Ophiolites

Ophiolites are a unique assemblage of rocks found in many mountain belts worldwide. They were formed in the oceans and subsequently transported to land during mountain-building processes. Ophiolites are useful to geologists as indicators of the location of ancient oceans. They are important to society because they are major sources of asbestos, nickel, and copper minerals.

Characteristics of Ophiolites

Ophiolites are unique assemblages of rocks that have fascinated geologists for centuries. The word “ophiolite” is derived from the Greek ophis, meaning snake or serpent. The term first appeared in the geological literature in the 1820s, when Alexandre Brongniartof France used it to describe rocks called serpentinite, which are made entirely of the mineral serpentine. The term “ophiolite” is appropriate because serpentinite, like some snakes, has a mottled green appearance.

The usage of the term “ophiolite” changed early in the twentieth century when the close association of serpentinite or serpentinized peridotite with deep-water sediments and with pillow lavas was noted. This assemblage of three rock types became known as the Steinmann Trinity. In the late 1960s, with the advent of plate tectonics as a unifying theory in geology, the definition of ophiolite again changed and was used to describe the Steinmann Trinity, plus other rocks arranged in a particular sequence. At the same time, the interpretation that ophiolites formed in the oceans became the cornerstone for reconstructing the history of mountains. As defined for modern usage by a Geological Society of America conference in 1972, “ophiolite” refers to a distinctive assemblage of mafic and ultramafic rocks. A completely developed ophiolite is 10 to 12 kilometers thick and covers an area of hundreds of square kilometers.

Rocks in ophiolites occur in layers in the following order (starting from the bottom and working up): peridotite, gabbro, sheeted dike complex, pillow lavas, and chert. Peridotite is an ultramafic igneous rock formed below the Earth's surface, dark in color, consisting of large grains (greater than 5 millimeters) of the minerals olivine (an olive-green mineral rich in magnesium, iron, silicon, and oxygen) and pyroxene (a black mineral rich in iron, magnesium, silicon, and oxygen). In many ophiolites, peridotite has been subjected to heat and hot water since its formation, resulting in some of the olivine minerals changing to serpentine, some of which is asbestos.

Gabbro is a mafic igneous rock formed below the Earth's surface that has a salt-and-pepper appearance. It consists of large grains (greater than 3 millimeters) of the minerals pyroxene and feldspar (a white mineral rich in calcium, sodium, aluminum, and oxygen). Sheeted dike complex is made up of mafic igneous rocks formed just below the Earth's surface, similar to gabbro but with smaller mineral grains (less than 3 millimeters). Pillow lavas are mafic igneous rocks formed on the Earth's surface as lava erupts into water, similar to gabbro but with much smaller grains (less than 1 millimeter). Chert is a sedimentary rock formed in deep water consisting of very small grains (some too small to be seen with a microscope) of various minerals and organic remains.

Not all ophiolites exhibit the complete sequence of rocks; some were formed with certain layers missing, and others were dismembered subsequent to their formation. A very important observation is that beneath all ophiolites there is a fault (fracture in the Earth) separating them from underlying rocks. Furthermore, there is evidence that ophiolites have probably moved hundreds of kilometers along those faults.

Formation of Ophiolites

Ophiolites can form in two different plate tectonic environments, both of which are in oceans. In one environment, such as in the middle of the Atlantic Ocean, two plates are moving away from each other. As they do so, magma from the interior of the Earth moves up to the bottom of the oceans and cools to form long mountain chains called oceanic ridges (for example, the Mid-Atlantic Ridge). The solidified rock becomes part of an outer layer of the Earth called the oceanic lithosphere. The newly formed oceanic lithosphere then moves away from the ridge at which it formed, resulting in widening of the ocean. (For example, the Atlantic Ocean is growing wider as North America moves westward, away from Europe.) Ophiolites are believed to be remnants of the top part of oceanic lithosphere that was created at ancient oceanic ridges. All the igneous rocks in ophiolites formed by the cooling of magma produced deep in the Earth and spewed forth at an oceanic ridge. Pillow lavas form when the magma erupts into seawater; sheeted dikes, gabbros, and peridotites form when that magma cools at various depths below the surface. Chert forms when fine particles, including mineral grains, animal parts, and plants, fall to the bottom of the newly created ocean floor to form sedimentary deposits.

A second environment where ophiolites may form is the region where two plates are moving toward each other. In this environment, oceanic lithosphere that formed at oceanic ridges disappears down into the interior of the Earth, resulting in melting at depth, the production of highly explosive volcanoes, and the generation of dangerous earthquakes. This general area is known as a subduction zone. When oceanic lithosphere has just begun to subduct (descend into the Earth's interior) beneath another oceanic lithospheric plate, ophiolites may form in the region above the subducting plate. The processes involved in formation of these ophiolites are the same as at an oceanic ridge, though the environments are different.

Movement of Ophiolites

Another important part of the geological history of ophiolites is how they are moved from their place of formation in the oceans to their current location on continents. The key lies in understanding how mountains are built. Ophiolites have been described from all the world's major mountain belts: the Appalachian Mountains of eastern North America, the Rockies in western North America, the Alps in Europe, the Himalaya in Asia, the Ural Mountains, the Andes in South America, and the mountains of Papua, New Guinea. They range in age from about 600 million to 15 million years old. The movement of ophiolites from the ocean floor to mountains is related to plate tectonics. Subduction zones, where plates are moving toward each other, provide the mechanism for emplacement of ophiolites.

