Stop #1: Peridotite
Location: Confluence Of The Middle And South Forks Of The Smith River
We begin our ophiolite trip in the mantle (refer to the figure on the left). The rock at this first outcrop is peridotite (mostly harzburgite). This rock is composed of the minerals olivine and pyroxene. The dense, iron- and magnesium-rich peridotite is interpreted as uppermost mantle material. The peridotite (harzburgite) is often altered to the rock serpentine (“serpentinized”) during the convergent process. Serpentine is a hydrated (water added) iron/magnesium-rich rock that is thought to form when peridotite is squeezed upward along faults. 
Geologists believe that the uppermost mantle (at depths of 10-50 km) is made of this dense, iron-magnessium-rich peridotite rock. This rock is interpreted to have been pushed upward from these great depths during plate convergence. The Smith River area is the site of one of the world's largest exposures of peridotite. To find out more about ophiolites, click on the introduction listed below. 
This photo shows both the orange surface of the peridotite and areas where the green, olivine- and pyroxene-rich rock is exposed. Any ideas why surface exposures of peridotite are orange? In many areas the peridotite has been "serpentinized". Hydration of peridotite during metamorphism results in this less dense metamorphic rock.

 
This photo shows the exterior texture of the peridotite. Note the rough appearance where more resistant pyroxene crystals extrude from the rock. Also note the regular jointing pattern of the outcrop.

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  • Geologic Map of the Area
  • An Introduction To Ophiolites
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