Earth Systems & Resources

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.

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
• Soil: Texture, Horizons, and Why It Erodes17 min · 3 objectivesRead a soil texture triangle and connect texture to water and nutrient behavior · Identify soil horizons and explain what forms each · Explain the causes of erosion and evaluate conservation practices
• El Niño, Watersheds, and Global Circulation17 min · 3 objectivesExplain Hadley cells and the resulting global distribution of deserts and rainforests · Describe El Niño and La Niña and their regional consequences · Define a watershed and connect land use within it to downstream water quality

Formulas in Unit 4

Every term in Unit 4

All 34 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.

Divergent, convergent and transform boundaries

Divergent plates spread apart, creating new crust at mid-ocean ridges and rift valleys; convergent plates collide, producing subduction, trenches and mountains; transform plates slide past each other, producing earthquakes without volcanism.

Subduction and volcanic arcs

Denser oceanic crust sinks beneath continental crust, melts, and rises as magma, building a chain of volcanoes parallel to the trench. The Pacific Ring of Fire is the sum of these arcs.

Soil formation factors

Parent material, climate, topography, organisms and time. Change any one and the resulting soil differs, which is why the same rock yields thin acidic soil in cold wet uplands and deep fertile soil in warm grassland.

What each soil horizon holds

O is surface litter; A is topsoil, dark with humus and where most roots live; E is leached of minerals; B is subsoil where those minerals accumulate; C is weathered parent rock. Erosion removes O and A — the two that took longest to build.

Loam and why it is agriculturally ideal

Roughly 40% sand, 40% silt, 20% clay. Sand supplies drainage and aeration, clay supplies water and nutrient retention, and loam has enough of each — pure sand drains too fast to hold water, pure clay holds water but suffocates roots.

Water-holding capacity vs permeability

Clay has high water-holding capacity and low permeability; sand is the reverse. A crop can fail from either extreme, so a soil test that reports texture is really reporting how the field will behave in a drought and in a downpour.

Watershed divide

The ridge line separating two watersheds. Everything that falls on one side drains to one river system, which is why upstream land use in one county determines water quality in another and why watersheds rarely match political boundaries.

Recharge zone

The surface area where water infiltrates to replenish an aquifer. Paving or contaminating a recharge zone damages an aquifer that may be pumped a hundred kilometers away, so protecting it is groundwater policy even though it looks like land-use policy.

Confined vs unconfined aquifer

An unconfined aquifer has a permeable surface above it and recharges directly; a confined aquifer sits between impermeable layers, recharges slowly and far away, and can be under enough pressure to produce an artesian well.

Why the troposphere cools with height and the stratosphere warms

The troposphere is heated from below by the surface, so temperature falls with altitude. The stratosphere is heated from within by ozone absorbing UV, so temperature rises with altitude — which makes it stable and traps pollutants below it.

Why the equator is wet and 30° latitude is dry

Intense equatorial heating drives rising air that cools, condenses and rains — the rainforest belt. That air descends near 30° north and south, warming and drying as it sinks, which is where the world's great deserts are.

Trade winds and the ITCZ

Surface air flowing back toward the equator is deflected west by the Coriolis effect, producing the trade winds. Where they converge — the intertropical convergence zone — air rises and rain is heaviest; the ITCZ migrates seasonally, driving wet and dry seasons.

La Niña conditions

Trade winds strengthen, pushing more warm water west and intensifying upwelling off South America. Fisheries do well, Indonesia and Australia turn wet, and the southern United States turns dry — roughly the mirror image of El Niño.

Thermohaline circulation

Density-driven deep ocean flow: cold salty water sinks in the North Atlantic and returns as a global conveyor over centuries. It carries heat poleward, and freshwater from melting ice is the mechanism by which warming could slow it.

Monsoon

A seasonal reversal of wind driven by land heating faster than ocean. Summer land heating draws in moist ocean air and torrential rain; winter reverses the flow and brings drought. Billions of people farm on this schedule.

Insolation and the seasons

Seasons come from axial tilt, not distance from the sun. Tilt changes the angle at which sunlight strikes a hemisphere and the day length, and both concentrate or spread the same energy over more or less surface area.

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.
• For any conservation-practice question, give the **physical mechanism**. "Contour plowing reduces erosion" restates the question; "furrows across the slope prevent water from running straight downhill and gaining speed" is the answer. Every practice on the list has a one-clause mechanism.

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

All 9 units of AP Environmental Science

1. Unit 1 · The Living World: Ecosystems
2. Unit 2 · The Living World: Biodiversity
3. Unit 3 · Populations
4. Unit 4 · Earth Systems & Resources
5. Unit 5 · Land & Water Use
6. Unit 6 · Energy Resources & Consumption
7. Unit 7 · Atmospheric Pollution
8. Unit 8 · Aquatic & Terrestrial Pollution
9. Unit 9 · Global Change