| Term | Definition |
|---|
| asthenosphere | the weak layer of soft but solid rock comprising the lower part of the upper mantle (below the listhosphere) and over which the plates slide. Movement occurs by plastic deformation. |
| climate system | A geosystem that includes all parts of the earth system and all the interactions among these components needed to describe how climate behaves in space and time. |
| core | the central part of the earth below a depth of 2900 km, comprising a liquid outer core and a solid inner core. Composed of iron and nickel, with minor amounts of some lighter element, suck as oxygen |
| crust | The thin outer layer of the Earth, averaging about 8 km thick under the oceans to about 40 km thick under the continents, consisting of relatively light materials that melt at low temperatures. Continental crust consists largely of granite and gronodiorite. Oceanic crust is mostly basalt. |
| Earth system | All the parts of our planet and all their interactions, taken together. |
| fossil | Trace of an organism of past geologic ages that has been preserved in the crust. |
| geodynamo | The geosystem that sustains Earth's magnetic field, driven by convection in the outer core. |
| geosystem | A specialized subsystem of the Earth system that econmpasses specific types of terrestrial behavior. They iclude the plate tectonic system, the climate system, the geodynamo, and other smaller subsystems. The Earth system can be thought of as the collection of all these open, interacting geosystems. |
| inner core | The central part of the Earth from a depth of 5150 to 6370 km, composed of iron and nickel. A solid metallic sphere with a radius of 1220 km suspended within the liquid outer core. |
| lithosphere | The strong, rigid outer shell of the Earth that encases the asthenosphere and contains the crust and the uppermost part of the mantle down to an average depth of about 100 km and forms the rigid plates. |
| magnetic field | The region of influence of a magnetized body or an electric current. |
| mantle | The region that forms the main bulk of the solid Earth, between the crust and the core, ranging from depths of about 40 km to 2900 km. It is composed of rocks of intermediate density, mostly compounds of oxygen with magnesium, iron, and silicon. |
| outer core | A liquid zone, composed of iron, nickel, and minor amounts of some lighter element, suck as oxygen or sulfur, below the mantle, from a depth of 2890 to 5190 km, that surrounds a solid iron-nickel inner core. |
| plate tectonic system | A geosystem that includes all parts of the Earth system and all the interactions among these components needed to describe how plate tectonics works in space and time. |
| principle of uniformitarianism | The principle that geologic processes we see in action today have worked in much the same way throughout geologic time; that is, the present is key to the past. |
| scientific method | A general research plan, based on methodical observations and experiments, by which scientist propose and test hypotheses that explain some aspect of how the universe works. |
| seismic wave | A ground vibration produced by earthquakes or explosions. |
| topography | The general configuration of varying heights that gives shape to Earth's surface. |
| continental drift | The large-scale motion of continents across Earth's surface driven by plate tectonics. |
| geodesy | The ancient science of measuring the shape of the Earth and locating points on its surface. |
| island arc | A linear or arc-shaped chain of volcanic islands formed on the seafloor at a convergent plate boundary. The islands are formed in the overriding plate from rising melt derived from fluid-induced melting of the mantle wedge above the downgoing lishospheric slab. |
| isochron | A contour that connects points of equal age on the world's ocean floors as determined by magnetic reversal data and fossils from deap-sea drilling. |
| magnetic anomaly | A pattern of long, narrow bands of high and low magnetic fields on the seafloor that are parallel to and almost perfectly symmetrical with respect to the crest of a mid-ocean ridge. The detection of such patterns was one of the great discoveries that confirmed seafloor spreading and led to plate tectonics theory. |
| magnetic time scale | The detailed history of Earth's magnetic field reversals going back into geologic time, as determined by measuring the thermoremanent magnetization of rock samples. |
| Pangaea | A supercontinent that coalesced in the latest Paleozoic era and comprised all present continents. Its breakup began in Mesozoic time, as inferred from paleomagnetic and other data. |
| plate tectonics | The theory proposing that the lithosphere is broken into about a dozen large plates that move over Earth's surface. Each plate acts as a distinct rigig unit that rides on the asthenosphere, wich aslo is in motion. The theory attempts to explain seismicity, volcanism, mountain building, and evidence of paleomagnetism in terms of these plate motions. |
| relative plate velocity | The velocity at which one plate moves relative to another plate. |
| Rodinia | A supercontinent that fomred about 1.1 Ga and gegan to break up about 750 Ma. |
| seafloor spreading | The mechanism by which new seafloor is created along the rift at the crest of a mid-ocean ridge as adjacent plates move apart. The crust separates along the rift, and new seafloor forms as hot new crust upwells into these cracks. The new seafloor spreads laterally away from the rift and is replaced by even newer crust in a continuing process of plate creation. |
| spreading center | The region at the crest of a mid-ocean ridge, where new crust is being formed by seafloor spreading. |
| subduction | The sinking of an oceanic plate beneath an overriding oceanic or scontinental plate at a convergent plate boundary. |
| transform fault | A plate margin at which the plates slide past each other and lithosphere is neither created nor destroyed. RElative desplacement occurs along the fault as horizontal slip between the adjacent plates. |
