Cone sheet


A cone sheet is a type of high-level igneous intrusion of subvolcanic rock, found in partly eroded central volcanic complexes. Cone sheets are relatively thin inclined sheets, generally just a few metres thick, with the geometry of a downward-pointing cone. Viewed from above, their outcrop is typically circular to elliptical. They were originally described from the Ardnamurchan, Mull and other central complexes of the British Tertiary Volcanic Province.

Occurrence

Cone sheets are widely distributed at the lower levels of volcanic complexes.
NameLocationAgeDominant rock typeReference
ArdnamurchanScotlandPaleogenedolerite
TejedaGran CanariaMiocenetrachyte, phonolite
VallehermosoLa GomeraMiocenetrachyte, phonolite
Jabal ArknuLibyaTertiary
OtogeJapanMiocenealkali basalt, trachyandesite
ZarzaMexicoCretaceousgabbro
HoushihushanChinaCretaceousgranite porphyry
Boa VistaCape VerdeMiocenephonolite
Ruri HillsKenyaMiocenecarbonatite
BagstoweQueenslandlate Paleozoicrhyolite
ThverartindurIceland

Formation

Soon after cone sheets were first described, their formation was explained in terms of intrusion along conical fractures extending from the top of an intrusive body into the overlying rocks, caused by high magmatic pressure.