Unit 4 – Earth Systems and Resources

Layers of Earth (from surface to center)

• Crust: Outermost solid layer; thin (5-70 km); oceanic crust (denser, basaltic) and continental crust (less dense, granitic)
• Mantle: Thick layer (~2,900 km) of hot, solid rock that flows slowly; divided into upper mantle and lower mantle
• Outer Core: Liquid iron and nickel (~2,200 km thick); creates Earth's magnetic field through convection
• Inner Core: Solid iron and nickel sphere (~1,200 km radius); extremely hot but solid due to immense pressure

Key Concepts

• Lithosphere: Rigid outer layer (crust + uppermost mantle); broken into tectonic plates
• Asthenosphere: Partially molten, plastic layer of upper mantle beneath lithosphere; plates "float" and move on this layer
• Convection Currents: Heat from Earth's core causes mantle material to rise, cool, sink, and rise again - driving force behind plate movement

Divergent Boundaries (Constructive)

Definition: Plates move APART from each other; new crust forms as magma rises from below.

Features Created:

• Mid-Ocean Ridges: Underwater mountain ranges where seafloor spreading occurs (e.g., Mid-Atlantic Ridge)
• Rift Valleys: On continents where plate is splitting apart (e.g., East African Rift Valley, Iceland)
• New Oceanic Crust: Basaltic lava solidifies to form new seafloor

Geological Activity: Volcanic eruptions (mild), shallow earthquakes, hydrothermal vents

Examples: Mid-Atlantic Ridge, East Pacific Rise, East African Rift, Red Sea

Convergent Boundaries (Destructive)

Definition: Plates move TOWARD each other; one plate subducts (slides beneath) the other or plates collide and crumple.

Three Subtypes:

Geological Activity: Most powerful earthquakes, explosive volcanic eruptions, mountain building

Transform Boundaries (Conservative)

Definition: Plates slide PAST each other horizontally; crust is neither created nor destroyed.

Features Created:

• Fault Lines: Fractures where plates grind past each other
• Offset Features: Rivers, roads, fences displaced by movement
• No Volcanoes: No melting or magma generation

Geological Activity: Frequent shallow earthquakes (plates stick and suddenly slip)

Examples: San Andreas Fault (California), Alpine Fault (New Zealand)

Weathering Processes

Physical (Mechanical) Weathering: Breaking rock into smaller pieces without changing chemical composition

• Frost Wedging: Water freezes in cracks, expands, breaks rock apart
• Root Action: Plant roots grow into cracks and split rocks
• Abrasion: Wind, water, ice grind rocks against each other
• Thermal Expansion: Temperature changes cause rock to expand/contract and crack

Chemical Weathering: Alters rock's chemical composition through reactions

• Hydrolysis: Water reacts with minerals to form new compounds
• Oxidation: Oxygen reacts with minerals (rusting of iron-containing rocks)
• Carbonation: Carbon dioxide dissolved in water forms weak acid that dissolves limestone
• Acid Rain: Acidic precipitation accelerates chemical weathering

Factors Affecting Soil Formation (CLORPT)

• Climate: Temperature and precipitation affect weathering rate and organic matter decomposition; warm, wet climates = fast soil formation
• Living Organisms (Biota): Plants, animals, bacteria, fungi add organic matter, mix soil, create pore spaces
• Organic Matter: Decomposed plant/animal material (humus) enriches soil with nutrients
• Relief (Topography): Slope affects drainage and erosion; steep slopes = thin soil; flat areas = thick soil
• Parent Material: Underlying rock type determines initial mineral composition
• Time: Soil development is SLOW; older soils are more developed (deeper, more differentiated layers)

O Horizon (Organic Layer)

Surface layer of organic matter (leaf litter, decomposing plants, humus); dark brown/black; most biological activity

A Horizon (Topsoil)

Mix of organic matter and minerals; dark colored; rich in nutrients; most fertile layer; where most plant roots grow; zone of leaching (water carries materials downward)

E Horizon (Eluviation/Leaching Layer)

