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| A subdivision of the mantle situated below the lithosphere. This zone of weak material exists below a depth of about 100 kilometers and in some regions extends as deep as 700 kilometers. The rock within this zone is easily deformed. |
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| A theory that origanlly proposed that the continents are rafted about. It has essentially been replaced by the plate tectontics theory. |
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| Mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent. |
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| A boundary in which 2 plates move together, causing one of the slabs of lithosphere to be consumed into the mantle as it descends beneath on an overriding plate. |
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| The temperature above which a material loses its magnetization. |
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| An elongated depression in the seafloor produced by bending of oceanic crust during subduction. |
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| A region where the rigid plates are moving apart, typified by the mid-oceanic ridges. |
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| Rocks that formed thousands or millions years ago and contained a record of the direction of the magnetic poles at the time of their formation. |
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| Any break or rupture in rock along which no appreciable movement has taken place. |
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| A concentration of heat in the mantle capable of producing magma, which in turn extrudes onto Earth's surface. The intraplate volcanism that produced the Hawaiian Islands is one example. |
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| Island Arc (Volcanic Island Arc) |
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| A chain of volcanic islands generally located a few hundred kilometers from a trench where active subduction of one oceanic slab beneath another is occuring. |
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| The rigid outer layer of Earth, including the crust and upper mantle. |
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| A change in the Earth's magnetic field resulting in the magnetic north being aligned with the geographic south, and the magnetic south being aligned with the geographic north. |
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| measures the magnetic polarity of hundreds of lava flows and use radiometric dating techniques to establish their ages. |
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| A very sensitive instrument towed behind a boat to survey the ocean floor and construct a detailed map of seafloor topography. |
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| A mass of hotter-than-normal mantle material that ascends toward the surface, where it may lead to igneous activity. These plumes of solid yet mobile material may originate as deep as the core-mantle boundary. |
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| A magnetic field that is the same as that which exists at present. |
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| A continuous elevated zone on the floor of all the major ocean basins and varying in width from 500 to 5,000 kilometers (300 to 3,000 miles. The rifts at the crests of ridges represent divergent plate boundaries. |
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| The natural remnant magnetism in rock bodies. The permanent magnetization acquired by rock that can be used to determine the location of the magnetic poles and the latitude of the rock at the time it became magnetized. |
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| The proposed supercontinent that 200 million years ago began to break apart and form the present landmasses. |
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| The process by which most igneous rocks melt. Since individual materials have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results. |
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| Plate (lithospheric plate) |
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Definition
| A coherent unit of Earth's rigid outer layer that includes the crust and upper unit. |
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| The theory that proposes that Earth's outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself. |
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| A magnetic field opposite to that which exists at present. |
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| A mechanism that may contribute to plate motion of a seismograph. |
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| A deep downfaulted structure along the axis of some ridge sediments. |
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| The process of producing new seafloor between two diverging plates. |
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| A mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and "pulls" the trailing lithosphere along. |
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| Induced mantle circulation that pulls both the subducting and overriding plates toward the trench. This mantle flow tends to "suck" in nearby plates. |
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| A long, narrow zone where one lithospheric plate descends beneath another. |
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| A boundary in which two plates slide past one another without creating or destroying lithosphere. |
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| A chain of volcanic islands generally located a few hundred kilometers from a trench where active subduction of one oceanic slab beneath another is occuring. |
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| A large wedge-shaped mass of sediment that accumulates in subduction zones. Here, sediment is scraped from the subducting oceanic plate and accreted to the overriding crustal block. |
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| Active Continental Margin |
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Definition
| Usually narrow and consisting of highly deformed sediments. They occur where oceanic lithosphere is being abducted beneath the margin of a continent. |
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| A fold in sedimentary strata resembling an arch. |
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| A circular downfolded structure. |
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| Brittle Failure (Brittle Deformation) |
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| Deformation that involves the fracturing of rock. Associated with rocks near the surface. |
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| General term for the processes of folding, faulting, shearing, compression, or extension of rocks as the result of various natural forces. |
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| A fault in which the movement is parallel to the dip of the fault. |
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| A roughly circular upfolded structure similar to an anticline. |
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| A type of solid state flow that produces a change in the size and shape of a rock body without fracturing. Occurs at depths where temperatures and confining pressures are high. |
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| A break in rock mass along which movement has occurred. |
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| A mountain formed by the displacement or rock along a fault. |
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| A cliff created by movement along a fault. It represents the exposed surface of the fault prior to modification by weathering and erosion. |
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| A bent rock layer or series of layers that were originally horizontal and subsequently deformed. |
