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The term ophiolite was proposed in 1813 by the French mineralogist Alexandre Brongniart. It comes from the words ophios (snake) and lithos (rock), which allude to the snakeskin appearance of these rock associations, first found in the Italian Alps.
Later, in 1910, ophiolites were defined as rocks from submarine lava eruptions, emitted on the ocean floor during the Eocene period, about 56 million years ago. From this definition, they were recognized for more than half a century as rock associations originating from magma. However, between 1963 and 1973, various discoveries determined that they are also formed by rocks of the oceanic crust and that they constitute allochthonous fragments, that is, that they have moved from their origin as a result of tectonic movements. Plate tectonics is a theory according to which the lithospheric plates move very slowly on top of each other, thanks to the fact that they lie on a viscous layer of the upper mantle called the asthenosphere.
Currently, ophiolites are recognized as fragments of lithosphere or fossil oceanic crust, incorporated into orogenic ridges, that is, those formed by the action of tectonic movements.
Composition of ophiolites
Ophiolites are formed by a sequence of magmatic rocks (those formed from magma), sedimentary (those formed by the accumulation of mineral matter or biological remains) and metamorphic (those formed by the transformation of rocks pre-existing under certain physical or chemical conditions, or a combination of both). The complete sequence from the top to the bottom of an ophiolite column is as follows.
- Marine sedimentary rocks .
- Extrusive magmatic rocks , that is, they are formed by cooling lava on the earth’s surface.
- Intrusive dike complexes. The dikes constitute tubular or laminar rock formations; they are intrusive if during their formation they cool underground.
- Massive gabbros , which are plutonic magmatic rocks, i.e. intrusive.
- Banded ultramafic-mafic cumulates . Cumulates consist of associations of crystals in a magma; They are banded without bands or stripes, ultramafic if they are composed mainly of basic minerals, and mafic if they are composed mainly of iron and magnesium.
- Deformed peridotites , which are rocks from the mantle that changed their shape by the action of metamorphic agents such as temperature, pressure, density, among others.
formation of ophiolites
The discovery of ophiolites distributed along the oceanic ridges of the planet indicates that this would be the environment from which they originated (oceanic ridges are submarine ridges formed as a result of tectonic movements). However, some investigations have found indications that they also originate in geological arcs, which are formations such as volcanoes or islands, the product of plate tectonics.
Another possible origin of ophiolites could be the detachment of fragments of oceanic lithosphere that has been accreted. the margins of less dense continents and over island arcs in subduction zones, that is, at points where one plate sinks below the edge of another.
Ophiolite classification
A recent review of ophiolites proposed to classify them, according to their place of formation, into seven different types:
- Ligurian-type ophiolites formed during the early opening of an oceanic basin like that of today’s Red Sea, located between Africa and Asia.
- Mediterranean-type ophiolites formed during the interaction of two oceanic plates, such as the one that originated the Izu-Bonin fore-arc, located south of Tokyo. A fore-arc is the region between an oceanic trench and the associated volcanic arc.
- Sierran-type ophiolites reflect subduction events such as those that originated the Philippine Islands arc.
- The Chilean-type ophiolites formed in a zone of extension of a later arc, like that of the current Andaman Sea, south of Burma.
- Macquarie-type ophiolites formed around an oceanic ridge like the one that gave rise to Macquarie Island in the Southern Ocean, which is the body of water that surrounds Antarctica.
- Caribbean-type ophiolites represent subduction of oceanic plateaus or large igneous provinces.
- Franciscan-type ophiolites are accreted pieces of oceanic crust that are scraped from the subducted plate into the upper plate, such as in present-day Japan.
Why do ophiolites look like snakeskin?
Ophiolites have a snakeskin appearance because the rocks that originally compose them undergo a phenomenon called serpentinization, during which there is a conversion of the primary materials to those of the serpentine group. Serpentine is a mineral formed by the association of others such as antigorite, which is organized in sheets, and chrysolite, which forms masses that occupy the cracks in the rock.
Aspects of scientific and economic interest of ophiolites
The study of ophiolites raises surprising issues about plate tectonics, such as the processes by which oceanic crust can be emplaced on top of higher continental materials.
In addition, the analysis of ophiolites provides information on the formation process and the age of mountain ranges, volcanoes, mid-ocean ridges and islands. It also offers clues about the closure of oceans that were in the middle of colliding continental plates.
On the other hand, the components of the ophiolites have industrial and agricultural applications: the serpentine is often used in the carving of sculptures and as a substrate for the cultivation of nickel-accumulating plants. These plants have drawn the attention of scientists for their potential use as cleaners for soils contaminated by heavy metals.
Sources
Florentino Díaz García, Ricardo Arenas, José R. Martínez Catalán, José González del Tánago and Greg R. Dunning. The tectonic evolution of the Careon ophiolite (Varisco Orogen, NW Spain) . Department of Geology – University of Oviedo: 67-78, 1998.
Higher Institute of Geological Correlation. Mid-Ocean Ridges . Miscellany , 18:229-244, 2010.
Juteau, T. (March 18, 2009). The current and fossil oceanic crust (ophiolites). Historical evolution of the concept of ophiolite over more than two centuries of geology. [Second lecture]. Lecture Series of the Faculty of Science and Technology, University of the Basque Country/EHU. Department of Mineralogy and Petrology.