Light-colored; heavily leached of minerals and organic matter; often absent in young soils; minerals washed down to B horizon

B Horizon (Subsoil)

Accumulation zone; receives minerals leached from above; clay-rich; less organic matter; harder, more compact; zone of illuviation (deposition)

C Horizon (Parent Material)

Partially weathered bedrock; little organic matter; source of minerals for upper layers; not true soil

R Horizon (Bedrock)

Solid, unweathered parent rock; impenetrable to roots; no soil development

Causes of Soil Erosion

• Deforestation: Removing vegetation exposes soil to wind and rain
• Agriculture: Plowing breaks soil structure; monocultures provide poor cover; overgrazing removes protective vegetation
• Construction/Development: Removes vegetation and compacts soil
• Mining: Strips away topsoil and vegetation
• Climate Factors: Heavy rain, drought (dries soil), strong winds

Types of Soil Erosion

• Sheet Erosion: Thin, uniform layer of soil removed by water flowing across surface
• Rill Erosion: Small channels/grooves cut by concentrated water flow
• Gully Erosion: Large, deep channels that cannot be plowed over; severe stage
• Wind Erosion: Soil particles blown away; common in dry, bare areas (Dust Bowl 1930s)

Consequences of Soil Erosion

• Loss of Fertility: Nutrient-rich topsoil removed; reduced agricultural productivity
• Water Pollution: Sediment clogs waterways, smothers aquatic organisms
• Desertification: Severe erosion can turn productive land into desert
• Flooding: Sediment fills reservoirs and stream channels, reducing water-holding capacity
• Economic Costs: Billions in lost agricultural production and cleanup

Soil Conservation Methods

• Contour Plowing: Plowing along contour lines (not up/down slope) slows water runoff
• Terracing: Create step-like platforms on slopes to reduce runoff speed
• No-Till/Conservation Tillage: Leave crop residue on field; minimal soil disturbance
• Cover Crops: Plant vegetation during off-season to protect soil
• Windbreaks/Shelterbelts: Rows of trees to block wind erosion
• Crop Rotation: Vary crops to maintain soil health and structure
• Reforestation: Plant trees on eroded land to stabilize soil

• Sand: Largest particles (0.05-2.0 mm); feels gritty; large pore spaces; drains quickly; poor water/nutrient retention; well-aerated
• Silt: Medium particles (0.002-0.05 mm); feels smooth like flour; moderate drainage; good fertility
• Clay: Smallest particles (<0.002 mm); feels sticky when wet; tiny pore spaces; poor drainage; high water/nutrient retention; poor aeration; prone to compaction
• Loam: IDEAL mixture (~40% sand, 40% silt, 20% clay); balances drainage, retention, and aeration; best for agriculture

Soil pH

• Acidic (<7): Common in high-rainfall areas; leaching removes bases; some nutrients less available; aluminum/manganese may be toxic
• Neutral (~7): Most nutrients readily available; ideal for most plants
• Alkaline (>7): Common in dry areas; calcium carbonate accumulation; iron/phosphorus less available
• Management: Add lime to raise pH; add sulfur to lower pH

Permeability and Porosity

• Porosity: Percentage of soil volume that is pore space (air/water); clay has high porosity but small pores
• Permeability: Rate water moves through soil; sandy soil = high permeability; clay = low permeability
• Water-Holding Capacity: Clay > Loam > Sand

Cation Exchange Capacity (CEC)

Definition: Soil's ability to hold and exchange positively charged nutrient ions (cations like Ca²⁺, Mg²⁺, K⁺, NH₄⁺)

• Clay and organic matter have negative charges that attract and hold cations
• High CEC = high fertility (nutrients retained for plant use)
• Low CEC = nutrients easily leached away
• Clay > Organic Matter > Silt > Sand

Permanent Gases (constant proportion)

• Nitrogen (N₂): 78% - Most abundant; inert; essential for proteins (after nitrogen fixation)
• Oxygen (O₂): 21% - Essential for cellular respiration; produced by photosynthesis
• Argon (Ar): 0.93% - Noble gas; inert; no biological role
• Trace Gases: <0.1% - Neon, helium, methane, krypton, hydrogen

