Glossary

Here I’m building short pages for different rock types and terranes found in the SF Bay Area. The goal is to have short pages that are optimized for viewing on a cell phone.

Geologic provinces

Some of the major geologic areas in Northern California include:

• Coastal Range
  • Franciscan complex
• Great Valley Group
• Sierran batholith

Terranes

A terrane consists of a coherent suite of rocks that originated together in a single locale, bear no geological affinity to surrounding formations, and are delineated from adjacent rock units by fault boundaries. In the SF Bay Area, these were mostly created by accretion of suites of sedimentary rocks formed under the Pacific Ocean. As the Farallon Plate subducted under the North American Plate, these suites piled up and formed the terranes.^{1 or 2}

The following terranes make up much of the SF Bay Area:

• Alcatraz
• Marin Headlands
• Bolinas Ridge
• San Bruno Mountain

Artistic imagery

The diagram below shows an approximation of a cross-section of the collision between the Farallon Plate and the North American Plate, showing the creation of the Coast Ranges as ocean sediments are accreted onto the North American Plate. Portions of each terrane are subducted to various degrees and the heat and pressure cements the rocks and generates the chemical reactions that defines metamorphosis.^{3 and 4}

Rocks

• Blueschist
• Graywacke
• Pillow Basalt
• Radiolarian chert
• Sandstone
• Serpentine
• Shale

Footnotes