Free Unit Notes

​

I. Plate Tectonics (Topic 4.1)

  A. Earth’s moving outer shell

1. Earth has a rocky crust, a thick mantle, and a metal core with a liquid outer layer and solid inner layer.

2. The lithosphere includes the crust and rigid uppermost mantle. It is divided into moving tectonic plates.

3. The asthenosphere is the hot, mostly solid upper mantle beneath the lithosphere. It flows slowly, allowing plates to move.

4. Internal heat drives mantle movement. Sinking plates and gravity at raised ocean ridges also help move plates.

  B. Plate boundaries

1. At a divergent boundary, plates separate and magma forms new crust. Seafloor spreading creates mid-ocean ridges; continental separation creates rift valleys.
   a. The Mid-Atlantic Ridge and East African Rift are examples. Both earthquakes and volcanoes can occur.

2. At a convergent boundary, plates move toward each other. Dense oceanic lithosphere can sink beneath another plate through subduction.
   a. Ocean–continent convergence forms a trench and a volcanic mountain chain, such as the Andes.
   b. Ocean–ocean convergence forms a trench and volcanic island arc, such as the Aleutian Islands.
   c. Continent–continent collision thickens and folds crust into mountains, such as the Himalayas. Strong earthquakes occur, but a volcanic arc usually does not form.

3. At a transform boundary, plates slide sideways. Crust is neither created nor destroyed. Earthquakes occur along faults such as the San Andreas Fault.

  C. Earthquakes, volcanoes, and evidence

1. A fault is a break in rock where movement occurs. When stress overcomes a locked fault, rocks slip and release stored energy as seismic waves.

2. The focus is where an earthquake begins underground. The epicenter is the surface location directly above it.

3. Sudden vertical movement of a large seafloor area can displace water and cause a tsunami. Not every undersea earthquake causes one.

4. Earthquake and volcano belts often mark plate boundaries. The Pacific Ring of Fire contains many subduction zones.

5. A hot spot can produce volcanoes within a plate. Hawaii’s island chain records the Pacific Plate moving over a hot spot.

6. Matching fossils and rocks across continents, symmetrical magnetic bands, and ocean crust that gets older away from ridges support plate movement.

II. Soil Formation and Erosion (Topic 4.2)

  A. Soil development

1. Soil contains mineral particles, organic matter, water, air, and organisms. Its parent material is the rock or sediment from which it develops.

2. Physical weathering breaks rock into smaller pieces without changing its chemistry. Freezing water and growing roots can widen cracks.

3. Chemical weathering changes minerals through reactions with water, acids, or oxygen. Weak acids can dissolve limestone.

4. Erosion moves particles; deposition drops them elsewhere. Soil can develop over bedrock or in transported sediment.

5. Decomposers release nutrients from dead material. Humus, relatively persistent decomposed organic matter, helps hold water and nutrients.

6. Parent material, climate, organisms, topography, and time affect soil formation. Warm, wet conditions often speed weathering; heavy rain can increase nutrient loss.

  B. Soil horizons

1. A soil profile is a vertical view of soil. Its layers, called horizons, differ in composition and organic matter. Not all soils have every horizon.

2. The O horizon contains surface organic material. The A horizon, or topsoil, contains minerals mixed with organic matter and supports many roots.

3. The E horizon is a pale layer that loses clay, iron, and other materials as water moves downward.

4. The B horizon, or subsoil, can accumulate clay and minerals from above. The C horizon contains partly weathered parent material; the R layer is bedrock.

5. Leaching carries dissolved substances downward. Heavy leaching can remove nutrients from the root zone and reduce fertility.

  C. Erosion and water quality

1. Wind and flowing water erode exposed soil. Clearing vegetation, overgrazing, steep slopes, and intense rain can increase losses.

2. Topsoil loss removes nutrients, organic matter, and rooting material. Soil often forms much more slowly than it erodes.

3. Sediment clouds water, reduces light, and covers aquatic habitats. Attached nutrients and pesticides can also enter streams.

4. Roots hold soil, leaves reduce raindrop impact, and plant litter slows runoff. Contour planting and terraces slow water on slopes.

5. Soil can trap particles and help remove some pollutants from infiltrating water. Its filtering ability is limited; pollutants can still reach groundwater.

