In this article we shall discuss ophiolites, how to recognize them, how they originate, and how we know this.
The reader may find it helpful to re-read the article on igneous rocks before going any further.
What is an ophiolite?Edit
Here we shall describe a classic "Penrose type" ophiolite; geologists recognize variations on this type for reasons that we will discuss later in this article.
An ophiolite, then, is a sequence of rocks exposed above sea-level and usually emplaced on or in continental crust. This sequence consists of (reading from the top down):
- Marine sedimentary rocks such as chert, limestone, and rock formed from pelagic clay. This top layer is often not counted as being part of the ophiolite, since this term is usually reserved just for the igneous rocks involved.
- Basalt, which is by definition an extrusive rock; often in the form of pillow basalt, which the reader should remember is only formed underwater.
- A sheeted dike complex: dikes which do not (as they usually do) intrude into some other rock, but which rather stand side by side rather like books on a shelf.
- Ultramafic rocks.
This sequence is known as the pseudostratigraphy of an ophiolite.
The diagram to the right shows an cross-section of such an ophiolite. To be precise, it shows a cross-section of a vertical ophiolite, in which the layers are horizontal and the sheets are vertical. We may perfectly well find an ophiolite lying on its side, as a result of the tectonic events that emplace it; but the relationships between the rocks remain the same.
So, an ophiolite is recognized by being something which looks like that — but what is an ophiolite? According to geologists, an ophiolite is a section of oceanic lithosphere from the upper mantle through the igneous crust and up to the sediment on top. (You may note that this means that an ophiolite is a kind of terrane; the subject of ophiolites is of sufficient complexity that I have thought it best to give them their own article.)
To assert that ophiolites are slices of oceanic lithosphere is a bold claim, since it is not immediately apparent what such a thing would be doing emplaced on or in continental crust. It is therefore time to ask ...
Ophiolites: how do we know?Edit
The reason that geologists think that ophiolites are pieces of oceanic lithosphere is that this is exactly what they look like.
The marine sediments on the top of an ophiolite obviously speak of a marine environment. The basalt is the characteristic igneous rock of the sea-floor, and the fact that it is usually pillow basalt agrees with this, since pillow basalt only forms underwater.
The sheeted dikes became explicable with the discovery of sea floor spreading. Since the mid-ocean rifts are linear, the route between the magma and the surface would naturally be shaped like a sheet, resulting in the formation of a dike. As the two sides of the rift move apart, this would lead to more dikes intruding between the previous dikes.
The gabbro in ophiolites would represent material which was not extruded, moving out laterally from the magma chamber beneath the rift.
The ultramafic rocks would represent a portion of the upper mantle: as we have discussed in the article on the structure of the Earth, this is what we expect the mantle to be made of.
After the discovery of sea floor spreading and the significance of sheeted dikes, geologists were quick to realize that if ophiolites represented a cross-section of the oceanic crust, then this would account extremely well for the results obtained by seismic studies of the oceanic crust.
It has been possible to drill and sample the oceanic crust through the sediment, through the pillow basalt, through the sheeted dikes, and down into the region where the dikes give way to gabbro; this was achieved at Hole 1256D of the Ocean Drilling Project. At the time of writing, no-one has managed to drill all the way from the sea-floor sediment to the upper mantle, but as far as anyone has drilled the oceanic crust does indeed look just like an ophiolite — and not just in the large-scale pseudostratigraphy, but in fine detail (see here, for example, for a discussion of how closely gabbro recovered from the hole resembles gabbro from an ophiolite.)
Where drilling cannot take us, nature has obligingly offered us another way to look at oceanic crust: so-called tectonic windows such as Hess Deep. This is a rift in the ocean floor comparable in size to the Grand Canyon: submarine exploration of the Deep has shown that the sides of the rift display the same pseudostratigraphy as an ophiolite, with ultramafic rocks at the bottom, then gabbros, then sheeted dikes, and so forth.
For all these reasons, geologists identify ophiolites as portions of oceanic lithosphere. Now when oceanic lithosphere moves towards continental lithosphere it is usually subducted beneath it, as discussed in the article on subduction; but if for some reason some of it was obducted (thrust up onto a continent) perhaps because the zone of subduction was so close to the ridge where the crust was produced that the crust was still elevated and low in density, then what we would get would be an ophiolite.
Note on varieties of ophioliteEdit
I would be painting an inaccurate picture of ophiolites if I did not point out that they come in several varieties. The particular type of ophiolite I have been using as an example does not even constitute a majority of those known to geologists. I have used this type as an example because they shed an interesting light on sea floor spreading, but I should now mention some of the variations on the ophiolite theme.
In this article I have taken as paradigmatic those ophiolites which display a complete sequence of ultramafic rocks, gabbro, sheeted dikes, basalt, and marine sediment. But not all ophiolites have sheeted dike complexes, and some also lack the gabbro.
Likewise I have concentrated on one particular mode of production of ocean crust, at mid-ocean ridges. I have been describing MORB (mid-oceanic ridge basalt) ophiolites; but geologists also recognize other varieties such as LIP (large igneous province) ophiolites and SSZ (supra-subduction zone) ophiolites. In this article the reader can find out more about SSZ ophiolites, with special reference to why they don't have sheeted dike complexes.
Finally, I have mentioned only one way that ophiolites get attached to a continent, but geologists know of others. We should note that all such mechanisms must involve extremely rare events, since the world's total stock of ophiolites can only be a fraction of one percent of the total oceanic crust produced during the Earth's history. This explains why there is frequently controversy among geologists as to the mechanism by which particular ophiolites are emplaced; it is much easier to understand the general rule (i.e. subduction) than it is to understand what must necessarily be rare freak events.
If, then, I was to attempt to describe all the varieties of ophiolites in terms of both their igneous origin and their mode of emplacement, to explain how geologists tell one from the other, and to say why in certain cases there is debate, I should run the risk of unbalancing the textbook, which would have to be retitled Historical Geology and Ophiolites. Instead, let us try to say what ophiolites have in common.
In terms of their composition, what they have in common is that they contain igneous marine rocks, and so clearly originated as oceanic lithosphere.
In terms of their history, what they have in common is that they cry out for an explanation in terms of plate tectonics.
There are other reasons behind plate tectonics why marine sedimentary rocks are found in continents on top of continental crust, as we shall discuss in later articles, but when we find actual igneous oceanic crust emplaced in a continent, then we have to seek an explanation in terms of the forces we know of that move bits of the Earth's crust around.