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Ophiolites are distinctive sequences of rock, typically capped by sediments followed by basic rocks (pillow lavas, sheeted dykes, and gabbro) followed by ultramafics (eg. dunite).

Although the exact emplacement process has evolved over time, these are generally recognized as pieces of ocean crust which have been "obducted" onto continents.

How and when was it realized that ophiolites could be explained as pieces of ocean crust?

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The first place I looked, Wikipedia, gave a fairly good answer.

The term dates back to the 19th century, but it first came it to use in the current context only about 50 years ago, when Frederick Vine and Drummond Matthews used it

to represent the formation of new crust at the oceanic ridge and its subsequent symmetric spreading away from that ridge.

This was inspired by the discovery of

magnetic anomaly stripes on the seafloor

In other words, odd bits under the sea that shouldn't have been where they were, or with the same characteristics as they had, pertaining to magnetic fields.

More key work was done by Ian Graham Gass et. al., who found a piece of an ophiolite that

must have formed by repetitive extension of crust and intrusion of magma, resulting in a formation consisting of 100% dikes with no older wall rocks preserved within the complex.

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