III. Soil Composition and Properties (Topic 4.3)

  A. Texture and structure

1. Soil texture is the relative percentage of sand, silt, and clay. These mineral fractions total 100%; organic matter and gravel are excluded.

2. Sand has the largest particles, silt has intermediate particles, and clay has the smallest particles.

3. A soil texture triangle identifies texture from these percentages. Follow each percentage’s grid line to their intersection and read the named region.
   a. Clay lines run horizontally; sand lines parallel the triangle’s right side; silt lines parallel its left side.
   b. If two percentages are known, subtract their sum from 100 to find the third. A mixture of 40% sand, 40% silt, and 20% clay is loam.

4. Loam contains a useful balance of sand, silt, and clay. It supports many crops by retaining water while allowing drainage and air movement.

5. Soil structure describes how particles form clumps called aggregates. Organic matter and compaction affect pore spaces and water movement.

  B. Pore space and water movement

1. Porosity is the fraction of soil volume occupied by pores. Permeability describes how easily water moves through connected pores.

2. Sandy soils usually drain quickly through large pores and retain fewer nutrients. Clay-rich soils usually drain slowly and retain more water and nutrients.

3. Clay can have high total porosity but low permeability because its pores are small. Larger particles do not automatically mean greater total porosity.

4. Infiltration is water entering the surface; percolation is water moving downward through soil. Compaction reduces large pores, drainage, and root oxygen.

5. Water-holding capacity is the amount of water soil can retain under stated conditions. Some water in clay is held too tightly for roots to use.

6. Field capacity is water content after a thoroughly wetted soil drains freely. The permanent wilting point is the content at which plants cannot recover without added water.

7. Plant-available water equals water at field capacity minus water at the permanent wilting point. Loamy soils often provide more usable water than very sandy or clay-rich soils.

  C. Fertility and soil testing

1. Fertility is soil’s ability to supply plant nutrients. Nitrogen, phosphorus, and potassium are important, but nutrients alone cannot fix poor drainage or compaction.

2. Clay and organic matter hold positively charged nutrients. This ability, called cation exchange capacity, helps reduce nutrient loss.

3. pH measures acidity or basicity and affects nutrient availability. Below 7 is acidic; above 7 is basic. Each pH unit represents a tenfold change in hydrogen ion concentration.

4. Physical tests measure texture, drainage, and water retention. Chemical tests measure pH and nutrients. Organisms and microbial activity indicate biological conditions.

5. To compare drainage, use equal soil volumes, identical containers, equal packing, and the same water input and measurement time. Repeat trials and compare averages.
   a. Soil type is the independent variable; drainage volume or rate is the dependent variable.
   b. Average drainage rate = drained water volume ÷ elapsed time. A sample draining 60 mL in 5 minutes averages 12 mL/min under those conditions.

6. For initially dry soil, water retained = water added − water drained, if leaks and evaporation are negligible. State the drainage time; immediate retention is not necessarily field capacity.

7. Sample several locations at the same depth for a representative result. Use results to guide irrigation, fertilizer, and soil management decisions.

IV. Earth’s Atmosphere (Topic 4.4)

  A. Atmospheric composition

1. The atmosphere is the layer of gases surrounding Earth. Dry air near the surface is approximately 78% nitrogen, 21% oxygen, and 0.93% argon, with small amounts of other gases.

2. Water vapor varies by location and weather. Carbon dioxide and water vapor are greenhouse gases that absorb and emit infrared radiation.

3. Air pressure and density generally decrease with altitude because less air lies above each higher location.

  B. Layers and temperature patterns

1. The troposphere is the lowest layer and contains most weather and water vapor. Temperature generally decreases upward because Earth’s surface heats the air above it.

2. The stratosphere lies above it. Temperature increases upward as ozone absorbs ultraviolet radiation. The ozone layer protects life from much harmful UV.

3. In the mesosphere, temperature decreases upward. Many meteors burn up here.

4. In the thermosphere, temperature increases as sparse gases absorb high-energy solar radiation. Thin air transfers little heat despite its high temperature.

5. The exosphere is the outermost layer and gradually blends into space. Layer boundaries vary; temperature trends distinguish the main layers.

6. Ozone’s absorption of UV and greenhouse gases’ absorption of infrared radiation are different processes.

V. Global Wind Patterns (Topic 4.5)

  A. Uneven heating and the Coriolis effect

1. Strong equatorial heating warms air, making it less dense and causing it to rise. Rising air expands and cools; condensation can produce clouds and rain.

