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One of many millions of small, rocky, and/or metallic objects that orbit the Sun, consisting of fragments of once-larger planetesimals, or chunks of protoplanetary material; most lie in the region between Mars and Jupiter.


The layer of the mantle that lies between 100-150 km and 350 km deep; the asthenosphere is relatively soft and can flow when acted on by force.


A layer of gases that surrounds a planet.


Variation in depth.

big bang

A cataclysmic explosion that scientists suggest represents the formation of the Universe; before this event, all matter and all energy were packed into one volumeless point.


A ball of ice and dust, probably remaining from the formation of the Solar System, that orbits the Sun.


The dense, iron-rich center of the Earth.


The study of the overall structure of the Universe.


The rock that makes up the outermost layer of the Earth.


In the context of planet formation, the process by which a planet separates into a metallic core and a rocky mantle very early in its history.


A vibration caused by the sudden breaking or frictional sliding of rock in the Earth.

Earth System

The global interconnecting web of physical and biological phenomena involving the solid Earth, the hydrosphere, and the atmosphere.

expanding Universe theory

The theory that the whole Universe must be expanding because galaxies in every direction seem to be moving away from us.


A fracture on which one body of rock slides past another.


An immense system of hundreds of billions of stars.

gas-giant planet

The outer planets (Jupiter, Saturn, Uranus, Neptune) that are very large and consist mostly of volatile elements.

geothermal gradient

The rate of change in temperature with depth.


A solid in which atoms are not arranged in an orderly pattern.


Water that resides under the surface of the Earth, mostly in pores or cracks of rock or sediment.


The relatively rigid, non-flowable, outer 100- to 150-km-thick layer of the Earth; constituting the crust and the top part of the mantle.

lower mantle

The deepest section of the mantle, stretching from 670 km down to the core-mantle boundary.

magnetic field

The region affected by the force emanating from a magnet.


The thick layer of rock below the EarthÕs crust and above the core.


Molten (liquid) rock.


A solid composed almost entirely of atoms of metallic elements; it is generally opaque, shiny, smooth, and malleable, and can conduct electricity.


An object that has entered a planetÕs atmosphere and is glowing and evaporating as it streaks to the planetÕs surface.


A piece of rock or metal alloy that fell from space and landed on Earth.


A homogenous, naturally occurring, solid inorganic substance with a definable chemical composition and an internal structure characterized by an orderly arrangement of atoms, ions, or molecules in a lattice. Most minerals are inorganic.


The seismic-velocity discontinuity that defines the boundary between the EarthÕs crust and mantle.


A solid object of ice and/or rock and metal that orbits a planet.


A cloud of gas or dust in space.

nebula theory

The concept that planets grow out of rings of gas, dust, and ice surrounding a new-born star.

organic chemical

A carbon-containing compound that occurs in living organisms, or that resembles such compounds; it consists of carbon atoms bonded to hydrogen atoms along with varying amounts of oxygen, nitrogen, and other chemicals.


A relatively large, spherical object that orbits a star and has cleared its orbit of most debris.


Tiny, solid pieces of rock and metal that collect in a planetary nebula and eventually accumulate to form a planet.


A body that grows by the accumulation of planetesimals but has not yet become big enough to be called a planet.

protoplanetary disk

The flattened cloud of dust, gas, and ice that orbits a nascent star prior to the formation of planets.


A dense body of gas that is collapsing inward because of gravitational forces and that may eventually become a star.

red shift

The phenomenon in which a source of light moving away from you very rapidly shifts to a lower frequency; that is, toward the red end of the spectrum.


A coherent, naturally occurring solid, consisting of an aggregate of minerals or a mass of glass.


An accumulation of loose mineral grains, such as boulders, pebbles, sand, silt, or mud, that are not cemented together.

Solar System

The Sun and all the objects that orbit it (including planets, moons, comets, and asteroids).


A large sphere, composed dominantly of hydrogen and helium, in which fusion reactions are producing energy.

stellar wind

Particles that have been ejected from a star and are shooting through space.


A short-lived, very bright object in space that results from the cataclysmic explosion marking the death of a very large star; the explosion ejects large quantities of matter into space to form new nebulae.


Variations in elevation.

transition zone

The middle portion of the mantle, from 400 to 670 km deep, in which there are several jumps in seismic velocity.


The sum of all matter and energy making up the hundreds of billions of known galaxies.

upper mantle

The uppermost section of the mantle, reaching down to a depth of 400 km.


Space that contains very little matter in a given volume (e.g., a region in which air has been removed).


A means of transmitting energy from one location to another; waves can be vibrations that propagate through a material, or undulations of electromagnetic fields that can propagate either through a material or in a vacuum.