To get a better idea of how ophiolites may have been put into place in the past, scientists consider what might happen to the Pacific Ocean in the future. The Pacific plate (that is, the oceanic lithosphere beneath the Pacific Ocean) is moving westward toward, and subducting beneath, the Asian plate along the Japanese islands, resulting in volcanoes and earthquakes along the zone. The Pacific plate is also subducting eastward beneath western North America. Therefore, the Pacific Ocean will become smaller, the North American continent will move closer to the Asian continent, and they will eventually collide. During this time, probably 99.99 percent of the Pacific oceanic lithosphere (potential ophiolites) will be lost to the interior of the Earth at the subduction zones. At the time of the collision of North America and Asia, however, a small portion (less than 0.01 percent) of the oceanic lithosphere may not slip back into the interior but instead may be pushed up onto the continents, because when the two continents collide, mountains are being built, and some of the Pacific oceanic lithosphere, sandwiched in the zone of collision, may be pushed up with the mountains. These portions of the Pacific oceanic lithosphere will then be called ophiolites. In a similar fashion, the ophiolites that are now visible in mountain belts are tiny remnants of ancient oceanic lithosphere pushed onto continents during ancient mountain building.

The occurrence of ophiolites in mountains has helped geologists understand how mountains are built. Ophiolites lie in narrow bands that continue for thousands of kilometers along a mountain belt. This narrow band represents the zone where two continents have come together to build the mountains. Thus, by locating ophiolites, geologists can identify and then characterize the two (or more) continents that existed separately before the mountains were built. For example, in the Appalachian Mountains of eastern North America, a narrow band of ophiolites extends from western Newfoundland, Canada, through Quebec, Canada, into Vermont and as far south as Alabama in the United States. This zone may represent the boundary between the ancient North American continent to the west and the ancient African continent to the east; the two ancient continents were plastered together during the building of the Appalachians between 500 million and 350 million years ago.

Study of Ophiolites

Geologists have used various techniques to study ophiolites, including field mapping, seismic studies, electron microprobe techniques, and X-ray techniques. Field mapping of ophiolites involves identifying rock types, noting their locations on maps, and ascertaining the relationship of the ophiolites with neighboring rocks. It is the basic tool used by the geologist to analyze the origin of ophiolites, and it precedes any laboratory techniques.

Seismic studies have been employed to compare ophiolites to the modern oceanic lithosphere. Seismic waves travel through the Earth after earthquakes. They can also be created by human-made explosions or by hitting the ground hard with a sledgehammer. Seismic waves travel at different speeds through different rock types. Thus, if scientists can identify the velocities of seismic waves as they travel through different types of rock, the type of rock can be identified even though it cannot be seen. In the oceans, seismic velocity measurements through rocks beneath the ocean floor have shown that the modern oceanic lithosphere is made up of layers that correspond exactly in rock type and in thickness to what is observed in ophiolites, which is very good evidence to support the interpretation that ophiolites formed as portions of oceanic lithosphere.

X-ray techniques have been used to determine the chemical composition of rocks in ophiolites. In a technique called X-ray fluorescence, X-rays bombard a powdered rock sample and interact with electrons of the chemical elements to produce secondary X-rays. The secondary X-rays generated by specific elements can be isolated and counted to determine the amounts or percentages of specific elements present in the rock. Results of such studies on ophiolites have shown that pillow lava and sheeted dike rocks are similar to lavas erupted in modern oceans. In particular, low percentages of potassium oxide and light rare-earth elements are common to both ophiolitic and modern oceanic rocks.

The electron microprobe is also used extensively to help interpret the origin of ophiolites. In the electron microprobe, electrons hit a rock sample at high speed and interact with electrons in chemical elements in the rock to produce X-rays. As with X-ray techniques, the amounts of chemical elements can be determined. In electron microprobe work, the rock sample, rather than being a crushed or powdered specimen, is a thin section (0.03 millimeter thick) of an intact rock that can be viewed through a microscope. Thus, minerals in the rock can be seen, selected, and analyzed for their chemical constituents.

Chemical analyses of minerals are most useful in the peridotite and gabbro layers of ophiolites. For example, chemical analyses of olivine, pyroxene, and feldspar from gabbro layers and upper parts of peridotite layers have shown that these layers precipitated from a magma of the same composition as the pillow lavas and sheeted dikes. Therefore, nearly all the igneous rocks in ophiolites can be related to one magma, part of which cooled at shallow depths (about five kilometers below the surface) to produce gabbro and peridotite and some of which was injected into higher levels in the Earth to produce the sheeted dikes and pillow lavas. The lower parts of the peridotite layer, however, must have originated in a different way. The amounts of aluminum in the mineral pyroxene reveal that the lower peridotite layer formed under high pressure at considerable depth, as much as 30 kilometers below the surface of the Earth—good evidence for the suggestion that ophiolites that now exist on the surface of the Earth must have been moved since the time of their formation.

Principal Terms

igneous rock: a rock formed when magma cools and forms minerals; it can form on the surface of the Earth when volcanoes erupt, or it can form at depth, without reaching the Earth's surface

lithosphere: the upper, rigid 100 kilometers of the Earth that forms the moving plates; beneath oceans, it is called oceanic lithosphere, and beneath continents, it is called continental lithosphere

mafic: a rock rich in iron and magnesium minerals; ultramafic rocks are very rich in these minerals

olivine: a mineral consisting of magnesium, iron, silicon, and oxygen

peridotite: an igneous rock consisting of minerals rich in magnesium; serpentinized peridotite has been heated such that the mineral olivine is partially converted into serpentine

pillow lava: the lava that is formed when molten rock erupts into water and cools in the shape of a pillow

plate tectonics: the theory that the upper part of the Earth consists of a number of “plates,” or rigid parts, that move relative to one another across the surface

serpentine: a mineral consisting of magnesium, iron, silicon, oxygen, and water

Bibliography

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