2. Cooler, denser air tends to sink. Rising air is associated with low surface pressure; sinking air is associated with high surface pressure.

3. Convection transfers heat through fluid movement. Surface pressure differences drive wind from higher toward lower pressure.

4. Earth’s rotation causes the Coriolis effect, which deflects motion to the right in the Northern Hemisphere and left in the Southern Hemisphere. Deflection is weakest at the equator.

  B. Circulation cells and rainfall belts

1. Hadley cells extend from about 0° to 30° in each hemisphere. Air rises near the equator, moves poleward aloft, sinks near 30°, and returns toward the equator at the surface.

2. The intertropical convergence zone (ITCZ) is where trade winds meet and moist air rises. Frequent rainfall supports tropical forests; seasonal shifts produce wet and dry seasons.

3. Air sinking near 30° compresses and warms, lowering relative humidity. These dry, high-pressure belts help explain many subtropical deserts.

4. Ferrel cells occur around 30°–60°. Their average surface flow moves poleward; air tends to rise where it meets colder polar air near 60°, supporting storms.

5. Polar cells extend from about 60° to 90°. Cold air sinks near the poles and flows toward 60° at the surface. Polar regions are dry because sinking, cold air provides little moisture.

  C. Prevailing surface winds

1. Trade winds blow toward the equator from the northeast in the Northern Hemisphere and southeast in the Southern Hemisphere.

2. Westerlies blow generally west to east between 30° and 60°. Polar easterlies blow generally east to west between 60° and 90°.

3. Wind names identify where winds come from. Mountains, continents, and seasons modify these broad patterns.

4. Winds help drive surface ocean currents, which move heat and affect regional temperature, rainfall, and marine nutrient supplies.

VI. Watersheds (Topic 4.6)

  A. Drainage and watershed characteristics

1. A watershed, or drainage basin, is the land area that drains runoff to a common outlet. It includes the land between streams, and smaller watersheds can lie within larger ones.

2. A drainage divide is high land separating neighboring watersheds. Water on opposite sides can enter different river systems.

3. Area determines how much rain a watershed can collect. Length and shape affect runoff travel time; steep slopes often deliver water faster.

4. Permeable soil favors infiltration and groundwater recharge, the addition of water to underground storage. Saturated or compacted soil often produces more runoff.

5. Vegetation intercepts rain, slows runoff, and holds soil. Wetlands and floodplains can store water and reduce peak flow.

  B. Land use, pollution, and calculations

1. Impervious surfaces, such as pavement and roofs, allow little infiltration. Development often increases rapid runoff, flooding, and streambank erosion while reducing recharge.

2. Runoff carries sediment, fertilizer, pesticides, and oil downstream. Vegetated stream buffers trap sediment and absorb some nutrients.

3. Rainfall volume = rainfall depth × watershed area, using compatible units. Runoff volume = rainfall volume × the stated runoff fraction.
   a. A 0.020 m rainfall over 3,000,000 m² supplies 60,000 m³ of water. If 30% becomes direct runoff, runoff volume is 18,000 m³.

4. A hydrograph shows stream discharge over time. Development often creates an earlier, higher peak for a comparable storm.

5. Discharge is water volume passing a point per unit time, commonly m³/s. It differs from total runoff volume, measured in m³.

VII. Solar Radiation and Earth’s Seasons (Topic 4.7)

  A. Solar angle and energy input

1. Insolation is incoming solar radiation. Energy received per unit area per unit time can be measured in W/m².

2. Direct rays concentrate energy over a smaller area. Low-angle rays spread it over a larger area and travel through more atmosphere.

3. Over the year, low latitudes generally receive more solar energy per unit area than high latitudes. Uneven heating drives atmospheric and ocean circulation.

4. Day length also affects daily energy input. Seasonal changes in solar angle and daylight change how much energy a location receives.

  B. Axial tilt and seasons

1. Earth’s axis is tilted about 23.5° from a line perpendicular to its orbital plane. It points nearly the same way as Earth orbits the Sun.

2. A hemisphere tilted toward the Sun has more direct rays and longer days, producing summer. The other hemisphere has less direct rays and shorter days, producing winter.

3. At the June solstice, the most direct midday rays reach 23.5° N. The Northern Hemisphere has its longest daylight period; the Southern Hemisphere has its shortest.

