5.7: Detailed Figure Descriptions

The following key terms and their definitions should help with your study.

Term	Definition
angular unconformity	A geologic relationship in which parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers.
biozone	An interval of geological strata that are defined on the basis of their characteristic fossil taxa.
country rock	A body of rock that receives or hosts an intrusion of a viscous geologic material
disconformity	A unconformity between parallel layers of sedimentary rocks which represents a period of erosion or nondeposition
Eon	The largest (formal) geochronologic time unit.
Epoch	The smallest geochronologic unit which subdivides Periods.
Era	The second largest geochronologic time unit.
geochronology	Science of absolute age dating in geology.
half-life	The time required for a quantity (of substance) to reduce to half of its initial value.
index-fossil	The fossilized remains or traces of particular plants or animals that are characteristic of a particular span of geologic time or environment, and can be used to identify and date the containing rocks.
nonconformity	A boundary between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock.
Period	A major rank below an Era and above an epoch.
Principle of Baked Contacts	The principle that states that the heat of an intrusion will bake (metamorphose) the rocks in close proximity to the intrusion.
Principle of Chilled Margins	A principle that states that the portion of an intrusion that has cooled and crystallized next to cold surrounding rock will form smaller crystals than the portion of the intrusion that cooled more slowly deeper in the intrusion, which will form larger crystals.
Principle of Cross-cutting relationships	A principle of geology that states that the geologic feature which cuts another is the younger of the two features.
Principle of Faunal Succession	A principle of geology that states that fossilized flora and fauna preserved in sedimentary rocks succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances.
Principle of Inclusion	A geologic principle that states that clasts in a rock are older than the rock itself.
Principle of Lateral Continuity	A principle of geology that states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous
Principle of Original Horizontality	A principle of geology that states that layers of sediment are originally deposited horizontally under the action of gravity
Principle of Superposition	A principle of geology that states that in undeformed stratigraphic sequences, the oldest strata will lie at the bottom of the sequence, while newer material stacks upon the surface to form new deposits over time.
radioisotope	An isotope that has excess nuclear energy, usually due to the number of neutrons in its nucleus, making it unstable.
radiometric dating	The fundamental method of geochronology.
relative dating	A process by which the age of one geologic unit is determined relative to that of another.
terrane	A fault-bounded area or region with a distinctive stratigraphy, structure, and geological history.
unconformity	A buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous.