Earth's lithosphere is divided into ~15 major tectonic plates that float on the semi-fluid asthenosphere. Plate movement is driven by convection currents in the mantle. Interactions at plate boundaries cause earthquakes, volcanism, and mountain building.
| Boundary Type | Plate Motion | Features Created | Examples |
|---|---|---|---|
| Divergent | Plates move apart | Mid-ocean ridges, rift valleys, new crust | Mid-Atlantic Ridge, East African Rift |
| Convergent | Plates collide | Mountains, trenches, subduction zones, volcanoes | Himalayas, Andes, Mariana Trench |
| Transform | Plates slide past each other | Earthquakes, fault lines | San Andreas Fault, Alpine Fault (NZ) |
75% of the world's volcanoes and 90% of earthquakes occur along the Pacific Ring of Fire — a horseshoe-shaped zone of convergent/transform boundaries.
When oceanic plate (denser) slides under continental plate. Creates deep-ocean trenches, volcanic arcs, and tsunamis. Example: Japan, Cascadia.
New oceanic crust forms at mid-ocean ridges. Evidence: magnetic striping, age of ocean floor increases with distance from ridge.
Volcanic activity from mantle plumes WITHIN a plate (not at boundaries). Hawaiian Islands formed as Pacific Plate moved over a hot spot.
Plate tectonics connects to: soil formation (volcanic soils are fertile), natural hazards (earthquakes, tsunamis, eruptions), mineral resource distribution, and geothermal energy availability.
Volcanic island arcs, deep-ocean trenches, and frequent earthquakes are most commonly associated with which type of plate boundary?
❌ Convergent vs. divergent confusion: Convergent = plates collide (mountains, trenches, volcanoes). Divergent = plates separate (mid-ocean ridges, rift valleys, new crust). Remember: converge = come together, diverge = move apart.
❌ Thinking hot spots are plate boundaries. Hot spots (e.g., Hawaii) occur WITHIN a plate from a mantle plume — they are NOT at plate boundaries.
❌ Forgetting that transform boundaries produce earthquakes but NOT volcanoes. The San Andreas Fault is transform — lots of earthquakes, no volcanic chain.
Soil forms through the weathering of parent material (bedrock) combined with the addition of organic matter. It takes 500-1,000 years to form 1 inch of topsoil, making it essentially a non-renewable resource on human timescales.
| Horizon | Name | Characteristics |
|---|---|---|
| O | Organic | Decomposing leaves, twigs, humus. Dark, rich in nutrients. Thin in most soils. |
| A | Topsoil | Mix of organic matter and minerals. Where most roots grow. Most fertile layer. Dark brown. |
| E | Eluviation | Leached zone where minerals wash downward. Light-colored, sandy. Present in forest soils. |
| B | Subsoil | Accumulation of leached minerals (clay, iron, aluminum). Reddish-brown. Less organic matter. |
| C | Parent Material | Partially weathered bedrock. Little biological activity. |
| R | Bedrock | Unweathered solid rock. Foundation of soil formation. |
Key practices: contour plowing (across slopes), terracing (step-cut hillsides), cover crops (plant between harvests), no-till farming (don't plow), windbreaks/shelterbelts (trees blocking wind), strip cropping (alternate crop rows).
Which soil horizon contains the highest concentration of organic matter and is most important for plant growth?
❌ Confusing O and A horizons. O = purely organic (leaf litter); A = organic + mineral mix (topsoil).
❌ Forgetting the E horizon exists. It only appears in certain soils (especially forest soils with heavy rainfall).
Soil is composed of four components: minerals (45%), water (25%), air (25%), and organic matter (5%). Soil texture is determined by the proportions of sand, silt, and clay particles.
| Particle | Size | Water Retention | Nutrient Holding | Drainage |
|---|---|---|---|---|
| Sand | Largest (0.05-2mm) | Low | Low | High (drains quickly) |
| Silt | Medium (0.002-0.05mm) | Medium | Medium | Medium |
| Clay | Smallest (<0.002mm) | High | High (large surface area) | Low (waterlogged) |
| Loam | Equal mix of all three | Optimal | High | Balanced — best for agriculture |
Loam is the ideal agricultural soil — roughly equal parts sand, silt, and clay. It balances drainage (sand), nutrient holding (clay), and water retention (silt). Soil pH also matters: most crops prefer pH 6.0-7.0.
