This list includes 81 Earth science words that start with A, from “Ablation” to “Azimuth”. These entries cover processes, landforms, rocks, minerals, and atmospheric phenomena you encounter in classrooms, fieldwork, and maps.
[TERM] are terms that describe Earth’s materials, processes, and features across geology, hydrology, and the atmosphere. For example, “Ablation” appears often in glacier studies and climate history discussions.
Below you’ll find the table with the columns: “Term”, “Definition”, “Category”, “Example”, and “Further reading”.
Term: Shows the word itself so you can scan names quickly and jump to entries you already know or want to learn.
Definition: Gives a concise one- or two-sentence meaning so you can grasp the term’s basic sense at a glance.
Category: Identifies the field (for example geology, meteorology, or oceanography) so you can filter words by topic or curriculum need.
Example: Offers a short real-world context showing how the term appears in maps, reports, or classroom lessons.
Further reading: Provides a link or citation for deeper articles, datasets, or classroom resources when you want more detail.
Earth science words that start with A
| Term | Category | Typical units/scale | Synonyms/Related terms |
|---|---|---|---|
| Ablation | Process (glaciology) | m/yr, seasonal | ice loss, melting |
| Abrasion | Process (erosion) | m/yr, mm | scouring, attrition |
| Absolute dating | Method (geochronology) | yr, ka, Ma | radiometric dating, numerical dating |
| Absolute humidity | Atmospheric property | g/m³ | moisture content |
| Abyssal plain | Landform (oceanography) | m depth, km² | deep-sea plain |
| Abyssal hill | Landform (oceanography) | m–km, m height | seafloor hill |
| Accelerator mass spectrometry | Instrumentation | counts, isotopic ratios | AMS, radiocarbon accelerator |
| Accretion | Process (tectonics/planetary) | mm–cm/yr, Ma | growth, buildup |
| Accretionary wedge | Landform (tectonics) | km, km³ | accretionary prism |
| Acoustic Doppler current profiler | Instrumentation (oceanography) | m/s, m depth | ADCP |
| Active margin | Tectonic setting | km, Ma | convergent margin |
| Active layer | Permafrost term | m seasonal | seasonally thawed layer |
| Aeolian | Process/landform (geomorphology) | m–km, yrs | eolian, wind-blown |
| Age (geologic) | Concept | yr, ka, Ma | geologic age |
| Aggradation | Process (fluvial) | m, mm/yr | sediment buildup |
| Air mass | Meteorology | km, days | airmass |
| Air parcel | Meteorology | m, km, s–days | parcel |
| Air pressure | Atmospheric property | Pa, hPa | barometric pressure |
| Air-sea exchange | Process (ocean–atmosphere) | mol/m²/s, W/m² | gas exchange |
| Aerosol | Atmospheric particle | µg/m³, optical depth | particulate matter |
| Aerosol optical depth | Atmospheric property | unitless | AOD |
| Albedo | Surface property | unitless, % | reflectivity |
| Alfisol | Soil order | cm, soil horizon | forest soil |
| Alkalinity | Water chemistry | meq/L, mg/L | buffering capacity |
| Alkali feldspar | Mineral/rock group | % modal, hardness | potassium feldspar |
| Allochthonous | Sediment/ecosystem term | m, kg | transported |
| Alluvial fan | Landform (fluvial) | m–km, m thickness | fan |
| Alluvial plain | Landform | km², m | floodplain |
| Alluvium | Sediment | m, cm grain size | river deposits |
| Alpine glacier | Glaciology | m length, m thickness | mountain glacier |
| Altimetry | Instrumentation/method | m, cm | radar altimetry |
| Altocumulus | Cloud type | km altitude | mid-level cloud |
| Altostratus | Cloud type | km altitude | mid-level sheet cloud |
| Amphibole | Mineral group | mineral formula, hardness | hornblende group |
| Amphibolite | Metamorphic rock | m, km | hornblende schist |
| Anemometer | Instrumentation (meteorology) | m/s | wind meter |
| Angular unconformity | Geologic structure | m–km, Ma | erosional surface |
| Anisotropy | Property (geophysics) | unitless, % | directional dependence |
