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Weathering & Soil Formation

Weathering & Soil Formation. Rocks exposed at the Earth’s surface are constantly being destroyed and transported elsewhere. These are the processes of weathering and erosion. The Great Falls of the Potomac River. Mechanical Weathering.

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Weathering & Soil Formation

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  1. Weathering & Soil Formation

  2. Rocks exposed at the Earth’s surface are constantly being destroyed and transported elsewhere. These are the processes of weathering and erosion. The Great Falls of the Potomac River

  3. Mechanical Weathering Mechanical weathering is when rocks break down as the result of physical phenomena such as ice, wind, water, gravity, plants and animals.

  4. The grinding and wearing away of rock surfaces by other rock or sand particles. Abrasion Wind weathering Water weathering Gravity weathering

  5. Wind blows sand and silt against exposed rock wearing away the surface. Water causes rocks and pebbles to roll along the bottom of swiftly flowing rivers, bumping them into other rock. Gravity causes rocks to fall on to each other. Abrasion by Wind, Water, and Gravity

  6. As water trapped within cracks freezes, it expands, pushing against the sides of the crack forcing it to open wider Ice Wedging (Freeze-Thaw)

  7. Roots can work their way into cracks and as the roots grow they may cause the cracks to grow and multiply Root Wedging Root wedging

  8. Both ice and root wedging can happen at the same time and in the same location. Ice + Root Wedging

  9. Animals burrow through the soil and move the soil. The mixing and digging that the animals do also contributes to chemical weathering. Animals

  10. Chemical Weathering Chemical weathering is the decomposition of rocks and minerals as a result of chemical and biochemical reactions that form new products. Lichen (fungi and algae) produce acids that can weather rocks

  11. Types of Chemical Weathering Hydrolysis: chemical reaction in which water is involved Carbonation: chemical reaction in which Carbon Dioxide is involved

  12. Oxidation Oxidation is a chemical reaction where an element combines with oxygen to form an oxide. Example: iron + oxygen = iron oxide (rust) When rocks weather, the iron is oxidized, producing red staining from minerals

  13. Precipitation that contains more acid than normal (due to air pollution); pH < 5.6 Acid Precipitation Picture: Weathering due to mildly acidic rainfall (hydrolysis) over the past century

  14. Caves Acidic groundwater (carbonation & hydrolysis) can dissolve limestone forming gigantic caverns

  15. SinkholesAlso form from acidic groundwater percolating through the soil.

  16. Different rocks weather at different rates. • Factors that control the rate of weathering: • Climate • Elevation • Type of rock Rates of Weathering

  17. Softer, less resistant rocks weather faster than hard rocks made of more stable minerals. Differential Weathering Devils Tower is a volcanic neck. Why is this part the last to weather away?

  18. Breaking rocks up (physical weathering) increases the ratio of surface area to volume. More surface area = Faster weathering Surface Area / Volume Twice as much surface area

  19. Climate and Weathering • Climate (combination of temperature and rainfall) strongly influences weathering processes: • Mechanical weatheringdominates in low rainfall, cold climate • Chemical weatheringdominates in high rainfall, warm climate

  20. Rocks at high elevations (mountains) are exposed to more wind, rain and ice than rocks at low elevations. Steep slopes add to the rapid weathering and removal of material. Weathering and Elevation

  21. This obelisk remained in Egypt for almost 3500 years in excellent condition. After only 75 years in NYC’s Central Park, some of the hieroglyphics carved into the obelisk were nearly weathered away…Why? Cleopatra’s Needle

  22. Soil

  23. Soil is: • Mineral material (from rocks) • Organic Material • Water • Air • Soils containing 20-30% humus are excellent for growing Soil Composition (% will vary)

  24. Parent Rock – source of mineral fragments in the soil Bedrock – layer of rock beneath soil Parent Rock & Bedrock

  25. Residual Soil – Soil that remains above its parent rock (it has NOT moved) Transported Soil – Soil blown or washed away from parent rock (it HAS moved) Residual Soil & Transported Soil

  26. Soil Texture • Soil Texture – the soil quality that is based on the proportions of soil particles.

  27. Soil Structure – the arrangement of soil particles. Soil Structure

  28. Soil naturally forms layers known as horizons Soil Horizons

  29. Leaching occurs as rain water dissolves minerals and nutrients as it travels through top soil. Leaching

  30. Soils can be acidic or basic. The pH scale ranges from 0 – 14 7 is neutral pH below 7 is acidic pH above 7 is basic Soil pH

  31. Climate and Parent Rock Climate and parent rock are the most important factors in soil formation and composition.

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