A farmer notices that water pools on the surface of a field after rainfall and drains very slowly. The soil most likely has a high proportion of
❌ Confusing weathering with erosion: Weathering = breaking down rock IN PLACE (physical, chemical, biological). Erosion = MOVING the broken material (by water, wind, ice, gravity). Weathering happens first, then erosion transports the pieces.
❌ Thinking sandy soil is best for farming. Sand drains too fast and holds few nutrients. LOAM (balanced mix of sand, silt, clay) is ideal for agriculture.
Earth's atmosphere is divided into layers based on temperature changes with altitude. The composition is 78% N₂, 21% O₂, 0.9% Ar, 0.04% CO₂, plus trace gases and water vapor.
| Layer | Altitude | Temperature Trend | Key Features |
|---|---|---|---|
| Troposphere | 0-12 km | Decreases with altitude | Weather occurs here; contains 75% of atmospheric mass; greenhouse effect |
| Stratosphere | 12-50 km | Increases with altitude | Contains ozone layer (O₃); absorbs UV radiation; jet aircraft fly here |
| Mesosphere | 50-80 km | Decreases | Coldest layer (-90°C); meteors burn up here |
| Thermosphere | 80-700 km | Increases sharply | Very thin air; auroras; ISS orbits here; up to 2,000°C but feels cold (few molecules) |
Know that the ozone layer is in the stratosphere (not troposphere). Also: ground-level ozone (troposphere) is a pollutant; stratospheric ozone is protective. The greenhouse effect occurs in the troposphere.
In which atmospheric layer does weather occur and where is most of the atmosphere's mass concentrated?
❌ Ozone layer placement: The ozone layer is in the STRATOSPHERE (12-50 km), not the troposphere. Ground-level ozone in the troposphere is a harmful POLLUTANT (smog component), while stratospheric ozone is PROTECTIVE (blocks UV radiation).
❌ Thinking the thermosphere is "hot" in the usual sense. It has temperatures up to 2,000°C but very few molecules — you'd actually feel cold because there isn't enough matter to transfer heat.
Global wind patterns are driven by uneven solar heating of Earth's surface and the Coriolis effect (deflection caused by Earth's rotation). Three convection cells in each hemisphere create predictable wind belts.
| Wind Belt | Latitude | Direction (N. Hemisphere) | Characteristics |
|---|---|---|---|
| Trade Winds | 0°-30° | NE → SW | Steady, reliable; historically used for sailing trade routes |
| Westerlies | 30°-60° | SW → NE | Dominant mid-latitude winds; drive most US/European weather systems |
| Polar Easterlies | 60°-90° | NE → SW | Cold, dry air flowing from poles; weaker than other wind belts |
Winds deflect RIGHT in Northern Hemisphere, LEFT in Southern. Caused by Earth's rotation. Does NOT affect winds at the equator.
Intertropical Convergence Zone at equator where trade winds meet. Rising warm, moist air → heavy rainfall → tropical rainforests.
Moist air rises on windward side of mountains (orographic lift) → rain. Dry air descends on leeward side → desert. Example: Sierra Nevada/Death Valley.
Descending dry air at 30° N/S creates deserts (Sahara, Australian Outback, Sonoran). High pressure zones with calm winds.
Wind patterns determine: biome distribution (deserts at 30°, rainforests at equator), ocean current direction, pollutant transport, and precipitation patterns. Rain shadow effect is a frequent FRQ topic.
Major deserts such as the Sahara and Australian Outback are located near 30° latitude primarily because
❌ Seasons caused by distance from sun: Seasons are caused by Earth's 23.5° axial tilt, NOT by distance from the sun. In fact, Earth is CLOSEST to the sun in January (Northern Hemisphere winter).
❌ Thinking the Coriolis effect deflects winds in the same direction everywhere. Winds deflect RIGHT in the Northern Hemisphere and LEFT in the Southern Hemisphere. At the equator, there is no Coriolis deflection.
A watershed (drainage basin) is an area of land where all precipitation drains to a common body of water (river, lake, ocean). Watersheds are separated by ridgelines (divides). Everything that happens in a watershed affects water quality downstream.
