What Is Weathering?
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Weathering is the break up (disintegration) and decay (decomposition) of rocks in
the place where they formed. It does not involve the removal of that rock. (learn
this!)
There are three main types of weathering, each with their own methods of breaking down
rock.
1. Mechanical/Physical Weathering
This is the break up of rock by physical changes without any change in the chemicals in
the rock.
a) Frost Shattering or Freeze thaw Weathering
Frost shattering, or freeze-thaw weathering, is mainly found in mountainous regions such
as the Himalayas. The main cause of this is a temperature fluctuation from below
freezing point to above it.
Frost shattering is caused by water filling cracks in rock during the day. The water
freezes at night and exerts pressure as it expands, causing the crack to break further.
The ice melts in the daytime, and the whole process begins again, hence the name freezethaw.
When the rock is finally broken into small pieces, it leaves the edges of mountains jagged.
Near the foot of the mountains, there may be piles of rock fragments which have been
broken by frost shattering weathering. These are known as scree.
A diagram to show frost shattering
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b) Exfoliation or Onion-Skin Weathering
This has nothing to do with human skin. Exfoliation in rocks involves a change in
temperature like frost shattering, but in a different range.
Exfoliation occurs mainly in hot desert areas. This includes places like Arizona in the USA
and Alice Springs in Australia. In these places, the temperature can get as high as 40°C.
As rock is made up of layers (especially types like sandstone and marble), in the intense
heat, the top layer of rock expands. However, the inner layers remain cool. At night, when
temperatures drop, the top layer contracts. Repeated heating and cooling causes the
surface of the rock to peel off like an onion skin.
c) Pressure release
If pressure is released from the ground, like when a glacier melts, the rocks underneath
are no longer under pressure and they break up.
d) Salt crystal growth
When temperatures are 26-28 degrees salt crystals on the ground expand and as water
evaporates they are left behind to attack rocks. This often occurs in deserts.
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Not all weathering is caused by nature's processes. Weathering can be stimulated by
human causes, as the next type of weathering shows.
2. Chemical Weathering
Chemical weathering is the decomposition of rock by a chemical change in the rock. It
often occurs in warm wet conditions.
There are 2 types of chemical weathering that you just need to be able to recognize.
a) Carbonation - Rainwater contains small amounts of carbonic acid, but in many places
affected by so-called acid rain, such as Scandinavia, chemical weathering is a real
problem. Sulphur dioxide emissions can increase the acidity of water.
When rainwater comes into contact with rock, such as sandstone or limestone, a chemical
reaction takes place, causing the rock to crumble. This is visible on stone buildings such as
old churches, and in cemeteries, where the features of stone gargoyles and gravestones
have been weathered away. Water and heat can speed up this process, so it mainly occurs
in warm and wet places, such as Britain.
The main rocks that this happens in are sandstone and limestone as they contain.
b) Oxidation – occurs when iron reacts with oxygen to form rust. Often in warm, wet
conditions.
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3. Biological Weathering
Biological weathering is caused by the actions of plants and animals which break up the
rock (without moving it).
Tree Power - One of the ways a plant spreads its seed is by the wind. Dandelion seeds
are an example of this, as are sycamore 'helicopter' seeds. Seeds carried by the wind
may drop into cracks in rock. The crack proves to be an ideal place for the seed to
germinate as the crack unwittingly provides moisture and shelter.
On growing into a young plantlet, the plant's roots delve deep into the rock. This causes
the crack to widen, and eventually the rock falls apart.
Unfortunately for the plant, it is now exposed to...
Animal Antics - Burrowing creatures such as badgers, moles and rabbits also cause rock
to shatter. By burrowing into a crack, sometimes for food, it causes the rock to split and
break.
Can Anything Be Done?
Not really. Short of destroying every plant and animal in the world or destroying all the
rock in the world, realistically, the only solution is to cut back on carbon dioxide
emissions. However, it may be too late to stop acid rain and rising temperatures.
On the other hand, a moral which could be squeezed from this is that if you want the
headstone of your grave to be readable in a hundred years time, have it made out of
granite and facing west (away from the direction of the Sun and the cold northerly wind)
and in a country which has a temperate climate. That would slow the weathering process
in the short-term, but it won't help the rest of the world.
