Earth Systems and Resources — AP Environmental Science APES Study Guide

For: AP Environmental Science candidates sitting AP Environmental Science.

Covers: the four core subtopics of the Earth Systems and Resources unit: plate tectonics and natural hazards, atmospheric layers and weather, soil composition and properties, and the hydrologic cycle and water resources.

You should already know: Algebra 1, basic biology and chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Environmental Science style for educational use. They are not reproductions of past College Board papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official College Board mark schemes for grading conventions.

1. What Is Earth Systems and Resources?

Earth Systems and Resources describes the interconnected geophysical, atmospheric, hydrological, and biological components of the planet that support all life, as well as the natural and anthropogenic processes that alter these components. It is the third unit of the AP Environmental Science CED, accounting for 10-15% of your final exam score. Common synonyms include geosphere-biosphere interactions, Earth’s life support systems, and natural resource fundamentals. Questions on this unit appear in both multiple-choice and free-response sections, often paired with data interpretation or case study prompts.

2. Plate tectonics and natural hazards

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.

There are three core plate boundary types, each associated with distinct geologic features and hazards:

1. Divergent boundaries: Plates move apart, creating new crust as magma rises to fill the gap. Examples include the Mid-Atlantic Ridge and East African Rift Valley, associated with low-magnitude earthquakes and volcanic activity.
2. Convergent boundaries: Plates collide. Ocean-ocean convergence forms volcanic island arcs and deep ocean trenches; ocean-continental convergence forms volcanic mountain ranges (e.g. the Andes) and trenches; continental-continental convergence forms non-volcanic mountain ranges (e.g. the Himalayas). These boundaries produce high-magnitude earthquakes, volcanic eruptions, and tsunami risk.
3. Transform boundaries: Plates slide horizontally past each other (e.g. the San Andreas Fault in California). They produce shallow, high-magnitude earthquakes with no associated volcanic activity.

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.

Worked Example

Question: A seismograph records an earthquake with a Richter magnitude of 5.8. A second earthquake in the same region releases 32768x more energy than the first. What is the magnitude of the second earthquake? Solution: We know each 1 magnitude increase = 32x energy. Let $n$ = number of magnitude steps: $$32^n=32768$$ $$n=5,\text{ since }32^5=32768$$ Final magnitude = $5.8+5=10.8$ Exam tip: Examiners frequently test the logarithmic nature of the Richter scale, so never treat it as a linear measurement.

3. Atmospheric layers and weather

Earth’s atmosphere is divided into 5 distinct layers ordered by altitude, each with unique temperature and chemical properties:

1. 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. The tropopause is the upper boundary where temperature stops decreasing.
2. 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.
3. Mesosphere (50-85km): The coldest atmospheric layer, where temperature decreases with altitude, and meteors burn up as they collide with gas particles.
4. Thermosphere (85-600km): The hottest layer, where gas molecules absorb X-ray and UV radiation, producing auroras from interactions with solar wind.
5. 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 (trade winds, westerlies, polar easterlies) that distribute heat and moisture across the planet.

Worked Example

Question: A weather balloon records a temperature of $22^{\circ }C$ at ground level (0km altitude) in the troposphere. What temperature would you expect it to record at an altitude of 5km, assuming standard lapse rate? Solution: Total temperature drop = $5\times 6.5=32.5^{\circ }C$ Temperature at 5km = $22-32.5=-10.5^{\circ }C$

4. Soil composition and properties

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.

Soil is organized into distinct horizons, ordered from top to bottom:

• 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.

Worked Example

Question: A soil sample has 30% sand, 60% silt, 10% clay. Identify its texture class and explain whether it would be suitable for growing water-intensive crops like rice. Solution: The sample is classified as silt loam. It has moderate permeability and high water holding capacity, making it suitable for rice, which requires consistent soil moisture to grow.

5. Hydrologic cycle and water resources

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.

Worked Example

Question: A 10km² watershed receives 1300mm of annual precipitation. Annual evapotranspiration is 620mm, and surface runoff is 430mm. Calculate the annual volume of infiltrated water in cubic meters. Solution:

1. Convert units: 1300mm = 1.3m, 620mm = 0.62m, 430mm = 0.43m, 10km² = $10\times 10^6=1\times 10^7{m}^2$
2. Calculate infiltration rate: $I=P-ET-R=1.3-0.62-0.43=0.25m/year$
3. Total infiltration volume: $0.25\times 1\times 10^7=2.5\times 10^6{m}^3$

