Geology and Minerals

Archean felsic volcanic rocks

Archean felsic volcanic rocks are felsic volcanic rocks that were formed in the Archean Eon (4 to 2.5 billion years ago). The term "felsic" means that the rocks have silica content of 62–78%. Given that the Earth formed at ~4.5 billion year ago, Archean felsic volcanic rocks provide clues to the Earth's first volcanic activities on the Earth's surface started 500 million years after the Earth's formation. As the Archean Earth was hotter than the present, formation of felsic volcanic rocks may differ from the modern plate tectonics. Archean felsic volcanic rocks are distributed only in the preserved Archean greenstone belts, where deformed sequences of volcanic-sedimentary rocks are common. Felsic volcanic rocks are rare in the early Earth and only contribute to less 20% of rocks in the Archean greenstone belts worldwide. In contrast, mafic volcanic rocks (such as basalt and komatiite, silicate content <52%) occupy about 50% in the greenstone belts. Thus, felsic volcanic rocks are rare members in the Archean terranes. Archean felsic volcanic activities commonly occur in submarine environments. The composition of Archean felsic volcanic rocks are equivalent to a spectrum between dacite and rhyolite. They can be distinguished by their mineral assemblages, rock chemistry and rock layer relationship in the sequences. Archean felsic volcanic rocks are utilised to date the timing of geological events and match distant rock units in separated Archean cratons. They are important to reconstruct Archean geological environments. Felsic granitoids are the most prevalent rock type in Archean terranes. These intrusive felsic igneous rocks include TTG suites (Tonalite-trondhjemite-granodiorite) that contributes over half the portion of Archean cratons. They have implications in finding how the felsic volcanic rocks were formed and related to the granitoids.

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See also

• ▸Beaver Valley, Delaware and Pennsylvania
• ▸Islay Lifeboat Station
• ▸Hokkaido Koma-ga-take

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