Denudation: Weathering and erosion
Weathering: breakdown and decay of rocks, material not moved
Erosion: Breakdown and removal of rocks
This happens in a number of ways:
- Frost as an agent of mechanical weathering.
- Water as a agent of chemical weathering
Settings
Local buildings and graveyards.
Burren
Glaciated regions (honours only)
Tropical regions (honours only)
Types of weathering
Mechanical weathering: Breaks up rocks into smaller pieces
1. Frost
2. Sudden temperature changes
3. Plants and animals
Chemical weathering: Causes rock to dissolve or otherwise decompose.
1. Rain
How frost can weather rocks
Frost action occurs in areas where;
1. Temperatures frequently rise above and fall below freezing point.
2. Precipitation (rain, snow etc.) is sufficient to provide plenty of water.
3. Frost action is particularly severe in the great-glaciated mountain regions of the world. Examples: The
Himalayas, the Alps, and the Rockies.
4. Rainwater fills the cracks in rocks. At night the temperature falls below freezing causing the water to freeze
and expand. During the day the ice thaws again. This constant freezing and thawing weakens the rock
which eventually breaks off. (Mechanical weathering)
Chemical weathering
The Burren Co. Clare is a famous karst area – a place where the soluble limestone is exposed at the surface.
How limestone is weathered by rainwater.
1. As rainwater passes through the atmosphere it takes in carbon dioxide and becomes a weak carbonic acid.
This acid reacts with calcium carbonate in limestone and dissolves it slowly. This is called carbonation.
2. Limestone is permeable so it allows water to pass through it. It is easy for the rainwater to pass through
many of the vertical joints and horizontal bedding planes.
Beneath the surface in the Burren
1. Underground rivers and seeping rainwater continue to dissolve the limestone beneath the surface of the
Burren, gradually forming passages and caves. These caves contain features such as stalactites,
stalagmites, pillars and curtains.
2. All of these features are formed when carbonic acid dissolves the calcium carbonate in limestone. Drops hang
from the ceiling of the cave (the water seeps down through the ground). When a drop falls a tiny speck of
calcite stays behind. The next drop also leaves a speck of calcite. After hundreds of years the calcite builds
up into a solid peak hanging from the ceiling (A stalactite - C for ceiling).
3. Stalagmites - g for ground) are formed if the calcite falls with the drop and builds up on the floor of the
cave.
4. A pillar forms when a stalactite and a stalagmite grow together.
Attractions of the Burren
1.
2.
3.
4.
5.
Geographers study the limestone features.
Potholers and cavers explore the caves.
Botanists and naturists examine plants.
Tourists admire the unusual Karst bare limestone surface) scenery.
The monuments and archaeological sites are of historical interest.
The Burren national park;
Attracts visitors, creates employment, improves local services and recreational facilities.
However, tourists may damage local flora and the rock surface, air pollution increases with visitor traffic, there is an
increased risk to monuments.
Chemical weathering in the tropics (Amazon basin)
Wet regions in the tropics suffer so much chemical weathering that the rocks in these regions can be decomposed to
a depth of 60m. This happens because these regions have;
1. Large amounts of rainfall.
2. High temperatures (this speeds up the chemical reactions of water)
3. Large amounts of vegetation (producing acids which weather rock).
Acid rain
1. When fossil fuels are burned gases are released into the air (sulphur dioxide and nitrogen oxide).
2. In the air these gases either react with moisture or linger in dry form.
They then fall to earth as dust, rain, hail, snow or mist.
3. The term acid deposition is more accurate as it includes both wet and dry forms.
4. Millions of tonnes of these gases are released into the atmosphere each year; the majority of these come
from factories, industrial furnaces, power stations and vehicle emissions. Most of these gases find there way
into the atmosphere through tall chimneystacks.
The effects of acid deposition (rain)
Human
1.
2.
3.
health
Acid deposition is thought to be responsible for pseudo-croup, a sickness, which can be fatal to children.
It is thought to be responsible for increased incidence of lung cancer and bronchitis.
It aggravates breathing problems and asthma, there is also a possibility that it can effect heart problems.
Buildings and monuments
1. Acid rain corrodes sandstone, limestone, historic monuments, statues and stained glass.
2. Calcium carbonate used in sewer pipes in Victorian times dissolves in acid, they started collapsing in Britain
and had to be replaced by plastic.
3. Acid rain speeds up the weathering process. In Greece the Acropolis has corroded more in the last 20 years
than in the last 2000.
Lakes, rivers and streams
1. Lakes, rivers and streams can only absorb a certain amount of acid. In Scandinavia and most of Britain the
acid has overcome the natural defence mechanisms, the result is that lakes become more acidic and fish and
plant life die.
2. In the winter acid snow forms and the acid becomes more concentrated. When this layer melts a “pulse” of
acid is released having a major impact on the lakes.
3. In Canada 100,000 lakes are affected, 50,000 lakes have no fish at all.
Forests and crops
1. The evidence suggests that acid rain kills forests on an enormous scale.
2. The forest death syndrome is most advanced in Eastern Germany and Poland. Germany were one of the
worst polluters in Europe but when they saw what effect acid pollution was having on their forests they
immediately made strict anti-pollution laws and joined with other European countries in an effort to reduce
nitrogen and sulphur emissions.
3. In Britain damage to crops is estimated to cost over 200 million each year.
4. Dissolved aluminium leached from the soil by acidic deposition poisons the water and can cause fish kills.
Read the information above carefully. For each heading you should look up your book and the diagrams for each
feature. If you are unsure of the diagrams you need to draw them onto this revision sheet.
When you have studied the information see if you can answer the questions below. Remember the format:
STATEMENT – DEVELOPMENT – EXAMPLE
Questions:
1.
List they two types of weathering you have studied and give one example of each.
2.
Describe using a clearly labelled diagram how frost causes weathering.
3.
Explain using an example you have studied how chemical weathering occurs.
4.
Explain using a fully labelled diagram how pillars are formed.
5.
Explain how acid rain is formed.
6.
Describe three different aspects of life on Earth which are affected by acid rain.
Now go to studyclix.ie. Select Junior cert geography, then higher level, finally the section on rocks and
weathering. Attempt each question then check your answers. If you come across a question you can't do,
ask in class.