Physical Geology 1st Year PDF

Physical Geology First Year – Faculty of Education – 2024 Dr. Mohamed Omran  Introduction  Our earth is made up rocks.  Rocks are composed minerals.  Our earth itself is changing.  Then, rocks are getting changed.  Weathering  Weathering is the physi...

Physical Geology First Year – Faculty of Education – 2024 Dr. Mohamed Omran  Introduction  Our earth is made up rocks.  Rocks are composed minerals.  Our earth itself is changing.  Then, rocks are getting changed.  Weathering  Weathering is the physical disintegration or chemical alteration of rocks at or near the Earth’s surface.  Erosion is the physical removal and transportation of weathered material by water, wind, ice, or gravity. Controlling Factors of Weathering  Composition and structure of rock  Nature of ground slope  Climatic variations  Floral effects Agents of Weathering  Wind  Glacier  Running water  Sea waves  under ground water Types of Weathering  Physical weathering  Chemical weathering  Biological weathering  Physical Weathering Also known as mechanical weathering. Mechanical (physical) weathering is the physical disintegration and reduction in the size of the rocks without changing their chemical composition.  Examples: exfoliation, frost wedging, salt wedging, temperature changes, and abrasion Physical weathering happens due to:  Changing of temperature  Freezing action of water  Roots growing plants which disintegrate rocks  Mechanical Weathering Mechanical weathering is the physical disintegration and reduction in the size of the rocks without changing their chemical composition.  Exfoliation  Frost Wedging  Salt Wedging  Temperature Changes  Abrasion 5  Mechanical Weathering: Frost Wedging  Frost wedging is a mechanical weathering process caused by the freezing action of water that is trapped between cracks in the rock.  When water freezes, it expands and applies pressure to the surrounding rock forcing the rock to accommodate the expansion of the ice.  This process gradually weakens, cracks, and breaks the rock through repetition. 7  Temperature Changes  Daily and seasonal temperature changes affect certain minerals and facilitates the mechanical weathering of bedrock.  Warmer temperatures may cause some minerals to expand, and cooler temperatures cause them to contract.  This expansion and contraction weakens the rock causing it to break apart into smaller fragments or to fracture.  Common in desert climates because they experience extreme fluctuations in daily temperature changes.  Not the dominant form of weathering, but instead tend to accelerate other forms of weathering already occurring. 8  Mechanical Weathering: Salt Wedging  Salt wedging occurs when salts crystallize out of solution as water evaporates. As the salt crystals grow, they apply pressure to the surrounding rock weakening it, until it eventually cracks and breaks down, enabling the salt crystal to continue growing.  Common in drier climates, such as deserts. Copyright © Michael 9 Collier  Mechanical Weathering: Abrasion  Abrasion occurs when rocks collide against each other while they are transported by water, ice, wind, or gravitational force.  During abrasion, rocks may also weather the bedrock surface they are coming into contact with as well as breaking into smaller particles and eventually individual grains. 10  Chemical Weathering  Decomposition and disintegration of rocks due to chemical reaction.  Water causes a change in the chemical composition of rocks.  Carbonation  Hydrolysis  Hydration  Oxidation  Solution Types of Chemical Weathering Solution: The dissolution of soluble particles and minerals from the rocks. Oxidation: The reaction of atmospheric oxygen to form oxides. Carbonation: The reaction of carbonate or bicarbonate ions with minerals. Hydration: The process of hydration is related to the addition of water to the minerals.  Chemical Weathering: Carbonation  Carbonation is a process by which carbon dioxide and rainwater or moisture in the surrounding environment chemically react to produce carbonic acid, a weak acid, that reacts with carbonate minerals in the rock.  This process simultaneously weakens the rock and removes the chemically weathered materials. 12  Chemical Weathering: Hydrolysis  Hydrolysis is a chemical reaction between H+ and OH- ions in water and the minerals in the rock. The H+ ions in the water react with the minerals to produce weak acids.  The reaction creates new compounds which tend to be softer and weaker than the original parent rock material. 14  Chemical Weathering: Hydration  Hydration is a process where mineral structure in the rock forms a weak bond with H20 which causes the mineral grains to expand, creating stress which causes the disintegration of the rock.  Produces a new mineral compound that is larger than the original compound. The increased size expanse the rock and can lead to decay. 15  Chemical Weathering: Hydration  Hydration can also lead to color changes in the weathered rock surface.  An example of hydrolosis: Anhydrite (CaSO4) can absorb two water molecules to become gypsum (CaSO4·2H2O). 15  Chemical Weathering: Hydration Hydrolysis Hydration Hydrolysis is an example of a decomposition reaction. This is because Hydration is the chemical process in water is one of the reactants in this which a water molecule is attached to reaction and water is known to break the another substance. chemical bond in the other reactants. Bond cleavage takes place in an organic Bond cleavage takes place in the process compound, but no cleavage takes place in of hydrolysis. inorganic compounds. The formation of saturated compounds The formation of hydrated compounds occurs from unsaturated compounds. occurs from dehydrated compounds 15  Chemical Weathering: Oxidation  Oxidation occurs when oxygen and water react with iron- rich minerals and weaken the structure of the mineral.  During oxidation the minerals in the rock will change colors, taking on a ‘rusty’, reddish-orange appearance. 16  Chemical Weathering: Solution  Solution occurs when minerals in rock dissolve directly into water.  Solution most commonly occurs on rocks containing carbonates such as limestone, but may also affect rocks with large amount of halite. 17  Biological Weathering  The action of plants and animals leads to breaking of rocks.  Roots cause disintegration of rocks.  The roots of the trees penetrate into the cracks of the rocks. Types of Biological Weathering  Burrowing animals  Quarrying  By human excreta  Biological Weathering The disintegration or decay of rocks and minerals caused by chemical or physical agents of organisms.  Organic activity from algae  Rock disintegration by plant growth  Burrowing and tunneling organisms  Secretion of acids  Organism Activity  Some animals may burrow or tunnel into rocks or cracks in rocks and cause the rock to break down and disintegrate. Small animals, worms, and other insects, often contribute to this form of biological weathering.  Some organisms, such as snails attach themselves to rocks and secrete acid, that chemically dissolve the rock surface. 19  Plant Roots The most common form of biological weathering is when plant roots penetrate into cracks of rocks and cause the rock to split or break into smaller particles through mechanical weathering. 20  Lichen, Algae, and Decaying Plants  This bio-chemical weathering process leaches minerals from the rock causing it to weaken and breakdown.  The decaying of plant materials can also produce acidic compounds which dissolve the exposed rock.  The presence of organisms growing or expanding also exerts a small amount of abrasion and pressure that cause the mechanical weathering of the rock as the organisms extract various minerals. 21  Differential Weathering  Weathering rates will not only vary depending on the type of weathering process, whether it is mechanical, chemical, or biological, but they will also vary depending on the rock material.  Some rocks are harder than other rocks, and will weather slower than softer rocks.  The differences in rates of weathering due to different types of rocks and textures is referred to as differential weathering.  Climate can also produce differential weathering responses for the same rock type. For example, limestone weathers more quickly in wet climates than dry climates. 22