The main cause of weathering The different types of weathering How to identify landforms formed as a result of one or more types of weathering 8 8 Weathering

Chapter 8: Weathering Weathering is the breaking down of rocks that are exposed on the Earth’s surface. The causes of weathering include:  Water  Changing temperatures  Animals  Plant life

Three types of weathering:  Mechanical  Chemical  Biological Chapter 8: Weathering

1.Mechanical weathering  Mechanical weathering occurs when rocks are broken into smaller pieces  Also known as physical weathering

Four types of mechanical weathering: i.Freeze-thaw action ii.Exfoliation iii.Pressure release (unloading) iv.Salt crystallisation Chapter 8: Weathering

1. Freeze-thaw action  Temperatures rise above and fall below freezing (0°C)  Plentiful supply of water  Occurs more often at higher altitudes  Day: water seeps into cracks or fissures in rocks  Night: water freezes and expands straining surrounding rock  Rocks crumble and break down  Also known as frost action

II. Exfoliation  Also known as onion-peeling  Large diurnal temperature range (two extremes of temps in a day)  Rock expands during the heat of the day and contracts at night  Exerts pressure on the rock and the outer layers of the rock shatter Chapter 8: Weathering

III. Pressure release  Also known as unloading  Denudation removes the rock material  Underlying rock is exposed and pressure is released  Outer layers of the rock expand which creates stresses on the rock  Leads to fracturing parallel to the rock surface  Sheets of rock break away from the exposed rocks along fractures Chapter 8: Weathering

IV. Salt crystallisation  Associated with arid climates  High temperatures cause strong evaporation  Causes the breaking down of rocks when a salt solution (i.e. water containing salt) seeps into cracks  Evaporate leaving salt crystals behind  Salt crystals expand when they are heated up, thus exerting pressure on the rock which leads to its disintegration Chapter 8: Weathering

2. Chemical weathering  Composition of rocks change  Break down of rock  Occurs in regions where the climate is warm and wet

Chapter 8: Weathering Carbonation  Caused by rainwater  Absorbed carbon dioxide forming weak carbonic acid  Dissolves certain minerals in rocks  e.g. south-west China (Guilin) karst region

Other types of chemical weathering i.Hydrolysis: chemical reaction between minerals in the rock and hydrogen in rain water The hydrogen in the rainwater reacts with minerals in the rock causing the rock to break down. Rocks like granite absorb and react with water turning the rock into clay. Chapter 8: Weathering

Other types of chemical weathering i.Oxidation: oxygen combines with water and minerals in the rock, e.g. calcium and magnesium Rocks like red sandstone have a high amount of iron oxide. Oxygen in rainwater reacts with iron oxide to ‘rust’ the rock. Chapter 8: Weathering

Other types of chemical weathering i.Hydration: minerals in the rock absorb water and expand creating stress Chapter 8: Weathering

3. Biological weathering  The main agents of biological weathering are:  Plants  Animals  Humans

Chapter 8: Weathering Weathering in the Wicklow Mountains case stud y  Granite is the main rock type  Granite is subject to both physical (mechanical) and chemical weathering  An igneous rock composed of mineral such as quartz, feldspar and mica  Granite weathers – feldspar undergoes hydrolysis and forms kaolin  Quartz remains as residual mineral  Weathered rock becomes an ingredient of soil

Weathering in the Wicklow Mountains case study (continued)  Quartz will become eroded and its sediment is transported to the sea, along with clays  Move downslope under the influence of gravity  Wicklow Mountains are rounded and have gradual slopes  Tors have formed on the Wicklow Mountains  Wicklow Mountains had shale and sandstone  These rocks were then eroded over time, releasing downward pressure on the underlying granite Chapter 8: Weathering

Weathering in the Wicklow Mountains case study (continued)  Granite was then able to expand upwards causing joints to appear in the rock  Joints appeared at points of weakness in the granite rock  Susceptible to hydrolysis, and the feldspar in the granite was converted to kaolin clay, as a result of chemical weathering  Freeze-thaw action continued to attack the joints  Under the extremes of diurnal temperature changes, the joints were further enlarged

Weathering in the Wicklow Mountains case study (continued)  Weathered regolith moved downslope under the force of gravity  Tors left behind upslope ‒ blocks of more resistant rock, often roughly rectangular in shape  Examples of tors are found on Dartmoor, and Bodmin Moor, in south-west England Chapter 8: Weathering