Variable Gases (proportion changes)

• Water Vapor (H₂O): 0-4% - Varies by location/weather; greenhouse gas; drives weather; source of precipitation
• Carbon Dioxide (CO₂): 0.04% (420 ppm) - Greenhouse gas; essential for photosynthesis; increasing due to fossil fuel combustion
• Ozone (O₃): Variable - Stratospheric ozone shields UV radiation; tropospheric ozone is air pollutant
• Methane (CH₄), Nitrous Oxide (N₂O): Trace greenhouse gases with potent warming effects

1. Troposphere (0-12 km)

• Characteristics: Lowest layer; where we live; contains ~80% of atmospheric mass; all weather occurs here
• Temperature Trend: DECREASES with altitude (~6.5°C per km) - gets colder as you go up
• Composition: Most water vapor, clouds, air pollution
• Boundary: Tropopause - top of troposphere where temperature stops decreasing
• Importance: Life support, weather systems, greenhouse effect

2. Stratosphere (12-50 km)

• Characteristics: Contains ozone layer (15-35 km altitude); very dry; jet aircraft fly in lower stratosphere
• Temperature Trend: INCREASES with altitude - gets warmer as you go up (due to ozone absorbing UV radiation)
• Ozone Layer (O₃): Absorbs harmful UV-B and UV-C radiation; protects life on Earth; depletion caused by CFCs
• Boundary: Stratopause - top of stratosphere
• Weather: Stable; no vertical mixing; no weather or clouds

3. Mesosphere (50-85 km)

• Characteristics: Coldest layer of atmosphere; meteors burn up here creating "shooting stars"
• Temperature Trend: DECREASES with altitude - gets colder as you go up (coldest at top: -90°C)
• Boundary: Mesopause - top of mesosphere (coldest point in atmosphere)
• Importance: Protects Earth from most meteoroids

4. Thermosphere (85-600+ km)

• Characteristics: Outermost layer; extremely thin air; satellites orbit here; aurora borealis/australis occur here
• Temperature Trend: INCREASES with altitude - can reach 1,500°C+ (but feels cold because air is so thin)
• Ionosphere: Lower part of thermosphere; contains ions; reflects radio waves enabling long-distance communication
• Exosphere: Uppermost portion; gradually transitions to space; no clear boundary
• Importance: International Space Station orbits here; protects from solar radiation

Formation and Function

• Formation: UV radiation splits O₂ into oxygen atoms; atoms combine with O₂ to form O₃ (ozone)
• Protection: Absorbs 97-99% of harmful UV-B and UV-C radiation from sun
• Without Ozone Layer: Increased skin cancer, cataracts, immune suppression; damage to phytoplankton, crops
• Location: Primarily in stratosphere (15-35 km altitude)

Ozone Depletion

• Cause: Chlorofluorocarbons (CFCs) - formerly used in refrigerants, aerosols, foam products
• Mechanism: CFCs release chlorine atoms in stratosphere; one chlorine atom destroys 100,000+ ozone molecules
• Ozone Hole: Seasonal thinning over Antarctica (September-November); some over Arctic
• Solution: Montreal Protocol (1987) - international treaty banning CFCs; phased out production
• Recovery: Ozone layer slowly recovering; expected to return to 1980 levels by 2060-2070

1. Unequal Solar Heating

• Equator: Receives direct, concentrated sunlight year-round → HOT
• Poles: Receive indirect, spread-out sunlight → COLD
• Result: Temperature gradient creates pressure differences that drive air movement
• Basic Principle: Warm air rises (low pressure); cool air sinks (high pressure); air flows from high to low pressure

2. Coriolis Effect

Definition: Deflection of moving objects (air, water, projectiles) caused by Earth's rotation. Objects moving across Earth's surface appear to curve rather than travel in straight lines.

