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AP Environmental Science · Unit 4 of 9
Earth Systems & Resources
10–15% of the exam4 lessons · 50 min18 terms
What this unit covers
The topics below follow the published Env. Science course framework for Unit 4. This unit is worth 10–15% of the exam, so budget your time against that rather than against how long the unit takes to teach.
Plate tectonicsSoilWatershedsEl Niño
Lessons in this unit
- Plate Tectonics12 min · 3 objectivesDescribe Earth’s internal layers and the driving force behind plate motion · Distinguish the three types of plate boundaries and their landforms · Calculate a plate’s spreading rate from seafloor age and distance data
- Soil Formation & Properties13 min · 3 objectivesOrder and describe the major soil horizons in a soil profile · Use the soil texture triangle to classify a soil from its sand, silt, and clay content · Relate soil texture to porosity, permeability, and water-holding capacity
- Watersheds & Water Resources12 min · 3 objectivesDefine a watershed and describe how divides direct the flow of water · Distinguish surface water from groundwater and describe aquifer recharge · Calculate the discharge of a stream from its cross-section and flow velocity
- Atmospheric Circulation & El Niño13 min · 3 objectivesExplain how uneven solar heating and the Coriolis effect create global wind belts · Describe how the El Niño–Southern Oscillation alters normal Pacific circulation · Predict the regional weather impacts of El Niño and La Niña conditions
Formulas in Unit 4
Rate of plate movement
rate = distance from ridge / age of that seafloor
Newly formed crust sits at the ridge (age 0). The farther the seafloor and the older it is, the faster the plate has spread. Convert units carefully — 1 km = 100,000 cm.
Texture, drainage, and retention
sand → high permeability, low water-holding · clay → low permeability, high water-holding · loam → balanced
Larger particles (sand) leave larger gaps, so water drains fast but little is retained. Smaller particles (clay) pack tightly, holding water but draining slowly.
Stream discharge
Q = A × v = (width × depth) × velocity
Q is discharge (volume per time, e.g. m³/s); A is the cross-sectional area of the channel; v is the flow velocity. More water, a deeper channel, or faster flow all raise discharge.
Normal Pacific vs. El Niño
normal: strong trade winds → upwelling off South America · El Niño: weak/reversed winds → upwelling shuts down
Follow the trade winds. Strong winds push warm water west and pull cold nutrient-rich water up in the east; weakened winds let warm water return east and suppress that upwelling.
Every term in Unit 4
All 18 terms we publish for Earth Systems & Resources, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.
- Plate tectonics
- Convergent boundaries build mountains and volcanoes, divergent create new crust, transform cause earthquakes. Driven by mantle convection.
- Soil horizons
- O organic litter, A topsoil, E leached zone, B subsoil accumulation, C weathered parent material, R bedrock.
- Soil texture triangle
- Proportions of sand, silt and clay. Loam holds water and nutrients while draining adequately, which is why it is best for agriculture.
- Soil permeability and porosity
- Porosity is the fraction of pore space; permeability is how easily water moves through. Sand is highly permeable, clay is porous but not permeable.
- Watershed
- The land area draining into a common body of water. Land use anywhere in it affects water quality downstream.
- Earth's atmospheric layers
- Troposphere (weather), stratosphere (ozone layer), mesosphere, thermosphere. Temperature alternates direction between layers.
- Coriolis effect
- Earth's rotation deflects moving air right in the Northern Hemisphere and left in the Southern, producing prevailing wind belts.
- Hadley, Ferrel and Polar cells
- Atmospheric convection cells producing rainforests at the equator and deserts near 30° latitude where dry air descends.
- Rain shadow effect
- Air rises over a mountain, cools and drops its moisture on the windward side, leaving the leeward side dry.
- El Niño and La Niña
- El Niño weakens trade winds, warming the eastern Pacific and suppressing upwelling; La Niña strengthens them, intensifying upwelling and cooling.
- Upwelling
- Deep, nutrient-rich water rising to the surface, supporting some of the world's most productive fisheries.
- Albedo
- The fraction of sunlight reflected. Ice has high albedo, so melting exposes darker ocean, which absorbs more heat — a positive feedback.
- Soil erosion causes
- Removing vegetation, overgrazing, tilling and steep-slope cultivation. Contour plowing, terracing and windbreaks reduce it.
- Cation exchange capacity
- Soil's ability to hold positively charged nutrients. Clay and organic matter have high CEC; sandy soils lose nutrients to leaching.
- Soil pH and nutrient availability
- Most nutrients are most available near pH 6–7. Acidic soils leach base cations and free toxic aluminum; lime raises pH.
- Aquifer and water table
- A permeable rock layer holding groundwater; the water table is its upper surface. Over-extraction lowers it and can cause subsidence.
- Cone of depression
- The local drop in water table around a heavily pumped well, which can dry neighboring shallower wells.
- Saltwater intrusion
- Over-pumping coastal aquifers lets seawater move inland into the freshwater lens, contaminating wells.
What examiners penalize here
- For spreading-rate problems, the unit conversion is where points are lost: 1 km = 1,000 m = 100,000 cm. Set up rate = distance ÷ age, keep the units attached through the whole calculation, and state the final answer in the units the question asks for (usually cm/yr).
- To read the soil texture triangle, follow all three axes (percent sand, silt, and clay) and find where they intersect — the percentages must add to 100%. Loam sits near the middle. Sandy soils plot toward the sand corner (fast drainage); clay soils toward the clay corner (slow drainage, high retention).
- For discharge problems, remember Q = A × v and that area = width × depth. Keep units consistent (meters and seconds give m³/s). A frequent follow-up asks how discharge changes if the stream gets deeper or faster after a storm — both raise Q proportionally.
- Two reliable exam anchors: sinking dry air at ~30° latitude → deserts, and El Niño → weakened trade winds → collapsed upwelling → failed fisheries plus flipped rainfall (wet Americas, dry Australia). Being able to trace those cause-and-effect chains earns the free-response points.
Practice Env. Science
Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.
Questions about this unit
How much of the AP Environmental Science exam is Unit 4?
Unit 4, Earth Systems & Resources, is worth 10–15% of the Env. Science multiple-choice section according to the published course framework. Across all 9 units that makes it a substantial share — heavier than an even split would give it.
What topics are covered in Env. Science Unit 4?
Earth Systems & Resources covers Plate tectonics, Soil, Watersheds and El Niño. We publish 18 terms with definitions for this unit, all of them on this page.
How should I study Env. Science Unit 4?
Read the 4 lessons below first — about 50 minutes — then drill the 18 terms in cram mode until you can produce each definition from memory rather than just recognize it. Recognition is what makes a unit feel finished when it is not. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.
All 9 units of AP Environmental Science
Unit names, topics and exam weights follow the published College Board course framework for AP Environmental Science. AP® is a trademark registered by the College Board, which does not endorse this site.