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Physical Geography and Landforms

Earth's Structure and Materials​

1. Layers of the Earth​

Crust:

  • Outermost solid layer (5-70 km thick)
  • Continental crust: thicker, less dense, granitic
  • Oceanic crust: thinner, denser, basaltic
  • Composed of rocks and soils
  • Made of various elements (oxygen, silicon, aluminum, iron)

Mantle:

  • Below crust, very hot (800-4000°C)
  • Rigid upper portion (lithosphere)
  • Plastic/ductile lower portion (asthenosphere)
  • Composed of ultramafic rocks (peridotite)
  • Convection currents drive plate tectonics

Core:

  • Inner core: Solid (temperature ~5200°C)
  • Outer core: Liquid iron and nickel
  • Responsible for Earth's magnetic field
  • High pressure and temperature

2. Plate Tectonics​

Continental Drift:

  • Alfred Wegener's theory (1912)
  • Continents move gradually (2-10 cm/year)
  • Evidence: fossils, rock types, coastline fit
  • Pangaea: supercontinent 200 million years ago
  • Plates still moving today

Plate Boundaries:

  • Convergent: Plates collide (subduction, collision, mountains form)
  • Divergent: Plates separate (rift valleys, new crust formed)
  • Transform: Plates slide past each other (earthquake zones)
  • Movement causes earthquakes, volcanoes, mountains

Plate Movement Consequences:

  • Mountain formation (Himalayas, Andes)
  • Ocean basin creation (Atlantic widening)
  • Volcanic arcs (Ring of Fire)
  • Earthquake zones
  • Material recycling (subduction)

3. Rock Cycle​

Igneous Rocks:

  • Form from cooling magma/lava
  • Intrusive: Slow cooling → large crystals (granite)
  • Extrusive: Fast cooling → small crystals (basalt)
  • Composition varies (acidic to basic)

Sedimentary Rocks:

  • Form from compressed sediments
  • Clastic: From rock fragments (sandstone, shale)
  • Chemical: From dissolved minerals (rock salt, limestone)
  • Organic: From remains (coal, fossil limestone)
  • Contain fossils

Metamorphic Rocks:

  • Form under heat and pressure
  • Foliated: Banded (slate, gneiss, schist)
  • Non-foliated: Uniform texture (marble, quartzite)
  • Original rock structure transformed

Weathering and Erosion​

1. Weathering Processes​

Mechanical Weathering:

  • Rock broken without chemical change
  • Freeze-thaw: Water enters cracks, freezes, expands, breaks rock
  • Exfoliation: Outer layers peel off (pressure release)
  • Abrasion: Rock particles rub together
  • Salt crystallization: Crystals form, expand
  • Common in cold, dry climates

Chemical Weathering:

  • Rock chemically altered
  • Oxidation: Iron combines with oxygen (rust)
  • Hydrolysis: Water breaks bonds (feldspar → clay)
  • Carbonation: Rainwater (weak acid) dissolves limestone
  • Hydration: Minerals absorb water and expand
  • Faster in warm, wet climates

Biological Weathering:

  • Living organisms break rock
  • Plant roots penetrate and widen cracks
  • Acids from decaying matter
  • Burrows and tunnels
  • Lichen and fungi produce acids

2. Erosion and Transportation​

Erosion Definition:

  • Breakdown and removal of rock/soil
  • Different from weathering (no transport)
  • Requires agents: water, wind, ice, gravity
  • Rate depends on rock type and climate

Erosion Agents:

River Erosion:

  • Vertical erosion: Deepens valley (youthful stage)
  • Lateral erosion: Widens valley (mature stage)
  • Load transport: Suspended, solution, traction
  • Deposition: Where velocity decreases

Glacial Erosion:

  • U-shaped valleys (unlike V-shaped river valleys)
  • Drumlins, moraines (deposition features)
  • Glacial lakes and outwash plains
  • Striations and polished surfaces
  • Significant landscape transformation

Wind Erosion:

  • Deflation: Fine particles blown away
  • Abrasion: Wind-blown particles smooth surfaces
  • Significant in arid regions
  • Loss of valuable topsoil
  • Dune formation from deposition

Coastal Erosion:

  • Wave erosion: Hydraulic pressure and abrasion
  • Caves, arches, stacks form
  • Beach erosion and cliff retreat
  • Human activity increases rates (sea walls, dams)

3. Deposition and Landform Building​

Deposition Processes:

  • Material settles where erosion energy decreases
  • River deposition: Deltas, floodplains, terraces
  • Glacial deposition: Tills, moraines, eskers
  • Wind deposition: Dunes, loess
  • Coastal deposition: Beaches, spits, bars

Landform Development:

  • Alluvial fans: Debris from mountain slopes
  • Bajadas: Coalesced alluvial fans
  • Levees: Natural and artificial barriers
  • Oxbow lakes: Curved river meanders become isolated

Mountains and Upland Areas​

1. Mountain Formation​

Fold Mountains:

  • Formed by compressional forces at convergent boundaries
  • Rock layers folded (synclines, anticlines)
  • Examples: Himalayas, Alps, Rockies, Andes
  • Inside continental collisions primarily
  • Highest mountains often fold mountains

Block Mountains:

  • Formed by faulting (tension/shear)
  • Large rock blocks uplift or subside
  • Graben (sunken block), horst (raised block)
  • Examples: East African Rift, Basin and Range
  • Steep sides, flat tops often

Volcanic Mountains:

  • Built from lava/pyroclastic material accumulation
  • Cone volcanoes (steep sides)
  • Shield volcanoes (gentle slopes)
  • Examples: Mount Kilimanjaro, Mount Fuji
  • Form at plate boundaries and hotspots

