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A carbonate mineral, consisting of calcium carbonate (CaCO3). Calcite is a common constituent of sedimentary rocks, particularly limestone.

calcium carbonate

A chemical compound with the formula CaCO3, commonly found in rocks in the mineral forms calcite and aragonite, as well as the shells and skeletons of marine organisms.


A collapsed, cauldron-like volcanic crater formed by the collapse of land following a volcanic eruption.


A zone of cemented material within soil, formed when water infiltrates the soil, dissolves soluble materials, and evaporates, leaving behind precipitates (particularly calcium carbonate) in the pore space between soil grains. Layers of caliche accumulate to tens of feet in some locations. Caliche is commonly collected for use as an additive in cement.


The process by which ice breaks off from the end of a glacier (sometimes into a lake or ocean, sometimes over the edge of a cliff).


The head of a crinoid.


A geologic time period lasting from 541 to 485 million years ago. During the Cambrian, multicellular marine organisms became increasingly diverse, as did their mineralized fossils.
The Cambrian is part of the Paleozoic Era.

Canadian Shield

The stable core of the North American continental landmass, containing some of the oldest rocks on Earth. The shield has experienced very little tectonic activity ( faulting or folding) for millions of years. As the stable cores of all continents, shields are often covered by layers of younger material.

capstone, caprock

A harder, more resistant rock type that overlies a softer, less resistant rock. The harder rock typically helps to control the rate of erosion.

carbonate rocks

Rocks formed by accumulation of calcium carbonate, often made of the skeletons of aquatic organisms such as corals, clams, snails, bryozoans, and brachiopods. These organisms thrive in warm, clear shallow waters common to tropical areas, therefore modern carbonate rocks are observed forming in places such as the Florida Keys and the Bahamas. They are also one of the dominant rock forms of the bottom of the ocean, where sediments form from the skeletons of planktonic organisms such as foraminifera.
Carbonate rocks include limestone, dolostone, and dolomite.


A geologic time period that extends from 359 to 299 million years ago. It is divided into two subperiods, the Mississippian and the Pennsylvanian. By the Carboniferous, terrestrial life had become well established.
The name Carboniferous means "coal-bearing," and it is during this time that many of today’s coal beds were formed.
The Carboniferous is part of the Paleozoic.


Pertaining to rocks made up of cemented fragments that originated from the mechanical breakdown of rock associated with plate tectonics. Cataclastic rocks form in regions that have undergone intense metamorphism and are associated with features such as folds and faults. They typically contain bent, broken, and granular minerals.


The precipitation of minerals, such as silica and calcite, that bind together particles of rock, bones, etc., to form a solid mass of sedimentary rock.


The geologic time period spanning from 66 million years ago to the present. The Cenozoic is also known as the age of mammals, since extinction of the large reptiles at the end of the Mesozoic allowed mammals to diversify.
The Cenozoic includes the Paleogene, Neogene, and Quaternary periods.


A marine invertebrate animal belonging to the Class Cephalopoda in the Phylum Mollusca, and characterized by a prominent head, arms, and tentacles with suckers, and jet propulsion locomotion.
Cephalopods are swimming predators with beak-shaped mouthparts. The shells of cephalopods range from long straight cones to spirals, but some have internal shells or no significant shell at all, such as the octopus.The group includes belemnites, ammonoids, nautilus, squid, and octopuses.
A mass extinction between the Cretaceous and Paleogene eliminated many varieties of cephalopods.


A crystalline silicate mineral that is a microcrystalline variety of quartz.


A yellow mineral consisting of a copper-iron sulfide (CuFeS2). Chalcopyrite is the most common and important source of copper, and can also be called copper pyrite.


A soft, fine-grained, easily pulverized, white-to-grayish variety of limestone, composed of the shells of minute planktonic single-celled algae.

chemical fossils

Chemicals produced by an organism that leave behind an identifiable trace in the geologic record. Chemical fossils provide some of the oldest evidence for life on Earth.

chemical reaction

A process that involves changes in the structure and energy content of atoms, molecules, or ions but not their nuclei.


A sedimentary rock composed of microcrystaline quartz. It is often found as nodules or concretions in limestone and other marine sedimentary rocks. As these rocks form, water moving through them transports small amounts of silicon dioxide that accumulate into clumps of microscopic crystals. The resulting rocks are extremely strong and have no planes of weakness.
For thousands of years, humans exploited these qualities, breaking chert nodules into blades and other tools.


A variety of pumpellyite with a distinctive “turtleback” pattern created by its interlocking green crystals.