The horizontal difference between two adjacent wave troughs or two adjacent crests.

absolute plate velocity

The movement of a plate relative to a fixed point in the mantle.

abyssal plain

A broad, relatively fl at region of the ocean that lies at least 4.5 km below sea level.

active continental margin

A continental margin that coincides with a plate boundary.

apparent polar-wander path

A path on the globe along which a magnetic pole appears to have wandered over time; in fact, the continents drift, while the magnetic pole stays fairly fixed.


The layer of the mantle that lies between 100-150 km and 350 km deep; the asthenosphere is relatively soft and can flow when acted on by force.

black smoker

The cloud of suspended minerals formed where hot water spews out of a vent along a mid-ocean ridge; the dissolved sulfide components of the hot water instantly precipitate when the water mixes with seawater and cools.


The process of two buoyant pieces of lithosphere converging and squashing together.

continental drift hypothesis

The idea that continents have moved and are still moving slowly across the Earth's surface.

continental rift

A linear belt along which continental lithosphere stretches and pulls apart.

convergent plate boundary

A boundary at which two plates move toward each other so that one plate sinks (subducts) beneath the other; only oceanic lithosphere can subduct.

divergent plate boundary

A boundary at which two lithosphere plates move apart from each other; they are marked by mid-ocean ridges.

fracture zone

A narrow band of vertical fractures in the ocean floor; fracture zones lie roughly at right angles to a mid-ocean ridge, and the actively slipping part of a fracture zone is a transform fault.

global positioning system (GPS)

A satellite system people can use to measure rates of movement of the Earth's crust relative to one another, or simply to locate their position on the Earth's surface.

hot spot

A location at the base of the lithosphere, at the top of a mantle plume, where temperatures can cause melting.

hot-spot track

A chain of now-dead volcanoes transported off the hot spot by the movement of a lithosphere plate.


The relatively rigid, nonflowable, outer 100- to 150-km-thick layer of the Earth; constituting the crust and the top part of the mantle.

lithosphere plate

A portion of the outer, relatively rigid layer of the Earth; most seismic activity happens at the boundaries of plates, while the interior of a plate is relatively stable.

magnetic anomaly

The difference between the expected strength of the Earth's magnetic field at a certain location and the actual measured strength of the field at that location.

magnetic declination

The angle between the direction a compass needle points at a given location and the direction of true north.

magnetic dipole

A magnetic entity that has a north and south end.

magnetic inclination

The angle between a magnetic needle free to pivot on a horizontal axis and a horizontal plane parallel to the Earth's surface.

magnetic pole

The north or south ends of a magnet; field lines point straight down at the pole.

magnetic reversal

The change of the Earth's magnetic polarity; when a reversal occurs, the field flips from normal to reversed polarity, or vice versa.

magnetic-reversal chronology

The history of magnetic reversals through geologic time.

mantle plume

A column of very hot rock rising up through the mantle.

marine magnetic anomaly

An unusually strong or unusually weak magnetic field, as measured over the sea floor; in map view, they look like stripes that are parallel to the mid-ocean ridge.

mid-ocean ridge

A 2-km-high submarine mountain belt that forms along a divergent oceanic plate boundary.


The record of ancient magnetism preserved in rock.


The supposed position of the Earth's magnetic pole in the past, with respect to a particular continent.


A supercontinent that assembled at the end of the Paleozoic Era.

passive margin basin

A thick accumulation of sediment along a tectonically inactive coast, formed over crust that stretched and thinned when the margin first began.


One of about twenty distinct pieces of the relatively rigid lithosphere.

plate boundary

The border between two adjacent lithosphere plates.

relative plate velocity

The movement of one lithosphere plate with respect to another.

ridge-push force

The force that drives plates away from a mid-ocean ridge; it is caused by the fact that the ridge is elevated relative to the regions of oceanic plate away from the ridge.


The process by which continental lithosphere stretches and breaks apart; rifting produces normal faults and, commonly, volcanism.

sea-floor spreading

The gradual widening of an ocean basin as new oceanic crust forms at a mid-ocean ridge axis and then moves away from the axis.


An isolated submarine mountain.

slab-pull force

The force that downgoing plates (or slabs) apply to oceanic lithosphere at a convergent margin.


The process by which one oceanic plate bends and sinks down into the asthenosphere beneath another plate.

transform plate boundary

A boundary at which one lithosphere plate slips laterally past another.


A deep elongate trough bordering a volcanic arc; a trench defines the trace of a convergent plate boundary.

triple junction

A point where three lithosphere plate boundaries intersect.

volcanic arc

A curving chain of active volcanoes formed adjacent to a convergent plate boundary.

Wadati-Benioff zone

A sloping band of seismicity defined by intermediate- and deep-focus earthquakes that occur in the downgoing slab of a convergent plate boundary.

carbonate rocks

Rocks containing calcite and/or dolomite.