• Northern Hemisphere: Objects deflect to the RIGHT
• Southern Hemisphere: Objects deflect to the LEFT
• Equator: No Coriolis effect (zero deflection)
• Poles: Maximum Coriolis effect
• Result: Winds curve, creating circular patterns (not straight north-south flow)

Hadley Cell (0° - 30° latitude)

• Location: Tropical regions on both sides of equator
• Process: Intense heating at equator → warm, moist air rises → moves poleward at high altitude → cools and sinks at ~30° latitude
• Surface Winds: Trade Winds - blow from east to west (NE in N.Hem; SE in S.Hem)
• At Equator (0°): Intertropical Convergence Zone (ITCZ) - rising air, low pressure, heavy rainfall, tropical rainforests
• At 30°: Subtropical High - sinking air, high pressure, dry conditions, MAJOR DESERTS (Sahara, Arabian, Kalahari, Australian)

Ferrel Cell (30° - 60° latitude)

• Location: Mid-latitude regions (temperate zones)
• Process: Air descends at 30°, flows poleward at surface, rises at 60°, returns equatorward at altitude
• Surface Winds: Westerlies - blow from west to east (SW in N.Hem; NW in S.Hem)
• Weather: Variable; collision zone between tropical and polar air masses; storms, fronts
• At 60°: Subpolar Low - rising air, low pressure, stormy, wet conditions

Polar Cell (60° - 90° latitude)

• Location: High-latitude regions (Arctic and Antarctic)
• Process: Cold, dense air sinks at poles → flows equatorward at surface → warms and rises at 60° → returns poleward at altitude
• Surface Winds: Polar Easterlies - blow from east to west (NE in N.Hem; SE in S.Hem)
• At Poles (90°): Polar High - sinking air, high pressure, very cold, very dry (polar deserts)
• Weather: Extremely cold, minimal precipitation

• Tributaries: Smaller streams and rivers that feed into larger rivers
• Headwaters: Small streams at the source; often in mountains or high elevations
• Main Channel: Primary river that receives water from all tributaries
• Mouth: Where watershed drains into larger water body (ocean, lake)
• Divide/Ridge: High ground separating adjacent watersheds; determines drainage direction
• Floodplain: Low-lying areas adjacent to rivers that flood during high water
• Groundwater: Water infiltrating into soil becomes part of watershed system

Precipitation → Multiple Pathways

• Surface Runoff: Water flows over land into streams (fastest path); carries pollutants, sediment
• Infiltration: Water soaks into soil; filtered by soil layers
• Percolation: Water moves deeper to groundwater/aquifers
• Groundwater Flow: Slow movement through aquifers; eventually reaches streams/lakes
• Evapotranspiration: Water evaporates from surfaces or transpires from plants back to atmosphere

Pollution Sources

• Point Source: Pollution from identifiable, single location (factory pipes, sewage treatment plants) - easier to regulate
• Nonpoint Source: Pollution from diffuse, widespread sources (agricultural runoff, urban stormwater, parking lots) - harder to control
• Key Principle: Anything that enters watershed eventually reaches outlet; pollution upstream affects everyone downstream

Common Watershed Pollutants

• Sediment: From erosion; smothers aquatic habitats, increases turbidity
• Nutrients: Nitrogen and phosphorus from fertilizers, sewage; cause eutrophication
• Pathogens: Bacteria, viruses from sewage, animal waste; health risks
• Toxic Chemicals: Pesticides, heavy metals, industrial chemicals
• Petroleum Products: Oil, gasoline from roads, parking lots
• Thermal Pollution: Heated water from power plants reduces dissolved oxygen

Watershed Management Strategies

• Riparian Buffers: Vegetated zones along waterways filter runoff, stabilize banks
• Wetland Restoration: Wetlands act as natural filters and sponges
• Stormwater Management: Green infrastructure, retention ponds, permeable pavement
• Reforestation: Trees reduce erosion and increase infiltration
• Best Management Practices (BMPs): Techniques to minimize pollution (cover crops, no-till farming, buffer strips)
• Integrated Watershed Management: Coordinate land use across entire watershed, not just individual properties

