Physical Geology Geo 303 Lecture Notes PDF

# Physical Geology Geo 303 ## Instructor Dr. Fares Khedr Faculty of Science Geology Department ## Geological Process * **Epigene process:** * Air, wind, water, glacier and organic activity. * Examples: 1. Denudation (weathering, erosion, transportation) 2. Sedimentation (m...

# Physical Geology Geo 303 ## Instructor Dr. Fares Khedr Faculty of Science Geology Department ## Geological Process * **Epigene process:** * Air, wind, water, glacier and organic activity. * Examples: 1. Denudation (weathering, erosion, transportation) 2. Sedimentation (mechanical, chemical, organic). * **Hypogene process:** * Examples: 1. Earth movements (earthquakes, mountains building). 2. Magmatic activity (volcanic eruption, lava flow). 3. Metamorphism (igneous and sedimentary rocks). # Weathering ## Definition Weathering is the decomposition and disintegration of rocks and minerals at the earth's surface. This material accumulates where it forms and overlies unweathered bedrock. ## Processes These four processes (weathering, erosion, transportation, and deposition) work together to modify the earth's surface: 1. **Weathering:** Fragments are loosened from exposed rocks. 2. **Erosion:** Weathered fragments are removed by rain, streams, and other forces. 3. **Transportation:** Eroded particles are carried downstream. 4. **Deposition:** Transported particles accumulate on a delta. ## Types * **Mechanical weathering:** * Reduces solid rock to rubble. * Does not alter the chemical composition of rocks and minerals. * Five major processes: 1. Pressure-release fracturing: The pressure from the weight of overlying rocks increases with depth. When the overlying rock erodes away, the pressure on the buried rock decreases (unloading pressure) causing the rock to expand and fracture. For example, cracks in granite cliffs are due to pressure-release fracturing. 2. Frost wedging: Freezing water in a crack expands, pushing the rock apart. In temperate climates, water freezes at night and thaws during the day (freeze-thaw cycle). Ice cements the rock fragments and when it melts, the fragments tumble from a steep cliff. Talus slopes or cones are large piles of loose angular rocks that lie beneath many cliffs as a result of frost wedging. 3. Abrasion: The mechanical wearing and grinding of rock surfaces by friction and impact. Many rocks along streams or beaches are rounded and smooth due to collisions with rocks (that tumbled downstream), silt, and sand carried by moving water. Wind hurls sand and other small particles against rocks, often sandblasting unusual and beautiful landforms. Glaciers cause much abrasion as they drag particles ranging in size from clay to boulders across bedrock. In this case, both the rock fragments embedded in the ice and the bedrock are abraded. 4. Thermal expansion and contraction: Rock at the earth's surface is exposed to daily and yearly of heating and cooling. They expand when heated and contract when cooled (daily heating-cooling cycles). When the temperature changes rapidly, the surface of a rock heats or cools faster than the interior. This causes the surface to expand or contract faster than the interior and results in forces that fracture the rock. Campfire, forest fires, and brush fires break rocks and represent important agents of mechanical weathering. 5. **Organic activity:** The roots of plants work their way into cracks, expand, and push the rock apart. City dwellers often see the results of organic activity in sidewalks where the roots push from underneath. This raises the concrete and frequently cracks it. * **Chemical weathering:** * **Occurs when air and water chemically react with rocks:** The important processes of chemical weathering are dissolution, hydrolysis, and oxidation. Water, carbon dioxide, acids & bases, and oxygen are common substances that cause these processes to decompose rocks. * **Dissolution:** Halite crystals (rock salt) dissolve rapidly in water and the ions disperse to form a solution, so it is rare in moist environments. (Minerals dissolve with no chemical reaction with the solution) - For example, HCl (hydrochloric acid) dissociates to release H+ and Cl- ions, NaOH to Na+ & OH- and H2O to H+ & OH-. The solution becomes acidic when the concentration of H+ ions increases and the solution becomes basic