Built by a geologist
Weathering, Erosion & Sediment
Weathering, Erosion & Sediment The moment a rock is exposed at Earth's surface, it begins to break down. This process — weathering — is the first step in creating sediment, soil, and eventually new sedimentary rocks. Weathering: Breaking Down Rock Physical Mechanical Weathering Rock breaks into smaller pieces without changing composition: - Frost wedging : Water freezes in cracks, expands by 9%, and splits rock apart - Thermal expansion : Repeated heating and cooling causes surface layers to flake off exfoliation - Root wedging : Plant roots grow into cracks and pry rock apart - Abrasion : Wind-blown sand or tumbling river rocks grind surfaces smooth - Unloading : Removal of overlying rock reduces pressure, causing expansion cracks Chemical Weathering Rock's minerals are chemically altered or dissolved: - Dissolution : Water dissolves soluble minerals limestone caves form this way - Oxidation : Iron-bearing minerals rust hematite staining = "iron hat" in exploration - Hydrolysis : Water reacts with silicate minerals to form clay feldspar → kaolinite - Carbonation : CO₂-rich water dissolves carbonate minerals Chemical weathering is faster in warm, wet climates. Physical weathering dominates in cold, dry environments. Erosion and Transport Once weathered, material must be moved. Erosion agents include: - Water rivers, waves, rain : The most powerful erosion agent globally - Wind : Important in deserts and coastlines - Ice glaciers : Extremely effective — can move house-sized boulders - Gravity mass wasting : Landslides, rockfalls, creep Deposition When transport energy decreases, sediment is deposited. The heaviest particles settle first gravel , then sand, then silt, then clay. This sorting creates the characteristic layers of sedimentary rocks. Soil: The Living Skin Soil is the product of weathering plus biological activity: - O horizon : Organic matter humus - A horizon : Topsoil — mixture of organic and mineral material - B horizon : Subsoil — accumulated clays and minerals - C horizon : Partially weathered parent rock - R horizon : Unweathered bedrock