Weathering
Learning outcomes:
- Define weathering.
- Explain the difference between physical and chemical weathering.
- Name examples of physical and chemical weathering and explain how they work.
- Explain what type of weathering occurs where, give examples.
Definitions:
Weathering:
The breakdown of rocks without movement.
Physical / Mechanical Weathering:
The breakdown of rocks through direct contact with
atmospheric conditions such as heat, water, ice and pressure.
Chemical Weathering:
The breakdown of rocks by chemicals, it will change the
chemical composition of the rock.
Weathering Types:
Physical Weathering Types
Thermal Stress
- Rocks heat up during the day and expand
- They cool and contract during the cold night
- The movement causes the rock to break
E.g. In deserts the days are very hot and the nights
are very cold this causes thermal weathering. This process needs a climate with
high temperature differences between day and night. E.g. Sahara Desert
Freeze-Thaw action
- Water moves into cracks and between layers of rock
- When it gets cold the water freezes and expands
- The pressure from the frozen water breaks the rocks apart —
E.g. On a high mountain the days are warm during the day,
the snow melts and the water flows into a crack in the rock. At night the
temperatures go below zero and the water freezes. The ice expands and pushes
the rock apart. This process needs a wet climate with temperatures moving
around zero degrees. E.g. on top of Mt. Kilimanjaro
Salt - Crystal
- Very high temperature causes high evaporation
- Salts in the rocks begin to crystallise
- The growing salt crystals break the rock apart
E.g. In dry areas near the sea the temperatures are high and
lots of salt can be found. When the sun heats the rocks the water evaporates
from inside the rock and salt, which was inside the water, crystallises. The
growing salt crystals break the rock apart. This process needs a hot and dry
climate. E.g. near the Dead Sea
Pressure release
- Rocks deep in the earth are under a lot of pressure and squeezed together
- If the rocks and soil on top are removed the rock expands and breaks apart
E.g. Many million years ago magma was placed deep inside
other rocks and it cooled there to form a granite pluton (bubble). Now the rock
above it has been eroded away and the granite is on the surface. As there is
less pressure on the surface than deep inside the earth the rock expands and
breaks apart. This process can happen everywhere in the world. E.g. Wicklow
mountains, Ireland
Chemical Weathering Types
Dissolution & carbonation
- Water & CO 2 combine to form a weak acid (chemical reaction)
- Carbonic acid
- This acid attacks the rock and dissolves it
E.g. Limestone, a carbonate rock, formed many million years
ago and is now at the surface. Rain falls and the water combines with the carbon
dioxide in the atmosphere to form carbonic acid. The acid dissolves the limestone
rock forming cracks and caves. This process needs a wet climate and carbonate
rock. E.g. Burren, Ireland
Oxidation
- Oxygen and metals combine (chemical reaction, especially with iron)
- The reaction forms a new material – oxide (rust)
- The oxide breaks down easily
E.g. Water falls on a rock that contains many metal minerals
e.g. iron and the rock begins to ‘rust’ rust is weak and breaks apart easy.
This process needs a wet climate and minerals such as iron. E.g. Uluru,
Australia
Hydration
- Water and minerals combine (chemical reaction)
- A new material is formed – hydride
- Volume increases and the rock breaks easier because of more pressure
E.g. Water falls on rocks, containing such minerals as
olivine or gypsum (a type of salt), that have the potential to react with water.
The minerals change into a different chemical substance that is bigger (as it
has taken in water) and the pressure causes the rock to break apart. This
process needs a wet climate and minerals that can react with water. E.g. Gypsum
deposit in England.
Special Considerations
Biological Weathering
Plants (and animals) cause weathering
- Directly; physical: roots of a tree grow into a crack in the rock and push it apart
- Indirectly; chemical: rabbits make a burrow; water can get to the rocks underneath and cause dissolution
These processes can occur everywhere.
Physical action
Wind, waves and rivers push rocks against other rocks so
they break apart
Attention! Don’t confuse with erosion … weathering does not
include the transport by water and wind!
Gravity
Rocks fall down so the chemicals and mechanics can work on
the next layer of rock. Not a type of wreathing but helps by exposing new rock.
Where and what?
- Hot and wet climates have more chemical weathering.
- Cold and wet climates have more physical weathering.
- Hot and dry climates have very little weathering.
- Cold and dry climates have little weathering.
