Transcript Unit 3 - Rocks and Minerals - Igenous Rock Vocabualry

Igneous Rock Vocabulary
Pluton - an igneous intrusive body of rock that is discordant (cuts across existing rock).
Dike - an igneous intrusive body of rock that is rather small in geographic area, usually
is a stem off of a pluton and cuts across the country rock making it discordant.
Sill – an igneous intrusive body of rock that is rather small in geographic area, usually
is a stem off of a pluton and moves along bedding planes of country rock. It is
considered to be concordant.
Batholith – a large pluton that covers several kilometers, that cuts across the existing
country rock making it discordant.
Stock – a smaller pluton that covers a rather small geographic area that cuts across the
existing country rock making it discordant.
Crosscutting Relationships - the association of geologic rock bodies with other rock
bodies and geologic features to determine which event occurred first. A thick layer of
sedimentary rock may have been intruded by a pluton, then faulted, then a dike cuts
across all of them. We use these relationships to piece the geologic history of an area.
Inclusions (Xenoliths, Country Rock, Exotic Blocks or Wall Rock) – these are
pieces of existing rock that has been broken off and trapped in the intruding magma.
Unconformity – this is a break sediment deposition, with an erosional surface or a
break in deposition shown, usually by an eroded surface as a contact between the units
above and below the unconformity.
Nonconformity – break in geologic deposition between an igneous or metamorphic
rock body, that is overlain by sedimentary rocks. It represents a period of erosion of the
igneous or metamorphic body prior to the deposition of the sedimentary rocks or the
volcanic “sediments”. Rock types are totally different above and below the break in
sequence they are (Not alike).
Disconformity – is a break in geologic deposition between two sedimentary layers of
rock (ash fall from a volcano can be considered sedimentary in deposition). The break
in deposition shows that the deposition was discontinuous.
Angular Unconformity – break in geologic deposition where the lower sedimentary
rock beds have been tilted and an erosional break is shown with horizontal strata
overlaying the tilted beds.
Baked Zone – the zone of contact metamorphism around an intrusive magma. The
localized heat creates the “baked zone” forming melted and re-crystallized rock material
creating a metamorphic rock body.