1. Ch. 5- Weathering and Erosion
Mrs. Giblin
Earth Science

2. Weathering – “breaking of rocks”.
2 Types of Weathering:
Mechanical Weathering – physical forces break rocks without changing its chemical composition.
Causes:
1. Frost Wedging- When water gets into a crack and freezes.
-Freezing causes water to expand by 9%!
-Causes rocks to split apart.

3. Unloading – The pressure over an igneous rock formation gets released.
exfoliation – outer layers of rocks pop off.
Biological Activity –
Tree roots growing into rocks.
Burrowing animals.
Humans-driving/walking over surfaces.

4. Chemical Weathering – Transformation of rock into 1 or more compounds.
Water – water absorbs gases (oxygen, CO2) and creates “chemicals”.
-CO2 + H2O = Carbonic acid (acid rain)
-spheroidal weathering – water running over rocks wears them into spheres.

5. Rates of Weathering: Rock Characteristics: Climate:
Mineral composition
Cracks
Climate:
Temperature
Moisture
Causes frost wedging!
MORE chemical weathering occurs when the climate is warm and moist!

6. Regolith- layer of rock and mineral fragments
Regolith- layer of rock and mineral fragments. Soil- Part of the regolith that supports plant growth.
4 Components of Soil:
-mineral matter – broken down rocks.
-organic matter – “humus” – decaying plants and animals.
-water
-air } Both are found in pore spaces between particles.
Soil Texture – Proportions of different particle sizes.
-The USGS has categorized soils by texture.
Soil Structure – How easily soils can be cultivated and eroded.
-How easily water & nutrients move through the soil.

7. Soil Formation: Parent Material – source of mineral matter.
Soil formed on bedrock-residual soil.
Soil formed on unconsolidated sediments – transported soil.
Transported soil forms quickly.
Regolith – layer of rock & mineral fragments.
Time – the longer the soil has been forming, the thicker it becomes.
Climate – greatest factor of formation.
Variations in temperature and precipitation affect soil type.
Affects what plants and organisms live there.
Organisms – Add to organic matter in the soil.
Decompose dead material.
Bacteria release nitrogen.
Burrowing organisms mix soil.
Slope – Steep slopes have more erosion and less water content.
Flat areas have less erosion and less drainage.
Direction of slope-facing sun?

8. C Horizon – partially weathered parent material.
Horizons: Layers of soil classified by differences in composition, texture, structure, and color. Soil Profile: A vertical section of the horizons.
A Horizon – topsoil
Made mostly of organic matter.
Full of insects, fungi, and microorganisms.
B Horizon – subsoil
Contains clay washed through the A horizon by water.
Clay that accumulates forms “hardpan”.
Lower limit for plant roots and burrowing animals.
C Horizon – partially weathered parent material.

9. Soil Types:
Pedalfer – forms in temperate areas that receive > 63 cm. of rain/year.
Usually brown-red in color.
Pedocals – forms in drier areas with grasses and brush.
Usually light grey-brown.
Laterite – forms in hot, wet tropical areas.
Usually orange-red.

10. Soil Erosion: Water, wind, other agents.
Sheet erosion – thin sheets of H2O move the particles.
Human activities – farming, construction, and logging.
Soil Deposition:
Sediments deposited in rivers may block entrances for ships.
Contamination of sediments causes contamination of streams.
Controlling Erosion:
Rows of trees – windbreaks
Terracing hillsides
Rotating crops
Cover crops

11. Mass Movements: Transfer of rock and soil down-slope due to gravity.
Triggers of Mass Movements:
Water – Soil becomes saturated causing the particles to slide past each other easily.
Oversteepened slope – when the angle of a slope is too great to keep the soil stable.
Caused by construction of roads or streams undercutting valley wall.
Removal of Vegetation – Roots help to hold soil together.
Earthquakes – Aftershocks can cause large amounts of soil to dislodge.

12. Types of Mass Movements:
Rockfalls – rocks fall freely through the air.
Slides – Block of material moves suddenly along a flat, inclined surface.
Rock slide – (includes pieces of bedrock)
Land slide
Slumps – movement of material along a curved surface.
Flows – mass movements containing a lot of H2O.
Mudflows
Earthflows
Creep – movement of only a few mm or cm per year. Usually caused by freezing and thawing.