Why Equator is Hotter than Poles

• Angle of Incidence: Equator receives DIRECT sunlight (rays hit perpendicular to surface); poles receive INDIRECT sunlight (rays hit at low angle, spread over larger area)
• Concentration: Same amount of solar energy concentrated over smaller area at equator vs. spread over larger area at poles
• Atmosphere Thickness: Sunlight travels through MORE atmosphere to reach poles (more scattering, absorption, reflection) vs. LESS atmosphere at equator
• Result: Temperature gradient from hot equator to cold poles drives atmospheric and oceanic circulation

Albedo (Reflectivity)

Definition: The fraction of solar radiation reflected by a surface (expressed as percentage or decimal 0-1)

• High Albedo (reflects more): Fresh snow (~90%), ice (~50-70%), clouds (~40-90%), deserts (~30-40%)
• Low Albedo (absorbs more): Forests (~10-15%), oceans (~6-10%), asphalt (~5%)
• Positive Feedback Loop: Ice melts → darker surface exposed → absorbs more heat → more melting (climate change concern)
• Average Earth Albedo: ~30% (meaning 70% absorbed, 30% reflected back to space)

Earth's Axial Tilt (23.5°)

Key Concept: Seasons are caused by Earth's 23.5° tilt on its axis, NOT by Earth's distance from the sun (orbit is nearly circular).

• Effect of Tilt: As Earth orbits sun, different hemispheres tilt toward or away from sun during the year
• When tilted TOWARD sun: Longer days, more direct sunlight, higher sun angle → SUMMER
• When tilted AWAY from sun: Shorter days, more indirect sunlight, lower sun angle → WINTER
• Opposite Hemispheres: When Northern Hemisphere has summer, Southern Hemisphere has winter (and vice versa)

Common Misconception

FALSE: "Seasons occur because Earth is closer to/farther from sun"

TRUE: Earth's distance from sun varies slightly, but this is NOT what causes seasons. In fact, Earth is CLOSEST to sun (perihelion) in January when Northern Hemisphere has WINTER!

Proof: If distance caused seasons, both hemispheres would have same season simultaneously - but they don't!

Summer Solstice (~June 21)

• Northern Hemisphere: Longest day, shortest night; first day of summer; sun directly overhead at Tropic of Cancer (23.5°N)
• Southern Hemisphere: Shortest day, longest night; first day of winter; sun lowest in sky
• Arctic Circle (66.5°N): 24 hours of daylight (midnight sun)
• Antarctic Circle (66.5°S): 24 hours of darkness (polar night)

Winter Solstice (~December 21)

• Northern Hemisphere: Shortest day, longest night; first day of winter; sun directly overhead at Tropic of Capricorn (23.5°S)
• Southern Hemisphere: Longest day, shortest night; first day of summer; sun highest in sky
• Arctic Circle (66.5°N): 24 hours of darkness (polar night)
• Antarctic Circle (66.5°S): 24 hours of daylight (midnight sun)

Spring (Vernal) Equinox (~March 21)

• Both Hemispheres: Equal day and night (12 hours each) everywhere on Earth
• Sun Position: Directly overhead at Equator (0°)
• Northern Hemisphere: First day of spring (moving toward summer)
• Southern Hemisphere: First day of fall/autumn (moving toward winter)

Fall (Autumnal) Equinox (~September 23)

• Both Hemispheres: Equal day and night (12 hours each) everywhere on Earth
• Sun Position: Directly overhead at Equator (0°)
• Northern Hemisphere: First day of fall/autumn (moving toward winter)
• Southern Hemisphere: First day of spring (moving toward summer)

1. Latitude

• Effect: Distance from equator determines solar radiation intensity and seasonal variation
• Low Latitudes (0-23.5°): Tropics - hot year-round, minimal seasonal temperature change
• Mid-Latitudes (23.5-66.5°): Temperate zones - moderate temperatures, distinct four seasons
• High Latitudes (66.5-90°): Polar regions - cold year-round, extreme seasonal daylight variation

