Rhyolite and Rhyolite Tuff EarthCache
Rhyolite and Rhyolite Tuff
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This is the first of two earthcaches I have placed along the Oat Hill Mine Trail. It will take you about 1.5 miles uphill to an area where the trail has cut into the hill exposing rocks characteristic of the volcanic activity which has occurred in this area. Beware of the considerable poison oak close to the edge of the trail and watch out for the mountain bikers who also use this trail. There is parking near the trail head at the parking coordinates.
When I placed this cache in late April following a wet winter and spring, the hills were green and teemed with flowers and the temperature was mild. In the summer it will be HOT. So be sure to carry plenty of water.
The trail passes through both public and private land on an easement allowing passage along the trail. Please remain on the trail.
The Oat Hill Mine Road was constructed in the late 1800s as a road connecting the cinnabar (mercury) mines well to the north with Calistoga. The mines operated more or less continuously until the late 1960s when they became unprofitable. Today the rough track is managed as a trail by the Napa County Regional Park and Open Space District.
You are in a volcanic field which is about 60 miles long by 20 miles wide and known as the Sonoma Volcanics. It is one of a complex series of lava flows which stretch along the California Coast Range. Its age is estimated at around 3 million years. Distinct volcanoes didn't form here. But rather eruptions occurred along northwest-trending faults zones. The initial eruptions were explosive in nature, but eventually gave way to more passive andesite flows. You will encounter this rock on the lower portions of the trail. It is highly weathered and covered with blue-gray iron and manganese oxide stains. Freshly broken surfaces are dark gray and fine-grained. This rock was quarried for use as a road base in the Calistoga area.
As the trail rises you will pass out of the andesite layer into layers of tan-colored rhyolite and, at the cache site, rhyolite tuff. Unlike andesite, rhyolite is rich in silica. This makes the resulting lava pasty, rather like peanut butter. Dissolved volcanic gases such as water vapor and hydrogen sulfide cannot escape easily, so eruptions of this material tend to be explosive. These explosions create vast quantities of volcanic debris which consolidate into layers known as tuff. Unlike the dark gray andesite and tan rhyolite which are rather homogenous, rhyolite tuff looks like lots of broken rock cemented together.
LOGGING THE CACHE
Observe the rhyolite tuff and E-MAIL me the answers to the following questions. DO NOT POST YOUR ANSWERS.
What is the approximate size of the rock fragments in the tuff?
Are the fragments sharp-edged or smooth and rounded? Why?
From your GPS, what is the elevation at the cache site?
If you proceed a short distance up the trail, you will cross a layer of rhyolite. Note how different this layer is from the rhyolite tuff. You will also have some fine views of the Napa Valley along this section of the trail.
Continuing up the trail an additional 2 miles will bring you to my second earthcache titled Volcanic Mud Flows. You will climb a total of 1600 ft from the trailhead along some very rough trail to reach the final cache.
REFERENCE
Trailside Geology of the Oat Hill Road, Calistoga, Napa County, California by Dean A. Enderlin, March 1995; available at Napaoutdoors.org.
Additional Hints
(No hints available.)