Geology: Geologic Time, Weathering, Mass Wasting, Streams, and Groundwater

Comprehensive vocabulary flashcards covering fundamental concepts in relative and radiometric dating, physical and chemical weathering, mass wasting, stream processes and flood hazards, and hydrogeology.

Relative dating

Determining the order of geologic events without assigning exact numerical ages.

Law of superposition

In undisturbed strata, lower layers are older than the layers above.

Original horizontality

Sediments are generally deposited in approximately horizontal layers.

Lateral continuity

Sedimentary layers originally extend sideways until they thin out or meet a boundary.

Cross-cutting relationships

A fault or intrusion is younger than the rocks it cuts.

Principle of inclusions

Rock fragments enclosed in another rock are older than their host rock.

Unconformity

A surface representing missing geologic time due to erosion or nondeposition.

Angular unconformity

Younger beds overlie older tilted or folded and eroded beds.

Nonconformity

Sedimentary strata overlie eroded igneous or metamorphic rocks.

Disconformity

An erosional or nondepositional gap between approximately parallel sedimentary beds.

Index fossil

A widespread fossil restricted to a short geologic time interval.

Rock correlation

Matching rock units of comparable age between separated locations.

Isotope

Atoms of the same element with equal proton counts but different neutron counts.

Atomic number

The number of protons in an atom's nucleus.

Mass number

The sum of protons and neutrons in the nucleus.

Half-life

Time required for half the radioactive parent atoms in a sample to decay.

Three half-lives

After three half-lives, one-eighth ($12.5\%$) of the parent isotope remains.

Four half-lives

After four half-lives, one-sixteenth ($6.25\%$) of the parent isotope remains.

Alpha decay

A nucleus emits two protons and two neutrons, reducing atomic number by two and mass number by four.

Beta-minus decay

A neutron becomes a proton while emitting an electron; atomic number rises by one.

Electron capture

A proton captures an inner electron and becomes a neutron; atomic number falls by one.

Carbon-14 dating

Dating once-living organic material, generally up to about $50{,}000$ years old.

Open system

A sample that can gain or lose parent or daughter isotopes after its clock starts.

Closure temperature

The temperature below which a mineral effectively retains the isotopes used for dating.

Radiometric age

A numerical age inferred from radioactive parent-to-daughter isotope decay.

Initial daughter abundance

The amount of daughter isotope already present when a mineral's isotopic clock began.

Physical weathering

Breaking rock into smaller pieces without changing mineral chemistry.

Chemical weathering

Altering minerals through chemical reactions, often creating new minerals or dissolved ions.

Frost wedging

Water freezes and expands in fractures, progressively prying rock apart.

Root wedging

Growing roots widen cracks and mechanically break rock.

Unloading and exfoliation

Removal of overlying rock reduces pressure and forms sheetlike fractures roughly parallel to the surface.

Granite exfoliation

Sheet joints are especially conspicuous in massive granite exposed by unloading.

Cooling joints

Fractures that form as cooling igneous rock contracts.

Hydrolysis

Water reacts with silicate minerals, commonly changing feldspar into clay minerals.

Oxidation

Oxygen reacts with minerals such as iron-bearing silicates, producing oxidized compounds.

Quartz resistance

Quartz is relatively resistant to chemical weathering near Earth's surface.

Surface area effect

Smaller rock fragments expose more surface area and often weather chemically faster.

Soil horizons

Distinct soil layers produced by organic accumulation, leaching, and mineral accumulation.

Arid-region soils

Dry climates often produce thin soils with limited leaching and possible carbonate accumulation.

Humid tropical soils

Warm wet climates encourage intense chemical weathering and iron-rich reddish soils.

Mass wasting

Downslope movement of rock or soil under the influence of gravity.

Soil creep

Extremely slow downslope movement of soil, often bending trees or tilting posts.

Tilted fence posts

A common visible clue to gradual soil creep on a slope.

Slump

Movement of a coherent mass along a curved, rotational slip surface.

Debris flow

Rapid downslope movement of water-saturated sediment and rock fragments.

Pore-water pressure

Water pressure in sediment pores that can reduce frictional resistance to sliding.

