Rosemary, thyme, basal, sanadine, & rhyolite
In California, United States
How Geocaching Works
Look to the road cut to the east. The lighter colored bed is a “welded rhyolite tuff.” What does this mean? Rhyolite is a light volcanic rock. It is a light because the relatively small amounts of iron (and higher silica content) which form dark colored minerals. A welded tuff is formed when hot, recently erupted volcanic ash fuses together. Think of sticky, warm chocolate chips piled together that fuse together when they cool. This band of rock is associated with the volcanic rocks of the Grizzly Peak basalts, which were formed mostly by darker color lava flows. These darker volcanic rocks have higher iron content then this rhyolite tuff.
It would be really good if you had some sort of hand lens or strong magnifying glass. Break a small piece of this light colored rock. Look at it with you lens. You will see a few dark minerals, possibly pyroxenes or hornblende. Also note the clear mineral grains. Many a geology student is fooled thinking these clear minerals are quartz. I was. Typically quartz either forms hexagonal “rods” or if cooled more quickly more shapeless “blobs.” However, you will notice this mineral has a regular angular structure. The mineral in this rock is a form of feldspar called sanadine, which in this rock is “water-clear.”
In accordance with Earthcahcing guidelines, to log you listing please send you answers to the cache owner:
Looking at the sanadine (clear) crystals with a hand lens, why type of 3D structure does it have? The angles do you observe between the crystal faces? This will tell you what the angle between crystal faces for all types of feldspar minerals.
Look at the rock beds above and below this "welded rhyolite tuff", what color to the rocks bounding it have? Based on the bulk color of basalt beds bounding the rhyolite tuff, the color of the tuff itself, and the discussion above, which do you think has the lower iron content?
Congrats to the team of Gealabhan and Scpet on FTF.
This is one of my first two Earthcaches I ever submitted. Hope you like it.
(No hints available.)
Silica Rich Welded Volcanic Ash
Last Updated: on 2/2/2013 6:48:33 PM (UTC-08:00) Pacific Time (US & Canada) (2:48 AM GMT)
Coordinates are in the WGS84 datum