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Stop 12, Jarbo Gap Ophiolite: The Setting

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Stop 12 is at 39°43.20´N, 121°30.96´W, just beyond the Big Bend Road turnoff in this roadcut near a busy highway. (more below)
A Sierra ophiolite
Photo (c) Andrew Alden, licensed to About.com (fair use policy)
Ophiolites are large chunks of deep rocks of the oceanic crust. Usually these dense, dark rocks are subducted into the mantle, but occasionally they are trapped near the surface where they attract geologists. Here the ophiolitic rocks consist of serpentinite and lavas that have been intruded more than once by dikes dating from the Jurassic. The details of this exposure suggest that it was once part of an oceanic fracture zone, a long crack that forms at spreading ridges where they are offset in transforms (see Transforms in a Nutshell for an explanation).

Day 1, Coast Range: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

Day 2, Sierra Nevada: 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18

Day 3, Sierra Nevada: 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28

Day 4, Coast Range: 29 | 30 | 31 | 32 | 33

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