| Anorthite | Mineral (feldspar) | % composition | Ca-feldspar |
| Anorthosite | Igneous rock | km³, % plagioclase | plagioclase-rich rock |
| Anoxia | Environmental condition | mg/L, % O₂ | hypoxia, oxygen depletion |
| Anticline | Structural geology | m–km, km | upfold, arch |
| Anticyclone | Meteorology | km, days | high pressure system |
| Anthropocene | Geologic epoch (proposed) | yr, decades | human epoch |
| Anthropogenic | Descriptor | emissions, % change | human-caused |
| Aphotic zone | Ocean ecology | m depth | dark zone |
| Apatite | Mineral | mm–cm crystals | phosphate mineral |
| Aquifer | Hydrogeology | m, m³/day, L/s | groundwater reservoir |
| Aquifer recharge | Hydrology | mm/yr, m³/day | infiltration |
| Aquiclude | Hydrogeology | m, permeability | impermeable layer |
| Aquitard | Hydrogeology | m, permeability | semi-permeable layer |
| Arc (volcanic) | Tectonics/volcanology | km length, km depth | volcanic arc, island arc |
| Arete | Glacial landform | m height, m length | ridge |
| Arkose | Sedimentary rock | % feldspar, m | feldspathic sandstone |
| Aridity index | Climate metric | unitless | dryness index |
| Arctic amplification | Climate phenomenon | °C, decades | polar amplification |
| Aragonite | Mineral | CaCO₃ (%), µm crystals | carbonate mineral |
| Arcuate delta | Landform (coastal) | km, km² | fan-shaped delta |
| Aridity | Climate descriptor | mm/yr, index | dryness |
| Arsenic contamination | Environmental geochemistry | µg/L | As pollution |
| Artesian well | Hydrogeology | m depth, L/s | confined well |
| Asperity | Seismology | m–km | fault patch |
| Asthenosphere | Earth structure | km depth | upper mantle |
| Atmosphere | Earth system | km thickness, Pa | air |
| Atmospheric boundary layer | Meteorology | m–km height, hrs | ABL, planetary boundary layer |
| Atmospheric circulation | Climate/ocean coupling | km, months–yrs | general circulation |
| Atmospheric river | Meteorology | km wide, kg/s | AR |
| Attenuation | Geophysics/remote sensing | dB, m−1 | damping, decay |
| Auger | Field instrument (geomorphology/soil) | cm–m depth | soil auger |
| Autogenic process | Sedimentology/system | yrs–ka | internal forcing |
| Authigenic | mineral/sediment term | µm–mm | formed in place |
| Avalanche | Natural hazard | m, secs–mins | snow slide |
| Avulsion | Fluvial process | m–km, yrs–decades | channel switching |
| Azimuth | Geospatial/mapping | degrees | bearing, heading |
| Acidification | Environmental process | pH units, µmol/kg | acidify |
| Acid rain | Atmospheric deposition | pH,<5.6 | acid deposition |
| Aeration zone | Hydrology (vadose) | m depth | vadose zone |
| Anoxia event | Paleoceanography | yr, ka | black shale event |
| Aphelion | Orbital parameter | AU, km | furthest orbital point |
| Ash plume | Volcanology | km height, kg/s | eruption plume |
| Aquifer recharge area | Hydrogeology | km², mm/yr | recharge zone |
Descriptions
Ablation
Removal of ice or snow from a glacier or snowpack by melting, sublimation, or calving; measured in m/yr and crucial for glacier mass-balance.
Abrasion
Mechanical wearing and smoothing of rock surfaces by particles carried by water, ice, or wind; common in rivers, glaciers, and coastal zones.
Absolute dating
Techniques that determine the numeric age of rocks or fossils (e.g., radiometric methods), giving ages in years, thousands, or millions of years.
Absolute humidity
Mass of water vapor per unit volume of air (grams per cubic meter); a direct measure of atmospheric moisture distinct from relative humidity.
Abyssal plain
Vast, flat regions of the deep ocean floor typically between 3,000 and 6,000 m depth formed by fine sediment accumulation.
Abyssal hill
Small volcanic or tectonic hills on the ocean floor, commonly 100–1,000 m high and widespread on abyssal plains.
Accelerator mass spectrometry
Sensitive instrument for measuring rare isotopes (e.g., 14C) in small samples; widely used for radiocarbon and isotope geochemistry.
Accretion
Gradual addition or growth of material by deposition or tectonic addition, used for sediments, continents, and planetary formation.
Accretionary wedge
Pile of deformed sediments scraped off a subducting oceanic plate and stacked at a convergent margin; common near subduction zones.
Acoustic Doppler current profiler
Sonar instrument that measures water current velocities across depths using Doppler shifts; standard for oceanographic flow profiling.
Active margin
Plate boundary where oceanic and continental plates interact with deformation, volcanism, and earthquakes (e.g., Pacific coasts).
Active layer
Top layer of soil above permafrost that seasonally thaws and refreezes, important for carbon cycling and infrastructure.
Aeolian
Processes and landforms formed by wind, including dunes, loess deposits, and wind erosion in arid and coastal settings.
Age (geologic)
Numeric or relative time assigned to rock units or events, expressed in years (yr), thousands (ka), or millions (Ma) of years.
Aggradation
Upward building of a riverbed or floodplain by sediment deposition, raising local land surface elevation.
Air mass
Large body of air with fairly uniform temperature and humidity that forms over source regions and influences weather.
Air parcel
A theoretical volume of air used to model thermodynamic changes (rising, sinking) without mixing with surroundings.
Air pressure
Force exerted by the atmosphere per unit area; commonly measured in hectopascals (hPa) and drives weather patterns.
Air-sea exchange
Transfer of heat, gases, and momentum between the ocean and atmosphere, controlling climate and biogeochemistry.
Aerosol
Tiny solid or liquid particles suspended in air from natural or human sources; affect air quality, clouds, and climate.
Aerosol optical depth
Measure of aerosol amount in a column of atmosphere; higher values indicate more light scattering or absorption.
Albedo
Fraction of incoming solar radiation reflected by a surface; snow and ice have high albedo, affecting Earth’s energy balance.
Alfisol
Temperate, moderately leached soils with clay-enriched subsoil, common beneath deciduous forests in many regions.
Alkalinity
Water’s capacity to neutralize acids, often measured as bicarbonate/carbonate concentration; important for aquatic health and buffering pH.
Alkali feldspar
Group of K- and Na-rich feldspars common in plutonic and volcanic rocks, important in petrology and weathering.
Allochthonous
Material or organic matter transported into a system from elsewhere, e.g., river-borne sediments or imported leaf litter.
Alluvial fan
Cone-shaped deposit of sediment formed where a high-gradient stream slows abruptly, typically at mountain front.
Alluvial plain
Flat area formed by river sediment deposition over time; fertile and often heavily cultivated.
Alluvium
Unconsolidated sediments (sand, silt, clay) deposited by running water in channels, floodplains, and fans.
Alpine glacier
Glacier confined within mountain valleys that flows under its own weight and shapes alpine landscapes.
Altimetry
Use of radar or laser from aircraft or satellites to measure surface elevation of land or sea surface height.
Altocumulus
Mid-level cloud composed of rounded clumps or rolls, usually 2–6 km altitude; can signal changing weather.
Altostratus
Mid-level, widespread gray cloud layer that often precedes continuous precipitation.
Amphibole
Silicate minerals with characteristic double-chain structure found in igneous and metamorphic rocks.
Amphibolite
Medium- to high-grade metamorphic rock dominated by amphibole and plagioclase, common in orogenic belts.
Anemometer
Instrument that measures wind speed (and often direction) used in weather stations and field studies.
Angular unconformity
A contact where tilted or folded older strata are overlain by younger, more horizontal layers, indicating a gap in time.
Anisotropy
Variation of a property (e.g., seismic velocity, permeability) with direction; important for interpreting subsurface measurements.
Anorthite
Calcium-rich end-member of plagioclase feldspar important in igneous petrology and geochemistry.
Anorthosite
Igneous rock dominated by plagioclase feldspar, often forming large plutons on continents and the Moon.
Anoxia
Condition of very low or absent dissolved oxygen in water or sediments, causing stress or mortality in aquatic life.
Anticline
Fold in rock layers that arches upward with the oldest beds at its core; common trap for hydrocarbons.
Anticyclone
Large-scale high-pressure system with clockwise circulation (Northern Hemisphere) bringing stable, fair weather.
Anthropocene
Informal term for a proposed epoch recognizing significant global Earth-system changes driven by human activities since the Industrial Revolution.
Anthropogenic
Refers to processes or materials resulting from human activity, e.g., greenhouse gas emissions or land-use change.
Aphotic zone
Ocean zone below the depth where sufficient sunlight penetrates for photosynthesis, typically below ~200 m.
Apatite
Common phosphate mineral used in geochronology (fission-track, U–Pb) and as a recorder of geological histories.
Aquifer
Permeable underground layer that stores and transmits groundwater, supplying wells and springs.
Aquifer recharge
Process by which water infiltrates the ground to replenish an aquifer, via precipitation, rivers, or managed recharge.
Aquiclude
Geologic unit with negligible permeability that prevents water flow and confines aquifers.
Aquitard
Layer of low permeability that restricts but does not completely prevent groundwater flow between aquifers.
Arc (volcanic)
Curved chain of volcanoes above a subduction zone formed by melting of subducted slab and mantle wedge.
Arete
Sharp, narrow ridge separating adjacent glacial valleys formed by cirque erosion.
Arkose
Coarse-grained sandstone rich in feldspar fragments, indicating rapid erosion and deposition near source.
Aridity index
Ratio of mean annual precipitation to potential evapotranspiration; used to classify dryness and drought risk.
Arctic amplification
Enhanced warming of the Arctic region relative to global average due to feedbacks like sea-ice loss and albedo changes.
Aragonite
A high-pressure polymorph of calcium carbonate common in marine shells and precipitates; sensitive to ocean chemistry.
Arcuate delta
Delta with a rounded, arc-shaped shoreline formed where wave action redistributes river sediments.
Aridity
Degree to which a climate lacks moisture, typically defined by precipitation versus evapotranspiration.
Arsenic contamination
Presence of toxic arsenic in water or soils from natural sources or mining, monitored due to health risks.
Artesian well
Well tapping a confined aquifer under pressure that can cause water to flow above the aquifer level, sometimes to the surface.
Asperity
Highly stressed patch on a fault that can lock and later release in an earthquake; controls rupture behavior.
Asthenosphere
Partially molten, mechanically weak layer of the upper mantle beneath the lithosphere that allows plate motion.
Atmosphere
Layer of gases enveloping Earth, composed mainly of nitrogen and oxygen and driving weather and climate.
Atmospheric boundary layer
Lowest part of the atmosphere directly influenced by Earth’s surface on timescales of an hour or less, controlling near-surface turbulence.
Atmospheric circulation
Large-scale movement of air that redistributes heat and moisture, including Hadley, Ferrel, and polar cells.
Atmospheric river
Narrow corridor of intense water vapor transport in the atmosphere that can deliver heavy precipitation and flooding.
Attenuation
Decrease in amplitude or intensity of waves (seismic, acoustic, electromagnetic) as they travel through a medium.
Auger
Hand or powered drill used to collect soil, sediment, or ice cores for subsurface sampling.
Autogenic process
Process driven by internal system dynamics (e.g., channel migration) rather than external changes in climate or tectonics.
Authigenic
Minerals or sediments that formed in place within a depositional environment rather than being transported from elsewhere.
Avalanche
Rapid downslope flow of snow (or debris) that can be triggered by overload, steep slopes, or weak layers; major mountain hazard.
Avulsion
Sudden shift of a river channel to a new route across its floodplain, creating new depositional patterns and terraces.
Azimuth
Angular measurement in degrees clockwise from north used to describe direction in mapping and field surveying.
Acidification
Decrease in pH of water or soil due to natural or human inputs (e.g., CO₂-driven ocean acidification).
Acid rain
Precipitation made acidic by atmospheric pollutants (SO₂, NOx) that can harm ecosystems, buildings, and water quality.
Aeration zone
Zone between land surface and groundwater table where pore spaces contain both air and water; important for infiltration and root respiration.
Anoxia event
Geologic interval of widespread low-oxygen waters in oceans or lakes, often linked to mass extinctions and organic-rich deposits.
Aphelion
Point in Earth’s orbit when it is farthest from the Sun; influences but does not dominate seasonal climate variations.
Ash plume
Column of volcanic ash and gas ejected into the atmosphere during eruptions, posing hazards to aviation and health.
Aquifer recharge area
Surface region where water infiltrates to replenish an aquifer; protecting these areas is vital for groundwater sustainability.