California Rock StoriesLinking tectonics to rock formation

Ellen Metzger

BAESI – October 8, 2011

California’s Rocks

• California, including the Bay Area, has a greater variety of rocks than do other regions of the United States.

• This reflects the state’s complex tectonic/geologic history.

Mineral Hazards

• Serpentinite California state rock Metamorphosed ultramafic rocks from the

mantle (can you find these rocks on your California geologic map?)

There are several minerals in the serpentine groupChrysotile is asbestiform

California’s Mineral Resources

Mineral Hazards: Mercury in the Environment

• Mercury is a neurotoxin – principal source for humans: consumption of mercury-contaminated fish

• Sources of mercury in the environmentNatural sources:

Volcanoes, hot springs, and natural mercury deposits (the mineral cinnabar (HgS)is an ore of mercury

Sources related to human activities: coal combustion, incineration of waste, industrial activities, mining

Source: California Geological Survey

Source: BLM

Bay Area Rocks

• Young sedimentary and volcanic rocks

• Mesozoic rocks Franciscan Complex Coast Range Ophiolite Great Valley Group

• Salinian basement

Bay Area Rocks

• Do your students bring you grungy, fine-grained, black and green rocks?

California Tectonics: Present

Source: USGS

Cross section of western North America at about 100 million years ago/www.nps.gov/prsf/naturescience/images/Subduction-animation_1.gif

California tectonics in the past: a subduction zone

The Rock Cycle

Source: USGS

Rock Description

• Color• Texture• Weathering/resistance to erosion

Unique Bay Area Rocks

• Igneous Sea floor basalt

Pillow lavasGreenstone (altered basalt)

• Sedimentary Graywacke (“dirty” sandstone) Radiolarian Chert

Radiolarians: Tiny ocean animals that make their skeletons of silica (SiO2) http://www.mdia.org/images/Radiolaria.jpg

Unique Bay Area Rocks

• MetamorphicGlaucophane schist (“blueschist”) formed

under high P-low T in a subduction zone.Serpentinite - hydrated mantle rocks

Mantle rock = ultramafic (Si02-poor), dense, dark Serpentinite = rock (CA State Rock) Serpentine = mineral Should serpentinite be “demoted” as our state

rock?

http://openlearn.open.ac.uk/file.php/3512/SXR260_1_007i.jpg

Formation of Blueschist in a Subduction Zone

Unique conditions: High pressure combined with low temperature

Note depressed isotherms.

Due to slow heating of cold, down-going oceanic plate