Environmental Geology: Earthquakes, Volcanoes, Rivers, Mass Wasting, Soil, and Water Resources

Comprehensive vocabulary flashcards covering the key concepts, hazards, processes, and terminology across Earthquakes, Volcanoes, Rivers & Flooding, Mass Wasting, Soil Resources, and Water Resources.

Liquefaction

A process where water-saturated, unconsolidated sediment experiences strong ground shaking, increasing pore-water pressure and reducing grain-to-grain contact so the sediment temporarily loses strength and behaves like a liquid.

Fault Displacement

The surface rupture and offset of the ground along a fault plane during an earthquake, which directly damages buildings, roadways, and infrastructure crossing the fault trace.

Tsunami

A series of massive, long-wavelength ocean waves generated by the sudden vertical displacement of the seafloor during an undersea earthquake, landslide, or volcanic eruption, capable of traveling at speeds of hundreds of kilometers per hour.

Seismic Gap

A segment along an active fault zone that has experienced little to no recent seismic activity and is therefore interpreted to be accumulating elastic strain that may be released in a future earthquake.

Probabilistic Forecasting

A seismic risk assessment method that estimates the statistical likelihood of an earthquake of a given magnitude occurring in a specific geographic region over a defined time window based on fault slip rates and historical seismicity.

Earthquake Early Warning System

A technical network that detects non-destructive initial seismic waves (P-waves) near an earthquake source and transmits electronic warnings faster than seismic wave propagation, providing seconds to tens of seconds of notice before damaging shaking arrives.

Recurrence Interval (Earthquakes)

The estimated average time span between earthquakes of a specific size or magnitude along a particular fault zone or within a given region.

Right-Lateral Strike-Slip Fault

A strike-slip fault where motion is horizontal and an observer looking across the fault plane observes the opposing crustal block moving to the right, such as the San Andreas Fault.

Decompression Melting

The generation of magma caused by the upward movement of hot mantle rock into regions of lower confining pressure without the addition of heat, commonly occurring at divergent plate boundaries and mantle hotspots.

Flux Melting

The melting of the mantle wedge above a subducting oceanic slab caused by the release of water and volatiles, which lowers the melting temperature of the surrounding rock.

Hot Spot

A persistent thermal anomaly in the mantle fed by a rising plume of hot mantle material that produces volcanic activity independently of tectonic plate boundaries.

Magma Viscosity

The resistance of magma to flow, primarily governed by temperature, silica ($\text{SiO}_2$) concentration, and mineral composition; higher silica and lower temperatures produce higher viscosity.

Volatiles

Gaseous chemical components dissolved in magma (principally $\text{H}_2\text{O}$, $\text{CO}_2$, and $\text{SO}_2$) that exsolve into expanding bubbles as confining pressure drops during magma ascent, driving volcanic explosions.

Tephra

A collective term for fragmented volcanic rock, glass, and mineral particles ejected into the air during an explosive eruption, ranging in size from fine ash to lapilli, blocks, and bombs.

Volcanic Explosivity Index (VEI)

A logarithmic scale ranging from $0$ to $8$ used to measure and classify the magnitude of explosive volcanic eruptions based on erupted tephra volume, eruption column height, and deposit extent.

Shield Volcano

A broad, gently sloping volcano built by successive eruptions of low-viscosity, fluid basaltic lava flows that travel long distances, typified by the Hawaiian volcanoes.

Stratovolcano (Composite Volcano)

A tall, steep-sided volcanic cone constructed from alternating layers of intermediate to felsic lava flows, pyroclastic debris, and volcanic ash, typically erupting explosively at subduction zones.

Cinder Cone (Scoria Cone)

A small, steep-sided volcano constructed predominantly from pyroclastic fragments (scoria/cinders) ejected from a single vent and deposited immediately around it during short-lived explosive eruptions.

Caldera

A large, basin-shaped volcanic depression formed by the inward collapse of the overlying volcanic roof structure into an underlying magma chamber that has been partially evacuated during massive eruptions.

Pyroclastic Flow

A rapidly moving, high-density, ground-hugging avalanche of extremely hot gases, ash, pumice, and fragmented rock that travels downslope from an erupting volcano at hundreds of kilometers per hour.

