Ms. Carr 5 th Year Geography

Understand the main causes of Weathering. Identify the different types of weathering. Recognise a landform created as a result of one type of weathering.

Weathering

Weathering is the breaking down of the rocks exposed on the earth’s surface. Causes of Weathering Water Fluctuating Temperatures Animals Plants

There are three main types of weathering: Mechanical / Physical Weathering is the break down of rocks into smaller pieces, such as Freeze-thaw action, Exfoliation, Pressure Release and Crystallisation of salts. Chemical Weathering is when a chemical reaction occurs in the rock causing it to decompose, such as Carbonation, Hydration, Oxidation and Hydrolysis. Biological Weathering is the break down of rocks due to the physical and/or chemical actions of living things, such as Plants, Animals and Humans.

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

1. Freeze-thaw action  Frequent temperature change  Temperatures rise above and fall below freezing (0°C)  Plentiful supply of water  Most pronounced 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  Scree, e.g. Croagh Patrick, Co. Mayo  Also known as frost action

Freeze-thaw Action

II. Exfoliation  Also known as onion-peeling  Large diurnal temperature range  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  e.g. the Kissing Camels, Garden of the Gods, Colorado, USA

Onion Weathering/Exfoliation

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

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

Crystallisation of Salts

 Composition of rocks change  Break down of rock caused by water and acid  Occurs in regions where the climate is warm and wet

Carbonation Hydrolysis Oxidation Hydration

Carbonation occurs when: Carbon dioxide is absorbed by the rainwater to form weak carbonic acid. This weak acid dissolves rock, especially Limestone. Rain + Carbon Dioxide = Weak Carbonic Acid Weak Carbonic Acid + Limestone = Calcium Bicarbonate Eg; The Burren, Co Clare

Carbonation

Hydrolysis involves the action of the hydrogen in the rainwater on rock- forming minerals, such as Feldspars. Feldspar is a mineral in granite which is converted into Kaolin by hydrolsis. Kaolin is used as pottery clay. In granite, the other minerals are resistant to hydrolysis and therefore fall out of the rock. It is more effective in hot conditions and therefore is more common in hot, wet regions.

Hydrolysis

Oxidation occurs when a mineral in the rock chemically combines with oxygen or air. Rocks which contain iron are easily oxidised. Iron oxides in rocks and appears as reddish or orange-yellow streaks leaching from the rock.

Oxidisation

Hydration is when the rock minerals combine with water. As the minerals hydrate they expand. This expansion puts pressure on the rock and causes it to shatter. This process is extremely similar to the process of crystallisation of salts but needs water to take place and is therefore a chemical process.

Hydration

The action of plants, animals and man can speed up both mechanical and chemical weathering. This can include burrowing animals, the growth of plant roots, quarrying and deforestation.

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

 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

 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

 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 also found on Dartmoor, and Bodmin Moor, in south-west England  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 also found on Dartmoor, and Bodmin Moor, in south-west England

Mechanical/Physical Weathering- Onion Weathering

Chemical Weathering - Carbonation

Mechanical/Physical Weathering- Freeze-thaw Action.

Exfoliation Carbonation Hydration Oxidation Hydrolysis

Choose one example of each type of weathering process and explain how it occurs. (30m) Weathering is the process where rock and soil is broken up and can be caused by; Freeze-thaw action Carbonation Describe and explain any one of these weathering processes. (30m)