AP Environmental Science Earth Systems and Resources — Worked Answer Explanations

Unit 4 · 12 questions explained

Below is a complete answer key for our AP Environmental Science Earth Systems and Resources practice questions. For each question you'll find the correct choice, a full written explanation of how to get there, and — for every wrong answer — a short note on exactly why it's tempting and where it goes wrong. Reading these straight through is one of the fastest ways to find the gaps in a unit before exam day.

Prefer to test yourself first? Take the timed Earth Systems and Resources practice test and come back here to review, or head back to the Earth Systems and Resources unit overview.

In-content ad

Question 1 · Easy
Which layer of the atmosphere contains the ozone layer that shields Earth from UV radiation?
- A
  Troposphere
  Why not A: The troposphere (0–12 km) is where weather occurs; ozone here is a pollutant, not a UV shield.
- B
  StratosphereCorrect
- C
  Mesosphere
  Why not C: The mesosphere (50–85 km) burns up meteors; it is above the stratospheric ozone layer.
- D
  Thermosphere
  Why not D: The thermosphere is where the ISS orbits; it is far above the ozone layer.
Explanation
The stratosphere (12–50 km altitude) contains the ozone layer, which absorbs 97–99% of the Sun's harmful UV-B and UV-C radiation. Temperature in the stratosphere increases with altitude (due to ozone absorbing UV). Note: ozone in the troposphere is a harmful pollutant; only stratospheric ozone is beneficial.

Key takeaway
Stratospheric ozone = UV shield (beneficial). Tropospheric ozone = pollutant (harmful to lungs and plants).
Question 2 · Easy
The horizon sequence O-A-B-C describes a soil profile. Which horizon is most important for plant growth and contains the highest concentration of organic matter?
- A
  O horizon (organic layer of surface litter)
  Why not A: The O horizon is surface litter — decomposing organic matter, but too little mineral content for most root growth.
- B
  A horizon (topsoil)Correct
- C
  B horizon (subsoil)
  Why not C: The B horizon accumulates leached minerals but has less organic matter and is less biologically active than topsoil.
- D
  C horizon (parent material)
  Why not D: The C horizon is weathered bedrock with minimal biological activity and little organic content.
Explanation
The A horizon (topsoil) is the most biologically active layer. It is rich in humus (decomposed organic matter), mineral particles, water, air, microorganisms, and plant roots. Loss of topsoil through erosion is a critical agricultural and environmental problem — topsoil takes hundreds to thousands of years to form.

Key takeaway
A horizon (topsoil) = highest organic matter, most biologically active, essential for plant growth.
Question 3 · Easy
Which type of rock forms from the cooling and solidification of magma or lava?
- A
  Sedimentary
  Why not A: Sedimentary rock forms from compression and cementation of sediment particles deposited by water, wind, or ice.
- B
  Metamorphic
  Why not B: Metamorphic rock forms when existing rock is transformed by heat and/or pressure without melting.
- C
  IgneousCorrect
- D
  Alluvial
  Why not D: Alluvial refers to sediment deposited by flowing water, not a rock type formed from cooling magma.
Explanation
Igneous rock solidifies from molten rock: intrusive igneous (e.g., granite) cools slowly underground and has large crystals; extrusive igneous (e.g., basalt, obsidian) cools rapidly at the surface and has small crystals or is glassy. The rock cycle connects igneous, sedimentary, and metamorphic rock through melting, erosion, deposition, and heat/pressure.

Key takeaway
Igneous = from magma/lava. Sedimentary = from compressed sediment. Metamorphic = existing rock changed by heat/pressure.
Question 4 · Easy
The water cycle returns precipitation to the atmosphere through two main processes from the land surface. Which pair correctly names these processes?
- A
  Condensation and sublimation
  Why not A: Condensation forms clouds/dew; sublimation is ice→vapor. Neither is a primary land-to-atmosphere water return process.
- B
  Evaporation and transpirationCorrect
- C
  Infiltration and runoff
  Why not C: Infiltration moves water into soil; runoff moves it over the surface — both move water within the land, not to the atmosphere.
- D
  Precipitation and percolation
  Why not D: Precipitation is water falling from the atmosphere; percolation is movement through soil — neither returns water to the atmosphere.
Explanation
Evapotranspiration combines two processes: evaporation (water vaporizes from open water surfaces, moist soil) and transpiration (plants release water vapor through their stomata). Together they are the dominant pathway returning water from land surfaces to the atmosphere, completing the water cycle. Deforestation reduces transpiration significantly, affecting regional precipitation patterns.

