Earth science

Ocean acidification Ocean acidification is the progressive lowering of seawater pH caused by absorbed atmospheric CO2, changing carbonate chemistry and threatening marine life, ecosystems, and coastal…

Ocean current Flowing masses of seawater driven by wind, Earth's rotation, density differences and tides; they shape climate, marine ecosystems, and sea navigation worldwide.

Ocean gyre: large-scale circular ocean current systems Ocean gyre An ocean gyre is a vast, circular system of surface currents driven by wind, Earth's rotation and ocean physics. Gyres shape climate, marine ecosystems and concentrate floating deb…

Ocean surface wave Overview of ocean surface waves: how they form, main physical properties and propagation, coastal effects, types (wind waves, swell, tsunamis), measurement, hazards and practical u…

Oceanic climate (maritime climate) Oceanic climate A temperate climate influenced by nearby oceans, with mild winters, cool summers, frequent precipitation year-round, and a narrow annual temperature range.

Oceanic crust: composition, formation, and lifecycle Oceanic crust Oceanic crust is the dense, mafic portion of Earth's outer shell beneath the seas. This article explains its structure, formation at mid‑ocean ridges, recycling by subduction, age…

Oceanic trench Deep, narrow depressions on the seafloor formed where one tectonic plate descends beneath another; sites of intense geology, unique ecosystems, seismic hazards, and deep‑sea explor…

Oceanography: The Science of the Seas and Marine Environment Oceanography Oceanography is the multidisciplinary study of the ocean—its physics, chemistry, biology, geology, methods, history, and applications from climate to resource management.

Oil field: structure, development, and production Oil field An oil field is an area containing accumulations of petroleum that are developed with multiple wells, infrastructure, and recovery methods to produce hydrocarbons for energy and in…

Oil sands (tar sands): composition, extraction, impacts, and global significance Oil sands Oil sands are deposits of sand, clay, water and dense bitumen used as an unconventional petroleum source; extraction is energy‑intensive and has important environmental and economi…

Ōjin Seamount: Extinct Guyot in the Hawaiian–Emperor Seamount Chain Ōjin Seamount Ōjin Seamount, an extinct flat-topped guyot in the Hawaiian–Emperor seamount chain, formed over a Pacific hotspot and last active about 55 million years ago; named for Emperor Ōjin…

Ojos del Salado: Highest Active Volcano and Second-Highest Peak in the Americas Ojos del Salado Discover Ojos del Salado, a 6,893 m stratovolcano on the Chile–Argentina border — the highest active volcano on Earth and the Americas' second-highest peak, famed for extreme aridi…

Ol Doinyo Lengai: Tanzania’s unique carbonatite volcano Ol Doinyo Lengai Ol Doinyo Lengai is an active volcano in northern Tanzania, notable for erupting rare carbonatite (natrocarbonatite) lava. It is geologically and culturally important and attracts…

Old Earth creationism Religious perspective that accepts an ancient Earth while maintaining divine creation; encompasses several interpretive approaches that reconcile geological time with scriptural be…

Old Faithful Geyser Old Faithful is an iconic cone geyser in Yellowstone National Park, Wyoming, known for frequent, relatively predictable eruptions and its importance to tourism and geothermal scien…

Old Red Sandstone A widespread sequence of Devonian-age red-bed rocks deposited on the ancient continent Laurussia; notable for terrestrial sediments, early vertebrate fossils and its role in geolog…

Oldest dated rocks and minerals on Earth and the Moon Oldest dated rocks Survey of the oldest known dated materials: distinction between minerals and rocks, dating methods, notable examples (Jack Hills zircons, Acasta Gneiss, Isua, lunar samples), and t…

Oligocene epoch Oligocene A geological epoch from about 34 to 23 million years ago marked by global cooling, Antarctic glaciation and the rise of more modern mammal and plant communities during the Paleogen…

Olivine Olivine is a common green magnesium–iron silicate mineral (formula (Mg,Fe)2SiO4), important in igneous and mantle rocks, known in gem form as peridot and notable for its role in pl…

Onyx: banded chalcedony used as gemstone and ornamental stone Onyx Onyx is a variety of chalcedony (microcrystalline quartz) known for parallel banding, used historically and today for jewelry and decorative carving; often treated or confused with…

Ooid: small concentrically layered carbonate grain Ooid An ooid is a small, rounded sedimentary grain (≤2 mm) formed by concentric mineral precipitation around a nucleus; abundant in warm, agitated shallow waters and important in ooliti…

Opal: formation, properties, varieties and uses Opal Opal is a hydrated, amorphous form of silica prized as a gemstone for its iridescent play-of-color. This article explains its structure, formation, varieties, uses, care and cultur…

Ophiolite: exposed sections of oceanic crust and upper mantle Ophiolite Ophiolites are fragments of oceanic lithosphere—crust and upper mantle—emplaced onto continental margins. They reveal ocean basin history, rock sequences, formation processes and e…

Orbital forcing and Milankovitch cycles Orbital forcing Orbital forcing describes how slow changes in Earth's orbital shape, axial tilt and precession alter sunlight distribution, pacing glacial cycles and influencing long-term climate…

Orbiting Carbon Observatory (OCO) Orbiting Carbon Observatory NASA Earth science satellite planned to map global atmospheric carbon dioxide; lost in a 2009 launch failure. Its objectives and legacy led to a successful follow-up mission.