6. Common Pitfalls (and how to avoid them)

• Mistake: Treating the Richter scale as linear, e.g. assuming a magnitude 6 earthquake is 2x as powerful as a magnitude 3. Why: Most scales students encounter are linear, so they default to this assumption. Correct move: Remember each 1-point increase = 32x energy, so a magnitude 6 quake is $32^3=32768x$ more powerful than a magnitude 3.
• Mistake: Mixing up temperature trends in atmospheric layers, e.g. stating temperature increases with altitude in the troposphere. Why: Students memorize layer order but skip associated temperature changes. Correct move: Use the mnemonic: T(roposphere) = down, S(tratosphere) = up, M(esosphere) = down, T(hermosphere) = up ("Down Up Down Up").
• Mistake: Confusing leaching (A horizon) and accumulation (B horizon) in soil profiles. Why: Both terms refer to mineral movement, so students mix up direction. Correct move: Link "leaching" to "leaving" the upper A horizon, so minerals accumulate in the lower B horizon.
• Mistake: Forgetting most freshwater is locked in ice caps, not groundwater or surface water. Why: Students associate freshwater with accessible sources they use daily. Correct move: Memorize the freshwater breakdown: 68.7% ice caps, 30.1% groundwater, 0.3% surface water.
• Mistake: Misapplying Coriolis effect direction, e.g. stating air deflects left in the Northern Hemisphere. Why: Students mix up hemisphere rules. Correct move: Use the mnemonic "North Right, South Left" (N R S L = "No Rain, So Lazy").

7. Practice Questions (AP Environmental Science Style)

Question 1 (Multiple Choice)

A geologist records two earthquakes: the first has a Richter magnitude of 4.7, the second has a magnitude of 7.7. How much more energy does the second earthquake release than the first? A) 3x B) 100x C) 96x D) 32768x

Solution

Step 1: Calculate magnitude difference: $7.7-4.7=3$ points. Step 2: Each 1-point increase = 32x energy, so total energy increase = $32^3=32768x$. Correct answer: D. Exam note: The most common wrong answer is B, which uses amplitude instead of energy, so always read the question carefully to confirm the measurement requested.

Question 2 (Free Response Part A)

A farmer in Ohio tests their soil and finds it is 70% sand, 20% silt, 10% clay. (i) Identify one disadvantage of this soil texture for crop growth. (ii) Describe one modification the farmer could make to improve crop productivity.

Solution

(i) Sandy soil has very low water holding capacity and low CEC, so it dries out quickly and cannot retain enough nutrients to support healthy crop growth, leading to frequent irrigation and fertilizer requirements. (ii) The farmer could add compost and clay to the soil: compost increases organic matter to boost CEC and water retention, while clay reduces permeability to reduce water loss from drainage.

Question 3 (Free Response Calculation)

A 22km² suburban watershed receives 1050mm of annual precipitation. Annual evapotranspiration is 580mm, and 35% of precipitation becomes surface runoff. (i) Calculate the annual volume of infiltrated water in cubic meters. (ii) If local municipalities withdraw 25% of infiltrated water for residential use, calculate the volume of water left to recharge the underlying aquifer annually.

Solution

1. Unit conversions: 1050mm = 1.05m, 580mm = 0.58m, 22km² = $22\times 10^6=2.2\times 10^7{m}^2$ (i) Surface runoff depth = $0.35\times 1.05=0.3675m/year$ Infiltration rate = $1.05-0.58-0.3675=0.1025m/year$ Total infiltration volume = $0.1025\times 2.2\times 10^7=2.255\times 10^6{m}^3/year$ (ii) Aquifer recharge volume = $(1-0.25)\times 2.255\times 10^6=1.691\times 10^6{m}^3/year$ Exam tip: Always show all unit conversions and calculation steps to earn full marks on FRQ calculation questions.

8. Quick Reference Cheatsheet

9. What's Next

This unit is the foundation for almost all later content in the AP Environmental Science syllabus. For example, plate tectonics and soil properties tie directly to later units on land use and agriculture, where you will learn about soil erosion, deforestation, and sustainable farming practices designed to preserve soil fertility. Atmospheric layers and weather connect to units on air pollution, climate change, and ozone depletion, where you will analyze how anthropogenic emissions like CFCs and greenhouse gases alter atmospheric composition and disrupt global weather patterns. The hydrologic cycle and water resources are core to units on water pollution, water use, and sustainability, where you will evaluate strategies to reduce global water scarcity and protect freshwater ecosystems from contamination.

To reinforce your understanding of these concepts, practise more exam-style questions, and explore connections to later syllabus content, you can ask Ollie, our AI tutor, for customized quizzes, flashcards, or step-by-step explanations of any concept you find confusing. You can also find more study resources and full-length practice tests aligned to the College Board AP Environmental Science CED on the homepage.