Lahar

A destructive mudflow or debris flow composed of a slurry of pyroclastic debris, rocky sediment, and water mobilizing rapidly down the slopes of a volcano or along river valleys.

Drainage Basin

The entire geographic area of land surface that collects and directs precipitation and runoff into a particular river, stream network, or standing body of water.

Stream Discharge

The volume of water flowing past a given cross-sectional point in a stream channel per unit time, mathematically calculated as $\text{Discharge} = \text{Area} \times \text{Velocity}$.

Stream Capacity

The total maximum quantity of sediment and dissolved mineral load that a stream is physically capable of transporting under given flow conditions.

Bedload

Coarse sediment particles, such as sand, gravel, and boulders, transported along the stream bottom by rolling, sliding, or bouncing (saltation).

Suspended Load

Fine sediment particles, primarily silt and clay, kept aloft within the moving water column by fluid turbulence.

Dissolved Load

The portion of a stream's total sediment load comprised of soluble chemical ions and minerals carried in aqueous solution.

Base Level

The lowest topographic elevation to which a stream or river can chemically and mechanically erode its channel, commonly represented globally by sea level.

Alluvial Fan

A fan-shaped accumulation of sediment deposited where a steep mountain stream abruptly decelerates and spreads out onto a broad, low-gradient valley floor or plain.

Delta

A fan-shaped or lobate deposit of sediment formed where a sediment-laden river empties into a quiet body of standing water, such as an ocean or lake, and drops its sediment load.

Cut Bank

The steep, actively retreating riverbank located on the outer curve of a meander bend, where water velocity is highest and active erosion occurs.

Point Bar

A low-gradient, arcuate accumulation of sand and gravel deposited along the inner bend of a meandering river where localized water velocity drops.

Oxbow Lake

A curved, crescent-shaped body of water formed when a meandering stream cuts through the narrow neck of a meander loop during high discharge, isolating the abandoned bend.

Braided Stream

A stream pattern consisting of a network of small, interwoven channels separated by transient gravel and sand bars, typically forming where sediment load is exceptionally high relative to stream discharge.

Floodplain

A broad, low-lying, flat valley floor bordering a stream channel constructed of deposited alluvium that periodically receives floodwaters during overbank flow.

Flood Stage

The stage or surface elevation of water in a stream channel at which water spills over the banks onto adjacent dry ground.

Hydrograph

A continuous graphical record plotting stream discharge or stage height against elapsed time for a specific gauging location.

Recurrence Interval (Floods)

The estimated average time span in years between flood events of a specified magnitude, calculated using the formula $R = \frac{N+1}{M}$, where $N$ is the number of years of record and $M$ is the flood's magnitude rank.

Retention Pond

A manmade surface basin designed to collect and temporarily impound urban runoff, releasing it at a controlled rate to diminish peak downstream discharge.

Artificial Levee

An engineered ridge or earthen embankment built along the margins of a river channel to elevate the banks and prevent floodwaters from inundating the floodplain.

Mass Wasting

The downslope movement of soil, regolith, and rock under the direct pull of gravitational force, occurring independently of flowing water, ice, or wind transport agents.

Angle of Repose

The steepest angle relative to the horizontal plane at which a pile of loose, unconsolidated sediment or rock particles remains stable without slipping downslope.

Shear Stress (on Slopes)

The parallel component of gravitational force ($G_s$) acting tangent to a slope that drives material downward and opposes resisting frictional forces.

Creep (Mass Wasting)

The imperceptibly slow, continuous downslope movement of soil and regolith caused by gravity and thermal expansion-contraction cycles, visibly manifested by tilted trees and fence posts.

Rockfall

An extremely rapid form of mass wasting in which detached bedrock blocks free-fall, bounce, or roll down a steep cliff face under gravity.

Translational Slide

A landslide in which a coherent mass of rock or debris moves downslope along a planar, non-curved failure surface such as a joint, bedding plane, or fault.

Slump

A rotational slide involving the downward and backward rotational movement of a cohesive block of rock or unconsolidated sediment along a concave, spoon-shaped slip surface.

Debris Avalanche

An exceptionally fast, chaotic, and destructive downslope flow of rock, soil, vegetation, water, and ice, capable of traveling great distances at velocities exceeding $100\,\text{km/hr}$.

