Weathering, Soil, and Erosion

1. Weathering, Soil, and Erosion Chapter 5

2. Weathering • Def: the breakup of rock due to exposure to processes that occur at Earth’s surface • 2 types: • Mechanical- the breakdown of rock that takes place when a rock is broken into smaller pieces of the same material without changing its composition • Chemical- the breakdown or decomposition of rock that takes place when minerals are changed into different substances

3. Mechanical Weathering • Processes • Frost wedging • Def: a process in which water freezes in the cracks of a rock and wedges it apart • Mostly in porous rocks and in rocks with many cracks • In places where there are frequent freezes and thaws • Ice Heaving- creates potholes; ice lifts pavement up and it collapses when it thaws

4. Examples of Frost Wedging and Ice Heaving

5. Mechanical Weathering cont… • Abrasion • Def: the wearing away of rock material by grinding action • Water, wind and ice are capable of moving rocks. • Sand is a product of abrasion.

6. Mechanical Weathering cont… • Plants and Animals • Growth contributes • Roots wedged into tiny pores and crevices • Burrowing contributes • Digging holes into soil • Bringing rock fragments to surface

7. Mechanical Weathering cont… • Upward Expansion • Exfoliation- the peeling of surface layers from exposed bedrock • Rock is uplifted by tectonic forces (ex: granite) • Overlying rock is worn away which reduces pressure • Upward expansion causes the granite to break along curved joints • Large sheets of loosened rock break away

8. Upward Expansion Examples

9. Chemical Weathering • Water • Hydrolysis- the chemical weathering by reaction of water with other substances • Acids: seeps into the ground and reacts chemically with many common minerals • Calcite • Dissolves completely • Underground caverns

10. Chemical Weathering cont… • Acid Rain • Def: rainwater that contains unusually high amounts of acids that can be traced back to pollutants • Increases the rate of chemical weathering

11. Chemical Weathering cont… • Oxidation • Def: chemical reaction of oxygen with other substances • Effective with minerals containing iron • Formation of rust, or iron oxides

12. Rates of Weathering • Weathering is usually a slow process • Factors that affect the rate of weathering: • Surface Exposure • More surface exposed, weathers more quickly • Composition of Rock • Various rocks are affected differently by weathering processes • Climate • Warm/wet climates: both chemical and mechanical • Cold/dry climates: mainly mechanical

13. Soil • Def: loose, weathered rock and organic material in which plants with roots can grow • Formed by weathering • Contains air, water, organic material, mineral matter • Parent material • Def: material from which a soil is formed • Residual soil- soil whose parent material is the bedrock below • Transported soil- soils formed from transported materials • Deposits left by winds, rivers, and glaciers

14. Soil Profile • Def: cross section of earth exposed by the digging • Soil horizons- three distinct zones of mature soil • A-horizon- topsoil • Darker, contains humus (organic material), sandy • B-horizon- subsoil • Red or brown, iron oxides, clay from topsoil, calcium and magnesium • C-horizon • Weathered parent material, rock fragments

16. Soil Composition • 3 noticeable parts: sand, silt, and clay • Affects soil’s ability to hold water and air • Factors that affect composition: • Time • Parent material • Plants and animals • Topography • Climate- MOST IMPORTANT!

18. 12.3 Mass Movements and Erosion • Mass movements- the downward transportation of weathered materials • Gravity causes materials to fall, slide, or move at slow speeds to lower levels • Erosion- the removal and transport of materials by natural agents such as wind and running water

19. Mass Movements • Talus- rock fragments that have been weathered from a cliff and pulled down by gravity • Landslide- movement of a mass of bedrock or loose soil and rock down the slope of a hill, mountain, or cliff • Steep slopes • Regions near volcanoes and in earthquake-prone regions

20. Mass Movements • Creep- slow, imperceptible movement of soil down a slope • Causes fence posts, poles and other objects fixed in soil to lean downhill • Slump- a block of land tilts and moves downhill along a surface that curves into the slope • Tends to occur because bottom of slope can no longer support top of slope

21. Mass Movements • Earthflows- the downslope movement of a mass of earth materials that have been saturated with water • Slower and less fluid than a mudflow; velocity affected by amount of water present, the composition of the soil, and the steepness of the slope • Mudflows- the downslope movement of water that contains large amounts of suspended clay and silt • Rapid movement; capable of moving rocks, boulders, trees, and houses; lahars- volcanic mudflows

22. Erosion • Topography- uplifting and weathering/erosion are happening at same time • More uplifting- rugged and sharp (Himalayas) • More erosion- smooth and rounded (Appalachians) • Climate- in humid areas, water is primary agent and causes rounded topography • Composition of rock- some types are more resistant than other rock types • Volcanic neck- the solidified lava filling the center vent of an extinct volcano (picture pg 270)