when the hydroxyl ion (OH-) increases. H+ and OH- ions are chemically reactive and therefore acids & bases are much more corrosive than pure water. - If the bonds between an atom and the crystal are stronger than the attraction of the atom to its outside environment, then the crystal remains intact. If outside attractions are stronger, they pull the atom away from the crystal and the mineral dissolves. - Halite dissolves in water because the attractions between the water molecules and the sodium and chloride ions are greater than the strength of the chemical bonds in the crystal. - Calcite, the mineral that comprises limestone and marble, weathers in natural environments in a three-step process. In the first two steps, water reacts with carbon dioxide in the air to produce carbonic acid, which dissociates to release hydrogen ions. $CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$ (Carbon dioxide + Water --> Carbonic Acid --> Hydrogen ion + Bicarbonate ion) - In the third step, calcite dissolves in the carbonic acid solution. $CaCO_3 + H^+ \rightarrow Ca^{2+} + HCO_3^-$ (Calcite + Hydrogen ion --> Calcium Ion +Bicarbonate Ion) - Acid rain is formed as sulfur reacts with oxygen to form sulfur dioxide. Sulfur dioxide then reacts with water and oxygen to form sulfuric acid. $S + O_2 \rightarrow SO_2$ (Sulfur + Oxygen --> Sulfur Dioxide) . $2 SO_2 + 2 H_2O + O_2 \rightarrow 2 H_2SO_4$ (Sulfur Dioxide + Water +Oxygen --> Sulfuric Acid) - Flowing water carries the dissolved ions away from the site of weathering and produces caverns in limestone (Stalactites & Stalagmites). - Most solution reactions are reversible and can proceed in either direction of conditions change. - Calcite dissolves readily in acid to form a solution. If a base is added to the solution, solid calcite precipitates again. * **Hydrolysis:** During hydrolysis, water reacts with a mineral to form a new mineral with the water incorporated into its crystal structure. - For example, feldspar is hydrolyzed to clay. $2KAlSi_3O_8 + 2H^+ + H_2O \rightarrow Al_2Si_2O_5(OH)_4 + 2K^+ + 4SiO_2$ (Orthoclase feldspar + Hydrogen ion + Water --> Clay mineral + Potassium ion +Silica). * **Oxidation:** Oxidation is properly defined as the loss of electrons from a compound or elements during a chemical reaction. In the weathering of common minerals, this usually occurs when the mineral reacts with molecular oxygen. - Pure metals are rare in the earth's crust, and most metallic elements exist in nature as compounds. - Few metals, such as gold, silver, copper, and platinum commonly occur in their pure states. - Iron is abundant in many minerals (olivine, pyroxene & amphibole), and when oxidizes, the mineral decomposes. - Many metallic elements (iron, copper, lead & zinc) occur as sulfide minerals in ore deposits, and when oxidize, the sulfur reacts to form sulfuric acid, a strong acid. For example, pyrite (FeS2) oxidizes to form sulfuric acid and iron oxide. - Many natural ore deposits generate sulfuric acid when they weather. ## Chemical and Mechanical Weathering Operating Together * Mechanical processes crack rocks, thereby exposing more surface area for chemical agents (chemical weathering) to work on. - More surface area is available for a chemical attack on the corners and edges of a cube than on a face. - Therefore, corners and edges are rounded during weathering (Spheroidal Weathering) - Both mechanical and chemical processes have weathered this boulder along old fractures. - The outer layer continues to peel away, like layers of an onion. * **Salt cracking:** The growing crystal of dissolved salt exerts tremendous forces, enough to widen a crack and fracture a rock, a process called salt cracking. - Salt cracking is a mechanical process (thermal expansion or pressure release) followed by chemical weathering (dissolution). Finally, the expanding salt crystals mechanically push the rock apart. - Salt cracking forms pits & depressions in sea cliffs. * **Exfoliation:** A process in which large plates (few centimeters to a few meters thick) or shells split away like the layers of an onion. - Exfoliation is a mechanical process (pressure release fracturing) followed by chemical weathering (hydrolysis) forming clay (from feldspars), and the mechanical expansion of the clay contributes to the exfoliation fractures. # Questions?