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Glossary
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Below you will find key words and concepts that you should remember from this chapter.
angular unconformity  An unconformity in which younger strata overlie an erosion surface on tilted or folded layered rock.
(See page(s) 182)
Archean Eon  The oldest eon of Earth’s history.
(See page(s) 194)
Cenozoic Era  The most recent of the eras; followed the Mesozoic Era.
(See page(s) 188)
contacts  Boundary surface between two different rock types or ages of rocks.
(See page(s) 176)
correlation  In geology, correlation usually means determining time equivalency of rock units. Rock units may be correlated within a region, a continent, and even between continents.
(See page(s) 183)
cross-cutting relationship  A principle or law stating that a disrupted pattern is older than the cause of disruption.
(See page(s) 178)
disconformity  A surface that represents missing rock strata but beds above and below that surface are parallel to one another.
(See page(s) 182)
eon  The largest unit of geological time.
(See page(s) 194)
epoch  Each period of the standard geologic time scale is divided into epochs (e.g., Pleistocene Epoch of the Quaternary Period).
(See page(s) 187)
era  Major subdivision of the standard geologic time scale (e.g., Mesozoic Era).
(See page(s) 187)
faunal succession  A principle or law stating that fossil species succeed one another in a definite and recognizable order; in general, fossils in progressively older rock show increasingly greater differences from species living at present.
(See page(s) 186)
formation  A body of rock of considerable thickness that has a recognizable unity or similarity making it distinguishable from adjacent rock units. Usually composed of one bed or several beds of sedimentary rock, although the term is also applied to units of metamorphic and igneous rock. A convenient unit for mapping, describing, or interpreting the geology of a region.
(See page(s) 176)
fossil assemblage  Various different species of fossils in a rock.
(See page(s) 187)
half-life  The time it takes for a given amount of a radioactive isotope to be reduced by one-half.
(See page(s) 191)
inclusion  A fragment of rock that is distinct from the body of igneous rock in which it is enclosed.
(See page(s) 182)
index fossil  A fossil from a very short-lived species known to have existed during a specific period of geologic time.
(See page(s) 186)
isotope  Atoms (of the same element) that have different numbers of neutrons but the same number of protons.
(See page(s) 189)
isotopic dating  Determining the age of a rock or mineral through its radioactive elements and decay products (previously and somewhat inaccurately called radiometric or radioactive dating).
(See page(s) 189)
lateral continuity  Principle that states that an original sedimentary layer extends laterally until it tapers or thins at its edges.
(See page(s) 178)
Mesozoic Era  The era that followed the Paleozoic Era and preceded the Cenozoic Era.
(See page(s) 188)
nonconformity  An unconformity in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.
(See page(s) 182)
numerical (or absolute) age  Age given in years or some other unit of time.
(See page(s) 176)
original horizontality  The deposition of most water-laid sediment in horizontal or near-horizontal layers that are essentially parallel to Earth’s surface.
(See page(s) 177)
Paleozoic Era  The era that followed the Precambrian and began with the appearance of complex life, as indicated by fossils.
(See page(s) 188)
period  Each era of the standard geologic time scale is subdivided into periods (e.g., the Cretaceous Period).
(See page(s) 187)
Phanerozoic Eon  Eon of geologic time. Includes all time following the Precambrian.
(See page(s) 194)
physical continuity  Being able to physically follow a rock unit between two places.
(See page(s) 183)
Pleistocene Epoch  An epoch of the Quaternary Period characterized by several glacial ages.
(See page(s) 188)
Prearchean (Hadean) Eon  The earliest Eon of Earth's history.
(See page(s) 194)
Precambrian  The vast amount of time that preceded the Paleozoic Era.
(See page(s) 188)
Proterozoic Eon  Eon of Precambrian time.
(See page(s) 194)
Quaternary Period  The youngest geologic period; includes the present time.
(See page(s) 188)
radioactive decay  The spontaneous nuclear disintegration of certain isotopes.
(See page(s) 189)
Recent (Holocene) Epoch  The present epoch of the Quaternary Period.
(See page(s) 188)
relative time  The sequence in which events took place (not measured in time units).
(See page(s) 176)
standard geologic time scale  A worldwide relative scale of geologic time divisions.
(See page(s) 187)
superposition  A principle or law stating that within a sequence of undisturbed sedimentary rocks, the oldest layers are on the bottom, the youngest on the top.
(See page(s) 177)
unconformity  A surface that represents a break in the geologic record, with the rock unit immediately above it being considerably younger than the rock beneath.
(See page(s) 182)
uniformitarianism  Principle that geologic processes operating at present are the same processes that operated in the past. The principle is stated more succinctly as “The present is the key to the past.” Also, see actualism.
(See page(s) 176)







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