(1) The tendency of a mineral to break along preferred planes; (2) a type of foliation in low-grade metamorphic rock.

conchoidal fracture

Smoothly curving, clamshell-shaped surfaces along which materials with no cleavage planes tend to break.


A single, continuous piece of a mineral bounded by flat surfaces that formed naturally as the mineral grew.

crystal face

The flat surface of an euhedral mineral grain.

crystal habit

The general shape of a crystal or cluster of crystals that grew unimpeded.

crystal lattice

The orderly framework within which the atoms or ions of a mineral are fixed.

crystal structure

The internal arrangement of atoms or ions within a crystal.


The flat surface of a cut gemstone; facets are produced by grinding.


A mineral or form of a mineral that is particularly beautiful and/or rare, and thus has value.


A cavity in which euhedral crystals precipitate out of water solutions passing through a rock.


In mineralogy, hardness refers to the resistance of a mineral to scratching; a harder mineral can scratch a softer mineral.


The way a mineral surface scatters light.


A homogenous, naturally occurring, solid inorganic substance with a definable chemical composition and an internal structure characterized by an orderly arrangement of atoms, ions, or molecules in a lattice. Most minerals are inorganic.


The study of minerals.

Mohs hardness scale

A list of ten minerals in a sequence of relative hardness, with which other minerals can be compared.


Two minerals that have the same chemical composition but a different crystal lattice structure.

Silicate minerals

Minerals composed of silicon-oxygen tetrahedra linked in various arrangements; most contain other elements as well.

silicon-oxygen tetrahedron

The basic building block of silicate minerals; it consists of one silicon atom surrounded by four oxygen atoms.

specific gravity

A number representing the density of a mineral, as specified by the ratio between the weight of a volume of the mineral and the weight of an equal volume of water.


The color of the powder produced by pulverizing a mineral on an unglazed ceramic plate.


The condition in which the shape of one part of an object is a mirror image of the other part.


The process of magma contamination in which blocks of wall rock fall into a magma chamber and dissolve.


A vast composite, intrusive, igneous rock body up to several hundred kilometers long and 100 km wide, formed by the intrusion of numerous plutons in the same region.

Bowen's reaction series

The sequence in which different silicate minerals crystallize during the progressive cooling of a melt.

crystalline igneous rock

A rock that solidifies from a melt and consists of interlocking crystals.


A tabular (wall-shaped) intrusion of rock that cuts across the layering of country rock.

extrusive igneous rock

Rock that forms by the freezing of lava above ground, after it flows or explodes out (extrudes) onto the surface and comes into contact with the atmosphere or ocean.

fractional crystallization

The process by which magma becomes progressively more silicic as it cools, because early-formed crystals settle out.

fragmental igneous rock

Fragments of igneous material that have been stuck together to form a coherent mass.


The change in temperature with depth in the Earth.

glassy igneous rock

Igneous rock consisting entirely of glass, or of tiny crystals surrounded by a glass matrix.

igneous rock

Rock that forms when hot molten rock (magma or lava) cools and freezes solid.

intrusive igneous rock

Rock formed by the freezing of magma underground.


A shallow igneous intrusion that has a blister-like shape.


Molten rock that has flowed out onto the EarthÕs surface.

lava flow

Sheets or mounds of lava that flow onto the ground surface or sea floor in molten form and then solidify.


Molten rock beneath the EarthÕs surface.


An igneous rock consisting of a solid mass of volcanic glass.

partial melting

The melting in a rock of the minerals with the lowest melting temperatures, while other minerals remain solid.


A coarse-grained igneous rock containing crystals of up to tens of centimeters across and occurring in dike-shaped intrusions.


An irregular or blob-shaped intrusion; can range in size from tens of meters across to tens of kilometers across.


A glassy igneous rock that forms from felsic frothy lava and contains abundant (over 50%) pore space.

pyroclastic rock

Rock made from fragments blown out of a volcano during an explosion that were then packed or welded together.


A glassy mafic igneous rock containing abundant air-filled holes.


A nearly horizontal table-top-shaped tabular intrusion that occurs between the layers of country rock.


A process by which magma intrudes; blocks of wall rock break off and then sink into the magma.


A pyroclastic igneous rock composed of volcanic ash and fragmented pumice, formed when accumulations of the debris cement together.


Open holes in igneous rock formed by the preservation of bubbles in magma as the magma cools into solid rock.

volcanic ash

Tiny glass shards formed when a fine spray of exploded lava freezes instantly upon contact with the atmosphere.

volcanic breccias

A rock composed of angular chunks of volcanic debris that have been cemented together.


(1) A vent from which melt from inside the Earth spews out onto the planetÕs surface; (2) a mountain formed by the accumulation of extrusive volcanic rock.

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