Igneous - Geological Field Techniques - page 293

Appendix A7: Igneous

Table A7.1 Names for igneous intrusions, based on shape and relationship with country rocks.

Name Description

Batholith Grouping of more-or-less contiguous plutons

Central
complex

Assemblage of largely intrusive rocks, with screens and other masses of extrusive and country
rocks, arranged around one or more focal points. Considered to represent the interior or
shallow roots of a volcano

Cone-sheet Cone-shaped curtain (sheet) intrusion dipping inwards towards a central focal point.

Essentially a dyke, with roughly circular outcrop pattern, dipping inwards

Cupola Dome-shaped upward protuberance from the top of an intrusion

Diapir Dome-shaped body of igneous rock, inferred to have deformed and ruptured the country rock

during ascent

Diatreme Breccia-fi lled volcanic pipe

Dyke (US
spelling is dike)

Discordant curtain of igneous rock, originally intruded into a steep or vertical fracture.
Typically tens of centimetres to several metres in width

Dyke swarm A number of spatially associated dykes, typically radial, parallel or en échelon

Laccolith Intrusion that is roughly circular in plan, generally concordant with the country rock and

having a planar fl oor but a domed roof

Lopolith Large, generally concordant intrusion that is broadly saucer-shaped

Minor intrusion General informal term for any intrusion too small to be classed as a pluton

Neck Feeder pipe of a former volcano, occupied either by collapsed material from the vent or by a

cylindrical intrusion of magma also referred to as a 'plug'

Pluton Discordant intrusive body a kilometre or greater in scale. Of cylindrical, lenticular or tabular

shape, resulting in a roughly circular or elliptical outcrop pattern. May be simple (one
intrusion) or composite (supplied by more than one recognizable pulse of magma)

Ring complex Association of ring-intrusions, ring-dykes and cone-sheets

Ring-dyke Dyke with annular outcrop pattern. Either vertical or dipping steeply outwards (whereas a

cone-sheet dips inwards)

Ring-intrusion Intrusion emplaced within, or bounded by, a cylindrical ring-fracture

Sheet Intrusion with broadly parallel margins and one dimension much shorter than the other two

(such as a sill or a dyke). More aptly referred to as a curtain if steeply dipping. 'Inclined
sheet' is sometimes used for a dipping sheet that cannot be called a sill but is not steep
enough to be called a dyke

Sill Roughly horizontal sheet of intrusive rock with top and bottom generally concordant with

stratifi ed country rock. Local discordancies may occur where a sill steps from one horizon to
another. Typically metres to tens of metres thick

Vein Sheet of igneous rock narrower and more irregular than would be classifi ed as a sill, dyke or

sheet. (Beware: veins occupied by minerals precipitated from hydrothermal fl uids are not igneous)