| anion | A negatively charged ion. |
| atom | The smallest unit of an element that retains the element's physical and chemical properties. |
| atomic mass | The sum of an element's protons and neutrons. |
| atomic number | The number of protons in the nucleus of an atom. |
| bedding | The formation of parallel layers of sediment as particles settle to the bottom of the sea, a river, or a land surface. |
| cation | A positively charged ion. |
| chemical and biochemical sediments | New chemical substances that form by precipitation when some of a rock's components dissolve during weathering and are carried in river waters to the sea. |
| chemical reaction | The interaction of the atoms of two or more chemical elements in certain fixed proportions that produces a new chemical substance. |
| cleavage | The tendency of a crystal to break along flat planar surfaces. Also, the geometric pattern produced by such breakage. |
| contact metamorphism | Changes in the mineralogy and texture of rocks resulting from the heat and pressure in a small area, such as the rocks near and in contact with an igneous intrusion. |
| covalent bond | A bond between atoms in which the outer electrons are shared. |
| crystal | An ordered three-dimensional array of atoms in which the basic arrangement is repeated in all directions. |
| crystal habit | The shape in which a mineral's individual crystals or aggregates of crystals grow. |
| crystallization | The growth of a solid from a gas or liquid whose constituent atoms come together in the proper chemical proportions and ordered three-dimensional arrangement. |
| density | mass per unit volume of a substance |
| electron sharing | The mechanism by which a covalent bond is formed between the elements in a chemical reaction |
| electron transfer | the mechanism by which an ionic bond is formed between the elements in a chemical reaction |
| erosion | The set of processes that loosen soil and rock and move them downhill or downstream, where they are deposited as layers of sediment. |
| extrusive igneous rock | A fine-grained or glassy igneous rock formed from a rapidly cooled magma that erupts at the surace through a volcano. |
| foliation | A set of flat or wavy parallel planes produced by deformation. |
| fracture | The tendency of a crystal to break along irregular surfaces other than cleavage planes. |
| hydrothermal solution | A hot solution formed around bodies of molten rock when circulating groundwater comes into contact with a hot intrusion, reacts with it, and carries off significant quantities of elements and ions released by the reaction. These elements and their ions then interact with one another to deposit ore minerals, usually as the fluid cools. |
| igneous rock | A rock formed by the solidification of a magma, before or after it reaches the surface. |
| intrusive igneous rock | A coarse-grained igneous rock that crystallized slowly when magma intruded into country rock deep in Earth's crust. |
| ionic bond | A bond between atoms formed by electrical attraction between ions of the opposite charge. |
| isotope | One of two or more species of atoms of the same atomic number that have differing atomic masses. |
| lithification | The process that converts sediments into solid rock by compaction or cementation. |
| luster | The way in which the surface of a mineral reflects light to produce the shine of its surface, described by such subjective terms as dull, glassy, or metallic. |
| metallic bond | A covalent bond in which freely mobile electrons are shared and dispersed among ions of metallic elements, which have the tendency to lose electrons and pack together as cations. |
| metamorphic rock | A rock formed by the transformation of preexisting solid rocks under the influence of high pressure and temperature. |
| mineral | A naturally occuring, solid crystalline substance, generally inorganic, with a specific chemical composition. |
| mineralogy | The relative proportions of a rock's constituent minerals. |
| ore | A mineral deposit from which valuable metals can be recovered profitably. |
| polymorph | One of two or more alternative possible crystal structures for a single chemical compound; for example, the minerals quartz and cristobalite are polymorphs of silica (SiO2). |
| precipitate | The crystals that drop out of a saturated solution. |
| regional metamorphism | Metamorphism caused by high pressures and temperatures that extend over large regions, as happens where plates collide. |
| rock | A naturally occuring solid aggregate of minerals, or in some cases, nonmineral solid matter. |
| rock cycle | The set of geologic processes that convert each type of rock into the other two types. |
| sediment | A material deposited at Earth surface by physical agents (wind, water, ice), chemical agents (precipitation from oceans, lakes, and rivers), or biological agents (organisms, living and dead). |
| sedimentary rock | A rock formed as the burial product of layers of sediments (such as sand, mud, and calcium carbonate shells), whether they were laid down on the land or under the sea. |
| siliciclastic sediments | Clastic sediment produced by the weathering of rocks composed largely of silicate minerals. |
| specific gravity | the weight of a mineral divided by the weight of an equal volume of pure water at 4C. |
| streak | The color of the fine deposit of mineral dust left on an abrasive surface, such as a tile of unglazed porcelain, when a mineral is scraped across it. |
| texture | The sizes and shapes of a rock's mineral crystals and the way they are put together. |
| vein deposit | A sheetlike deposit of minerals precipitated in fractures or joints that are foreign to the host rock. |
| weathering | The general process that breaks up rocks into fragments of various sizes by a combination of physical fracturing and chemical decomposition. |
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