Serpentinite forms by addition of water to minerals in peridotite, changing them from olivine and/or pyroxene to the serpentine minerals antigorite, chrysotile and lizardite. Mount Diablo Serpentinite: Serpentinite is a rock frequently found in association with an ophiolite. Serpentinite is derived from the basal portion of the original ocean crust and uppermost part of the mantle, but has been metamorphosed by hydration from ocean water circulating through fractures in the ocean crust. The Mount Diablo Ophiolite underlies the mountain north of a line drawn from Long Ridge through Murchio Gap, encompassing the Zion Peak rock quarry, Mitchell Rock, and Eagle Peak. Radiometric and fossil-age determinations date the ophiolite as having been formed approximately 165 million years ago during the Mid-Jurassic. The rocks of this old ocean crust on Mount Diablo have been named the Mount Diablo Ophiolite and is considered a fragment of the Coast Range Ophiolite. Ophiolites generally form a uniform vertical rock sequence consisting, from bottom to top, of ultramafic peridotite from the top of the mantle, mafic intrusive gabbros and/or diabase that formed one or more miles below the sea floor, and mafic extrusive rocks, often in the form of pillow lava extruded beneath water. ![]() Ophiolites are thought to form at oceanic spreading centers in the middle of the oceans, associated with oceanic island chains (arcs), or in narrow oceans such as the Gulf of California. The oceanic crust caught between this subduction zone and anĮarlier shoreline in the ancient Sierra foothills was preserved as the Coast Range Ophiolite and later partially exposed. ![]() ![]() It is generally believed that near the close of the Jurassic a subduction zone developed along what is presently represented by the modern California coast.
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