Weathering
Look around at the landscape in your area. Whether you see mountains,
plains, river valleys, coasts, islands, lakes or ocean, what you see
is the product of the constant changing of the Earth's surface.
Mountains
are pushed up from the plains. Islands form. New land is created by
volcanic action, caused by the collision of tectonic plates and by the
uplift of shallow sea beds. Over thousands and millions of years, the
twin processes of weathering and erosion will flatten the mountains,
extend the plains and wash the highest peaks back to the ocean floor.
This topic investigates the two forms of weathering and how they
help change the surface of the Earth.
What is weathering?
Weathering
is the constant wearing away and the decomposition of the Earth's surface.
Rain, wind, ice, plants, animals, the temperature and chemicals all
play a part in breaking down the rocks into smaller particles which
form the basis of soil.
There are two categories of weathering; mechanical weathering and chemical
weathering. Each involves different mechanism of weathering.
Mechanical
weathering
Mechanical weathering involves the breakdown of rock
to smaller pieces by mechanical or physical means. The main types of
mechanical weathering are:
- Ice wedging - If the air temperature
drops below the freezing point of water, any water caught in a crack
in a rock will expand upon freezing. This can cause the rock to crack
even more and can even break off large chunks of rock starting the
process of erosion.
- Temperature change - The Sun warming
up the surface of a rock will make the rock's surface expand. The
underlying rock remains cold and does not expand. At this point, the
surface rock may split from the underlying rock, making it vulnerable
to further weathering and erosion.
- Plants and animals - The growth of
plant roots in cracks in the rock and animals burrowing around the
rocks, allows water to enter the rock and it surrounds again making
the rock vulnerable to further weathering and erosion. Both these
processes accelerate the breakdown of rock surfaces.
- Water - Rivers and streams carrying
small rocks and mud act as abrasives on rock surfaces, wearing away
the rocks in their path.
- Wind - Wind carries sand which can
impact surface rocks and sand blast them forming more sand and dust.
Chemical
weathering
Changes in the chemical composition of a rock will often weaken
it structurally, making it more vulnerable to mechanical weathering.
Chemical weathering occurs when air, water or acids react with rock
to change its chemical composition. The main types of chemical weathering
are:
- Water - Rain falling on a new rock
surface will remove some elements from the minerals forming the rock.
This process, known as "leaching", can dramatically alter
surface rock, allowing it to become more prone to mechanical weathering.
- Oxidation - Iron left out in the weather
soon rusts. Other elements within the rock minerals will also react
with oxygen in the air, quite often forming water-soluble minerals
that can then leach away.
- Carbonation - Rainwater combines with
carbon dioxide in the air to form a weak acid known as carbonic acid.
This acidic solution is particularly effective in weathering limestone
and marble. Some limestone caves and sinkholes are thought to have
been formed by the constant action of carbonation over thousands and
millions of years.
- Plant acids - Some plants and fungi
produce acids that dissolve the surface of rocks and minerals to help
their roots gain a hold in the rock.
- Acid rain - Acid rain has been a part
of the environment for about 4.5 billion years. Before the industrial
age, it was a result of the interaction between sulfur from volcanoes
and water in the atmosphere and in rivers and oceans. The industrial
age has brought with it an increase of acid rain, now produced by
sulfur dioxide being released when coal and oils are burnt. Acid rain
can be a very strong acid, killing fish and accelerating the destruction
of man made monuments and rock alike. The strong acids dissolve elements
from minerals in the rock, altering the surface of the rock in much
the same way as has been described in points 1, 2 and 4.
Once weathering has weakened the structure of rocks, the process of
erosion can now take place. Weathering structurally weakens the rocks,
while erosion removes and transports the weathered material.
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