Understanding Weathering: Impact and Processes Demystified

1. Chapter 14 – Weathering and Erosion Section 1 – Weathering Processes Section 2 – Rates of Weathering Section 3 – Soil Section 4 - Erosion

2. Section 1: Weathering Processes • Weathering – the physical breakdown or chemical decomposition of rock materials exposed at the earth’s surface.

3. Mechanical Weathering • Strictly a physical process. It does not change the composition of the rock . • Agents of mechanical weathering are ice, plants and animals, gravity, running water, and wind. • Granite is often formed deep in the earth. As overlying rocks erode, the pressure on the granite lessens as the rock is exposed. Due to less pressure, the granite expands. Small cracks, called joints, develop. When the joints are parallel to the surface of the rock, curved sheets peel away in a process called exfoliation.

7. Ice wedging – a type of mechanical weathering that happens in cold climates. Water seeps into the cracks in the rock and then freezes. When the water freezes, the volume increases by 10% and creates pressure on the surrounding rock. Every time the ice melts and refreezes, cracks in the rock widen. • Abrasions – caused by gravity, running water and wind. It is the collision of rocks against rock that breaks the rocks over time. • Organic activity – As plants grow, the roots grow and expand to create pressure that wedges rock apart. Burrowing animals also cause weathering.

8. Ice Wedging

13. Chemical Weathering • The process by which rock is broken down because of chemical interactions with the environment. The chemical reactions occur with the minerals of the rock. The reactions occur between rock, water, carbon dioxide, oxygen, and acids. • Chemical weathering changes both the chemical composition and physical appearance of the rock.

14. Types of Chemical Weathering • Oxidation – the process by which elements combine with oxygen. It commonly occurs in rocks and soils that contain iron. The oxygen dissolved in the water forms red iron oxide. • Hydrolysis – water causes a change in composition of minerals. Often this change form clay. The water can carry the dissolved minerals to lower parts of the rock in a process called leaching.

15. Flashflood – notice the red rock walls

20. Carbonation – When carbon dioxide in the air dissolves in water a weak acid is formed called carbonic acid. When certain minerals come in contact with carbonic acid, they are changed into a new mineral called a carbonate. Carbonic acid reacts with calcite, a major component of limestone, and forms calcium bicarbonate. Calcium bicarbonate dissolves easily in water, and so limestone weathers away. • Organic Acids – these are produced naturally by certain living things. Lichens and mosses grow on rocks and produce weak acids that can weather the surface of the rock. The acids seep down into the rock.

24. Acid Precipitation - (Acid Rain) – natural rainwater is slightly acidic, but when it combines with nitrogen oxides and sulfur dioxides found in the air (due to burning fossil fuels), nitrous acid or sulfuric acid is formed in the rainwater. When this acid rain falls to the ground, it weathers some rocks faster than normal precipitation. • Many historical monuments and sculptures have been damaged by acid rain.

26. Section 2: Rates of Weathering • The processes of mechanical and chemical weathering are usually very slow. • Carbonization of limestone dissolves only .2 cm every 100 years. • The rate at which rock weathers depends on a number of factors – including rock composition, climate, and topography. • Differential weathering – the process by which softer, less weather-resistant rock wears away and leaves harder, more resistant rock behind.

28. Rock composition – Limestone and other sedimentary rocks that contain calcite are weathered most rapidly due to carbonation. • Amount of Exposure – The more exposure to weathering agents a rock receives, the faster the rock will weather. Fractures and joints in the rock also help it weather more quickly. One reason why, is that there is a greater surface area available and exposed to the elements. • Climate – Climates with alternating periods of hot and cold weather, have faster rates of weathering. The slowest rates of weathering are in hot, dry climates.

29. Topography or elevation – temperatures are generally cold at high elevations and ice wedging is more common at high elevations. On mountainsides, weathered rock fragments are pulled downhill by gravity and washed out by heavy rain. Due to all this, new surfaces of the mountain are continually exposed to weathering. • Human activities – mining, construction, recreational activities such as hiking or riding all-terrain vehicles. • Plant and Animal Activities – rocks are disturbed and broken by plants and animals. The biological wastes (guano) can cause chemical weathering.

30. Section 3: Soil • Regolith – the layer of weathered rock fragments that covers much of Earth’s surface. • Bedrock – the solid, unweathered rock that lies beneath the regolith. • Soil – complex mixture of minerals, water, gases, and the remains of dead animals – very fine particles.

31. y • Characteristics of soil – • Depends mainly on the parent rock from which it came. If it stays near the parent rock, it is called residual soil. If the soil has been carried away by water, wind, or glaciers, it is called transported soil. • Parent rocks rich in feldspar and aluminum produces soils that contain large amounts of clay. • Black soils are rich in organic material • Red soil often form from iron-rich parent rocks • Soil texture – Three main types • Clay diameter from .004 mm • Silt diameter from .004-.06 mm • Sand diameter from .06-2 mm

32. Soil Profile is a cross-section of the soil and its bedrock. The different layers of the soil are called horizons. • Residual soils have 3 horizons • Horizon A – topsoil. It is a mixture of organic materials and small rock particles. Almost all organisms live in Horizon A. When the organisms die, they decompose and become humus. • Horizon B – subsoil. Contains minerals leached from the topsoil, clay, and sometimes humus. • Horizon C – contain s partially-weathered bedrock

33. Section 4: Erosion • When rock weathers, the rock particles do not always stay near the parent rock. The process where particles move is called erosion. • Farming and ranching increases soil erosion. Ground cover is destroyed and increases erosion rate. • Soil erosion is considered one of the major environmental problems of the day by some scientists.

34. Types of erosion • Gullying and sheet erosion – Gullying is where water runs over a surface with a small furrow. As the water increases speed, the furrow increases into size to a gully. Sheet erosion is where the entire layer of topsoil is washed away. • Rockfalls and landslides – the fastest kinds of mass movement. Rockfalls are the fall of rock from a steep cliff. Landslides are when masses of loose rock begin to slide down a steep slope.

35. Mudflows and slumps – A mudflow is the rapid movement of a large amount of mud usually in a dry, mountainous region after there has been a sudden, heavy rainfall. A slump is a large block of soil and rock that becomes unstable and moves downhill in one piece. This often occurs after being saturated by water. • Solifluction – the movement of water-saturated soil over hard or frozen layers – usually topsoil. • Creep – an extremely slow downhill movement of weathered rock • Talus – rock fragments that accumulate at the base of a slope

36. Soil Conservation Methods • Contour plowing – soil is plowed in curved bands that follow the contour or shape of the land • Strip-Cropping – crops are planted in alternating bands, usually with one band being a cover crop like alfalfa or clover. • Terracing – constructing step-like ridges that follow the contours of a sloped field • Crop Rotation – farmers plant one type of crop one year and a cover crop the next year

37. The three major landforms that are shaped by weathering and erosion are mountains, plains, and plateaus. • Erosion of Mountains – During early stages of a mountain, the mountain undergoes uplift. Uplifted mountains have sharp peaks and deep, narrow valleys. Weathering and erosion wear down the rugged peaks to rounded peaks and gentle slopes. • Erosion of plains – A plain is a relatively flat landform near sea level. • Erosion of plateaus – A plateau is a flat, broad landform that has a high elevation. It has more erosion than a plain. In a dry climate, resistant rock forms flat-topped mesas. As they continue to erode, the become buttes. Both have steep sides and flat tops.

40. Mesa in Arizona

42. Buttes