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| A valley formed by the downward displacement of a fault-bounded block. |
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| The gradual subsidence of mountains caused by lateral spreading of weak material located deep within these structures. |
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| An elongated, uplifted block of crust bounded by faults. |
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| The concept that Earth's crust is floating in gravitaional balance upon the material of the mantle. |
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| Process of establishing a new level of gravitational equilibrium. |
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| A fracture in rock along which there has been no movement. |
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| A one-limbed flexure in strata. The strata are unusally flat-lying or very gently dipping on both sides of the ___. |
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| A fault in which the rock above the fault plane has moved down relative to the rock below. |
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| The processes that collectively result in the formation of mountains. |
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| Passive Continental Margin |
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Definition
| Margins that consist of a continental shelf, continental slope, and continental rise. They are not associated with plate boundaries and therefore experience little vocanism and few earthquakes. |
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| A fault in which the material above the fault plane moves up in relation to the material below. |
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| A fault along which the movement is horizontal. |
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| A linear downfold in sedimentary strata; the opposite of anticline. |
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| A crustal block bounded by faults, whose geologic history is distinct from the histories of adjoining crustal blocks. |
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| A low-angle reverse fault. |
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| A major strike-slip fault that cuts through the lithosphere and accomodates motion between two plates. |
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| An unconformity in which the strata below dip at an angle different from that of the beds above. |
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| The second eon of Precambrian time, following the Hadean and preceding the Proterozoic. It extends between 3.8 billion and 2.5 billion years before the present. |
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| The concept that Earth was shaped by catastrophic events of a short-term nature. |
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| A span on the geologic time scale beginning about 65 million years ago following the Mesozoic era. |
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| Layers of rock that were deposited without interruption. |
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| Establishing the equivilance of rocks of similar age in different areas. |
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| Cross-Cutting Relationships |
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Definition
| A principle of relative dating. A rock or fault is younger than any rock (or fault) through which it cuts. |
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| A type of unconformity in which the beds above and below are parallel. |
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| The largest time unit on the geologic time scale, next in order of magnitude above era. |
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| A unit of the geologic calendar that is a subdivision of a period. |
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| A major division on the geologic calendar; eras are divided into shorter units called periods. |
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| The remains or traces of organisms preserved from the geologic past. |
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| Fossil organisms that succeed one another in a definite and determinable order, and any time period can be recognized by its fossil content. |
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| The division of Earth history into blocks of time - eons, eras, periods, and epochs. The time scale was created using relative dating principals. |
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| The time required for one half of the atoms of a radioactive substance to decay. |
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| A piece of one rock unit contained within another. ____ are used in relative dating. The rock mass adjacent to the one containing the ___ must have been there first in order to provide the fragment. |
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| A fossil that is associated with a particular span of geologic time. |
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| A span on the geologic time scale between the Paleozoic and Cenozoic eras from about 248 million to 65 million years ago. |
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| An unconformity in which older metamorphic or intrusive igneous rocks are overlain by younger sedimentary strata. |
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| Date that specifies that actual number of years that have passed since an event occurred. |
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| Layers of sediments are generally deposited in a horizontal or nearly horizontal position. |
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| The systematic study of fossils and the history of life on Earth. |
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| A span on the geologic time scale between the eons of the Precambrian and Mesozoic era from about 540 million to 248 million years ago. |
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| A basic unit of the geologic calendar that is a subdivision of an era. ___ may be divided into smaller units called epochs. |
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| That part of geologic time represented by rocks containting abundant fossil evidence. The eon extending from the end of the Proterozoic eon (about 540 million years ago) to the present. |
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| All geologic time prior to the Paleozoic era. |
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| The eon following the Archean and preceding the Pahnerozoic. It extends between about 2,500 million (2.5 billion) and 540 million years ago. |
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| The spontaneous decay of certain unstable atomic nuclei. |
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| The radioactive isotope of carbon, which is produced continuously in the atmosphere and is used in dating events from the very recent geologic past (the last few tens of thousands of years). |
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| The procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes. |
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| Rocks are placed in their proper sequence or order. Only the chronological order of events is determined. |
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| In any undeformed sequence of sedimentary rocks, each bed is older than the layers above and younger than the layers below. |
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| A surface the represents a break in the rock record, caused by erosion or nondeposition. |
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| The concept that the processes that have shaped Earth in the geologic past are essentially the same as those operating today. |
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