Weathering

Weathering

Learning outcomes:

1. Define weathering.
2. Explain the difference between physical and chemical weathering.
3. Name examples of physical and chemical weathering and explain how they work.
4. 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.