Cone sheet

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File:Cone Sheet Mingary Ardnamurchan 01.jpg
A cone sheet at Mingary, Ardnamurchan, Scotland
File:Cone sheet at Mingary - geograph.org.uk - 4378.jpg
Closer view of a cone sheet at Mingary, Ardnamurchan
File:Presa del Parralillo on Gran Canaria in Canary Islands 2011.jpg
Tejeda cone sheets on Gran Canaria. The lower two-thirds of the photo shows the Tejeda cone sheet swarm of Miocene age; the cone sheets dip down towards the bottom left. The cone sheets are overlain by Pliocene flat-lying lava flows and pyroclastic rocks.[1]

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 (now recognised as part of the North Atlantic Igneous Province).

Occurrence

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Cone sheets are widely distributed at the lower levels of volcanic complexes.

Examples of cone sheet complexes
Name Location Age Dominant rock type Reference
Ardnamurchan Scotland Paleogene dolerite [2][3]
Tejeda Gran Canaria Miocene trachyte, phonolite [4]
Vallehermoso La Gomera Miocene trachyte, phonolite [5]
Jabal Arknu Libya Tertiary [6]
Otoge Japan Miocene alkali basalt, trachyandesite [7]
Zarza Mexico Cretaceous gabbro [8]
Houshihushan China Cretaceous granite porphyry [9]
Boa Vista Cape Verde Miocene phonolite [10]
Ruri Hills Kenya Miocene carbonatite [11]
Bagstowe Queensland late Paleozoic rhyolite [12]
Thverartindur Iceland Pliocene [13]
Tehilla Sudan Cambrian–Ordovician granite, monzonite [14]

Formation

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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.[15][16]

References

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