Quick Clay

A sensitive clay deposited in a marine environment that maintains a delicate, open structural framework bound by salt ions, which abruptly liquefies into a fluid slurry when disturbed or shaken.

Terraced Roadcut

An engineered slope-stabilization method where a steep hillside is excavated into a succession of horizontal stepped benches to decrease the slope angle and capture rock debris.

Rock Bolt

A high-strength steel anchor rod drilled into a rock slope face to pin fractured, unstable surface blocks into deeper, competent bedrock.

Karst Topography

A distinctive landscape underlain by soluble bedrock (such as limestone, dolostone, or gypsum) characterized by sinkholes, caves, underground caverns, and disappearing streams formed by groundwater dissolution.

Sinkhole

A circular surface depression formed either by the gradual chemical dissolution of soluble bedrock at the surface or by the collapse of a subsurface cave roof.

Ground Subsidence

The settling or sinking of the land surface caused by subsurface volume reduction, commonly triggered by excessive groundwater extraction, mining operations, or void collapse.

Regolith

The mantle of fragmented, weathered rock debris, mineral grains, and loose sediment that lies directly above unweathered solid bedrock.

Soil (Scientific Definition)

A dynamic, natural surface mixture consisting of mineral matter, organic matter, water, air, and living organisms that has developed distinct horizons through biological, chemical, and physical weathering processes.

CLORPT

An acronym identifying the five state factors of soil formation: Climate, Organisms, Relief (topography), Parent material, and Time.

Eluviation

The mechanical leaching and downward washing of dissolved minerals, clays, and organic materials out of an upper soil layer (the E horizon) by percolating water.

Soil Texture

The relative volumetric proportion of sand ($0.05\text{ to }2\,\text{mm}$), silt ($0.002\text{ to }0.05\,\text{mm}$), and clay ($<0.002\,\text{mm}$) particles composing a soil sample.

Sheet Erosion

The broad, relatively uniform stripping and removal of a thin surface layer of topsoil by unchanneled overland water flow.

Rill Erosion

The erosion process by which concentrated overland water flow carves numerous small, distinct, shallow linear channels into bare soil.

Gully Erosion

An advanced form of water erosion where concentrated runoff carves deep, wide channels into the landscape that are too large to be erased by standard agricultural tillage.

Soil Salinization

The harmful concentration of water-soluble salts within the root zone of soils, typically occurring when irrigation water evaporates rapidly in arid climates without adequate drainage.

Desertification

The progressive degradation of land in arid, semiarid, and dry subhumid regions driven by climatic fluctuations and unsustainable land use practices, which strips vegetative cover and soil fertility.

Porosity

The percentage ratio of the total volume of void or open pore space within a rock or sediment mass to its total bulk volume.

Permeability

A quantitative measure of the ease with which a porous material, sediment, or rock transmits fluids through interconnected pore networks or fractures.

Specific Yield

The volume of water that drains freely from a saturated rock or sediment body under the force of gravity, defined mathematically as $\text{Specific Yield} = \text{Porosity} - \text{Specific Retention}$.

Water Table

The boundary surface separating the overlying unsaturated (vadose) zone from the underlying saturated (phreatic) zone, at which pore water pressure equals atmospheric pressure.

Aquifer

A permeable geologic rock, sediment, or soil unit that contains, stores, and yields economically and practically usable quantities of groundwater.

Aquitard

A low-permeability geologic stratum, such as shale or dense clay, that substantially impedes or restricts groundwater movement between aquifers.

Confined Aquifer

An aquifer bounded above and below by low-permeability aquitards, containing groundwater stored under hydrostatic pressure that rises above the aquifer boundary when tapped by a well.

Potentiometric Surface

An imaginary hydraulic surface representing the static level to which pressurized groundwater within a confined aquifer will rise inside an artesian well.

Cone of Depression

A localized, conical depression in the water table or potentiometric surface that forms immediately around a well while it is actively pumping groundwater.

Saltwater Intrusion

The landward and upward migration of saline ocean groundwater into fresh coastal aquifers, caused when excessive groundwater pumping lowers the freshwater hydraulic head.