Today’s episode focuses on one of those wonderful jargon words geologists love to use: Ophiolites.
It’s not a contrived term like cactolith nor some really obscure mineral like pararammelsbergite. Ophiolites are actually really important to our understanding of the concept of plate tectonics and how the earth works dynamically.
The word goes back to 1813 in the Alps, where Alexandre Brongniart coined the word for some scaly, greenish rocks. Ophiolite is a combination of the Greek words for snake and stone, and Brongniart was also a specialist in reptiles. So he named these rocks for their resemblance to snake skins.
Fast forward about 150 years, to the 1960s. Geophysical data, deep-sea sampling, and other work was leading to the understanding that the earth’s crust is fundamentally different beneath the continents and beneath the oceans—and we found that the rocks in the oceanic crust are remarkably similar to the greenish, iron- and magnesium-rich rocks that had been labeled ophiolites long ago and largely ignored except by specialists ever since.
Those rocks that form the oceanic crust include serpentine minerals, which are soft, often fibrous iron-magnesium silicates whose name is yet another reference to their snake-like appearance. Pillow basalts, iron-rich lava flows that solidify under water with bulbous, pillow-like shapes, are also typical of oceanic crust. The term ophiolite was rejuvenated to apply to a specific sequence of rocks that forms at mid-ocean ridges, resulting in sea-floor spreading and the movement of plates around the earth.
The sequence usually but not always includes some of the most mantle-like minerals, such as olivine, another iron-magnesium silicate, that may settle out in a magma chamber beneath a mid-ocean ridge. Shallower, relatively narrow feeders called dikes toward the top of the magma chamber fed lava flows on the surface – but still underwater, usually – that’s where those pillow lavas solidified.
There are certainly variations, and interactions with water as well as sediment on top of the oceanic crust can complicate things, but on the whole that’s the package. So why not just call it oceanic crust and forget the jargon word ophiolite? Well, we’ve kind of done that, or at least restricted the word to a special case.
|Pillow Lava off Hawaii. Source: NOAA.|
The word ophiolite today is usually used to refer to slices or layers of oceanic crust that are on land, on top of continental crust. But wait, you say, you keep saying subduction is driven by oceanic crust, which is denser, 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 oceanic crust to be pushed up over bits of continental crust, despite their greater density. One area where this seems to happen with some regularity is a setting called back-arc basins, which are areas of extension, pulling-apart, behind the collision zone where oceanic crust and continental crust come together with the oceanic plate mostly subducting, going down under the continental plate. It took some time in the evolution of our understanding of plate tectonics for the idea to come out that you can have significant pulling apart in zones that are fundamentally compression, collision, but they’re recognized in many places today, as well as in the geologic past.
It seems to me that back-arc basins are more likely to develop where the interaction is between plates or sub-plates that are relatively weak, or small, and more susceptible to breaking. An example is where two oceanic plates are interacting, with perhaps only an island arc between them. The “battle” is a closer contest than between a big, strong continent and weaker, warmer, softer, oceanic crust, so slices of one plate of oceanic crust may be squeezed up and onto the rocks making up the island arc. This happens in the southwest Pacific, where the oceanic Pacific Plate and the oceanic part of the Australian Plate are interacting, creating back-arc basins around Tonga and Fiji and elsewhere.
It also happens where continental material is narrower, or thinner, or where the interaction is oblique or complex. One example of this today is the back-arc basin in the Andaman Sea south of Burma, Myanmar, where the Indian Ocean plate is in contact with a narrow prong of continent, Indochina and Malaya.
We’ve now recognized quite a few ophiolites on land, emplaced there long ago geologically. At Gros Morne National Park in Newfoundland, the Bay of Islands ophiolite is of Cambrian to Ordovician age. The area is a UNESCO World Heritage Site for the excellent exposures of oceanic crust there, not to mention fine scenery.
On Cyprus, the Troodos Ophiolite represents breaking within the Tethys Oceanic plate as it was squeezed between Gondwana, or Africa, and the Anatolian block of Eurasia, which is today’s Turkey. The Troodos Ophiolite is rich in copper sulfides that were probably deposited from vents on a mid-ocean ridge. In fact, the name Cyprus is the origin of our word copper, by way of Latin cuprum and earlier cyprium.
On the island of New Caledonia, east of Australia and in the midst of the messy interactions among tectonic plates large and small, the ophiolite 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 third place as the world’s largest producer of nickel, after Indonesia and the Philippines.
There’s a huge ophiolite in Oman, the Semail Ophiolite, covering about a hundred thousand square kilometers. It’s one of the most compete examples anywhere, and it was pushed up on to the corner of the Arabian continental block during Cretaceous time, around 80 million years ago. Like the one in Cyprus, this one is also rich in copper as well as chromite, another deep-crustal or mantle-derived mineral.
The Coast Range Ophiolite in California is Jurassic, about 170 million years old, and formed at roughly the same time as the Sierra Nevada Batholith developed as a more standard response to subduction. It’s likely that western North America at that time was somewhat like the southwestern Pacific today, with strings of island arcs, small irregular continental blocks, and diverse styles of interaction – the perfect setting for a long band of oceanic crust to be pushed up and over other material. The whole thing ultimately got amalgamated with the main North American continent. I talked a bit more about these events in the episode on the Franciscan, November 7, 2014.
—Richard I. Gibson