An animal that possesses the following five traits during at least one stage of its development: a notochord (the flexible rod that, in vertebrates, becomes the backbone), a hollow dorsal nerve cord, pharyngeal gill slits, an endostyle (precursor to the thyroid gland), and a post-anal tail.


A lustrous, hard, steel-gray metallic element (Cr), resistant to tarnish and corrosion. Chromium is used as a component of certain pigments, as a component of steel (providing resistance and hardness), and in the production of chrome and stainless steel.

Cincinnati Arch

An uplifted region that existed between the Illinois Basin, the Michigan Basin, and the Appalachian Basin during the late Ordovician and Devonian. It stretched from southeastern Ontario all the way to northern Alabama.


A type of pyroclastic particle in the form of gas-rich lava droplets that cool as they fall.


A large bowl-shaped depression carved by glacial erosion and located in mountainous regions.


An anatomical structure used by sharks for mating.


The common name for a number of very fine-grained, earthy materials that become plastic (flow or change shape) when wet. Chemically, clays are hydrous aluminum silicates.


A physical property of minerals. Cleavage occurs when a mineral breaks in a characteristic way along a specific plane of weakness.
Mica and graphite have very strong cleavage, allowing them to easily break into thin sheets.


A description of the average temperature, range of temperature, humidity, precipitation, and other atmospheric /hydrospheric conditions a region experiences over a period of many years (usually more than 30). These factors interact with and are influenced by other parts of the Earth system, including geology, geography, insolation, currents, and living things.
The climate of a region represents the average weather over a long period of time.

climate change

See global warming


A combustible, compact black or dark-brown carbonaceous rock formed by the compaction of layers of partially decomposed vegetation.
By far the greatest abundance of coal is located in strata of Carboniferous age.


The process by which coal is formed from plant materials through compression and heating over long periods of time.


A marine phytoplankton with a skeleton made up of microscopic calcareous disks or rings, and forming much of the content of chalk rocks.

cold front

The boundary between the warm air and the cold air moving into a region. At this boundary, denser, colder air moves in, making the less dense, warm air rise. This displaced warm air cools as it rises because air pressure decreases with increasing height in the atmosphere. As the air cools, it becomes saturated with water vapor, and condensation begins to occur, eventually leading to dramatic rainstorms.

color (mineral)

A physical property of minerals. Color is determined by the presence and intensity of certain elements within the mineral.

color (soil)

A physical property of soils. Soil color is influenced by mineral content, the amount of organic material, and the amount of water it routinely holds. These colors are identified by a standard soil color chart called the Munsell chart.

Colorado Plateau

A physiographic region that covers an area of 337,000 square kilometers (130,000 square miles) of desert and forest within Colorado, New Mexico, Arizona, and Utah. Most of the area is drained by the Colorado River and its tributaries.

columnar joint

Five- or six-sided columns that form as cooling lava contracts and cracks. Columnar joints are often found in basalt flows, but can also form in ashflow tuffs as well as shallow intrusions. The columns are generally vertical, but may also be slightly curved.


A good for which there is demand, but which is treated as equivalent across all markets, no matter who produces it.

compression, compressional force

Forces acting on an object from all or most directions, resulting in compression (flattening or squeezing). Compressional forces occur by pushing objects together.


A hard, compact mass, usually of spherical or oval shape, found in sedimentary rock or soil. Concretions form when minerals precipitate around a particulate nucleus within the sediment.


A sedimentary rock composed of multiple large and rounded fragments that have been cemented together in a fine-grained matrix. The fragments that make up a conglomerate must be larger than grains of sand.


A woody plant ( tree ) of the division Coniferophyta. Conifers bear cones that contain their seeds.


An extinct, eel-shaped animal classified in the class Conodonta and thought to be related to primitive chordates. Originally, conodonts were only known from small phosphatic tooth-like micro fossils, which have been widely used for biostratigraphy. Knowledge about their soft tissues still remains limited.

Conservation of Energy

A principle stating that energy is neither created nor destroyed, but can be altered from one form to another.

contact metamorphism

The process by which a metamorphic rock is formed through direct contact with magma. Changes that occur due to contact metamorphism are greatest at the point of contact. The farther away the rock is from the point of contact, the less pronounced the change.


The rise of buoyant material and the sinking of denser material. In the mantle, variations in density are commonly caused by the melting of subducting materials.

convergent boundary

An active plate boundary where two tectonic plates are colliding with one another. Subduction occurs when an oceanic plate collides with a continental plate or another oceanic plate. If two continental plates collide, mountain building occurs.
See also: plate tectonics


A ductile, malleable, reddish-brown metallic element (Cu).
Copper is used extensively as wiring in the electrical industry as well as in alloys such as brass and bronze.


