Text Box: Newsletter of the Physical Sciences at Solano Community College

Metamorphic Rocks of Ring Mountain, Tiburon

At only 602 feet in elevation, Ring Mountain is hardly a mountain. But it is well known to geologist and biologist alike because of its unusual geology and the unique plants that grow in the serpentine soils on the Tiburon peninsula.

Serpentine is a metamorphic rock: a rock changed by heat and pressure. The high concentration of magnesium and calcium in the soils is usually toxic to plants, but some rare plants thrive [see: http://www.marin.cc.ca.us/~jim/ring/rplant.html].

In Spring 05, Geology Honors student Warren Gray collected and produced thin-sections of a variety of metamorphic rocks.

The metamorphic rocks in this area were produced by a collision of two tectonic plates: an ancient oceanic plate subducting beneath the North American continent. Such a process produced many types of metamorphic rocks, depending on the type of parent rock as well as the varying degrees of temperature and pressure.

One unusual rock that was found at Ring Mountain was the blueschist. As the name implies, the coloring of the rock is blue due to a rare type of blue mineral called glaucophane.

Glaucophane is only produced at subduction zones under very special conditions of low temperature and high pressure. The oceanic plate subducts so quickly that the temperature of the rocks remains fairly low while the depth (pressure) increases substantially.

Other types of metamorphic rocks that were collected and viewed under the microscope where: serpentinite, amphibolite, and metasediments each corresponding to different temperature and pressure regimes.

Hand samples and thin-sections were taken to UC Davis for detailed analysis with Prof. Howard Day, a specialist in igneous and metamorphic petrology. Using his digital camera, Warren took several photomicrographs of the samples collected and we studied in detail the processes involved in creating each of the rocks.

Warren is finishing his AA degree Spring 06 and will take a year off before continuing with his education.

-Article and Photos

by Mark Feighner