http://geology.about.com/od/platetectonics/a/ophiolite.htm

What is an Ophiolite?

by Andrew Alden

The earliest geologists were puzzled by a peculiar set of rock types in the European Alps like nothing else found on land: bodies of dark and heavy peridotite associated with deep-seated gabbro, volcanic rocks and bodies of serpentinite, with a thin cap of deep-sea sedimentary rocks.

In 1821 Alexandre Brongniart named this assemblage ophiolite ("snake stone" in scientific Greek) after its distinctive exposures of serpentinite ("snake stone" in scientific Latin). Fractured, altered and faulted, with almost no fossil evidence to date them, ophiolites were a stubborn mystery until plate tectonics revealed their important role.

A hundred and fifty years after Brongniart, the advent of plate tectonics gave ophiolites a place in the big cycle: they appear to be small pieces of oceanic crust that have been attached to the continents.

Until the mid-20th century deep-sea drilling program we didn't know just how the seafloor is constructed, but once we did the resemblance with ophiolites was persuasive. The seafloor is covered with a layer of deep-sea clay and siliceous ooze, which grows thinner as we approach the mid-ocean ridges. There the surface is revealed as a thick layer of pillow basalt, black lava erupted in round loaves that form in the deep cold seawater.

Beneath the pillow basalt are the vertical dikes that feed the basalt magma to the surface. These dikes are so abundant that in many places the crust is nothing but dikes, lying together like slices in a bread loaf. They clearly form at a spreading center like the mid-ocean ridge, where the two sides are constantly spreading apart allowing magma to rise between them. (See more at "Divergent Zones in a Nutshell.")

Beneath these "sheeted dike complexes" are bodies of gabbro, or coarse-grained basaltic rock, and beneath them are the huge bodies of peridotite that make up the upper mantle. (Have a look at all these rocks in the ophiolitic rocks picture gallery.) The partial melting of peridotite is what gives rise to the overlying gabbro and basalt (see "About the Earth's Crust"). And when hot peridotite reacts with seawater, the product is the soft and slippery serpentinite that is so common in ophiolites. (See more about serpentinization.)

This detailed resemblance led geologists in the 1960s to a working hypothesis: ophiolites are tectonic fossils of ancient deep seafloor.

Ophiolites differ from intact seafloor crust in some important ways, most notably in that they aren't intact. Ophiolites are almost always broken apart, so the peridotite, gabbro, sheeted dikes and lava layers don't stack up nicely for the geologist. Instead they are usually strewn along mountain ranges in isolated bodies. As a result, very few ophiolites have all the parts of the typical oceanic crust. Sheeted dikes are usually what is missing.

The pieces must be painstakingly correlated with each other using radiometric dates and rare exposures of the contacts between rock types. Movement along faults can be estimated in some cases to show that separated pieces were once connected.

Why do ophiolites occur in mountain belts? Yes, that's where the outcrops are, but mountain belts also mark where plates have collided. The occurrence and disruption were both consistent with the 1960s working hypothesis.

Since then, complications have arisen. There are several different ways for plates to interact, and it appears that there are several types of ophiolite.

The more we study ophiolites, the less we can assume about them. If no sheeted dikes can be found, for instance, we cannot infer them just because ophiolites are supposed to have them.

The chemistry of many ophiolite rocks does not quite match the chemistry of mid-ocean ridge rocks. They more closely resemble the lavas of island arcs. And dating studies showed that many ophiolites were pushed onto the continent only a few million years after they formed. These facts point to a subduction-related origin for most ophiolites, in other words near shore instead of the mid-ocean. Many subduction zones are areas where the crust is stretched, allowing new crust to form in much the same way as it does in midocean. Thus many ophiolites are specifically called "supra-subduction zone ophiolites."