Ophiolite
An ophiolite is a section of the Earth's oceanic crust. The underlying upper mantle has been uplifted and exposed above sea level. The rocks which make up ophiolites are an altered form of basalt.[1] The rocks are often green.
They are found in the mountain belts such as the Alps or the Himalayas. They show where former ocean basins were consumed by subduction. This was one of the discoveries of plate tectonics. Ophiolites have always played a central role in plate tectonic theory and the interpretation of ancient mountain belts.[2]
Stratigraphy and definition
The stratigraphic sequence seen in ophiolites is caused by lithosphere-forming processes at mid-oceanic ridges:
- Sediments: muds (black shale) and cherts deposited after the crust formed.
- Extrusive sequence: basalt "pillow lavas" show magma/seawater contact.
- Vertical, parallel dikes fed lavas above.
- Gabbros from minerals settling from a magma chamber.
- Peridotite: mantle rock minerals, heated and pressured in orogeny (mountain-building).
Notable ophiolites
- In the California Coast Ranges, from Santa Barbara through San Francisco Counties, California.
- In Oman and the United Arab Emirates, widely considered one of the best exposed ophiolite sequences
- Macquarie Island, Tasmania, Australia was named a UNESCO World Heritage Site in 1997, as the only known example of an ophiolite complex in the process of being formed and currently in its original geological setting.
- In Gros Morne National Park, Newfoundland, named a UNESCO World Heritage Site in 1987 because of its superbly exposed complete ophiolite stratigraphic sequence
Ophiolite Media
Ordovician ophiolite in Gros Morne National Park, Newfoundland
Chromitic serpentinite, Bay of Islands Ophiolite, Lewis Hills, Newfoundland
A simplified structure of an ophiolite suite:- axial magma chamber
- pelagic sediments
- pillow basalts
- sheeted basaltic dykes
- intrusive, layered gabbro
- dunite/peridotite cumulates
A pillow lava from an ophiolite sequence, Northern Apennines, Italy
_4.jpg)
References
- ↑ Ben-Avraham Z. et al 1982. The emplacement of ophiolites by collision. Journal of Geophysical Research: Solid Earth (1978-2012). 87 (B5) 3861-3867.
- ↑ Dilek Y. 2003. Ophiolite concept and its evolution. In Dilek Y. & Newcomb S. Ophiolite concept and the evolution of geological thought. Special Paper 373. Geological Society of America. pp. 1–16 [1][dead link]. ISBN 0813723736