Ore: naturally occurring mineral deposits containing extractable metals Ore An ore is a rock or mineral that contains metal or valuable elements in concentrations that can be profitably mined and processed. Covers origin, types, extraction, and uses.

Orogeny: the geological process of mountain building Orogeny Orogeny (orogenesis) is the set of geological processes that form mountain belts through plate interactions, magmatism, metamorphism, uplift and erosion over millions of years.

Orosirian Period (Palaeoproterozoic): 2.05–1.80 billion years ago Orosirian A summary of the Orosirian Period (2.05–1.80 Ga): timing, tectonics, major impacts, environmental context and geological significance during assembly of the supercontinent Columbia…

Orpiment (As2S3) — yellow arsenic sulfide mineral Orpiment Yellow-orange arsenic sulfide mineral (As2S3). Soft and toxic; historically a pigment and arsenic ore. Forms in hydrothermal and geothermal deposits and oxidizes to arsenic trioxid…

Outflow (meteorology): thunderstorm downdrafts, gust fronts and boundaries Outflow (meteorology) Outflow is the cool air pushed outward from a thunderstorm downdraft. It forms gust fronts and boundaries that affect weather, trigger new storms, create dust storms, and show dist…

Outwash plain (glacial outwash) Outwash plain A broad, flat landform made of sand and gravel deposited by meltwater from a retreating glacier; important for groundwater, soil development, and as a record of past glacial activi…

Ozone layer: structure, chemistry, history, and global importance Ozone layer The ozone layer is a region of the stratosphere rich in ozone (O3) that absorbs most harmful ultraviolet radiation, has been altered by human-made chemicals, and is monitored and r…

Pacific hurricane — tropical cyclones of the northeastern Pacific Ocean Pacific hurricane A Pacific hurricane is a tropical cyclone that forms in the northern Pacific east of the International Date Line. This article explains formation, structure, seasons, impacts, moni…

Pacific Plate Largest tectonic plate beneath the Pacific Ocean, covering much of the ocean basin; notable for its boundaries in the Ring of Fire, the Hawaiian hotspot, and its role in plate tect…

Pacific Ring of Fire A broad arc of tectonic boundaries encircling the Pacific Ocean where most of the world’s volcanic eruptions and large earthquakes occur; shaped by subduction and plate interaction…

Palaeocene The Palaeocene (66–56 million years ago) is the earliest epoch of the Paleogene, following the end-Cretaceous extinction and ending with the Paleocene–Eocene Thermal Maximum.

Palaeoclimatology: study of Earth's past climates Palaeoclimatology Palaeoclimatology reconstructs past climate from natural records (proxies) such as ice cores, sediments, tree rings and corals to understand variability, drivers, and impacts on li…

Palaeogene: early Cenozoic period of mammal diversification and climate change Palaeogene The Palaeogene (also Paleogene) spans about 66–23 million years ago; it follows the Cretaceous, precedes the Neogene, and includes the Paleocene, Eocene and Oligocene epochs.

Palaeoproterozoic Era Palaeoproterozoic The Palaeoproterozoic (≈2.5–1.6 billion years ago) was the earliest Proterozoic era, notable for major oxygenation, stromatolite proliferation, first eukaryote fossils, and early s…

Palaeozoic Era Palaeozoic The Palaeozoic Era (about 541–252 million years ago) witnessed the Cambrian explosion, colonization of land by plants and animals, formation of coal forests and Pangaea, and ended…

Paleontology: the science of fossils, past life and Earth's history Paleontology Paleontology studies fossils to reconstruct ancient life, evolutionary relationships, environments and geological contexts. Covers methods, subfields (zoology, botany, micropaleont…

Pangaea: the late Paleozoic–early Mesozoic supercontinent Pangaea Pangaea was a single global landmass that assembled through plate tectonics in the late Paleozoic, altered climate and biogeography, and began breaking apart in the early Mesozoic…

Pannotia: the late Neoproterozoic supercontinent Pannotia Pannotia, also called the Vendian or Greater Gondwanaland, is a proposed late Neoproterozoic supercontinent assembled before the Cambrian and fragmented into Laurentia, Baltica, Si…

Parent peak (topography) Parent peak Parent peak: the higher summit used to relate and measure a mountain's topographic prominence; different definitions exist and the choice affects how peaks are classified and compa…

Parinacota (Parina Quta) — stratovolcano on the Chile–Bolivia border Parinacota Volcano Parinacota is a large dormant stratovolcano on the Chile–Bolivia border in the central Andes, known for its twin peak Pomerape, high-altitude ecosystems and cultural significance t…

Particularly Dangerous Situation (PDS): meaning, use, and significance Particularly Dangerous Situation A Particularly Dangerous Situation (PDS) is intensified wording used by U.S. forecasters to flag an unusually high likelihood of catastrophic severe weather—most often major tornad…

Pedogenesis: Processes, Factors and Importance of Soil Formation Pedogenesis Pedogenesis is the natural process by which soils form and evolve under the influence of climate, organisms, parent material, topography and time; it underpins land use, ecology an…

Pennsylvanian: the Upper Carboniferous epoch in North America Pennsylvanian The Pennsylvanian (323–299 Ma) is the Upper Carboniferous epoch marked by vast equatorial coal-forming wetlands, distinctive flora and amphibian faunas, and the buildup of coal res…