Fore-Notes:
- From a list of geologist jokes:
You know you’re a geologist if you think road cuts are tourist attractions.
- At the top of the cul-du-sac or many places on your drive up, stop and admire the spectacular views of the San Francisco Bay and Golden Gate.
- At the top of the cul-du-sac one house has large blocks of the blueschist right on the edge of the sidewalk, these are pretty cool too.
First be aware the 2 foot high rock wall is imported rock unrelated to this Earthcache. You’ll want to look at the bedrock in the cut slope above the rock wall. The most distinctive exposures for you to examine or the rock face about 20 feet uphill of the bottom of the rock wall. There is another good exposure above the uphill side of the rock wall. You’ll need to get up close and examine the colors and texture of this rock from just a few inches away.
What you are looking at here is blueschist grade metavolcanic rocks. That is, this rock was once a volcanic rock that was metamorphosed in the a subduction zone. Greenstones and greenschist form when the original rock is exposed to a more “typical” pressure-temperate regime. That is as the rock is subducted it is buried (causing the pressure from the overburden) and heated from the mantle. However, blueschists experience more pressure relative temperature resulting a suite of minerals that give the rock it’s blue hue. This segment of blue schist is a thin sliver less than a 1,000 meters wide.
Through microscopic study minerals identified in this rock include sodic amphibole and lawsonite, which are both blue and mostly associated blueschist. Also present are jadeitic pyroxenes (green; aka jadeite, not “true” jade”), quartz, albite (plagioclase feldspar), and some relic (from it’s igneous origin, clinopyroxene). Microscope studies have identified these minerals which are generally about 0.3 mm in diameter, so not individually visible to the naked eye.
As with all EarthCaches you will need to send the Cache Owner (me) some answers to the questions below. Go ahead and log you cache immediately, but send me the answers in a timely manner, or I’ll be forced to delete your log. I honor all good faith attempts regardless of the accuracy of the answer.
- Name of the EC and the GC code
- There are some fractures in the rock that are filled with secondary quartz, large-ish white veins, you can ignore this. On the correct face you can see some fine banded , very particular, texture. Examine it closely. Describe the fine texture.
- Again ignoring the white quartz veins, the fine banding can have different colors. They are not vivid colors, more hues, but describe the colors you see in the fine bands. If you read the description above you'll see the some particular colors mentioned.
- Photos are not required but always welcome.
References:
Wakabayashi, John (1992) Metamorphism and Tectonic Origin of Franciscan Metabasites and a Field Trip Guide to Three Localities in the San Francisco Bay (see stop 1 page 8) In Field Guide to Geology and Metamorphism of the Franciscan Complex and Western Metamorphic Belt of Northern California by Schiffman, P and Wagner D.L.. California Divisions of Mines and Geology. Special Publication 114.
Congrats to NoETA4 and BerkeleyBoomers on FTF. 43 minutes from posting to logging the FTF