A porous, sometimes crumbly limestone, composed of fragments of shells and coral, and used as a building material.


A member of a group called the Cordaitales, which were closely related to or early members of the conifers. The best known taxon is the eponymous Pennsylvanian genus Cordaites. The group was prominent in swampy habitats during the Carboniferous, but went extinct by the end of the Triassic.

Cordilleran Ice Sheet

One of two continental glaciers that covered Canada and parts of the Western US during the last major Pleistocene ice age.


An aluminum oxide mineral (Al2O3) that is, after diamond, the hardest known natural substance. Corundum is best known for its gem varieties, ruby (red) and sapphire (blue).


The old, underlying portion of a continent that is geologically stable relative to surrounding areas. The portion of a craton exposed at the surface is termed a shield, while that overlain by younger layers is often referred to as a platform.
A craton can be thought of as the heart of a continent—it is typically the oldest, thickest, and most stable part of the bedrock. It is also usually far from the margins of tectonic plates, where new rock is formed and old destroyed. This rock has usually been metamorphosed at some point during its history, making it resistant to erosion.


The slow movement or deformation of a material under the influence of pressure or stress (such as gravity); the slow progression of rock and soil down a slope due to the interacting factors of gravity, vegetation, water absorption, and steepness.


A geologic time period spanning from 144 to 66 million years ago. It is the youngest period of the Mesozoic. The end of the Cretaceous bore witness to the mass extinction event that resulted in the demise of the dinosaurs.
"Cretaceous" is derived from the Latin word creta, meaning "chalk." The white ( chalk ) cliffs of Dover on the southeastern coast of England are a famous example of Cretaceous chalk deposits.


A deep crack in an ice sheet or glacier, which forms as a result of shear stress between different sections of the moving ice.


A marine invertebrate animal belonging to the Class Crinoidea of the Phylum Echinodermata, and characterized by a head (calyx) with a mouth on the top surface surrounded by feeding arms. Several groups of stemmed echinoderms appeared in the early Paleozoic, including crinoids, blastoids, and cystoids.
Crinoids have five-fold symmetry and feathery arms (sometimes held off the sea floor on a stem) that collect organic particles from the water. The stems, the most often preserved part, are made of a series of stacked discs. Upon death, these stems often fall apart and the individual discs are preserved separately in the rock.
The crinoid’s feathery arms make it look something like a flower on a stem. Thus, crinoids are commonly called “sea lilies,” although they are animals, not plants.


Layering within a bed in a series of rock strata that does not run parallel to the plane of stratification. Cross-beds form as flowing water or wind pushes sediment downcurrent, creating thin beds that slope gently in the direction of the flow as migrating ripples. The downstream slope of the ripple may be preserved as a thin layer dipping in the direction of the current, across the natural flat-lying repose of the beds. Another migrating ripple will form an additional layer on top of the previous one.


The uppermost, rigid outer layer of the Earth. Two types of crust make up the lithosphere, which is broken into tectonic plates. Oceanic crust is denser but significantly thinner than continental crust, while continental crust is much thicker but less dense (and therefore buoyant).
When continental crust collides with oceanic crust, the denser oceanic crust will be dragged ( subducted ) under the buoyant continental crust. Although mountains are created by these oceanic/continental crust collisions due to the compression of the two plates, much taller ranges are produced by continental/continental collisions. When two buoyant continental crusts collide, there is nowhere for the crust to go but up! The modern Himalayas, at the collision site of the Asian and Indian plates, are a good example of very tall mountains formed by a collision between two continental crusts.


A geologic period lasting from 850 to 635 million years ago, during the Precambrian. During this period, the Earth was subject to a 200-million-year-long ice age.

crystal form

A physical property of minerals, describing the shape of the mineral’s crystal structure (not to be confused with cleavage ). A mineral might be cubic, rhomboidal, hexagonal, or polyhedral.


A group of bacteria, also called "blue-green algae," that obtain their energy through photosynthesis.


A palm-like, terrestrial seed plant ( tree ) belonging to the Class Cycadopsida, and characterized by a woody trunk, a crown of stiff evergreen leaves, seeds without protective coatings, and no flowers. Cycads were very common in the Mesozoic, but are much reduced in diversity today, restricted to the tropical and subtropical regions of the planet.


Alternating sequences of marine and non-marine sedimentary rocks, usually including coal, and characterized by their light and dark colors.


Extinct, stalked echinoderms related to crinoids, but with an ovoid body and triangular pore openings.