Undercutting

Removal of support at a slope's base, increasing instability.

Slope drainage

Removing excess water from a slope to help lower the risk of failure.

Rockfall

Rapid free fall or bouncing of detached rock from a steep face.

Vegetation roots

Roots can reinforce shallow soil and help resist some slope failures.

Drainage basin

The land area from which water flows toward a particular stream outlet.

Stream discharge

The volume of water passing a cross section per unit time.

Discharge equation

Discharge equals average stream velocity multiplied by cross-sectional area ($Q = V \times A$).

Discharge calculation

A stream with area $4\,\text{m}^2$ and velocity $3\,\text{m/s}$ has discharge $12\,\text{m}^3/\text{s}$.

Stream gradient

The change in stream elevation divided by horizontal distance.

Bed load

Sediment moved along the streambed by rolling, sliding, or bouncing.

Suspended load

Fine sediment carried within flowing water rather than along the bed.

Dominant sediment load

Suspended sediment commonly makes up the greatest proportion of a river's transported sediment by volume.

Base level

The lowest elevation to which a stream can erode its channel.

Waterfall retreat

Erosion and undercutting can cause a waterfall to migrate upstream.

Base-level rise

A rise in base level generally encourages sediment deposition upstream.

Uplift and incision

Land uplift can steepen a stream and promote downcutting into its valley.

Dam sedimentation

A reservoir slows water and encourages deposition upstream of a dam.

Braided stream

A channel divided into multiple shifting threads around sediment bars.

Cut bank

The outside of a meander bend, where faster water erodes the bank.

Point bar

The inside of a meander bend, where sediment is deposited.

Oxbow lake

A crescent-shaped lake left when a meander loop is cut off from the river.

Flash flood

A flood that rises rapidly, often with little warning, after intense rainfall.

Narrow canyon hazard

A confined valley can concentrate floodwater and produce dangerous rapid rises.

Urban runoff

Pavement and roofs reduce infiltration and speed runoff into channels.

Big Thompson flood

A deadly 1976 Colorado flash flood associated with intense rain in a steep canyon.

Rapid City flood

A deadly 1972 South Dakota flash flood caused by extreme rainfall and rapid runoff.

Flooded-road safety

Avoid driving or walking through floodwater; turn around and seek higher ground.

Floodplain

Low land beside a river that is periodically inundated.

100-year flood

A flood magnitude with a $1\%$ probability of being equaled or exceeded in any given year.

Recurrence interval

An estimate of average time between floods of a specified magnitude, based on flood-frequency statistics.

Floodplain map

A map identifying land expected to be inundated under specified flood scenarios.

Levee

An embankment built to contain high river flows, but which can fail or shift risk downstream.

Detention basin

An area designed to temporarily store stormwater and reduce peak runoff.

Groundwater

Water stored beneath the surface in pores and fractures of rock and sediment.

Porosity

The percentage of a material's total volume made up of pore spaces.

Permeability

The ability of a material to transmit water through connected openings.

Clay and permeability

Clay may have substantial pore space but low permeability because its pores are tiny.

Aquifer

A permeable geologic unit that stores and transmits usable groundwater.

Aquitard

A geologic unit that restricts groundwater flow relative to an aquifer.

Water table

The upper boundary of the saturated zone in an unconfined aquifer.

Drought and water table

Reduced recharge during drought can cause the water table to decline.

Hydraulic head

A measure of groundwater potential energy related to elevation and pressure.

Hydraulic gradient

The change in hydraulic head divided by distance along groundwater flow.

Cone of depression

A lowered, cone-shaped water table or potentiometric surface around a pumping well.

Artesian well

A well tapping a confined aquifer in which pressure raises water above the aquifer top.

Spring

A location where groundwater naturally emerges at Earth's surface.

Overpumping

Withdrawing groundwater faster than it is replenished, causing declines in water levels.

Ogallala Aquifer

A major High Plains groundwater resource stressed by heavy irrigation and slow recharge in many areas.

Limestone cave

A void produced mainly by dissolution of carbonate rock by weakly acidic groundwater.

Karst landscape

Terrain with dissolution features such as sinkholes, caves, and underground drainage.