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Concept
Ophiolite
Summary
An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks.
The Greek word ὄφις, ophis (snake) is found in the name of ophiolites, because of the superficial texture of some of them. Serpentinite especially evokes a snakeskin. The suffix lite from the Greek lithos means "stone". Some ophiolites have a green color. The origin of these rocks, present in many mountainous massifs, remained uncertain until the advent of plate tectonic theory.
Their great significance relates to their occurrence within mountain belts such as the Alps and the Himalayas, where they document the existence of former ocean basins that have now been consumed by subduction. This insight was one of the founding pillars of plate tectonics, and ophiolites have always played a central role in plate tectonic theory and the interpretation of ancient mountain belts.
The stratigraphic-like sequence observed in ophiolites corresponds to the lithosphere-forming processes at mid-oceanic ridges. From top to bottom, the layers in the sequence are:
Pelagic sediments: mostly siliceous oozes, calcareous oozes and red clays deposited since the crust formed.
Extrusive sequence: basaltic pillow lavas show magma/seawater contact.
Sheeted dike complex: vertical, parallel dikes that fed lavas above.
High level intrusives: isotropic gabbro, indicative of a fractionated magma chamber.
Layered gabbro, resulting from settling out of minerals from a magma chamber.
Cumulate peridotite: dunite-rich layers of minerals that settled out from a magma chamber.
Tectonized peridotite: harzburgite/lherzolite-rich mantle rock.
A Geological Society of America Penrose Conference on ophiolites in 1972 defined the term "ophiolite" to include all of the layers listed above, including the sediment layer formed independently of the rest of the ophiolite.
Official source
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