Earth Systems and Resources
Unit 4 of AP Environmental Science, worth 10–15% of the exam. 13 questions below, each with the working. Every answer was checked by a second pass before it was published.
Plate tectonics, soil formation and composition, the atmosphere, global wind patterns, watersheds, solar radiation and seasons, El Niño and La Niña.
How this unit is tested
Most Unit 4 questions test whether you can connect a physical mechanism to its observable effect: a plate boundary type to the landform it produces, a soil horizon to what happens to it chemically, a pressure belt to the climate underneath it. Before drilling flashcards, build a mental chain for each topic — cause, mechanism, effect — rather than memorizing isolated facts.
Start with the two topics that generate the most quantitative reasoning on the exam: solar angle calculations and Coriolis-driven wind belts. Practice the insolation formula until you can plug in any latitude and declination without hesitation, and be able to explain in words why a higher solar angle means more concentrated energy per unit area. For wind patterns, draw the three-cell circulation model from memory (Hadley, Ferrel, Polar) and label the pressure belts and the deserts/rainforests that result — this single diagram answers a large share of exam questions on climate zones.
For soil, learn the horizons top to bottom (O-A-B-C-R) and tie each one to a process (organic accumulation, leaching, mineral deposition, weathered parent rock, bedrock). For plate tectonics, sort every boundary into divergent, convergent, or transform and know the landform and hazard associated with each. For watersheds, always think in terms of the whole drainage basin, not just the visible waterway.
End with El Niño/La Niña, since the FRQs often ask you to reason forward from a described change in trade winds or sea surface temperature to its downstream effects on weather and fisheries — practice explaining the mechanism in your own words rather than just naming the phenomenon.
What you have to know
Plate boundary types
Divergent boundaries pull apart and create new crust (mid-ocean ridges, rift valleys); convergent boundaries collide and destroy crust (subduction zones, trenches, mountain building); transform boundaries slide past each other, creating neither new nor destroyed crust but generating earthquakes.
Coriolis effect
Earth's rotation deflects moving air and water to the right of their path in the Northern Hemisphere and to the left in the Southern Hemisphere, curving global wind belts and ocean currents rather than letting them flow straight from high to low pressure.
Three-cell atmospheric circulation
Each hemisphere has three convection cells — Hadley (0°–30°), Ferrel (30°–60°), and Polar (60°–90°) — whose rising and sinking air create alternating bands of high rainfall (equator, 60°) and aridity (30°, poles).
Angle of insolation
Solar angle at solar noon = $90^\circ - |\phi - \delta|$, where $\phi$ is latitude and $\delta$ is the sun's declination (+23.5° at summer solstice, 0° at equinox, -23.5° at winter solstice in the Northern Hemisphere). A higher angle spreads the same solar energy over a smaller surface area, delivering more energy per unit area.
Seasons are caused by axial tilt, not distance
Earth's 23.5° axial tilt changes the angle and duration of sunlight each hemisphere receives as Earth orbits the sun; Earth is actually closest to the sun (perihelion) during Northern Hemisphere winter, so distance does not explain the seasons.
ENSO (El Niño–Southern Oscillation)
El Niño occurs when trade winds weaken, allowing warm surface water to shift toward the eastern Pacific and suppressing upwelling; La Niña occurs when trade winds strengthen, pushing warm water west and intensifying upwelling of cold, nutrient-rich water in the eastern Pacific.
13 practice questions
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Which type of plate boundary is responsible for the formation of new oceanic crust at features like the Mid-Atlantic Ridge?
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Answer. Divergent boundary
At divergent boundaries, plates move apart and magma rises from the mantle to fill the gap, cooling into new oceanic crust. This process creates mid-ocean ridges and rift valleys. -
Deep ocean trenches and chains of active volcanoes typically form at which type of plate boundary?
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Answer. Convergent boundary (subduction zone)
When one plate is denser than the other, it subducts beneath it, creating a deep trench; the melting subducted material rises to form a volcanic arc, such as the Andes or the Cascades. -
The San Andreas Fault produces frequent earthquakes but no volcanic activity. Explain why, based on the type of plate boundary it represents.
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Answer. It is a transform boundary, where two plates slide horizontally past each other without one being subducted or crust being created, so the stress builds and releases as earthquakes rather than melting rock into magma.
Volcanism requires either magma generation from subduction (convergent) or from decompression melting as plates separate (divergent). Transform boundaries only involve lateral sliding friction, which produces seismic energy but not the melting needed for volcanism. -
Name two factors from the CLORPT model that affect the rate of soil formation, and explain how each speeds up or slows down the process.
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Answer. Climate and time (also acceptable: parent material, organisms, topography)
Climate affects rate through temperature and precipitation — warm, wet climates speed chemical weathering and soil formation, while cold, dry climates slow it. Time simply means older land surfaces have had longer for weathering and horizon development to occur, producing deeper, more developed soils than young surfaces. -
A soil sample has a much higher proportion of clay than sand. Compared to a sandy soil, this sample will most likely have:
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Answer. Higher water retention and lower permeability
Clay particles are extremely small and pack tightly, leaving little pore space for water to drain through, so water is held longer but drains more slowly, increasing the risk of waterlogging compared to coarse, porous sandy soils. -
Which layer of Earth's atmosphere contains the ozone layer that absorbs incoming ultraviolet radiation?
