocks that dissolve in acid easily are
Limestone and marble.
CaCO3 + HCl  CO2 + CaCl2 + H2O
HCO3-
Another important chemical reaction is oxidation
Iron minerals in the rock react with oxygen to form iron oxides
This often appears as red stains in the rock
Fe 2+ + O2  Fe2O3
Plant action:
1.Tiny root hairs seek out small cracks and pits in rock.
2.Once the root hairs find a place they grow and expand.
3.The expansion causes great pressure and cracks the rock.
Factors controlling Chemical Weathering:
•Surface area of rock
•Climate
•Rock type
Surface area:
The greater the surface area the more area of the rock that is
exposed to the elements and so it weathers faster.
Rocks with many joints and fractures will weather faster.
Climate and the speed of weathering
The wetter and warmer the climate the faster chemical
weathering will occur.
So where in the world we you find the fastest weathering?
Tropical regions with high rainfall and warm
temperatures.
Weathering here can extend to depths of several
tens of meters.
In cold areas and deserts it will only go down a
few centimeters!
Soil:
What is soil? Soil is a mixture of:
Air,
water and its solutes,
mineral particles from parent bedrock,
decaying organic matter,
And organism responsible for its decay.
Soil is a living skin!
Soil Horizons
Soil Horizons:
O
composed mainly of organic material
a few centimeter thick
not fully decomposed
A
Top soil
organic matter
biological activity, bacteria, plant roots, fungi, worms etc.
minerals can be leached by water into lower layers
B
sub-soil
fewer organisms
soluble minerals build up here
C
partially altered bedrock with little organic matter
The most important factor effecting soil formation is Climate
Different climates produce different soils
Soils and climate:
Tropical climate have laterite soils.
These are deep, but not not very fertile as the minerals have been
leached out. Nutrients are in the O horizon.
The soil is often a deep red color from weathering.
If soil is rich in aluminum, then leached aluminum hydroxide may
accumulate in the B horizon. as bauxite (aluminum ore).
Humid areas form pedalfer soils.
usually rich in organic matter
abundant water has leached most soluble minerals from A
horizon
Pedocal soils are found in arid and semi-arid areas
Western and southwest regions of the USA
Contain less organic matter than pedalfers
Soil Degradation:
Soil is one of the most important factors in human civilization.
Its mismanagement and destruction has been the collapse of
many civilizations in the past.
E.g.
Sumeria in Iran/Iraq
Easter Island
Teotihuacan
Maya
The decrease in soil productivity or loss of soil to erosion is called
soil degradation