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
LINKS:
