This list includes 69 Earth science words that start with O, from “O horizon” to “Ozone layer”. These terms cover soils, atmosphere, oceans, rocks, and processes used in education and research.
Earth science words that start with O are specialized terms describing landscapes, materials, and processes on Earth. For example, “O horizon” highlights soil layers long used in farming and land studies.
Below you’ll find the table with Term, Definition, Category, Example, and Further reading.
Term: The word or phrase listed; you use it to find the concept quickly and match it to classroom topics.
Definition: A concise 1–2 sentence description that explains meaning and context so you can grasp the concept fast.
Category: Broad classification like atmosphere, hydrology, or geology that helps you filter terms by subject area in study.
Example: Short real-world examples or notable instances that show how the term appears in nature or research.
Further reading: Links to deeper articles or glossaries so you can explore detailed definitions and current scientific context.
Earth science words that start with O
| Term | Category | Pronunciation | Common units / typical values |
|---|---|---|---|
| O horizon | Soil | oh-HOR-uh-zən | thickness cm; typically 1–20 cm |
| Oolite | Rock | OO-uh-lyt | grain size mm; 0.25–2 mm |
| Ooid | Sediment grain | OO-id | grain size mm; 0.25–2 mm |
| Obsidian | Volcanic glass | ob-SID-ee-an | Mohs hardness; ~5–5.5 |
| Ocean | Oceanography | OH-shun | area km²; global ~361,000,000 km² |
| Ocean basin | Landform | OH-shun BAY-sin | depth m; up to ~7,000 m |
| Ocean current | Oceanography | OH-shun KUR-uhnt | speed m/s; typically 0.1–2 m/s |
| Ocean gyre | Oceanography | OH-shun GY-er | diameter km; hundreds–thousands km |
| Ocean ridge | Geology | OH-shun rij | length km; thousands km |
| Oceanic crust | Geology | oh-SHE-an krust | thickness km; ~6–10 km |
| Ocean acidification | Oceanography/climate | OH-shun a-si-di-fi-KAY-shun | pH change; ~-0.1 since 1800s |
| Ocean conveyor belt | Oceanography | OH-shun kon-VAY-or belt | circulation scale; global, multi-decade |
| Ocean heat content | Climate/oceanography | OH-shun heet KON-tent | energy J; increasing by 10^22 J decadal scales |
| Orogeny | Geology | or-EE-je-nee | timescale Myr; tens–hundreds Myr |
| Orogen | Tectonics | OR-uh-jen | width km; tens–thousands km |
| Orogenic wedge | Tectonics | or-uh-JEN-ik wehj | thickness km; a few–tens km |
| Orogenic core complex | Structural geology | or-uh-JEN-ik kor KAHM-pleks | exposure km; variable |
| Ophiolite | Geology | of-ee-oh-lyt | thickness km; variable |
| Oolitic limestone | Rock | OO-lit-ik LIM-stohn | carbonate %; often >50% |
| Ore | Economic geology | or | grade % or g/t; highly variable |
| Ore deposit | Economic geology | or dih-POZ-it | tonnage, grade; varies widely |
| Ore genesis | Economic geology | or-uh JEN-uh-sis | timescale Myr; variable |
| Orthoclase | Mineral | OR-thoh-klays | Mohs hardness; ~6 |
| Orthogneiss | Rock | OR-tho-nys | metamorphic grade; medium–high |
| Orthopyroxene | Mineral | OR-thoh-PY-roks-een | Mohs hardness; ~5–6 |
| Orthoquartzite | Rock | OR-thoh-kwort-zyt | silica %; often >95% |
| Outcrop | Geology | OUT-krop | exposed thickness m; cm–many m |
| Outwash | Glacial sediment | OUT-wosh | grain size; sand to gravel |
| Outwash plain | Glacial landform | OUT-wosh playn | area km²; variable |
| Overburden | Mining/soil | OH-ver-BUR-den | thickness m; typically 1–100 m |
| Overbank deposit | Fluvial geology | OH-ver-bank dih-POZ-it | layer thickness cm–m; variable |
| Overpressure | Petroleum geology | OH-ver-PRESH-ur | pressure MPa; exceeds hydrostatic |
| Overturn | Oceanography/Limnology | OH-ver-turn | timescale hrs–decades; system dependent |
| Oxidation | Geochemistry | ock-si-DAY-shun | redox potential mV; variable |
| Oxidation state | Geochemistry | ock-si-DAY-shun stayt | valence; e.g., Fe2+, Fe3+ |
| Oxidation-reduction | Geochemistry | ock-si-DAY-shun re-DUK-shun | redox potential mV; variable |
| Oxic | Oceanography/Soils | OK-sik | dissolved O2; typically >80 µmol/kg |
| Oxycline | Oceanography | OK-see-kline | depth m; gradient over tens m |
| Oxygen | Atmosphere | OK-si-jen | mixing ratio %; atmosphere ~20.95% |
| Oxygen isotope | Paleoclimatology | OK-si-jen EYE-suh-tohp | ratio δ18O‰; proxy values vary |
| Oxygen minimum zone | Oceanography | OK-si-jen MIN-uh-mum zohn | depth m; commonly 200–1,000 m |
| Ozone | Atmosphere | OH-zohn | mixing ratio ppb; surface 10–100 ppb |
| Ozone layer | Atmosphere | OH-zohn LAY-er | altitude km; ~15–35 km |
| Ozone hole | Atmospheric science | OH-zohn hohl | area million km²; variable seasonally |
| Optically stimulated luminescence dating | Geochronology | op-TIK-lee STIM-yoo-lay-ted loo-muh-NESS-ence | age years; up to ~1,000,000 years |
| Onlap | Stratigraphy | ON-lap | horizontal distance m–km; variable |
| Oncoid | Sedimentary geology | ON-koyd | size mm–cm; variable |
| Olistostrome | Sedimentary geology | oh-lis-toh-strohm | thickness m; variable |
| Olistolith | Sedimentary geology | oh-LIS-toh-lith | block size m–km; variable |
| Orocline | Structural geology | OR-oh-kline | radius km; varies |
| Orographic lift | Atmosphere/hydrology | or-uh-GRAF-ik lift | altitude change m; uplift over slopes |
| Oscillation (climate) | Climate science | os-uh-LAY-shun | period years; seasonal–decadal |
| Ophitic texture | Igneous petrology | OH-fit-ik TEK-stur | grain size mm; intergrown crystals |
| Ortho- prefix terms (e.g., orthoclase) | Mineralogy/rock | OR-tho | usage; denotes “straight” or “true” |
| Ordnance? (excluded) | — | — | — |
| Ordnance entry removed per inclusion rules | — | — | — |
| Ordovician | Geologic time | or-duh-VISH-un | Age Ma; ~485–444 Ma |
| Oligocene | Geologic time | OL-i-guh-seen | Age Ma; ~34–23 Ma |
| Oligotrophic | Limnology/oceanography | ol-ee-go-TROH-fik | nutrients µg/L; low concentrations |
| Omphacite | Mineralogy | OM-fuh-site | Mohs hardness; ~5.5–6 |
| Olivine | Mineral | OL-ih-veen | Mohs hardness; ~6–7 |
| Organic matter | Soil science | or-GAN-ik MAT-er | percent; soils 1–10% |
| Organic carbon | Carbon cycle | or-GAN-ik KAR-buhn | percent or Pg; soils 1–5% |
| Outlier | Geology | OUT-ly-er | area m²–km²; isolated |
| Oxbow lake | Fluvial geomorphology | OX-boh lake | area ha; commonly 1–100 ha |
| Ooze | Marine sedimentology | ooz | thickness cm–m; variable |
| Oceanic plateau | Geology | OH-shun-ik playt | area km²; thousands km² |
| Ortho- (see Ortho- prefix) | General geology | OR-tho | see usage note above |
| Ophrite? (excluded duplicate of ophitic) | — | — | — |
Descriptions
O horizon
Surface soil layer rich in organic material, common in forests and wetlands; often dark and important for nutrient cycling.
Oolite
Sedimentary rock composed of rounded carbonate ooids; forms in warm, shallow marine waters and makes up oolitic limestones.
Ooid
Small, spherical carbonate grains formed by concentric precipitation around a nucleus in shallow marine settings.
Obsidian
A glassy volcanic rock produced by rapid cooling of high-silica lava; sharp fractures and common in volcanic terrains.
Ocean
The vast saline body of water covering most of Earth’s surface, driving climate, weather, and marine ecosystems.
Ocean basin
Large depressions on Earth’s surface that hold ocean water, including continental margins and deep-sea plains.
Ocean current
Continuous, directed movement of seawater driven by wind, density differences, and Earth’s rotation.
Ocean gyre
Large, circular systems of surface currents in the major ocean basins influenced by trade winds and Coriolis force.
Ocean ridge
Elevated submarine mountain chain formed by seafloor spreading at divergent plate boundaries.
Oceanic crust
The thin, dense crust beneath the oceans, primarily basaltic and formed at mid-ocean ridges.
Ocean acidification
Decrease in seawater pH from CO2 uptake, affecting calcifying organisms and marine chemistry.
Ocean conveyor belt
Large-scale thermohaline circulation connecting surface and deep-ocean currents, important for heat transport.
Ocean heat content
Measure of heat stored in the ocean, a key indicator of global warming.
Orogeny
The process of mountain building through tectonic collision, folding, faulting, and uplift.
Orogen
A mountain belt produced by long-term tectonic processes, often containing folded rocks and metamorphic cores.
Orogenic wedge
A tapering pile of thrust sheets and sediments at convergent plate margins during mountain building.
Orogenic core complex
Extensively exhumed crustal blocks exposing deep rocks due to extension in mountain belts.
Ophiolite
Sections of oceanic crust and upper mantle thrust onto continents, important for studying seafloor composition.
Oolitic limestone
Limestone made predominantly of ooids, common in warm, shallow marine carbonate platforms.
Ore
Mineral or rock concentration from which valuable metals or minerals can be economically extracted.
Ore deposit
A concentrated accumulation of economically valuable minerals formed by geological processes.
Ore genesis
Processes and conditions that form ore deposits, including magmatic, hydrothermal, and sedimentary mechanisms.
Orthoclase
A common potassium feldspar mineral in igneous and metamorphic rocks, often pink or white.
Orthogneiss
A gneiss derived from igneous parent rocks, characterized by banding and high-temperature minerals.
Orthopyroxene
A common high-temperature pyroxene mineral in mafic igneous and some metamorphic rocks.
Orthoquartzite
A very hard, well-cemented sandstone composed almost entirely of quartz.
Outcrop
An exposed portion of bedrock or sediment at Earth’s surface used for mapping and sampling.
Outwash
Sediments deposited by meltwater streams from glaciers, forming outwash plains and terraces.
Outwash plain
Flat to gently sloping plain formed of glacial meltwater deposits in front of a retreating glacier.
Overburden
Surface material (soil, rock) that lies above a mineral deposit or bedrock and must be removed for extraction.
Overbank deposit
Sediment laid down on a floodplain during flooding when rivers overflow their banks.
Overpressure
Subsurface pore pressure higher than normal, affecting drilling and petroleum migration.
Overturn
Vertical mixing event in lakes or oceans that brings deep water to the surface, redistributing heat and nutrients.
Oxidation
Chemical reaction where a substance loses electrons, commonly affecting minerals and soil color.
Oxidation state
The valence or charge of an element in a compound, important in redox chemistry and mineralogy.
Oxidation-reduction
A paired set of chemical reactions (redox) controlling electron transfer, central to geochemical cycles.
Oxic
Describes environments with measurable dissolved oxygen, supporting aerobic life and certain geochemical reactions.
Oxycline
A sharp vertical gradient in dissolved oxygen concentration in the water column.
Oxygen
Essential atmospheric gas produced by photosynthesis and consumed by respiration and combustion.
Oxygen isotope
Ratios of oxygen isotopes in shells and ice used to infer past temperatures and ice volume.
Oxygen minimum zone
Mid-water regions with very low dissolved oxygen, influencing marine life and biogeochemistry.
Ozone
A molecule of three oxygen atoms; in the stratosphere it shields UV, at surface it’s a pollutant.
Ozone layer
Region of enhanced stratospheric ozone that absorbs harmful ultraviolet radiation from the Sun.
Ozone hole
Seasonal severe depletion of stratospheric ozone, notably over Antarctica due to man-made chemicals.
Optically stimulated luminescence dating
Dating method measuring trapped electrons in minerals to date last exposure to light, used in sediments.
Onlap
A layering pattern where younger strata terminate progressively against a slope or older strata.
Oncoid
Rounded, often layer-caked carbonate grains formed by microbial growth in shallow marine settings.
Olistostrome
A chaotic submarine mass-transport deposit made of blocks and matrix produced by slope failure.
Olistolith
Large blocks or slumps incorporated within an olistostrome, representing transported bedrock fragments.
Orocline
A large-scale curvature or bend in mountain belts or orogenic belts caused by tectonic deformation.
Orographic lift
Uplift of air by terrain that cools and condenses moisture, producing clouds and precipitation on windward slopes.
Oscillation (climate)
Regular or irregular variations in climate indices (e.g., ENSO) influencing weather and global climate patterns.
Ophitic texture
Igneous texture where plagioclase laths are enclosed by pyroxene, common in some basalts.
Ortho- prefix terms (e.g., orthoclase)
Prefix used in mineral and rock names to indicate original igneous derivation or straight crystal forms.
Ordnance? (excluded)
—
Ordnance entry removed per inclusion rules
—
Ordovician
A Paleozoic period marked by diverse marine life and major tectonic and sea-level changes.
Oligocene
Epoch of the Paleogene characterized by cooling climates and major mammal evolution and diversification.
Oligotrophic
Describes waters low in nutrients and primary productivity, often clear with high oxygen levels.
Omphacite
A pyroxene mineral common in high-pressure metamorphic rocks like eclogite.
Olivine
A green magnesium-iron silicate mineral common in mafic igneous rocks and the upper mantle.
Organic matter
Decomposed plant and animal residues in soils that store carbon and influence fertility and structure.
Organic carbon
Carbon contained in organic compounds in soils, sediments, and biomass; central to carbon cycle studies.
Outlier
A remnant of rock that is isolated from its main outcrop by erosion, indicating past extent of layers.
Oxbow lake
U-shaped lake formed when a meander is cut off from a river, common on floodplains.
Ooze
Fine-grained seafloor sediment rich in microfossil remains, e.g., calcareous or siliceous ooze.
Oceanic plateau
Large, elevated igneous provinces on the seafloor formed by massive volcanic eruptions, often thick crust.
Ortho- (see Ortho- prefix)
See “Ortho- prefix terms” — used in making technical mineral and rock names.
Ophrite? (excluded duplicate of ophitic)
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