Unit 4: Earth Systems and Resources is one of the most content-rich units on the APES exam. It covers geological processes, soil composition, atmospheric circulation, and climate patterns, often tying these concepts to environmental problems and human impacts. Expect a mix of MCQs, FRQs, and data analysis.
In this RevisionDojo review, we’ll break down all the high-priority topics and give you AP exam strategies.
📌 Key Topics in Unit 4
- Earth’s structure (core, mantle, crust)
- Plate tectonics and boundaries
- Rock cycle and mineral resources
- Soil formation and properties
- Soil conservation techniques
- Earthquakes, volcanoes, and natural hazards
- Atmosphere layers and properties
- Global wind patterns and Coriolis effect
- Watershed and drainage basins
- El Niño–Southern Oscillation (ENSO)
🌍 Earth’s Structure & Plate Tectonics
- Core – Inner (solid) and outer (liquid iron/nickel).
- Mantle – Convection drives plate movement.
- Crust – Oceanic and continental plates.
Plate Boundaries:
- Divergent → plates move apart (mid-ocean ridges).
- Convergent → plates collide (mountains, trenches).
- Transform → plates slide past (earthquakes).
🪨 Rock Cycle
- Igneous – from cooled magma/lava.
- Sedimentary – from compacted sediments.
- Metamorphic – transformed by heat & pressure.
🌱 Soil Science
- Soil Horizons: O (organic), A (topsoil), B (subsoil), C (parent material), R (bedrock).
- Properties: texture (sand, silt, clay), permeability, porosity, nutrient content.
- Threats: erosion, desertification, salinization.
Soil Conservation: crop rotation, contour plowing, terracing, agroforestry.
🌬 Atmosphere & Circulation
- Layers: Troposphere, Stratosphere (ozone layer), Mesosphere, Thermosphere, Exosphere.
- Wind patterns shaped by Coriolis effect and convection cells (Hadley, Ferrel, Polar).
🌊 Watersheds & Water Resources
- Watersheds funnel precipitation to rivers and lakes.
- Management is key to controlling flooding, pollution, and water quality.
🌎 Climate Patterns – ENSO
- El Niño → warmer Pacific waters, altered weather patterns, reduced upwelling.
- La Niña → cooler Pacific waters, opposite impacts.
⚠ Natural Hazards in Unit 4
- Earthquakes (seismic activity at plate boundaries)
- Volcanoes (lava, ash, gases)
- Tsunamis (underwater earthquakes/volcanoes)
🧠 APES Exam Tips for Unit 4
- Memorize plate boundary types and examples.
- Be ready to interpret soil texture triangles.
- Practice FRQs that involve ENSO data interpretation.
- Link geological processes to resource availability and human impacts.
💡 FAQ – Unit 4: Earth Systems and Resources
Q: Which plate boundary creates mountains?
A: Convergent (continental-continental collision).
Q: What is the best soil for agriculture?
A: Loam — balanced sand, silt, clay, good drainage and nutrients.
Q: How often does El Niño occur?
A: Roughly every 3–7 years.
Final Word from RevisionDojo:
Unit 4 combines Earth science fundamentals with environmental problem-solving. Mastering this unit will help you handle both MCQs and FRQs with confidence.
