Remember the “Kilroy was here” meme and doodle?
It originated in World War II as a doodle by G.I.s. It was a way of saying, “An American G.I. was here.”
Ophiolites are roughly analogous to Kilroy.
Ophiolites are a characteristic geologic formation which says to a geologist, “An ocean was here.” And they turn up in some pretty astonishing places.
Ophiolites are an assemblage of rocks, a column, that have a characteristic structure. An idealized ophiolite looks something like this:
From the bottom up, Periodite is the stuff of the Earth’s mantle, the layer under the crust.
The next layer up is Gabbro, which is slowly cooled and crystallized crustal rock. Think of it as basalt that never emerged on the surface.
Sheeted dykes are the “plumbing” that connected the then-molten Gabbro with the pillow lavas above them. Sheeted dykes are the geologic signature of a spreading center, a place like the Mid-Atlantic Rise where oceanic crust is being made. They are the “veins” where the molten rock pushed its way up to the ocean bottom. They are vertical tabular layers, quite thin in relation to their length and breadth, and represent the sequential spreading of the seafloor.
Pillow lavas are what happens when lava flows into water. They are thick sequences of discontinuous solid pillow-shaped masses, commonly up to one meter in diameter. Pillow lavas are being made off the coast of Hawai’i as lava from Kiliuea Volcano runs into the Pacific Ocean.
Lastly, the pillow lavas are covered with a layer of marine sediments. The composition and thickness of the marine sediment layer depends on where the seafloor originated, how old it is and where it has been.
If you think about it for a moment, most of the ocean floors are composed of ophiolites, generated at a spreading center and migrating across the ocean floor to a subduction zone.
In the field, ophiolites are pretty variable. The sheeted dykes may be absent – think of the eruptions on the surface of Icealnd, for example. The gabbro may be modified by exposure to hot water under pressure, transformed into serpentine, for example. Nor are all ophiolites created at spreading centers. Geology, as WC has said before, is messy. But ophiolites are always created on seafloor.
So when you find ophiolites far inland, it means that at some point in the geologic past an ocean was nearby. In California, for example, there are ophiolites in the Coastal ranges and ophiolites in the foothills of the Sierra Nevada, documenting two instances when island arc complexes, transported by plate tectonics, came in from the Pacific Ocean and attached themselves to what is now California. More surprisingly, the Ural Mountains, in the middle of Eurasia, contain many ophiolite sequences, documenting a long-vanished ocean.
Ophiolites show that much of what we think of as continents are mostly assemblages of chunks of geology that have been transported from elsewhere by the inexorable mechanisms of plate tectonics.
WC could prattle on a long time about the different origins of ophiolites, back arcs and forearcs, about the ways the chemistry of the rocks can tell you about when and where they originated and about the geologic mechanisms that put chunks of seafloor on continents. But you don’t need to know any of that to grasp the principle and importance of ophiolites: they are the geologic signature of an ocean, just as “Kilroy was here” is the signature of a G.I. And more proof that geology is cool.