2. Elevation (Altitude)

• Effect: Temperature DECREASES with increasing elevation (~6.5°C per 1,000 m or 3.5°F per 1,000 ft)
• Reason: Air pressure decreases at higher elevations; lower pressure = lower temperature
• Example: Mount Kilimanjaro (Tanzania) has snow at summit despite being near equator
• Result: Mountains have cooler climates than surrounding lowlands at same latitude

3. Proximity to Water Bodies (Continentality)

• Water Moderates Temperature: Water has high specific heat - absorbs/releases heat slowly; land heats/cools quickly
• Maritime (Coastal) Climates: Mild temperatures year-round; smaller temperature range; moderate seasons; higher humidity
• Continental (Interior) Climates: Extreme temperatures; large daily/seasonal temperature ranges; hot summers, cold winters; lower humidity
• Example: San Francisco (coastal) vs. Kansas City (interior) - both ~39°N but very different temperature ranges

4. Ocean Currents

• Warm Currents: Transfer heat from tropics to higher latitudes; warm adjacent coastal areas
• Example: Gulf Stream warms Western Europe (London warmer than same-latitude cities in Canada)
• Cold Currents: Transfer cold water from poles toward equator; cool adjacent coastal areas; reduce precipitation
• Example: California Current cools California coast; Humboldt Current creates Atacama Desert (Chile)
• Upwelling: Cold, nutrient-rich water rises to surface; high biological productivity but cool, foggy conditions

How Rain Shadow Forms (Step-by-Step)

1. Windward Side (upwind): Moist air approaches mountain and is forced upward
2. Orographic Lifting: As air rises, it cools (adiabatic cooling - ~6.5°C per km)
3. Condensation: Cool air can't hold as much moisture; water vapor condenses forming clouds
4. Precipitation: Heavy rain or snow falls on windward side; air loses moisture
5. Leeward Side (downwind): Now-dry air descends mountain's other side
6. Warming: Descending air warms (adiabatic warming); relative humidity drops further
7. Desert Formation: Leeward side receives little precipitation; creates rain shadow desert

Examples: Sierra Nevada (windward: wet California coast; leeward: Nevada desert), Cascades (windward: wet Seattle; leeward: dry eastern Washington), Himalayas (windward: wet India; leeward: dry Tibetan Plateau)

• Trade Winds: Blow east to west across tropical Pacific (from South America toward Asia/Australia)
• Warm Water Pool: Trade winds push warm surface water westward; accumulates near Indonesia/Australia (western Pacific)
• Cold Water Upwelling: Cold, nutrient-rich water rises along South American coast (Peru, Ecuador); supports rich fisheries
• Thermocline: Boundary between warm surface water and cold deep water slopes downward from east to west
• Precipitation: Heavy rainfall over warm western Pacific (Indonesia, Philippines); dry conditions along South American coast
• Atmospheric Pressure: Low pressure over warm western Pacific; high pressure over cool eastern Pacific

🔥 El Niño Conditions

❄️ La Niña Conditions

✓ Must-Know Concepts

• 3 plate boundary types & features
• Soil horizons (O-A-E-B-C-R)
• Soil texture (sand/silt/clay/loam)
• 4 atmospheric layers & temp trends
• 3-cell wind model (Hadley/Ferrel/Polar)
• Coriolis effect (right in N, left in S)
• Watershed components & pollution
• 23.5° tilt causes seasons
• Rain shadow effect mechanism
• El Niño vs. La Niña differences

⚠️ Common Mistakes to Avoid

• Continental-continental = NO volcanoes
• A horizon = topsoil (most fertile)
• Loam is ideal, not clay
• Stratosphere temp INCREASES (ozone)
• Deserts at 30° (sinking air)
• Windward = wet; leeward = dry
• Distance ≠ causes seasons (tilt does!)
• Weather ≠ climate (time scale)
• El Niño = warm (not cold) Pacific
• Confusing El Niño/La Niña impacts