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Chromite
Chromite, sold industrially as chrome ore, is an iron chromium oxide mineral of the spinel group and the only commercial source of chromium, smelted mostly into ferrochrome for stainless steel.
Chronostratigraphy
Chronostratigraphy is the branch of stratigraphy that organizes the rocks of the Earth's crust into named units representing intervals of geologic time, formalized at Bologna in 1881.
Chrysoberyl
Chrysoberyl is a rare beryllium aluminum oxide mineral, BeAl₂O₄, prized as a gemstone and best known for its alexandrite variety, which changes color, and its chatoyant cat's-eye.
Chrysotile
Chrysotile, also called white asbestos, is a fibrous serpentine mineral and the most widely used asbestos in commerce, found in cement, brakes, and insulation until banned by the EPA in 2024.
Cinder cone
A cinder cone, also called a scoria cone, is a steep conical volcanic hill built from solidified lava fragments around a vent, the most common volcanic landform on Earth.
Cinnabar
Cinnabar is a bright red mercury sulfide mineral, HgS, the chief ore from which mercury is extracted by roasting and the source of the pigment vermilion.
Circles of latitude between the 25th parallel north and the 30th parallel north
The circles of latitude between the 25th parallel north and the 30th parallel north are five whole-degree parallels from 25°N to 29°N, just north of the Tropic of Cancer.
Circles of latitude between the 65th parallel north and the 70th parallel north
The circles of latitude between the 65th and 70th parallels north are six whole-degree parallels that span the subarctic-to-Arctic transition and contain the Arctic Circle at 66°34′ N.
Circulation model (Earth science)
A circulation model is a numerical solver of the equations of fluid motion, used in Earth science to simulate ocean currents, atmospheric flow, and climate.
Circumhorizontal arc
The circumhorizontal arc, often called a fire rainbow, is a colorful band of light formed when sunlight refracts through flat, plate-shaped ice crystals in cirrus clouds, visible only where the sun climbs above 58°.
Cirrocumulus cloud
Cirrocumulus (Cc) is one of the ten cloud genera: a thin, white, high-level cloud of small grains or ripples, mostly ice crystals and usually short-lived.
Cirrostratus cloud
Cirrostratus is one of the ten WMO cloud genera, a transparent whitish veil of ice crystals at 20,000 to 40,000 ft that generally produces halos and often signals an approaching warm front.
Cirrus cloud
Cirrus is a genus of thin, high-altitude clouds made of ice crystals, appearing as delicate white filaments or bands and covering more than half the planet.
Cirrus cloud thinning
Cirrus cloud thinning is a proposed climate intervention that would seed high cirrus clouds so they thin and more heat escapes to space, cooling the planet.
Citrine
Citrine, from the French citron meaning lemon, is the transparent yellow to brownish orange gem variety of quartz colored by trace iron, and a November birthstone with topaz.
Civil defense siren
A civil defense siren, also called an air raid siren or tornado siren, is a fixed outdoor siren governments use to warn the public of attack or natural hazards.
Claiborne Group
The Claiborne Group is a sequence of middle Eocene sands, clays, and lignite-bearing strata across the Gulf Coastal Plain, named from Claiborne Bluff on the Alabama River.
Clastic rock
A clastic rock is a sedimentary rock composed of clasts, fragments of pre-existing rock or mineral grains, bound by compaction and cementation, and classified mainly by grain size.
Clathrate gun hypothesis
The clathrate gun hypothesis, also known as the Big Burp Theory, proposes that abrupt warming episodes, including the PETM, were triggered by methane released from seafloor clathrate hydrates.
Clay
Clay is a fine-grained earthy material made of layered hydrous silicate minerals, also defined by particle size below 2 micrometers; it forms by weathering near the Earth's surface.
Clean Development Mechanism
The Clean Development Mechanism (CDM) is a Kyoto Protocol carbon offset mechanism that lets developing-country projects earn credits industrialized countries use toward their emission targets.
Clean Power Plan
The Clean Power Plan was a 2015 EPA rule establishing state guidelines to cut carbon dioxide emissions from existing power plants; it was stayed, repealed, and never took effect.
Clearwater Formation
Clearwater Formation: Lower Cretaceous (Albian) Mannville Group unit in Alberta; Wabiskaw Member at base; cap rock for Athabasca oil sands, Cold Lake bitumen reservoir, and fast-growing multilateral heavy oil play.
Cleavage (crystal)
Cleavage, or mineral cleavage, is the tendency of a crystal to break along flat planes where atomic bonds are weakest, a key property for identifying minerals.
Cliff
A cliff is a steep, high rock face, called a sea cliff at the coast; cliffs form about 75% of the world's coastline and retreat landward through wave erosion.
Climate
Climate is the long-term pattern of weather at a place, described through averages of temperature, precipitation, and wind over decades, typically summarized in 30-year Climate Normals.
Climate Action Tracker
The Climate Action Tracker (CAT) is an independent scientific project tracking government climate action against the Paris Agreement, run by Climate Analytics and NewClimate Institute since 2009.
Climate Analytics
Climate Analytics is a global climate science and policy institute founded in 2008 in Potsdam, Germany, and headquartered in Berlin, best known for the Climate Action Tracker project.
Climate and Clean Air Coalition
The Climate and Clean Air Coalition (CCAC) is a UNEP-convened partnership founded in 2012 to reduce short-lived climate pollutants such as methane and black carbon.
Climate change
Climate change, also called global warming, is a long-term shift in Earth's climate caused mainly by greenhouse gas emissions from burning fossil fuels, raising global temperature about 1.1 °C above pre-industrial levels.