Today’s episode focuses on one of thosewonderful jargon words geologists love to use: Ophiolites.

It’s not a contrived term like cactolith nor some reallyobscure mineral like pararammelsbergite. Ophiolites are actually really importantto our understanding of the concept of plate tectonics and how the earth worksdynamically.

The word goes back to 1813 in the Alps, where AlexandreBrongniart coined the word for some scaly, greenish rocks. Ophiolite is acombination of the Greek words for snake and stone, and Brongniart was also aspecialist in reptiles. So he named these rocks for their resemblance to snakeskins.

Fast forward about 150 years, to the 1960s. Geophysical data,deep-sea sampling, and other work was leading to the understanding that theearth’s crust is fundamentally different beneath the continents and beneath theoceans—and we found that the rocks in the oceanic crust are remarkably similarto the greenish, iron- and magnesium-rich rocks that had been labeledophiolites long ago and largely ignored except by specialists ever since.

Those rocks that form the oceanic crust include serpentineminerals, which are soft, often fibrous iron-magnesium silicates whose name is yetanother reference to their snake-like appearance.  Pillow basalts, iron-rich lava flows thatsolidify under water with bulbous, pillow-like shapes, are also typical ofoceanic crust. The term ophiolite was rejuvenated to apply to a specificsequence of rocks that forms at mid-ocean ridges, resulting in sea-floor spreadingand the movement of plates around the earth.

The sequence usually but not always includes some of themost mantle-like minerals, such as olivine, another iron-magnesium silicate,that may settle out in a magma chamber beneath a mid-ocean ridge. Shallower, relativelynarrow feeders called dikes toward the top of the magma chamber fed lava flowson the surface – but still underwater, usually – that’s where those pillow lavassolidified.

There are certainly variations, and interactions with wateras well as sediment on top of the oceanic crust can complicate things, but onthe whole that’s the package. So why not just call it oceanic crust and forgetthe jargon word ophiolite? Well, we’ve kind of done that, or at leastrestricted the word to a special case.

Pillow Lava off Hawaii. Source: NOAA. 
The word ophiolite today is usually used to refer to slicesor layers of oceanic crust that are on land, on top of continental crust. Butwait, you say, you keep saying subduction is driven by oceanic crust, which isdenser, diving down beneath continental crust, which is less dense. Well, yes –but I hope I didn’t say always.

Sometimes the circumstances allow for some of the oceaniccrust to be pushed up over bits of continental crust, despite their greaterdensity. One area where this seems to happen with some regularity is a settingcalled back-arc basins, which are areas of extension, pulling-apart, behind thecollision zone where oceanic crust and continental crust come together with theoceanic plate mostly subducting, going down under the continental plate. Ittook some time in the evolution of our understanding of plate tectonics for theidea to come out that you can have significant pulling apart in zones that arefundamentally compression, collision, but they’re recognized in many placestoday, as well as in the geologic past.

It seems to me that back-arc basins are more likely todevelop where the interaction is between plates or sub-plates that arerelatively weak, or small, and more susceptible to breaking. An example iswhere two oceanic plates are interacting, with perhaps only an island arcbetween them. The “battle” is a closer contest than between a big, strong continentand weaker, warmer, softer, oceanic crust, so slices of one plate of oceaniccrust may be squeezed up and onto the rocks making up the island arc. Thishappens in the southwest Pacific, where the oceanic Pacific Plate and theoceanic part of the Australian Plate are interacting, creating back-arc basinsaround Tonga and Fiji and elsewhere.

It also happens where continental material is narrower, orthinner, or where the interaction is oblique or complex. One example of this todayis the back-arc basin in the Andaman Sea south of Burma, Myanmar, where theIndian Ocean plate is in contact with a narrow prong of continent, Indochinaand Malaya.

We’ve now recognized quite a few ophiolites on land,emplaced there long ago geologically. At Gros Morne National Park inNewfoundland, the Bay of Islands ophiolite is of Cambrian to Ordovician age.The area is a UNESCO World Heritage Site for the excellent exposures of oceaniccrust there, not to mention fine scenery.

On Cyprus, the Troodos Ophiolite represents breaking withinthe Tethys Oceanic plate as it was squeezed between Gondwana, or Africa, andthe Anatolian block of Eurasia, which is today’s Turkey. The Troodos Ophioliteis rich in copper sulfides that were probably deposited from vents on amid-ocean ridge. In fact, the name Cyprus is the origin of our word copper, byway of Latin cuprum and earlier cyprium.  

On the island of New Caledonia, east of Australia and in themidst of the messy interactions among tectonic plates large and small, theophiolite is rich in another metal typical of deep-crust or mantle sources:nickel. There’s enough to make tiny New Caledonia tied with Canada for thirdplace as the world’s largest producer of nickel, after Indonesia and thePhilippines.

There’s a huge ophiolite in Oman, the Semail Ophiolite,covering about a hundred thousand square kilometers. It’s one of the mostcompete examples anywhere, and it was pushed up on to the corner of the Arabiancontinental block during Cretaceous time, around 80 million years ago. Like theone in Cyprus, this one is also rich in copper as well as chromite, anotherdeep-crustal or mantle-derived mineral.

The Coast Range Ophiolite in California is Jurassic, about170 million years old, and formed at roughly the same time as the Sierra NevadaBatholith developed as a more standard response to subduction. It’s likely thatwestern North America at that time was somewhat like the southwestern Pacifictoday, with strings of island arcs, small irregular continental blocks, anddiverse styles of interaction – the perfect setting for a long band of oceaniccrust to be pushed up and over other material. The whole thing ultimately gotamalgamated with the main North American continent. I talked a bit more about theseevents in the episode on the Franciscan, November 7, 2014.

—Richard I. GibsonLINKS: Nice images from Oregon State 

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