Unit 4: Earth Systems and Resources

Plate tectonics

The theory that Earth’s lithosphere is broken into tectonic plates that move relative to one another, concentrating earthquakes, volcanoes, and mountain building at plate boundaries.

Lithosphere

Earth’s rigid outer shell (crust plus uppermost mantle) that is broken into moving tectonic plates.

Asthenosphere

The softer, ductile layer beneath the lithosphere that flows slowly and allows plates to move above it.

Continental drift

Wegener’s early idea that continents were once joined and have moved over time, though he lacked a convincing mechanism.

Pangaea

The single supercontinent that included all present-day continents and began breaking apart about 200 million years ago.

Seafloor spreading

Process where new oceanic crust forms at mid-ocean ridges and moves outward; seafloor rock gets older with increasing distance from the ridge.

Mid-ocean ridge

An underwater mountain chain (spreading center) where plates diverge and magma rises to create new oceanic crust.

Magnetic striping

Alternating, symmetrical magnetic patterns (“stripes”) in seafloor rocks on both sides of mid-ocean ridges, supporting seafloor spreading.

Oceanic crust

Thin, dense crust that forms the ocean floor and commonly subducts beneath less dense plates at convergent boundaries.

Continental crust

Thicker, less dense crust that forms continents and typically resists subduction compared with oceanic crust.

Mantle convection

Heat-driven circulation in the mantle (hot rises, cool sinks) that helps drive plate motion.

Divergent boundary

A plate boundary where plates move apart, producing new crust, shallow earthquakes, and volcanic activity (e.g., mid-ocean ridges).

Convergent boundary

A plate boundary where plates move toward each other, causing subduction (often with volcanoes and trenches) or continental collision (mountain building).

Subduction zone

A convergent boundary where one plate slides beneath another into the mantle, producing deep trenches, strong earthquakes, and often volcanic arcs.

Transform boundary

A plate boundary where plates slide past each other, producing earthquakes with little volcanism (e.g., San Andreas Fault).

Hot spot

A volcanic source formed by a mantle plume melting through the lithosphere away from plate boundaries; can create a chain of volcanoes as a plate moves.

Earthquake

A sudden release of energy when rocks break and slip along a fault after stress builds up and overcomes friction.

Epicenter

The point on Earth’s surface directly above where an earthquake originates; shaking generally weakens with distance from it.

Fault

A fracture in Earth’s crust along which rocks can slip, producing earthquakes when stress exceeds friction.

Liquefaction

A process where saturated soils lose strength and behave like a liquid during strong shaking, increasing earthquake damage.

Volcano

A landform/vent where magma reaches the surface; eruptions can create new land and hazards such as ash and pyroclastic flows.

Rock cycle

The set of processes that continually transform rocks among igneous, sedimentary, and metamorphic types through melting, cooling, weathering, burial, heat, and pressure.

Soil

A living, structured mixture of mineral particles, organic matter, water, air, and organisms that supports plant growth and ecosystem functions.

Physical weathering

The mechanical breakdown of rock into smaller pieces without changing its chemical composition (e.g., freeze–thaw cracking).

Chemical weathering

The breakdown of minerals through chemical reactions (often involving water and acids) that alters rock’s composition.

Parent material

The rock and minerals from which a soil develops, either native to the area or transported by wind, water, or glaciers.

Soil profile

A vertical cross-section of soil showing its layered structure (horizons) from the surface downward.

A horizon (topsoil)

The fertile soil layer rich in organic matter, organisms, and minerals; critical for agriculture and commonly thick in grasslands.

Humus

Dark, decomposed organic material that improves soil structure, supports nutrient supply, increases water retention, and helps stabilize pH.

Soil texture

The relative proportions of sand, silt, clay (and sometimes gravel) in a soil, strongly influencing water and nutrient behavior.

Loam

A balanced soil mixture (roughly equal sand, silt, clay, plus humus) that is typically nutrient-rich and favorable for crops.

Porosity

The amount (fraction) of total pore space in a soil, affecting how much water it can hold when saturated.

Permeability

How easily water and air move through soil based on how well connected the pores are (clay can be porous but low-permeability).

Infiltration

The process of water entering the soil from the surface during rainfall or irrigation.

Leaching

The downward movement of dissolved substances (and sometimes very fine particles) through soil after saturation.

Soil erosion

The movement of soil by water, wind, and human activity; it removes topsoil, reduces fertility, and can pollute waterways with sediment and nutrients.

Troposphere

The lowest atmospheric layer (~0–10 km) containing most atmospheric mass and nearly all water vapor; weather occurs here.

Stratosphere

The atmospheric layer above the troposphere (~10–50 km) containing most protective ozone; temperature increases with altitude due to UV absorption.

Ozone layer

Stratospheric ozone (O3) that absorbs much of the Sun’s harmful ultraviolet radiation, protecting life at Earth’s surface.

Greenhouse effect

Warming of the lower atmosphere as greenhouse gases absorb and re-emit infrared (longwave) radiation; can be natural or enhanced by human emissions.

Albedo

The fraction of incoming sunlight reflected by a surface; high for snow/ice and low for oceans/forests/asphalt.

Temperature inversion

A condition where warm air overlies cooler air near the surface, suppressing vertical mixing and trapping pollutants close to the ground.

Coriolis effect

The apparent deflection of moving air (right in the Northern Hemisphere, left in the Southern Hemisphere) due to Earth’s rotation; it changes wind direction, not the initial cause of wind.

Hadley cell

A convection cell from the equator to ~30° latitude where air rises near the equator (wet conditions) and sinks in the subtropics (dry conditions; many deserts).

Upwelling

The rise of deep, nutrient-rich water to the surface (often when winds and the Coriolis effect move surface water away from shore), boosting primary productivity and fisheries.

Thermohaline circulation

Deep-ocean circulation driven by density differences caused by temperature and salinity; cold, salty water is dense and sinks to help power global deep-water movement.

Watershed

A land area that drains rainfall and snowmelt into a common water body (lake, ocean, or aquifer), linking land use to downstream water quality.

El Niño

The warm ENSO phase when trade winds weaken, warm water spreads east, the eastern thermocline deepens, and upwelling decreases—often reducing nutrients and harming fisheries.

La Niña

The cool ENSO phase when trade winds strengthen, warm surface water is pushed farther west, and upwelling off South America often increases, cooling surface waters and shifting rainfall patterns.

Storm surge

A hurricane/tropical cyclone-driven rise in sea level caused largely by strong winds pushing seawater toward shore, increasing coastal flooding risk.