Key takeaway
Evaporation + transpiration = evapotranspiration — primary land-to-atmosphere water return in the water cycle.
Question 5 · Easy
In the context of Earth's energy budget, what is the greenhouse effect?
- A
  The process by which ozone absorbs incoming UV radiation in the stratosphere
  Why not A: Ozone absorption of UV is a stratospheric process distinct from the greenhouse effect.
- B
  The reflection of incoming solar radiation by clouds and snow back to space
  Why not B: Reflection of solar radiation is called the albedo effect, not the greenhouse effect.
- C
  The absorption of outgoing infrared radiation by atmospheric gases, re-radiating heat back to Earth's surfaceCorrect
- D
  The trapping of solar radiation inside agricultural greenhouses to grow plants
  Why not D: This describes a literal greenhouse structure; the atmospheric greenhouse effect works via infrared absorption, not physical enclosure.
Explanation
Earth's surface absorbs solar shortwave radiation and re-emits it as longwave infrared radiation. Greenhouse gases (CO₂, H₂O, CH₄, N₂O) absorb this outgoing infrared and re-emit it in all directions, including back toward Earth — warming the surface. The natural greenhouse effect is essential for life (~33°C warming); enhanced greenhouse effect from anthropogenic emissions is causing climate change.

Key takeaway
Greenhouse effect: GHGs absorb outgoing IR → re-radiate heat back to surface → warming. Enhanced by CO₂, CH₄, N₂O from human activities.
Question 6 · Medium
At a convergent boundary where an oceanic plate meets a continental plate, which of the following features is most likely to form?
- A
  A mid-ocean ridge
  Why not A: Mid-ocean ridges form at divergent boundaries where plates move apart and magma wells up.
- B
  A transform fault
  Why not B: Transform faults form where plates slide horizontally past each other, such as the San Andreas Fault.
- C
  A subduction zone with a volcanic arcCorrect
- D
  A rift valley
  Why not D: Rift valleys form at divergent boundaries on continents, not at convergent oceanic-continental boundaries.
Explanation
When a denser oceanic plate converges with a lighter continental plate, the oceanic plate subducts (sinks) beneath the continental plate. The subducting slab melts and releases water into the mantle wedge, triggering melting and volcanism above — forming a volcanic arc (like the Cascade Range). Deep ocean trenches also form at the subduction zone.

Key takeaway
Oceanic-continental convergence → subduction zone + volcanic arc + deep trench.
Question 7 · Medium
Soil texture is determined by the relative proportions of sand, silt, and clay particles. A soil with 40% sand, 40% silt, and 20% clay is classified as:
- A
  Sandy loam
  Why not A: Sandy loam has a much higher sand percentage (>50%) and lower silt content.
- B
  LoamCorrect
- C
  Clay
  Why not C: Clay soil has >40% clay; this sample has only 20% clay.
- D
  Silt loam
  Why not D: Silt loam has >50% silt; here silt is 40% and sand is equally represented.
Explanation
Loam is defined by roughly equal parts sand and silt with a moderate clay fraction (~20%). Loam is considered ideal agricultural soil because it retains moisture (clay and silt) while still draining well (sand) and has good aeration. Sandy soils drain too fast; clay soils compact and drain too slowly.

Key takeaway
Loam ≈ balanced sand + silt + moderate clay → ideal agricultural soil texture.
Question 8 · Medium
During an El Niño event, which of the following patterns is observed in the Pacific Ocean?
- A
  Strong trade winds push warm water westward, causing upwelling off South America.
  Why not A: Strong trade winds pushing warm water west describe normal (La Niña-like) conditions, not El Niño.
- B
  Trade winds weaken, warm water sloshes eastward, suppressing upwelling off Peru.Correct
- C
  Cold water replaces warm water in the central Pacific, causing droughts in Australia.
  Why not C: Cold central Pacific characterizes La Niña, which typically brings increased rainfall to Australia, not drought.
- D
  The polar jet stream strengthens, driving cold air into North America.
  Why not D: El Niño shifts the jet stream but is primarily defined by equatorial Pacific SST changes, not strengthening the polar jet.
Explanation
In a normal year, trade winds blow westward along the equator, piling warm water near Australia/Indonesia and allowing cold upwelling off Peru. During El Niño, trade winds weaken or reverse, warm water spreads eastward, suppressing the Peruvian upwelling. This devastates Peruvian fisheries (cold, nutrient-rich water disappears) and causes droughts in Australia/Indonesia and flooding in Ecuador/Peru.