4. At the December solstice, the most direct midday rays reach 23.5° S. Daylight patterns reverse. Polar regions can experience 24-hour daylight or darkness.

5. At the March and September equinoxes, the most direct rays reach the equator. Most places have approximately 12 hours each of daylight and darkness.

6. Seasons result from axial tilt, rather than changing Earth–Sun distance. The hemispheres have opposite seasons at the same distance from the Sun.

  C. Reflection and absorption

1. Albedo is the fraction of incoming radiation reflected by a surface. Snow and ice have high albedo; dark soil and forests generally have lower albedo.

2. Reflected energy = incoming energy × albedo. With negligible transmission, absorbed energy = incoming energy × (1 − albedo).

3. A surface receiving 500 W/m² with an albedo of 0.30 reflects 150 W/m² and absorbs 350 W/m².

VIII. Earth’s Geography and Climate (Topic 4.8)

  A. Weather, climate, and location

1. Weather is the short-term atmospheric condition at a place. Climate is its long-term pattern and variation, commonly described using about 30 years of data.

2. Latitude affects solar input. Elevation, mountains, prevailing winds, nearby water, and ocean currents modify temperature and precipitation.

3. Temperature generally decreases with elevation in the troposphere. Higher locations are often cooler than nearby lowlands at the same latitude.

  B. Mountains and rain shadows

1. Moist air forced up a mountain expands and cools. Condensation forms clouds, and precipitation can fall on the windward side, which faces the incoming wind.

2. Air descends the leeward side, compresses, and warms. Its relative humidity decreases, making further precipitation less likely.

3. A rain shadow is the dry region beyond the mountain. The Sierra Nevada helps create dry conditions farther inland.

  C. Oceans and regional climate

1. Water warms and cools more slowly than land. Coastal areas often have milder summers and winters than inland areas, especially where winds bring ocean air ashore.

2. Ocean currents move heat. Warm currents can warm nearby air and supply moisture; cold coastal currents can cool air and reduce evaporation.

3. Prevailing winds carry this heat and moisture inland. Places at the same latitude can have different climates because their geography differs.

IX. El Niño and La Niña (Topic 4.9)

  A. Neutral Pacific conditions

1. El Niño–Southern Oscillation (ENSO) is a recurring interaction among tropical Pacific ocean temperatures, winds, and air pressure. Events occur irregularly, often two to seven years apart.

2. Normally, trade winds push warm surface water west toward Indonesia and Australia. Rising moist air supports heavy rain in the western tropical Pacific.

3. In the eastern equatorial Pacific, upwelling brings colder, nutrient-rich deep water toward the surface. Nutrients support phytoplankton and marine food webs.

4. The thermocline is a layer where water temperature decreases rapidly with depth. Its depth affects the temperature and nutrient content of upwelled water.

  B. El Niño conditions and effects

1. During El Niño, trade winds weaken and warm surface water spreads east. The central and eastern equatorial Pacific become warmer than usual.

2. The eastern thermocline deepens, reducing the delivery of cold, nutrient-rich water. Lower nutrient supply can reduce phytoplankton growth and fish productivity.

3. Tropical rainfall shifts east. Indonesia and parts of Australia often face drought and wildfire risk; parts of western South America face heavier rain, flooding, and landslides.

4. During many El Niño winters, the southern United States tends to be wetter and the northern United States warmer than usual.

  C. La Niña conditions and effects

1. During La Niña, trade winds strengthen and push more warm water west. The central and eastern equatorial Pacific become cooler than usual.

2. The eastern thermocline becomes shallower and upwelling generally strengthens, increasing nutrient delivery and potentially marine productivity.

3. Indonesia and parts of Australia often become wetter, increasing flood risk. Parts of western South America may become drier.

4. During many La Niña winters, the southern United States tends to be warmer and drier, while the Pacific Northwest tends to be cooler and wetter.

5. ENSO changes global wind, rainfall, and ocean circulation patterns. Effects vary with place, season, and event strength; these patterns are tendencies, not guarantees.

6. ENSO-related rain changes can alter erosion, streamflow, groundwater recharge, and water availability. ENSO is an irregular ocean–atmosphere pattern, distinct from tilt-driven annual seasons.

© 2026 The Knowledge Cannon®. All rights reserved.

AP® is a trademark registered by the College Board, which is not affiliated with, and does not endorse, this product.