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Ophiolite Complex
Cuesta Ridge Ophiolite

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Idealized Diagram of an Ophiolite Complex
Source: Winters, John; Intro to Igneous and Metamorphic Petrology

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Ophiolite Complexes

    Ophiolite complexes (Fig1) are a suite of rocks that form at spreading centers, or divergent plate boundaries.  As the plates move away from each other, upwelling magma makes its way up through the fracture in the oceanic crust, and forms several different types of rock along the way.  At the bottom, (about 21.75 miles below the surface of the Earth) is the lowest and ultramafic layer (See Fig2), comprised of the densest, mantle material.  Overlying the ultramafics, and still below the Moho (the boundary between the Earth’s crust and mantle), are the gabbros.  They can be layered (but is not the case in the Cuesta Ridge Ophiolite) and/or zoned.  They range from dunite at the bottom to diorite at the top.  These rocks are intrusive in nature, so they exhibit phaneritic (coarsely crystalline) to cumulate (whole crystals that accumulate at the bottom of the chamber due to density) texture.  Separating the ultramafics and the next layer is the Moho.  Above the mantle is the sheeted dike complex (Fig3).  This is comprised of a series of dikes that feed into the fractures.  Overlying the sheeted dikes are the pillow basalts.  (Figs4 & 5) These are extrusive features that form when the hot magma encounters the cold ocean and cool instantly.  They are bulbous in shape and extend laterally as well as vertically.  The final layer in an ophiolite complex are the pelagic sediments.  This is a sedimentary layer originating from the tests of oceanic organisms.  This layer is usually deepwater shale, chert or limestone (Winters).

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Created by:  Mollie Laird
Last Updated:  December 2003