Rock Types & Exogenic Processes – Quick Review

Rock Fundamentals

  • Rock = naturally formed solid composed of ≥1 minerals
  • Classified by origin
    • Igneous – crystallized magma / lava\text{magma / lava}
    • Sedimentary – lithified sediments
    • Metamorphic – altered by heat & pressure

Igneous Rocks

  • Formation: cooling & crystallization of molten material
  • Key features: mixed minerals, light/dark colors, variable crystal size, possible vesicles or glassy texture
  • Textural classes
    • Fine-grained – rapid cooling (microscopic crystals)
    • Coarse-grained – slow cooling (visible crystals)
    • Porphyritic – large phenocrysts in fine matrix
    • Glassy – quenched melt, no crystals
  • Composition classes
    • Felsic – high feldspar + silica
    • Intermediate – between felsic & mafic
    • Mafic – high Mg\text{Mg} & Fe\text{Fe}
    • Ultramafic – very high Mg\text{Mg} & Fe\text{Fe}
  • Occurrence types
    • Extrusive (volcanic) – solidify at surface
    • Intrusive (plutonic) – solidify within crust

Sedimentary Rocks

  • Formation: lithification = compaction + cementation of deposited material
  • Hallmarks: layering, clasts, fossils, color variety, sedimentary structures (cross-beds, mud cracks)
  • Sub-types
    • Clastic – fragments of pre-existing rocks
    • Chemical – precipitation from solution
    • Organic – lithified biological matter
    • Bioclastic – skeletal debris (shells, corals)

Metamorphic Rocks

  • Origin: pre-existing rock altered by heat, pressure, and/or fluids (metamorphism)
  • Indicators: foliation/banding, recrystallized minerals, few pores, bent layers
  • Metamorphism types
    • Contact – heating by nearby magma
    • Burial – deep diagenesis
    • Regional (dynamothermal) – tectonic compression
    • Hydrothermal – hot, reactive fluids
  • Agents
    • Heat – drives new mineral growth
    • Pressure – compacts & reorients minerals
  • Rock categories
    • Foliated – layered/banded (slate, schist, gneiss)
    • Non-foliated – massive (marble, quartzite)

Rock Cycle (simplified)

Magma→CoolingIgneous→Weathering& ErosionSediment→LithificationSedimentary→Heat& PressureMetamorphic→MeltingMagma\text{Magma} \xrightarrow[Cooling]{} \text{Igneous} \xrightarrow[Weathering \&\ Erosion]{} \text{Sediment} \xrightarrow[Lithification]{} \text{Sedimentary} \xrightarrow[Heat \&\ Pressure]{} \text{Metamorphic} \xrightarrow[Melting]{} \text{Magma}

Exogenic (Surface) Processes

  • Collective term: denudation (stripping Earth’s surface)
  • Interaction of atmosphere, hydrosphere, biosphere, cryosphere, lithosphere

Weathering (in-place breakdown)

Physical (Mechanical)
• Pressure-release fracturing
• Frost wedging
• Abrasion (particle collision)
• Organic activity (root wedging)
• Thermal expansion/contraction
Chemical
• Dissolution – minerals dissolve in acidic water
• Hydrolysis – reaction forming new hydrated minerals
• Oxidation – reaction with O<em>2\text{O}<em>2, esp. Fe-bearing • Carbonation – CO</em>2+H2O→\text{CO}</em>2 + H_2O \rightarrow carbonic acid attacking carbonates

Erosion (removal & transport)

Glacial – plucking & abrasion by ice
Fluvial (water)
• Rain splash – impact detachment
• Sheet – thin-layer removal by runoff
• Rill – small channel cutting
Aeolian (wind)
• Deflation – lifting fine particles
• Abrasion – sand-blasting sculpting
Gravity (mass wasting)
• Coherent – unified mass (slumps, slides)
• Incoherent – particles (debris flow, mudflow)

Deposition

  • Settling of transported material; builds new landforms (e.g., river deltas)