Chapter 8 Weathering is the breaking down of

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

Chapter 8: Weathering 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

Chapter 8: Weathering Four types of mechanical weathering I. Freeze-thaw action II. Exfoliation III. Pressure release (unloading) IV. Salt crystallisation

Chapter 8: Weathering I. 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

Chapter 8: Weathering 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

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

Chapter 8: Weathering Other types of chemical weathering I. Hydrolysis: chemical reaction between minerals in the rock and hydrogen in rain water II. Oxidation: oxygen combines with water and minerals in the rock, e. g. calcium and magnesium III. Hydration: minerals in the rock absorb water and expand creating stress

Chapter 8: Weathering 3. Biological weathering • The agents of biological weathering are plants and animals • Biological weathering occurs when tree roots or animals widen spaces within the rock/soil • Humans can weather rocks, for example, by quarrying, i. e. extracting rock from the Earth’s surface

Chapter 8: Weathering in the Wicklow Mountains case study • 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

Chapter 8: 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 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

Chapter 8: 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