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Answer. The stratosphere
The stratosphere lies above the troposphere and contains a concentration of ozone (O3) that absorbs most of the sun's harmful UV-B and UV-C radiation, protecting life at the surface. -
Explain how the Coriolis effect causes prevailing winds to curve differently in the Northern Hemisphere compared to the Southern Hemisphere.
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Answer. Because Earth rotates eastward, moving air is deflected to the right of its path of motion in the Northern Hemisphere and to the left of its path in the Southern Hemisphere, curving winds that would otherwise flow straight from high to low pressure.
This deflection is what turns north-to-south pressure-driven airflow into the curved trade winds and westerlies, and it reverses direction across the equator because the hemispheres rotate with opposite apparent spin relative to the surface. -
Major deserts such as the Sahara are located around 30° latitude because this is where:
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Answer. Dry, descending air from the Hadley cell sinks and warms, suppressing precipitation
Air that rose and lost its moisture near the equator descends around 30°N/S as part of the Hadley cell. As it sinks, it compresses and warms, lowering relative humidity and creating the subtropical high-pressure belt associated with major deserts. -
Define a watershed and explain why increasing impervious surfaces (pavement, rooftops) within it increases the risk of flooding downstream.
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Answer. A watershed is the entire land area that drains into a common water outlet; impervious surfaces prevent water from infiltrating into the soil, so more rainwater becomes fast-moving surface runoff that reaches streams and rivers quickly, raising peak flow and flood risk.
Natural, vegetated land absorbs and slows rainwater through infiltration and plant uptake. Paving over that land within the same drainage basin removes this buffering capacity, so a given storm produces a higher and faster flood peak than it would over undeveloped land. -
Calculate the angle of insolation at 40°N latitude on the winter solstice, when solar declination is −23.5°, and explain what this angle indicates about winter conditions there.
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Answer. 26.5°; a much lower, more oblique angle than in summer, spreading the same solar energy over a larger area and delivering less energy per square meter, producing colder temperatures and shorter days
Using solar angle = 90° − |latitude − declination|: 90° − |40 − (−23.5)| = 90° − 63.5° = 26.5°. This low angle means sunlight is spread thin across the surface and travels through more atmosphere, both of which reduce the energy received and explain the cold of winter. -
During an El Niño event, what typically happens to trade winds and sea surface temperature in the eastern Pacific Ocean?
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Answer. Trade winds weaken and sea surface temperature in the eastern Pacific rises
Normally strong easterly trade winds push warm surface water toward the western Pacific and allow cold upwelling in the east. When these winds weaken during El Niño, that warm water sloshes back eastward, warming sea surface temperatures off South America and suppressing upwelling. -
Explain one way El Niño affects weather in North America and one way it affects Pacific fisheries.
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Answer. El Niño often brings wetter-than-normal winters to the southern United States and drier conditions to the Pacific Northwest; it also reduces fish populations off the western coast of South America because suppressed upwelling cuts off the supply of cold, nutrient-rich water that supports the marine food web.
The shift in the jet stream position during El Niño redirects storm tracks, altering precipitation patterns across North America. Simultaneously, the warm surface water covering the normally cold eastern Pacific blocks nutrient-rich upwelling, collapsing the phytoplankton base that anchovy and other fisheries depend on. -
Which set of conditions correctly describes a La Niña event?
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Answer. Strengthened trade winds, increased upwelling, and cooler-than-normal sea surface temperatures in the eastern Pacific
La Niña is essentially an intensification of normal conditions: trade winds strengthen, pushing warm water further west and allowing even stronger upwelling of cold, nutrient-rich water along the South American coast, which cools the eastern Pacific below average.
What people get wrong
- Assuming Earth's distance from the sun causes the seasons — instead, remember seasons come from the 23.5° axial tilt changing the sun's angle and day length; Earth is actually nearest the sun during Northern Hemisphere winter.
- Mixing up divergent and convergent boundaries when identifying landforms — instead, ask whether crust is being created (divergent, e.g., mid-ocean ridge) or destroyed (convergent, e.g., trench/subduction) before naming the landform.
- Applying the Coriolis deflection direction the same way in both hemispheres — instead, always check which hemisphere the question is set in: right-deflection in the Northern Hemisphere, left-deflection in the Southern Hemisphere.
- Reversing El Niño and La Niña conditions on multiple-choice questions — instead, anchor on trade wind strength first: weak trade winds and warm water in the east = El Niño; strong trade winds and cold upwelling in the east = La Niña.
- Treating a watershed as just the river itself — instead, define it as the entire land area that drains into a common outlet, since land-use changes anywhere in that basin (not just at the river) affect runoff and water quality.
- Assuming every soil profile has all five horizons in equal thickness — instead, remember horizon presence and thickness vary with climate, parent material, and time, so profiles differ by region.
Drill this unit until it sticks
These questions come back on a schedule built from what you get wrong, alongside the rest of AP Environmental Science. Free, and no account needed to start.