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IGCSE Style Questions
Goa – India
Oslo - Norway
Question 1: Using the evidence above what types of weathering will be more likely in
each of these locations and why?
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What is erosion?
Erosion is the wearing away of solids (soil, mud, rock and other particles) by the wind,
water (sea or river) or ice, with a downward or down-slope movement in response to
gravity or by living organisms. Unlike weathering, it involves the movement of the
decomposed rock and particles. (Learn this!!!!)
Erosion is a natural process but in many places it is increased by human land use. Poor land
use practices include deforestation, overgrazing, unmanaged construction activity and
road or trail building. However, improved land use practices can limit erosion, using
techniques like terrace-building and tree planting.
A certain amount of erosion is natural and, in fact, healthy for the ecosystem. For
example, gravels continually move downstream in watercourses. Excessive erosion,
however, can cause problems, such as soil entering water, ecosystem damage (including
dead fish) and outright loss of soil.
Causes of erosion
What causes erosion to be severe in some areas and minor elsewhere is a combination of
many factors, including the amount and intensity of precipitation, the texture of the soil,
the gradient of the slope, ground cover (from vegetation, rocks, etc.) and land use.
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In general, given the same kind of vegetative cover, you expect areas with highintensity precipitation, sandy or silty soils and steep slopes to be the most erosive.
Soils with a greater proportion of clay that receive less intense precipitation and
are on gentle slopes tend to erode less.
Often the wind, sea, rivers and glaciers erode. But there are other forms of erosion….
FIRES -The factor that is most subject to change is the amount and type of ground
cover. When fires burn an area or when vegetation is removed as part of timber
operations or building a house or a road, the susceptibility of the soil to erosion is higher.
ROADS - Roads are especially likely to cause increased rates of erosion because, in
addition to removing ground cover, they can significantly change drainage patterns. A
road that has a lot of rock and one that is "hydrologically invisible" (that gets the water
off the road as quickly as possible, mimicking natural drainage patterns) has the best
chance of not causing increased erosion.
LOGGING AND FARMING- Understandably, many human activities remove vegetation
from an area, making the soil easily eroded. Logging and heavy grazing can reduce
vegetation enough to increase erosion. Changes in the kind of vegetation in an area can
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also effect erosion rates. Different kinds of vegetation effect infiltration rates of rain
into the soil. Forested areas have higher infiltration rates, so precipitation will result in
less surface runoff, which erodes. Instead much of the water will go flow under the
ground, which are generally less erosive. Leaves on the ground and low shrubs are an
important part of the high infiltration rates of forested systems, the removal of which
can increase erosion rates. Leaf litter also shelters the soil from the impact of falling
raindrops, which is a significant agent of erosion. Vegetation can also change the speed of
flow over the ground, so grasses and shrubs can also be very important in this aspect.
One of the main causes of erosive soil loss in the year 2006 is the result of slash and
burn treatment of tropical forests. When the total ground surface is stripped of
vegetation and then seared of all living organisms, the upper soils are vulnerable to both
wind and water erosion. In a number of regions of the earth, entire sectors of a country
have been rendered unproductive. For example, on the Madagascar high central plateau,
comprising approximately ten percent of that country's land area, virtually the entire
landscape is sterile of vegetation, with gully erosive furrows typically in excess of 50
meters deep and one kilometer wide. Shifting cultivation is a farming system which
sometimes incorporates the slash and burn method in some regions of the world.
ANIMALS - When land is overused by animal activities (including humans), there can be
erosion. In the case of the animal kingdom, this effect would become material primarily
with very large animal herds stampeding such as the Blue Wildebeast in Africa. Even in
this case there are broader material benefits to the ecosystem, such as continuing the
survival of grasslands, that are indigenous to this region. This effect may be viewed as
anomalous or a problem only when there is a significant imbalance or overpopulation of one
species.
FOUR WHEEL DRIVE - Bank erosion is often started by four wheeler all-terrain vehicles.
TOO MANY PEOPLE - In the case of human use, the effects are also generally linked to
overpopulation. For when large numbers of hikers use trails or extensive off road vehicle
use occurs, erosive effects often follow, arising from vegetation removal and furrowing
of foot traffic and off road vehicle tires. These effects can also accumulate from a
variety of outdoor human activities, again simply arising from too many people using a
finite land resource.
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