Earth Systems: Geology, Hydrology, and Geohazards

A comprehensive vocabulary review of core geological, hydrological, and environmental concepts covering rock formation, seismology, volcanology, surface streams, mass wasting, and hydrogeology.

Elastic-Rebound Theory

The geological principle stating that rock bodies bend elastically under accumulating stress until friction is overcome, snapping back to their unstrained shape and abruptly releasing stored energy as seismic waves.

Hypocenter (Focus)

The subsurface point on a fault plane where slip initiates and seismic wave generation begins.

Epicenter

The point on Earth's surface situated directly above the earthquake's focus (hypocenter).

P-Waves (Primary Waves)

The fastest-moving compressional body waves that propagate through solids, liquids, and gases by contracting and dilating material parallel to the direction of wave travel.

S-Waves (Secondary Waves)

Shear body waves that displace particles perpendicular to the direction of wave propagation and can only travel through solids, stopping at liquid boundaries.

Modified Mercalli Intensity (MMI) Scale

A qualitative seismic scale that measures the intensity of an earthquake based on observed structural damage and human perception of ground shaking.

Moment Magnitude Scale ($M_w$)

A quantitative seismic measurement representing the total energy released by an earthquake, calculated using fault rupture dimensions, displacement, and rock shear rigidity.

Sediment Liquefaction

A geohazard occurring when intense seismic shaking causes loosely packed, water-saturated granular sediment to lose shear strength and behave as a fluid.

S-Wave Shadow Zone

The region between $103^\circ$ and $180^\circ$ from an earthquake epicenter where direct secondary waves cannot be detected because they cannot pass through Earth's liquid outer core.

Strike-Slip Fault

A fault fracture along which adjacent crustal blocks slide past one another horizontally under shear stress, typical of transform plate boundaries.

Decompression Melting

The melting of upward-convecting mantle rock that occurs when overburden pressure decreases while rock temperature remains elevated.

Flux Melting

Melting that occurs in the mantle wedge above a subducting oceanic plate when descending crust releases volatiles (primarily $\text{H}_2\text{O}$), lowering the melting point of the asthenosphere.

Heat-Transfer Melting

The melting of crustal wall rocks caused by thermal energy transferred from hotter magma that pools beneath or intrudes the crust.

Fractional Crystallization

The progressive separation and removal of newly formed mineral crystals from a cooling magma chamber, which progressively changes the residual melt's chemical composition according to Bowen's reaction series.

Partial Melting

The incomplete melting of rocks where silica-rich, lower-melting-temperature minerals melt first, producing a magma that is more felsic than the source rock.

Magma Assimilation

The process by which ascending magma incorporates and chemically dissolves blocks of surrounding country rock (wall rock), modifying the melt's composition.

Dike

A sheet-like, discordant tabular igneous intrusion that cuts vertically or obliquely across pre-existing rock layers.

Sill

A tabular, concordant igneous intrusion that pushes horizontally between pre-existing sedimentary or volcanic layers.

Batholith

A vast composite intrusive igneous rock body composed of multiple overlapping plutons exposed over a surface area greater than $100\,\text{km}^2$.

A'a'

A basaltic lava flow characterized by a rough, jagged, rubbly, and clinkery surface formed when viscous or cooling lava breaks into sharp fragments during movement.

Pahoehoe

A fluid, low-viscosity basaltic lava flow that cools to form a smooth, billowy, or ropy surface texture.

Shield Volcano

A broad, gently sloping volcanic mountain formed primarily from successive accumulations of low-viscosity mafic (basaltic) lava flows during effusive eruptions.

Cinder-Cone Volcano

A symmetrical, steep-sided volcanic cone composed of loose basaltic pyroclastic tephra (cinders) ejected during short-lived, gas-charged fountain eruptions.

Stratovolcano (Composite Volcano)

A large, steep-sided volcanic cone built of alternating layers of intermediate-to-felsic lava flows and explosive pyroclastic tephra.

Caldera

A large, steep-walled volcanic depression formed when the summit of a volcano collapses inward following the rapid emptying of its magma chamber.

Pyroclastic Flow

A fast-moving, lethal avalanche of superheated volcanic gas, ash, and incandescent rock debris that sweeps downslope during an explosive eruption.

Lahar

A destructive volcanic mudflow or debris flow formed when tephra and ash mix with water from torrential rain or rapidly melted glacial ice and snow.

Recrystallization

A metamorphic process where mineral grains change in shape and size without altering the chemical identity of the mineral.

