Earth Science – Week 3-4 Notes: Exogenic & Endogenic Processes

Exogenic Processes

Weathering

• Definition: Process of breaking down rocks present at Earth’s surface.
  • Operates under the action of rainwater, temperature extremes, and biological activity.
  • Initiates soil formation and prepares material for erosion and transport.

Mechanical (Physical) Weathering

• Occurs without altering chemical composition of the rock.
• Main sub-processes:
  1. Pressure Release / Unloading – expansion after overburden removal.
  2. Temperature Change – repeated expansion/contraction of minerals due to heating/cooling.
  3. Frost-Wedging – cycles of freezing & thawing; ice occupies ~$9 ext{–}10\%$ more volume, widening cracks.
  4. Biological Activity – roots, burrowing animals, lichens mechanically pry apart grains; makes rocks more susceptible to chemical attack.

Chemical Weathering

• Converts original minerals into new substances with different physical & chemical properties.
• Dominant in warm, moist climates.
• Main sub-processes:
  1. Dissolution – solid phases dissolve in a liquid; common in halite, carbonates.
  2. Hydrolysis – reaction of water (often as acids) with silicate minerals to form new clays + soluble ions.
  • Important acids: Carbonic, Sulfuric, Nitric.
  1. Oxidation – combination of O\textsubscript{2} with Fe, Mg, etc., producing oxides (e.g., hematite) that redden soils.

Erosion

• Definition: Transportation of weathered material.
• Primary agents: running water, groundwater, wind, wave currents, glaciers, gravity.

Types

1. Water Erosion – fluvial transport to streams/rivers, ultimately oceans.
2. Wind Erosion – deflation & abrasion move fine sediments (dust, sand, small pebbles).
3. Glacial Erosion – downhill movement of compacted ice (glaciers) plucks & abrades bedrock.

Causes & Enhancers

• Soil erodibility & texture.
• Overgrazing (livestock/indigenous fauna remove vegetation).
• Deforestation / cutting of trees.
• Excessive pesticide application, disturbing soil structure.

Endogenic Processes

Earth’s Internal Heat

• Drives plate tectonics, earthquakes, volcanism.
• Two major sources:
  1. Residual Heat
  • Extraterrestrial Impacts: During accretion (Nebular Theory), kinetic energy of colliding planetesimals converted to heat.
  • Gravitational Contraction (Kelvin-Helmholtz): Accreting mass increased self-gravity → cloud contracted, spinning faster; gravitational potential converted to heat.
  • Nebular Theory: Solar system formed $\approx 4.5 \text{ Ga}$ from a rotating nebula (H, He, dust).
  1. Radiogenic Heat
  • Produced by radioactive decay of unstable isotopes (e.g., $^{238}\text{U}$, $^{232}\text{Th}$, $^{40}\text{K}$).

Earth’s Thermal (Energy) Budget – Solar Component

• Tracks incoming vs. outgoing energy at surface/atmosphere.
  • $19\%$ absorbed by clouds & atmosphere.
  • $51\%$ absorbed by Earth’s surface.
  • $\textbf{70\%}$ total solar energy absorbed.
  • $6\%$ scattered by atmosphere.
  • $4\%$ reflected by surface.
  • $20\%$ scattered & reflected by clouds.
  • $\textbf{30\%}$ total reflected/scattered back to space.

Energy Sources

Fossil Fuels (Non-Renewable)

• Organic deposits combustible for energy: coal, oil, natural gas.

Coal

• Carbon-rich rock from burial & coalification of swamp vegetation.
• Coalification pathway:
  1. Plant debris accumulates in swamp → peat.
  2. Shallow burial → lignite.
  3. Deeper burial → sub-bituminous then bituminous.
  4. Deep burial + tectonic/contact metamorphism → anthracite.
• Ranks (low → high):
  • Lignite – brown, crumbly.
  • Sub-bituminous – “black lignite,” transitional.
  • Bituminous – shiny, hard, industrial fuel.
  • Anthracite – highest grade, conchoidal fracture.

Peat

• Partially decayed plant matter; precursor to coal (peatification).

Oil (Petroleum)

• Black, viscous liquid trapped between rock layers; extracted via wells.

Natural Gas

• Predominantly methane; transported in pipelines; considered cleanest fossil option.

Philippine Context (2013)

• $61.25\%$ of national energy consumption from fossil fuels; major input for power plants.

• Abundant, easy to locate, high energy density, simple transport.

• Air pollution, greenhouse emissions, non-renewable on human timescales.

Renewable vs. Non-Renewable

• Renewable: replenished naturally within anthropogenic time scales (e.g., geothermal, solar, wind).
• Non-Renewable: finite or replenish extremely slowly relative to use (e.g., fossil fuels).

Geothermal Energy (Renewable)

• Harvests Earth’s internal heat for power.
• Enabled by understanding of geothermal gradient (temperature increases with depth).

Harnessing Process

1. Drill boreholes into geothermal reservoirs.
2. Hot water/steam rises through well.
3. Steam drives turbines.
4. Turbine mechanical energy → electricity via generators.

Three Essential Elements

• Heat Source – magmatic intrusions.
• Reservoir – permeable, porous rocks storing hot fluids, overlain by impermeable caprock.
• Geothermal Fluid – water or steam transporting heat.

Philippine Geothermal Development

• First plant (Tiwi, Albay) began $1967$.
• Additional fields: Mt. Makiling (Laguna), Mt. Banahaw (Quezon), Bacon-Manito (Sorsogon).

Advantages

• Fuel-free, low pollutants, renewable, geographically widespread (esp. volcanic arcs).

Disadvantages / Challenges

• High capital cost, need for specialized equipment & expertise, extensive exploration, limited to region of resource, energy difficult to transport over long distances (best used locally).

Ethical & Practical Implications / Real-World Connections

• Weathering & erosion shape landscapes and affect agriculture, hazard management (e.g., landslides).
• Fossil-fuel reliance impacts climate change; underscores urgency for renewables like geothermal.
• Geothermal exploitation must balance energy needs with environmental stewardship (e.g., fluid re-injection, induced seismicity monitoring).