2. Weathering and Erosion in Mountains​

Alpine Environment:

  • Freeze-thaw very active (high diurnal range)
  • Rock falls and talus slopes
  • Glacial erosion reshapes valleys
  • Limited soil development
  • High energy system

Mountain Zones (Altitude Effects):

  • Periglacial: Just below glaciers
  • Glaciated: Glaciers and ice fields
  • Montane: Forest zone (varies by latitude)
  • Alpine: Above tree line
  • Vegetation and climate varies sharply

3. Mountain Hazards​

Geological Hazards:

  • Avalanches: Snow/rock slides down steep slopes
  • Landslides: Slope failure from weathering/water
  • Rockfalls: Individual or multiple rocks
  • Lahars: Mudflows from volcanic material

Mitigation:

  • Engineering: Terracing, walls, drainage
  • Land use: Avoiding hazard zones
  • Monitoring: Early warning systems
  • Reforestation: Stabilizes slopes
  • Building codes: Earthquake-resistant design

Earthquakes​

1. Earthquake Causes and Mechanics​

Plate Boundary Earthquakes:

  • Most common (80%)
  • Convergent: Subduction zone movement
  • Divergent: Extension and rifting
  • Transform: Strike-slip movement
  • Very large magnitude possible

Intraplate Earthquakes:

  • Within plate interior (rarer, less frequent)
  • Cause mechanisms unclear sometimes
  • Often smaller magnitude
  • Can be damaging despite smaller size

Pressure Build-up and Release:

  • Elastic rebound theory
  • Stress accumulates as plates move
  • At critical point, rupture occurs (earthquake)
  • Energy released as seismic waves
  • Aftershocks: Adjustment and settling

2. Earthquake Characteristics​

Focus and Epicenter:

  • Focus (hypocenter): Point of rupture
  • Epicenter: Point on surface directly above focus
  • Depth ranges: Shallow (0-70km), intermediate, deep
  • Deeper earthquakes less surface damage usually

Magnitude and Intensity:

  • Magnitude: Energy released (Richter scale)
  • Frequency: Large earthquakes rare, small frequent
  • Intensity: Damage caused (Modified Mercalli scale)
  • Magnitude fixed; intensity varies by location

Seismic Waves:

  • P (Primary) waves: Fast, compress rock
  • S (Secondary) waves: Slower, shake rock side-to-side
  • L (Love) and R (Rayleigh) surface waves: Largest, damaging

3. Earthquake Impacts and Management​

Primary Hazards:

  • Ground shaking: Most damage
  • Ground rupture: Offset surfaces
  • Liquefaction: Soil loses strength (water-saturated)
  • Tsunami: Ocean waves from underwater earthquakes

Secondary Hazards:

  • Building collapse (human casualties)
  • Landslides and rockfalls
  • Gas explosions from ruptured pipes
  • Fire from electrical damage

Earthquake Management:

  • Prediction: Difficult, some precursors studied
  • Preparedness: Education, planning, drills
  • Building design: Earthquake-resistant structures
  • Early warning: Seconds of warning
  • Recovery: Post-disaster reconstruction

Volcanoes​

1. Volcanic Processes​

Magma Formation:

  • Partial melting of mantle (decompression, water, heat)
  • Magma rises through crust (less dense)
  • Viscosity varies (low Si vs. high Si)
  • Gas content affects behavior

Eruption Types:

  • Effusive: Quiet flows, low viscosity (basalt, shield)
  • Explosive: Violent eruptions, high viscosity (rhyolite, pumice)
  • Hawaiian: Low-viscosity, lava fountains
  • Strombolian: Repeated explosions
  • Vulcanian: Explosive, dome growth
  • Plinian: Very powerful, ash columns

Volcanic Products:

  • Lava: Magma on surface (basalt, andesite, rhyolite)
  • Pyroclastic material: Ash, pumice, bombs
  • Gases: Carbon dioxide, sulfur dioxide, water vapor
  • Lahars: Volcanic mudflows (water-saturated debris)

2. Volcanic Landforms​

Volcano Types:

  • Shield volcanoes: Flat, low-angle (Hawaii)
  • Cinder cones: Small, steep sides (symmetrical)
  • Stratovolcanoes: Composite, layers (Mount Fuji)
  • Calderas: Large depression from collapse

Other Features:

  • Lava tubes: Tunnels where lava flowed
  • Hot springs and geysers: Heat from subsurface
  • Fumaroles: Volcanic gas vents
  • Geothermal fields: Energy potential

3. Volcanic Hazards and Monitoring​

Primary Hazards:

  • Lava flows: Slow moving usually, destructive
  • Pyroclastic flows: Fast, hot, deadly
  • Lahars: Dangerous mudflows
  • Ash: Fine particles, respiratory hazard, crop damage
  • Gases: Toxic, can be lethal

Monitoring:

  • Seismometers: Detect earthquakes and tremors
  • GPS: Measures ground deformation
  • Gas analysis: Composition changes
  • Thermal imaging: Heat variations
  • Early warning systems

Volcanic Management:

  • Evacuation planning and execution
  • Building codes (ash-resistant)
  • Lahars barriers (channels, dams)
  • Prediction: Some success with precursors
  • Recovery: Hazard assessment before rebuilding

Summary​

Physical geography covers:

  • Earth Structure: Crust, mantle, core, plate tectonics
  • Rocks: Igneous, sedimentary, metamorphic, rock cycle
  • Weathering and Erosion: Mechanical, chemical, biological processes
  • Landforms: Mountains, valleys, plateaus, plains
  • Earthquakes: Causes, types, impacts, management
  • Volcanoes: Processes, landforms, hazards, monitoring

Understanding physical geography explains landscape formation and natural hazards affecting human populations.