Inside This Unit: The Full Breakdown
Earth Systems and Resources covers the geologic processes and natural resources that shape the planet. This unit includes plate tectonics, soil formation, the rock cycle, and the atmosphere — the physical systems that support all life.
Why it matters
Understanding Earth's physical systems is essential for the AP Environmental Science exam. Soil science, plate tectonics, and atmospheric structure explain natural hazards, resource availability, and the physical basis for climate and weather.
Key concepts
- Plate tectonics explains earthquakes, volcanoes, and mountain building. Convergent, divergent, and transform boundaries produce different geological features.
- Soil is composed of mineral particles, organic matter, water, and air. Soil horizons (O, A, B, C, R) form through weathering and decomposition over time.
- The rock cycle connects igneous, sedimentary, and metamorphic rocks through processes of melting, cooling, weathering, erosion, and heat/pressure.
- Earth's atmosphere has distinct layers (troposphere, stratosphere, mesosphere, thermosphere) with different temperature profiles and functions.
Plate Tectonics
Earth's lithosphere is divided into large plates that float on the partially molten asthenosphere. These tectonic plates move due to convection currents in the mantle. At divergent boundaries, plates pull apart and new crust forms (mid-ocean ridges). At convergent boundaries, plates collide — oceanic plates subduct beneath continental plates, causing volcanic arcs and deep ocean trenches. Two continental plates colliding create mountain ranges like the Himalayas. At transform boundaries, plates slide past each other, causing earthquakes (like the San Andreas Fault). Plate tectonics explains the distribution of earthquakes, volcanoes, and mineral resources around the globe. Understanding plate boundaries is important for assessing natural hazard risks and locating geological resources.
Soil Science
Soil is a complex mixture of weathered rock particles, organic matter (humus), water, air, and living organisms. It forms through the physical and chemical weathering of parent rock material over hundreds to thousands of years. A mature soil profile has distinct horizons: the O horizon (organic litter), A horizon (topsoil, rich in organic matter), B horizon (subsoil, where minerals accumulate), C horizon (weathered parent material), and R horizon (bedrock). Soil texture — the relative proportions of sand, silt, and clay particles — determines water-holding capacity, drainage, and nutrient retention. Loam (a balanced mixture) is ideal for agriculture. Soil erosion, caused by wind, water, and poor farming practices, is a serious environmental problem because topsoil takes centuries to form but can be lost in years.
Atmosphere and Weather
Earth's atmosphere is divided into layers based on temperature patterns. The troposphere (0-12 km) contains most weather and atmospheric mass; temperature decreases with altitude. The stratosphere (12-50 km) contains the ozone layer, which absorbs UV radiation, causing temperature to increase with altitude. The mesosphere and thermosphere extend above. The atmosphere is primarily nitrogen (78%) and oxygen (21%), with trace gases including CO₂, methane, and water vapor that play outsized roles in the greenhouse effect. Weather is driven by unequal heating of Earth's surface, which creates convection cells (Hadley, Ferrel, Polar) that distribute heat from the equator toward the poles. The Coriolis effect deflects wind patterns, creating prevailing winds and influencing ocean currents.
AP exam tip
For soil questions on the AP exam, always identify the specific soil horizon (O, A, B, C) involved and its properties. When discussing soil erosion, name the specific cause (tillage, deforestation, overgrazing) and at least one conservation method (contour plowing, terracing, cover crops).
Connections to other units
- Unit 1 (Ecosystems): Soil quality directly affects terrestrial ecosystem productivity through nutrient availability.
- Unit 7 (Atmospheric Pollution): Atmospheric structure determines where pollutants concentrate and how they affect climate.
- Unit 9 (Global Change): Plate tectonics, atmospheric composition, and soil carbon storage all connect to long-term climate patterns.