Roads, parking lots, rooftops prevent infiltration → increased runoff, flooding, and pollution in streams. Urbanization increases impervious cover.
Diffuse pollution from broad areas: agricultural runoff (fertilizers, pesticides), urban runoff (oil, trash), construction sediment. Hardest to regulate.
Traceable to a single discharge point: factory pipes, sewage outfalls, oil spills. Easier to identify and regulate (Clean Water Act).
Vegetation along waterways filters runoff, stabilizes banks, provides habitat, and shades water (keeps it cool for aquatic organisms).
Which of the following is the most difficult type of water pollution to regulate?
❌ Point vs. nonpoint source confusion: Point source = single identifiable discharge (factory pipe, sewage outfall). Nonpoint source = diffuse, widespread (agricultural runoff, urban stormwater). Nonpoint is HARDER to regulate because you can't trace it to one location.
❌ Forgetting that impervious surfaces increase runoff. Pavement and rooftops prevent water from infiltrating soil, increasing both flood risk and pollutant transport to waterways.
ENSO (El Nino-Southern Oscillation) is a periodic climate pattern in the tropical Pacific that shifts between warm (El Nino) and cool (La Nina) phases every 2-7 years, affecting global weather patterns.
| Feature | El Nino (Warm Phase) | La Nina (Cool Phase) |
|---|---|---|
| Pacific SST | Warmer than normal (eastern Pacific) | Cooler than normal (eastern Pacific) |
| Trade Winds | Weaken or reverse | Strengthen |
| Upwelling (Peru) | Suppressed → fisheries collapse | Enhanced → productive fisheries |
| US West Coast | Warmer, wetter winters | Cooler, drier |
| Australia/SE Asia | Drought, wildfires | Flooding, heavy rain |
El Nino suppresses upwelling off Peru's coast. Normally, trade winds push warm surface water west, and cold, nutrient-rich deep water rises (upwelling) to feed phytoplankton → fish. During El Nino, this stops → fisheries collapse → cascading ecological effects.
During an El Nino event, the anchovy fishery off the coast of Peru typically declines because
❌ El Nino = warm water in EASTERN Pacific: During El Nino, warm water accumulates in the EASTERN tropical Pacific (near Peru). Don't say "warm water moves to the western Pacific" — that's normal conditions. El Nino REVERSES the normal pattern.
❌ Confusing El Nino and La Nina effects. El Nino = warm eastern Pacific, weak trade winds, reduced upwelling, wet US West Coast. La Nina = cool eastern Pacific, strong trade winds, enhanced upwelling, dry US West Coast. They are OPPOSITES.
A coastal city at 30°N latitude experiences arid conditions and is located on the leeward side of a mountain range.
(a) Explain why deserts commonly form at 30° latitude. Reference global atmospheric circulation in your answer.
(b) Describe how the rain shadow effect contributes to the city's aridity. Include the role of orographic lift.
(c) During an El Nino year, the city receives unusually heavy rainfall. Explain the mechanism by which El Nino alters precipitation patterns at this location.
(d) Identify ONE positive and ONE negative ecological effect of the increased rainfall during El Nino.
Freshwater systems are divided into lotic (flowing: streams, rivers) and lentic (still: lakes, ponds). Only 2.5% of Earth's water is freshwater, and only 0.3% is accessible surface freshwater.
| Lake Zone | Location | Characteristics |
|---|---|---|
| Littoral | Near shore, shallow | Most biodiverse; sunlight reaches bottom; rooted plants, frogs, insects |
| Limnetic | Open water, sunlit | Phytoplankton, zooplankton; primary production zone |
| Profundal | Deep, dark water | No photosynthesis; cold; low oxygen; decomposers dominate |
| Benthic | Bottom sediments | Decomposition zone; detritivores; nutrient recycling |
Oligotrophic: deep, cold, clear, low nutrients, high O₂, low productivity (pristine mountain lakes). Eutrophic: shallow, warm, murky, high nutrients, low O₂, high productivity (algal blooms). Cultural eutrophication accelerates this transition through nutrient pollution.