Key takeaway
El Niño: weakened trade winds → warm water shifts east → upwelling suppressed off Peru → fishery collapse + altered precipitation worldwide.
Question 9 · Medium
The Coriolis effect influences atmospheric circulation by causing winds in the Northern Hemisphere to deflect:
- A
  To the left (counterclockwise)
  Why not A: Left deflection characterizes the Southern Hemisphere due to the Coriolis effect.
- B
  To the right (clockwise in high pressure, counterclockwise in low pressure)Correct
- C
  Straight toward the poles
  Why not C: The Coriolis effect curves wind paths; it does not direct them straight poleward.
- D
  Vertically upward in all circulation cells
  Why not D: Vertical movement in Hadley/Ferrel cells is driven by temperature, not the Coriolis effect.
Explanation
Earth's rotation causes the Coriolis effect: in the Northern Hemisphere, moving air deflects to the right of its direction of travel. This makes high-pressure systems rotate clockwise (air spirals outward to the right) and low-pressure systems rotate counterclockwise (air spirals inward and deflects right). In the Southern Hemisphere, the deflection is to the left.

Key takeaway
Northern Hemisphere Coriolis: right deflection → highs clockwise, lows counterclockwise.
Question 10 · Medium
A valley along the California coast experiences frequent morning fog that burns off by midday, mild temperatures year-round, wet winters, and dry summers. This climate pattern is characteristic of a:
- A
  Humid continental climate
  Why not A: Humid continental climates have extreme seasons (hot summers, cold winters) and year-round precipitation — not dry summers.
- B
  Mediterranean climateCorrect
- C
  Tropical monsoon climate
  Why not C: Tropical monsoon has very high year-round temperatures and intense wet seasons — very different from mild California coasts.
- D
  Subarctic (boreal) climate
  Why not D: Subarctic climates have long, very cold winters and short cool summers — the opposite of mild Mediterranean.
Explanation
Mediterranean climates occur on the western sides of continents between 30°–45° latitude and are characterized by warm, dry summers and mild, wet winters. This pattern results from seasonal shifts of high-pressure systems. Regions include coastal California, the Mediterranean Basin, Chile, South Africa's Cape, and SW Australia — all biodiversity hotspots.

Key takeaway
Mediterranean climate: dry summers, wet mild winters, western continental coasts 30–45° latitude.
Question 11 · Medium
Which of the following best explains why the leeward (rain shadow) side of a mountain range is typically drier than the windward side?
- A
  Leeward air is colder and cannot hold as much moisture.
  Why not A: Leeward air is actually warmer (adiabatic heating as it descends), not colder.
- B
  Air rises on the windward side, cools adiabatically, loses moisture as precipitation, then descends dry and warm on the leeward side.Correct
- C
  Mountains deflect precipitation-bearing clouds away from the leeward side entirely.
  Why not C: Mountains don't physically redirect clouds; the orographic lift and adiabatic cooling/warming explain the difference.
- D
  Leeward vegetation consumes all available soil moisture before rain can accumulate.
  Why not D: Vegetation doesn't prevent rainfall; reduced vegetation is a result of low precipitation, not a cause.
Explanation
Orographic lift: moist air is forced up the windward slope, cools at the dry adiabatic lapse rate, then at the dew point at the wet adiabatic rate, releasing precipitation. Air reaching the summit has lost most moisture. Descending the leeward side, it warms adiabatically (dry rate) — becoming warm and very dry. This creates rain shadow deserts (e.g., Great Basin, Atacama).

Key takeaway
Rain shadow: windward = orographic lift + precipitation. Leeward = descending dry, warm air = desert conditions.
Question 12 · Hard
La Niña conditions are characterized by which oceanic and atmospheric pattern in the Pacific?
- A
  Weakened trade winds and eastward shift of warm surface water
  Why not A: Weakened trade winds and eastward warm water shift describe El Niño, not La Niña.
- B
  Stronger-than-normal trade winds, cooler central/eastern Pacific, enhanced upwelling off South AmericaCorrect
- C
  Reversal of the thermohaline circulation and shutdown of the Gulf Stream
  Why not C: Thermohaline circulation changes are linked to freshwater influx from ice melt, not La Niña specifically.
- D
  Warmer-than-normal Indian Ocean temperatures replacing cold Pacific anomalies
  Why not D: Indian Ocean warming can accompany ENSO events, but La Niña is defined by its Pacific cooling pattern, not Indian Ocean warming.
Explanation
La Niña is the cool phase of ENSO: trade winds strengthen, pushing warm water further west toward Asia/Australia, and cold upwelling intensifies off South America. Effects include enhanced fisheries off Peru (cold, nutrient-rich water), drought in the Americas' Pacific coast, and increased rainfall/flooding in Australia and Southeast Asia. La Niña often follows an El Niño event.

Key takeaway
La Niña: stronger trade winds → cooler eastern Pacific → enhanced upwelling → drought SW Americas, flooding Australia/SE Asia.