AP Environmental Science Unit 4 Notes: Geology and Soil Systems

Tectonic plates

Large, rigid pieces of Earth’s lithosphere that move slowly over the softer, ductile asthenosphere.

Lithosphere

Earth’s crust plus the uppermost solid mantle; the rigid layer broken into tectonic plates.

Asthenosphere

Hot, mostly solid mantle layer beneath the lithosphere that deforms plastically over long time scales (not a liquid magma ocean).

Ridge push

Force that helps drive plates away from mid-ocean ridges as newly formed crust cools, thickens, and slides downslope from the elevated ridge.

Slab pull

Force that helps drive plate motion when old, cold, dense oceanic lithosphere sinks at a subduction zone and pulls the rest of the plate along.

Divergent boundary

Plate boundary where plates move apart; magma rises to form new crust, producing mid-ocean ridges or continental rifts, basaltic volcanism, and shallow earthquakes.

Convergent boundary

Plate boundary where plates move together; may involve subduction (oceanic plate sinks) or collision (continent-continent), producing trenches, volcanoes (in subduction), mountains, and earthquakes.

Subduction zone

Region at a convergent boundary where one plate (usually oceanic) sinks into the mantle, often generating deep trenches, earthquakes from shallow to deep, and explosive volcanism.

Transform boundary

Plate boundary where plates slide horizontally past each other; characterized by shallow earthquakes and generally little to no volcanism.

Hot spot

Localized volcanic activity away from plate boundaries, often linked to a relatively stationary mantle upwelling; can form age-progressive chains of volcanoes as a plate moves.

Tsunami

Large sea wave often triggered by major undersea earthquakes at subduction zones, typically requiring significant vertical displacement of the seafloor.

Soil

Dynamic mixture of weathered mineral material, organic matter, water, air, and living organisms that forms a layer over bedrock and supports ecosystems and plant growth.

Parent material

The original rock or sediment that soil forms from; influences soil texture, mineral content, and potential fertility.

Weathering

Breakdown of rock at or near Earth’s surface; includes physical (mechanical) and chemical processes that contribute to soil formation.

Physical (mechanical) weathering

Breaks rock into smaller pieces without changing chemical composition (e.g., freeze–thaw, abrasion, root growth); increases surface area for chemical weathering.

Chemical weathering

Alters minerals through reactions (e.g., hydrolysis, oxidation, dissolution), releasing ions and helping form clays important for nutrients and soil properties.

CLORPT

Mnemonic for the five soil-forming factors: Climate, Organisms, Relief (topography), Parent material, and Time.

Soil horizons (O-A-E-B-C-R)

Common layered sequence in a developed soil profile: O (organic), A (topsoil), E (eluviation/leaching), B (illuviation/accumulation), C (weathered parent), R (bedrock).

Eluviation

Leaching/removal of dissolved or fine materials from upper soil horizons (often associated with the E horizon).

Illuviation

Deposition/accumulation of leached materials (such as clays or oxides) in lower soil horizons (often the B horizon).

Erosion

Transport of soil particles by wind, water, ice, or gravity; can be greatly accelerated by human activities, causing fertility loss and water-quality impacts.

Porosity

Amount of total pore space in soil (how much water/air it can store).

Permeability

How easily water moves through soil via connected pores (movement), which can differ from porosity.

Humus

Dark, stable organic material formed from decomposed plant/animal matter that improves water-holding capacity, soil structure, and nutrient availability.

Cation exchange capacity (CEC)

Measure of how well soil holds and exchanges positively charged nutrient ions (e.g., Ca²⁺, K⁺, Mg²⁺); generally increases with more clay and organic matter.