3-1    External process:

• A process that occurs at the Earth's surface.

• Weathering: disintegration, break down and decompose of rocks.

• Mass wasting: transfer of rock material under gravitational force.

• Erosion: wear away of rock mechanically and chemically.

3-2    Mechanical weathering or physical weathering:

due to changes in T, P, loading, and biological activity.

1. Frost wedging: involving freeze-thaw cycle. When water freeze, its volume increases, which pry the fracture larger => frost wedging.

2. Talus slope: rock pile deposited in front of unbroken rocks. Fig 3.3

3. Unloading: Exfoliation: 

4. Thermal expansion at days and contraction at night.

5. Results: weathering =>larger pieces breakdown into small debris. Change in shape and size, but mot composition.

3-3    Chemical weathering:

Involve in dissolution and precipitation. Thus, chemical composition changes, and mineral composition changes, too. Therefore, original rocks decompose to substances that are suitable for surface environment, e.g. feldspar + acid + water => clay minerals + K⁺.

3-3-1    Agent of chemical weathering:

• Water: very good solvent, able to dissolve a significant numbers of compounds.

• Carbonic acid: CO₂ has solubility, when dissolved in water, carbonic acid forms. Most substances have higher solubility under acidic environment.

3-3-2    What decomposed? What formed?

• Most minerals in igneous and metamorphic rocks are unstable under surface condition.

• For dark minerals, e.g. olivine, pyroxene, and amphibole, Fe²⁺ amd Mg²⁺ are released into solution.

• For lighter minerals: K⁺, Na⁺, Ca²⁺, or even SO₂ go into solution.

• Clay minerals and oxides are stable, thus, they will be formed.

3-3-3    Rate of weathering:

Depending on mineral composition, climate, and texture of rocks.

• Mineral composition: for silicates, weathering follow the same order as with crystallization.

• Climate: In humid climate, chemical weathering predominate. Why? Water humic acid, temperature. In dry weather, physical weathering prevail.

3-4    Soil:

3-4-1    What is soil?

Combination of minerals and organic matter, water and air. Fig 3.11 show the percentage of each constituent.

3-4-2    Soil texture:

represented by the portions of particles of different sizes. Fig 3.12 show three basic components: sand, silt, and clay.

3-4-3    Controls of soil formation:

• Parent material:

• Residual soil vs. transported soil. The forma is formed on top of bedrock while the latter experiences transport by water, wind, etc.

• For residual soil, its composition is a reflection of the bedrock.

• Regolith: Layer of rocks and mineral fragments due to weathering, no organic matter.

• Time: soil develops with time.

• Climate: affect the mineral type in soil.

• Plants and animals: contribute to organic matter of soil.

• Slope: soil develops well at flat slope. Poor drainage => dark soil that is rich in organic matter.

3-4-4    Soil profile:

Vertical sections of soil, can be classified as horizons. Fig 3.15. From top to bottom:

Mature soil vs. immature soil: The former has well developed zones or horizons.

3-4-5    Soil type:

Based on climate, vegetation, mineral or chemical composition, and color.

• Pedalfer: rich in Al and Fe, in B horizon, brown color. In humid area where extensive precipitation leached out most Ca, Na, and K. Only Fe and Al are left as residues due to their low solubility.

• Pedocal: rich in CaCo3, whitish color, in arid region. 

• Laterite: Only Al and Fe left. In tropical weather, red color.

3-4-6    Soil Erosion:

Due to running water. Different names are given based on the degree of erosion.

• Sheet erosion: surface water carrying away soil particles.

• Gullies formed when vertical cuts predominant, e.g. Fig 3.17.

Erosion rate: controlled by rate of running water and soil vegetation. When rate of erosion > the rate of formation, soil loses its productivity.

3-5    Mass wasting:

refers to downslope movement of rocks, regolith and soil under gravitational force.

3-5-1    Falls, slides, or flows:

• Falls: rolling stones, individual pieces.

• Slide: materials move as a whole part.

• Flow: water plays an important role.

3-5-2    Slump:

mass of soils moving downslope as a whole piece. Due to steep slope and overloaded top

3-5-3    Rockslide:

Block of rocks moving down slope, often due to special structural effect, e.g. due to weak clay formation, and triggered by extensive rain, earthquake, etc.

3-5-4    Mudflow:

Soil and water mixed to form high viscous liquid, often associated with steep slope and heavy rain.

Lahars: mudflow in volcanic ash terrain.

3-5-5    Earthflow:

soil saturated with water. Water and clay sized particles flowing down slope. Slower than mudflow, less water than mudflow.

3-5-6    Creep:

Slower movement downslope due to unstability caused by freezing and thaw cycle.

Homework:

• Read chapter summary

• Use your own word to explain the key terms on page 85.

• Answer the review questions.