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. The rocks are often green.

Cross-section of an ophiolite

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.

Stratigraphy and definition

Idealized stratigraphic sequence of an ophiolite

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