Earth Systems and Resources — AP Environmental Science
1. Plate Tectonics and Natural Hazards ★★☆☆☆ ⏱ 5 min
The theory of plate tectonics states that Earth’s lithosphere (the outermost 100km of rigid rock, including the crust and upper mantle) is split into 15 major plates that float on the semi-molten asthenosphere below. Plate movement is driven by mantle convection currents, where hot magma rises, cools near the surface, and sinks in a continuous cycle, moving plates at an average rate of 2-15cm per year.
- **Divergent boundaries**: Plates move apart, creating new crust as magma rises to fill the gap. Associated with low-magnitude earthquakes and volcanic activity (examples: Mid-Atlantic Ridge, East African Rift Valley).
- **Convergent boundaries**: Plates collide. Ocean-ocean convergence forms volcanic island arcs and deep ocean trenches; ocean-continental convergence forms volcanic mountain ranges and trenches; continental-continental convergence forms non-volcanic mountain ranges. Associated with high-magnitude earthquakes, volcanic eruptions, and tsunami risk.
- **Transform boundaries**: Plates slide horizontally past each other. Produce shallow, high-magnitude earthquakes with no volcanic activity (example: San Andreas Fault, California).
Natural hazards linked to plate movement include volcanic eruptions (which release ash, sulfur dioxide, and CO₂, causing short-term global cooling and crop failure) and earthquakes, measured on the logarithmic Richter scale: each 1-point magnitude increase equals a 10x increase in wave amplitude and 32x increase in energy release.
2. Atmospheric Layers and Weather ★★☆☆☆ ⏱ 4 min
Earth’s atmosphere is divided into 5 distinct layers ordered by altitude, each with unique temperature and chemical properties:
- **Troposphere (0-12km)**: Contains 75% of total atmospheric mass, where all weather occurs. Temperature decreases with altitude at a standard lapse rate of $6.5^\circ C$ per km.
- **Stratosphere (12-50km)**: Contains the ozone layer, which absorbs 99% of harmful UV-B and UV-C radiation. Temperature increases with altitude as ozone absorbs UV energy.
- **Mesosphere (50-85km)**: The coldest atmospheric layer, where temperature decreases with altitude, and meteors burn up as they collide with gas particles.
- **Thermosphere (85-600km)**: The hottest layer, where gas molecules absorb X-ray and UV radiation, producing auroras from interactions with solar wind.
- **Exosphere (600km+)**: The outermost layer, where light gas particles escape to space.
Weather describes short-term (hours to weeks) atmospheric conditions driven by uneven solar heating of Earth’s surface. The Coriolis effect, caused by Earth’s rotation, deflects moving air to the right in the Northern Hemisphere and left in the Southern Hemisphere, creating global wind patterns that distribute heat and moisture across the planet.
3. Soil Composition and Properties ★★★☆☆ ⏱ 5 min
Soil is the weathered outer layer of Earth’s crust, a dynamic mix of inorganic minerals, organic matter, water, air, and living organisms, formed over hundreds to thousands of years via mechanical/chemical weathering of rock and decomposition of organic material.
- **O horizon**: Top organic layer, made of leaf litter, decomposing plant and animal matter.
- **A horizon**: Topsoil, a mix of organic matter and mineral particles, the primary zone of leaching where water dissolves and carries nutrients and clay downward.
- **E horizon**: Eluviated layer, heavily leached of clay and minerals, light in color.
- **B horizon**: Subsoil, the zone of accumulation where leached clay, minerals, and nutrients collect.
- **C horizon**: Weathered parent material, partially broken down rock.
- **R horizon**: Bedrock, unweathered parent material.
Soil texture is determined by the relative percentage of sand (largest particles, high permeability, low water/nutrient holding capacity), silt (medium particles), and clay (smallest particles, low permeability, high water holding capacity and cation exchange capacity (CEC), a measure of soil’s ability to retain positively charged nutrient ions like calcium and potassium for plant use. Loam, a mix of 40% sand, 40% silt, 20% clay, is the ideal soil for agriculture, with balanced drainage, nutrient retention, and aeration.
4. Hydrologic Cycle and Water Resources ★★★☆☆ ⏱ 4 min
The hydrologic (water) cycle is the continuous movement of water between the atmosphere, hydrosphere, lithosphere, and biosphere, powered by solar energy and gravity. Key processes include:
- Evaporation: Liquid water converts to water vapor and rises into the atmosphere.
- Transpiration: Water vapor is released from plant stomata as part of photosynthesis.
- Evapotranspiration: Combined total of evaporation and transpiration from a land area.
- Condensation: Water vapor cools and converts to liquid droplets, forming clouds.
- Precipitation: Liquid or solid water falls to Earth as rain, snow, sleet, or hail.
- Runoff: Surface water flows over land into streams, rivers, and oceans.
- Infiltration: Water seeps into the ground to recharge underground aquifers.
- Percolation: Water moves downward through soil and porous rock to deeper aquifer layers.
Only 2.5% of Earth’s total water is freshwater, and just 0.024% is accessible surface freshwater (lakes, rivers, streams) for human use. 68.7% of freshwater is locked in ice caps and glaciers, and 30.1% is stored in underground aquifers, which supply 30% of global human water use. The water balance equation describes the distribution of water in a watershed:
P = ET + R + I
Where $P$ = precipitation, $ET$ = evapotranspiration, $R$ = surface runoff, $I$ = infiltration.
Common Pitfalls
Why: Most scales students encounter are linear, so they default to this incorrect assumption.
Why: Students memorize layer order but do not memorize associated temperature changes.
Why: Both terms refer to mineral movement, so students mix up direction of movement.
Why: Students associate freshwater with accessible sources they use daily.
Why: Students mix up hemisphere rules.