Unit 4 Convergent boundary Tap / Space to flip
Unit 4 Plates collide. Subduction trenches (oceanic-continental), volcanic arcs, or mountain belts (continental-continental).
Unit 4 · 10–15% of exam
Earth Systems and Resources
Plate tectonics, soil, atmosphere structure, watersheds, global wind, El Niño / La Niña.
Must-know content
- Plate tectonics:
- Divergent — plates pull apart; mid-ocean ridges, rift valleys.
- Convergent — plates collide; subduction trenches, volcanic arcs, mountain ranges.
- Transform — plates slide past each other; San Andreas. Earthquakes; no volcanism.
- Ring of Fire — Pacific rim convergent boundaries with frequent earthquakes & volcanoes.
- Soil:
- Horizons (top to bottom): O (organic litter), A (topsoil + humus), E (eluviation/leaching), B (subsoil/illuviation), C (parent material), R (bedrock).
- Texture: sand, silt, clay. Use the soil texture triangle to classify.
- Loam — ideal agricultural soil, ~40% sand, 40% silt, 20% clay. Drains well, holds nutrients.
- Atmosphere layers (bottom up):
- Troposphere (0-12 km) — weather, decreasing T with altitude.
- Stratosphere (12-50 km) — ozone layer, INCREASING T with altitude (ozone absorbs UV).
- Mesosphere (50-85 km) — coldest layer, meteors burn up.
- Thermosphere (85+ km) — hot but very thin; aurora.
- Global wind cells: Hadley (0–30°), Ferrel (30–60°), Polar (60–90°). Coriolis effect deflects winds.
- Watersheds — land area drained by a single river system. Pollutants entering anywhere flow downstream.
- Earth\'s seasons — caused by 23.5° axial tilt, NOT distance from sun.
- El Niño Southern Oscillation (ENSO):
- El Niño — weakened trade winds; warm Pacific water shifts east. Drought in Australia/SE Asia, heavy rain on west Americas, collapsed Peruvian fishery.
- La Niña — strengthened trade winds; cold upwelling off South America. Opposite weather effects.
Example questions
MCQ Which soil horizon is the topsoil rich in organic matter? (A) O (B) A (C) B (D) C
Answer: B. The A horizon is topsoil — a mix of weathered minerals and humus. The O horizon above it is loose leaf litter; B is subsoil; C is parent rock material.
FRQ Explain how El Niño disrupts the Peruvian anchovy fishery.
Answer: Normally, strong easterly trade winds drive warm surface water westward across the Pacific, causing cold, nutrient-rich deep water to upwell along the Peruvian coast. This upwelling supports rich phytoplankton growth, which feeds large anchovy populations. During an El Niño event, trade winds weaken, the warm pool shifts east, and upwelling is suppressed. Without nutrient supply, phytoplankton populations crash, the food chain collapses, and the anchovy fishery declines sharply.
MCQ Which boundary type produces the San Andreas Fault? (A) Convergent (B) Divergent (C) Transform (D) Subduction
Answer: C. The Pacific and North American plates slide horizontally past each other along the San Andreas — a transform boundary. Earthquakes, but no volcanism.
Drill flashcards
Unit 4 Divergent boundary Tap / Space to flip
Unit 4 Plates separate. Mid-ocean ridges (oceanic) or rift valleys (continental). New crust forms.
Unit 4 Transform boundary Tap / Space to flip
Unit 4 Plates slide past each other (e.g., San Andreas fault). Earthquakes; no volcanism.
Unit 4 Soil horizons Tap / Space to flip
Unit 4 O (organic), A (topsoil), E (eluviation/leaching), B (subsoil/illuviation), C (parent material), R (bedrock).
Unit 4 Loam Tap / Space to flip
Unit 4 Ideal agricultural soil — roughly equal parts sand, silt, and clay. Drains well and holds nutrients.
Unit 4 Troposphere Tap / Space to flip
Unit 4 Lowest layer (0-12 km). Contains weather; temperature decreases with altitude. We live here.
Unit 4 Stratosphere Tap / Space to flip
Unit 4 12-50 km. Contains the ozone layer; temperature increases with altitude due to ozone absorbing UV.
Unit 4 Watershed Tap / Space to flip
Unit 4 Land area drained by a single river system into a body of water. Pollutants entering anywhere flow downstream.