Which lake zone has the highest biodiversity due to abundant sunlight and rooted aquatic vegetation?
Wetlands are areas where land is saturated with water permanently or seasonally. They are among the most productive and valuable ecosystems on Earth, providing disproportionate ecosystem services relative to their area.
| Wetland Type | Characteristics | Examples |
|---|---|---|
| Marshes | Grasses, reeds, shallow water; freshwater or saltwater | Everglades (freshwater), salt marshes (coastal) |
| Swamps | Trees dominate; standing water | Cypress swamps, mangrove swamps (coastal) |
| Bogs | Acidic, peat-accumulating, sphagnum moss | Northern peatlands, cranberry bogs |
| Fens | Alkaline, groundwater-fed, more diverse than bogs | Calcareous fens in limestone areas |
Wetlands trap sediments and absorb pollutants (nitrogen, phosphorus, heavy metals). Called "nature's kidneys."
1 acre of wetland stores 1-1.5 million gallons of floodwater. Absorbs and slowly releases water, reducing downstream flooding.
Peatlands store ~30% of global soil carbon despite covering only 3% of land. Draining peatlands releases massive CO₂.
Support 40% of the world's species for breeding and migration. Nursery habitat for fish, shrimp, and many waterfowl.
Over 50% of global wetlands lost since 1900 due to: draining for agriculture, urbanization, dam construction, pollution, and climate change. Loss of wetlands increases flood risk and reduces water quality downstream.
Which ecosystem service is most directly reduced when coastal wetlands are drained for development?
❌ Undervaluing wetlands: Wetlands are NOT "wastelands." They provide flood control, water filtration, carbon storage, and biodiversity habitat. One acre stores 1-1.5 million gallons of floodwater.
❌ Confusing marshes with swamps. Marshes = grasses and reeds (Everglades). Swamps = trees (cypress swamps, mangroves). Both are wetlands but have different dominant vegetation.
A farming community is located upstream in a watershed. Downstream, a lake has experienced increasing algal blooms and fish kills over the past decade. Adjacent wetlands were drained for housing development five years ago.
(a) Explain the process of cultural eutrophication and how agricultural activities upstream contribute to algal blooms in the lake.
(b) Describe TWO ecosystem services that were lost when the wetlands were drained. Explain how each loss worsens the lake's water quality problems.
(c) Identify the lake zones most affected by eutrophication and explain why fish kills occur.
(d) Propose TWO soil conservation practices the farming community could implement to reduce nutrient runoff. Explain the mechanism of each.
Mixed MCQ and FRQ in AP APES exam style. Attempt each before revealing the answer.
Volcanic soils near convergent plate boundaries are often highly fertile for agriculture primarily because
During a La Nina event, which of the following changes would be expected along the western coast of South America?
A geologist studying a region discovers a sequence of soil horizons (O-A-B-C-R) developed on basaltic bedrock near a dormant volcano at a convergent plate boundary.
(a) Describe how the basaltic bedrock (R horizon) is transformed into the C and B horizons through weathering processes. Identify ONE physical and ONE chemical weathering mechanism.
(b) Explain why the A horizon in this volcanic region is likely to be highly fertile. Reference soil composition.
(c) A nearby mountain receives heavy rainfall on its windward side but has sparse vegetation on the leeward side. Explain how this affects soil development on each side of the mountain.
(d) Propose TWO land management practices to protect the fertile volcanic soil from erosion on sloped terrain.
A city plans to develop a 500-acre wetland area adjacent to a river for a shopping complex. Environmental groups oppose the project.
(a) Identify and explain THREE ecosystem services provided by the wetland that would be lost if it is developed.
(b) Predict TWO downstream effects on the river and its watershed if the wetland is removed.
(c) The development would replace the wetland with impervious surfaces. Explain how this change affects the local water cycle, specifically infiltration and surface runoff.
Focus study time on plate boundary types and their features (most frequently confused), soil horizons and composition (know O-A-E-B-C-R order), and El Nino/La Nina mechanisms (upwelling, trade winds, global effects). The AP exam frequently tests rain shadow effect, watershed pollution sources, and wetland ecosystem services in FRQ format. Practice distinguishing weathering from erosion and point from nonpoint pollution — these distinctions appear on nearly every exam.