caldera
A vast depression at the top of a volcanic cone, formed when an eruption substantially empties the reservoir of magma beneath the cone's summit. Eventually the summit collapses inward, creating a caldera. A caldera may be more than 15 kilometers in diameter and more than 1000 meters deep.
caliche
A white soil horizon consisting of calcium carbonate, typical of arid and semi-arid areas. Brief heavy rains dissolve calcium carbonate in the upper layers of soil and transport it downward; the rainwater then evaporates rapidly, leaving the calcium carbonate to form a new, solid layer of soil.
capacity
The ability of a given stream to carry sediment, measured as the maximum quantity it can transport past a given point on the channel bank in a given amount of time. See also competence.
capillary fringe
The lowest part of the zone of aeration, marked by the rising of water from the water table due to the attraction of the water molecules to mineral surfaces and other molecules, and to pressure from the zone of saturation below.
carbon-14 dating
A form of isotope dating that relies on the 5730-year half-life of radioactive carbon-14, which decays into nitrogen-14, to determine the age of rocks in which carbon-14 is present. Carbon-14 dating is used for rocks from 100 to 100,000 years old.
carbonate
One of several minerals containing one central carbon atom with strong covalent bonds to three oxygen atoms and typically having ionic bonds to one or more positive ions.
catastrophism
The hypothesis that a series of immense, brief, worldwide upheavals changed the Earth's crust greatly and can account for the development of mountains, valleys, and other features of the Earth. See also uniformitarianism.
cave
A naturally formed opening beneath the surface of the Earth, generally formed by dissolution of carbonate bedrock. Caves may also form by erosion of coastal bedrock, partial melting of glaciers, or solidification of lava into hollow tubes.
cementation
The diagenetic process by which sediment grains are bound together by precipitated minerals originally dissolved during the chemical weathering of preexisting rocks.
Cenozoic Era
The latest era of the Phanerozoic Eon, following the Mesozoic Era and continuing to the present time, and marked by the presence of a wide variety of mammals, including the first hominids.
chemical sediment
Sediment that is composed of previously dissolved minerals that have either precipitated from evaporated water or been extracted from water by living organisms and deposited when the organisms died or discarded their shells.
chemical weathering
The process by which chemical reactions alter the chemical composition of rocks and minerals that are unstable at the Earth's surface and convert them into more stable substances; weathering that changes the chemical makeup of a rock or mineral. See also mechanical weathering.
chert
A member of a group of sedimentary rocks that consist primarily of microscopic silica crystals. Chert may be either organic or inorganic, but the most common forms are inorganic.
chondrules
Small nuggets of rocky material that exist in certain meteorites. These droplets of matter are believed to have condensed from our solar system's original nebula about five billion years ago. Their primary element is iron.
cinder cone
A pyroclastic cone composed primarily of cinders.
cirque
A deep, semi-circular basin eroded out of a mountain by an alpine glacier.
cirque glacier
A small alpine glacier that forms inside a cirque, typ-ically near the head of a valley.
clastic
Being or pertaining to a sedimentary rock composed primarily from fragments of preexisting rocks or fossils.
cleavage
The tendency of certain minerals to break along distinct planes in their crystal structures where the bonds are weakest. Cleavage is tested by striking or hammering a mineral, and is classified by the number of surfaces it produces and the angles between adjacent surfaces.
coal
A member of a group of easily combustible, organic sedimentary rocks composed mostly of plant remains and containing a high proportion of carbon.
coast
The area of dry land that borders on a body of water.
col
A high mountain pass that forms when part of an arête erodes.
compaction
The diagenetic process by which the volume or thickness of sediment is reduced due to pressure from overlying layers of sediment.
competence
The ability of a given stream to carry sediment, measured as the diameter of the largest particle that the stream can transport. See also capacity.
composite cone
See stratovolcano.
compound
An electrically neutral substance that consists of two or more elements combined in specific, constant proportions. A compound typically has physical characteristics different from those of its constituent elements.
compression
Stress that reduces the volume or length of a rock, as that produced by the convergence of plate margins.
cone ofdepression
An area in a water table along which water has descended into a well to replace water drawn out, leaving a gap shaped like an inverted cone.
conglomerate
A clastic rock composed of particles more than 2 millimeters in diameter and marked by the roundness of its component grains and rock fragments.
contact metamorphism
Metamorphism that is caused by heat from a magmatic intrusion.
continental collision
The convergence of two continental plates, resulting in the formation of mountain ranges.
continental drift
The hypothesis, proposed by Alfred Wegener, that today's continents broke off from a single supercontinent and then plowed through the ocean floors into their present positions. This explanation of the shapes and locations of Earth's current continents evolved into the theory of plate tectonics.
continental ice sheet
An unconfined glacier that covers much or all of a continent.
continental platform
Continental platforms are the regions adjacent to and surrounding the continental shields. They are typically a relatively thin veneer of sedimentary rock that buries the edges of the shields.
continental shield
Broad areas of exposed ancient crystalline rocks in the cores of the Earth's continents. These rocks are typically the oldest on the continentes, many more than 2.5 billion years old.
convection cell
The cycle of movement in the asthenosphere that causes the plates of the lithosphere to move. Heated material in the asthenosphere becomes less dense and rises toward the solid lithosphere, through which it cannot rise further. It therefore begins to move horizontally, dragging the lithosphere along with it and pushing forward the cooler, denser material in its path. The cooler material eventually sinks down lower into the mantle, becoming heated there and rising up again, continuing the cycle. See also plate tectonics.
convergence
The coming together of two lithospheric plates. Con-vergence causes subduction when one or both plates is oceanic, and mountain formation when both plates are continental. See also divergence.
core
The innermost layer of the Earth, consisting primarily of pure metals such as iron and nickel. The core is the densest layer of the Earth, and is divided into the outer core, which is believed to be liquid, and the inner core, which is believed to be solid. See also crust and mantle.
correlation
The process of determining that two or more geographically distant rocks or rock strata originated in the same time period.
covalent bond
The combination of two or more atoms by sharing electrons so as to achieve chemical stability under the octet rule. Atoms that form covalent bonds generally have outer energy levels containing three, four, or five electrons. Covalent bonds are generally stronger than other bonds.
craton
The segment of the Earth's continents that have remained tectonically stable and relatively earthquake-free for a vast period of time. The craton is composed of the continental shield and the surrounding continental platform.
creep
The slowest form of mass movement, measured in millimeters or centimeters per year and occurring on virtually all slopes. cross bed A bed made up of particles dropped from a moving current, as of wind or water, and marked by a downward slope that indicates the direction of the current that deposited them.
crust
The outermost layer of the Earth, consisting of relatively low-density rocks. See also core and mantle.
crystal
A mineral in which the systematic internal arrangement of atoms is outwardly reflected as a latticework of repeated three-dimensional units that form a geometric solid with a surface consisting of symmetrical planes.
crystal structure
1. The geometric pattern created by the systematic internal arrangement of atoms in a mineral. 2. The systematic internal arrangement of atoms in a mineral. See also crystal.
