Actualism
The principle that the same processes and natural laws that operated in the past are those we can actually observe or infer from observations as operating at present. Under present usage, uniformitarianism has the same meaning as actualism for most geolog
Angular Unconformity
An unconformity in which younger strata overlie an erosion surface on tilted or folded layer rock.
Archean Eon
The oldest eon of Earth's history.
Cenozoic Era
The most recent of the eras; followed the Mesocoiz Era.
Contacts
Boundary surface between two different rock types or ages of rocks.
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.
Cross-cutting relationships
A principle or law stating that a disrupted pattern is older than the cause of disruption.
Disconformity
A surface that represents missing rock strata but beds above and below that surface are parallel to one another.
Eon
The largest unit of geologic time.
Epochs
Each period of the standard geologic time scale is divided into epochs (e.g., Pleistocene Epoch of the Quaternary Period).
Eras
Major subdivision of the standard geologic time scale (e.g., Mesozoic Era).
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.
Formations
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 or bed or several beds of sedimentary rock, although the term is also applied to units of meta
Hadean Eon
The oldest eon.
Half-life
The time it takes for a given amount of a radioactive isotope to be reduced by one-half.
Holocene (or Recent) Epoch
The youngest epoch, which began around 10,000 years ago and it continuing presently.
Inclusion
A fragment of rock that is distinct from the body of igneous rock in which it is enclosed.
Index Fossil
A fossil from a very short-lived species known to have existed during a specific period of geologic time.
Isotopes
Atoms (of the same element) that have different numbers of neutrons but the same number of protons.
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).
Lateral Continuity
Principle that states an original sedimentary layer extends laterally unit it tapers or thins at its edges.
Mesozoic Era
The era that followed the Paleozoic Era and preceded the Cenozoic Era.
Nonconformity
An unconformity in which an erosion surface on plutonic or metamorphic rock has been converged by younger sedimentary or volcanic rock.
Numerical Age
Age given in years or some other unit of time.
Original Horizontality
The deposition of most water-laid sediment in horizontal or near-horizontal layers that are essentially parallel to Earth's surface.
Paleozoic Era
The era that followed the Precambrian and began with the appearance of complex life, as indicated by fossils.
Periods
Each era of the standard geologic time scale is subdivided into periods (e.g., the Cretaceous Period).
Phanerozoic Eon
Eon of geologic time. Includes all time following the Precambrian.
Physical Continuity
Being able to physically follow a rock unity between two places.
Pleistocene Epoch
An epoch of the Quaternary Period characterized by several glacial ages.
Precambrian
The vast amount of time that preceded the Paleozoic Era.
Proterozoic Eon
Eon of Precambrian time.
Quaternary Period
The youngest geologic period; includes the present time.
Radioactive Decay
The spontaneous nuclear disintegration of certain isotopes.
Relative Time
The sequence in which events took place (not measured in time units).
Standard Geologic Time Scale
A worldwide relative scale of geologic time divisions.
Superposition
A principle or law stating that within a sequence of undistributed sedimentary rocks, the oldest layers are on the bottom, the youngest on the top.
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.
Uniformitarianism
Principle that geologic processes operating at present are the same 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.