Pressure Solution

A metamorphic deformation process where mineral grains selectively dissolve along boundaries subject to high differential stress and reprecipitate in lower-stress zones, forming flattened or elliptical grains.

Foliation

The preferred parallel planar orientation of platy mineral grains or compositional banding within a metamorphic rock resulting from differential tectonic stress.

Frost Wedging

A physical weathering mechanism where liquid water infiltrates rock joints, expands by approximately $9\%$ upon freezing, and wedges the rock apart.

Hydrolysis

A chemical weathering reaction in which acidic water dissociates into ions that react with silicate minerals, altering feldspars into clay minerals such as kaolinite ($2\text{NaAlSi}_3\text{O}_8 + 2\text{H}_2\text{CO}_3 + 9\text{H}_2\text{O} \rightarrow 2\text{Na}^+ + 2\text{HCO}_3^- + 4\text{H}_4\text{SiO}_4 + \text{Al}_2\text{Si}_2\text{O}_5(\text{OH})_4$).

Differential Weathering

The unequal rate of weathering and erosion across rock units caused by differences in mineral composition, jointing, and physical resistance.

Lithification

The geological transformation of loose, uncompacted sediment into solid sedimentary rock through burial, compaction by overburden, and precipitation of mineral cement.

Evaporite

A chemical sedimentary rock formed by the precipitation of dissolved minerals (such as halite and gypsum) directly out of hypersaline water undergoing evaporation.

Slumping

A mass-wasting process where cohesive blocks of regolith and rock slip downslope along a spoon-shaped or curved failure surface, producing a prominent slump scar.

Solifluction

A slow mass-wasting movement in permafrost terrains where thawed, saturated surface soil flows downslope in overlapping sheets across permanently frozen substrate.

Sheetwash

A continuous thin layer of unchanneled water flowing downslope across a ground surface during heavy rain before focusing into discrete rills.

Drainage Divide

A topographic ridge or highland boundary that separates surface runoff flowing into adjacent drainage basins (watersheds).

Stream Discharge

The total volume of water passing through a specified cross-section of a stream channel per unit of time ($\text{m}^3/\text{s}$ or $\text{ft}^3/\text{s}$).

Ephemeral Stream

A temporary stream channel (dry wash) that flows exclusively during and shortly after precipitation events and remains dry above the regional water table for most of the year.

Braided Stream

A stream pattern consisting of a network of interwoven, shallow channels dividing and reuniting around mobile sand and gravel bars, developing in settings with abundant bed load.

Oxbow Lake

A crescent-shaped lake formed in an abandoned meander loop after a river cuts through a narrow meander neck.

Delta

A depositional sediment wedge built where a sediment-laden stream empties into a standing body of open water, causing flow velocity to drop rapidly.

Base Level

The lowest elevation to which a running stream or river can downcut and erode its channel.

Permeability

The capacity of a porous rock, sediment, or soil to transmit fluids through its interconnected network of pore spaces.

Porosity

The total volume of empty space or pore voids within a body of rock or sediment, expressed as a percentage ($\%$) of the bulk volume.

Aquifer

A permeable, water-bearing rock or sediment formation that stores and readily transmits usable quantities of groundwater.

Aquitard

An impermeable or low-permeability geologic layer (such as shale or dense clay) that severely impedes the subterranean movement of groundwater.

Water Table

The subsurface boundary marking the top of the phreatic zone, separating the unsaturated zone above from the fully saturated zone below.

Artesian Well

A well tapping a confined aquifer under hydrostatic pressure, causing water to rise spontaneously above the top boundary of the aquifer without pumping.

Spring

A natural surface emergence point where groundwater discharges naturally onto the ground surface, often where an aquifer intersects an impermeable strata.

Wetland

A semi-terrestrial ecosystem characterized by soil that is saturated or inundated with water at or near the surface permanently or seasonally, fostering hydric soils and specialized vegetation.

Hydraulic Residence Time

The average duration that a given unit of water remains inside a specific wetland, reservoir, or hydrological catchment.

Gleyed Soil

A soil profile showing prominent bluish-gray or greenish-gray mineral horizons formed under persistent anaerobic, saturated conditions where iron is chemically reduced ($\text{Fe}^{2+}$).

Eutrophication

The over-enrichment of aquatic environments with excessive nutrients (such as nitrates and phosphorus), provoking rapid algal blooms that deplete dissolved oxygen upon bacterial decomposition.

Groundwater Contaminant Plume

A localized, expanding subterranean body of contaminated groundwater that migrates downgradient along groundwater flow paths away from a pollution source.