1. California Rock Stories
Rock and the Rock Cycle
California Rock Stories
Linking tectonics to rock formation
Ellen Metzger
BAESI – November 19, 2011

2. Rocks: Aggregates of Minerals

3. Beyond ID: Every Rock Tells a Story
How do geologists describe rocks?
How can you tell if it’s igneous, metamorphic, or sedimentary?

4. Rock Description
Color
Texture
Weathering/resistance to erosion

5. The Rock Cycle
Source: USGS

6. Igneous Rocks Andesite with porphyritic texture Gneiss Granite (USGS)
Gneiss
Andesite with
porphyritic texture
Granite
(USGS)

7. Igneous Rocks Form as magma or lava cools and crystallizes Two Types:
Rocks formed inside Earth are called plutonic or intrusive rocks
Coarse-grained = slow cooling
2) Rocks formed on the surface
Formed from lava (a material similar to magma, but without gas
Called volcanic or extrusive rocks
Fine-grained = rapid cooling

8. Igneous Rocks
Classification is based on the rock's texture and composition
Light
(felsic)
Medium
(intermediate)
Dark
(mafic)
Fine
rhyolite
andesite
basalt
Coarse
Granite
Diorite
Gabbro
+ Glassy rocks: pumice and obsidian (what glassy texture say
about cooling rate?) 
Basalt is the most abundant extrusive rock (ocean crust).
Granite and related rocks are the most abundant intrusive
rocks (continental crust).

9. Sedimentary Rocks http://stone-network.com/images/classification.gif
Sandstone (USGS)
Chert (NPS)

10. Sedimentary rocks Form from sediment (weathered products)
About 75% of all rock outcrops on the continents
Used to reconstruct much of Earth's history
Clues to past environments
Provide information about sediment transport
Rocks often contain fossils 10

11. Sedimentary rocks Economic importance Coal Petroleum and natural gas
Sources of iron and aluminum 11

12. Sedimentary rocks: Two groups
Detrital (clastic) rocks
Material is solid particles
Classified by particle size
Shale (most abundant)
Sandstone
Conglomerate
Chemical rocks
Derived from material that was once in solution and precipitates to form sediment
Directly precipitated as the result of physical processes, or
through life processes (biochemical origin)
Chert: SiO2
Limestone – made of the mineral calcite (CaCO3) 12

13. Sedimentary rocks Features of sedimentary rocks
Strata, or beds (most characteristic)
Bedding planes separate strata
Fossils
Traces or remains of prehistoric life
Are the most important inclusions
Help determine past environments
Used as time indicators
Used for matching rocks from different places 13

14. Metamorphic Rocks Gneiss Marble
Gneiss
Marble

15. Metamorphic rocks "Changed form" rocks Produced from preexisting
Igneous rocks
Sedimentary rocks
Other metamorphic rocks
Metamorphism
Takes place where preexisting rock is subjected to temperatures and pressures unlike those in which it formed.

16. Metamorphic Rocks Metamorphic agents Heat Pressure (stress)
From burial (confining pressure)
From differential stress during mountain building
Chemically active fluids
Mainly water and other volatiles
Promote recrystallization by enhancing ion migration

17. Development of foliation: directed pressure

18. 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.
What did you collect at the gravel bar?

19. 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 group
Chrysotile is asbestiform

20. Source: California Geological Survey
Mineral Hazards: Mercury in the Environment
Mercury is a neurotoxin – principal source for humans: consumption of mercury-contaminated fish
Sources of mercury in the environment
Natural 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: BLM
Source: California Geological Survey

21. Bay Area Rocks
Do your students bring you grungy, fine-grained, black and green rocks?

22. Bay Area Rocks Young sedimentary and volcanic rocks Mesozoic rocks
Franciscan Complex
Coast Range Ophiolite
Great Valley Group
Salinian basement

23. California Tectonics: Present
Source: USGS

24. California Tectonics: Past
Cartoon of the subduction zone present on the West Coast
100 million years ago showing position of the Franciscan accretionary complex. Source: National Park Service

25. Rocks of the Franciscan Complex An accretionary wedge Forms mélange

26. Unique Bay Area Rocks Igneous Sedimentary Sea floor basalt
Pillow lavas
Greenstone (altered basalt)
Sedimentary
Graywacke (“dirty” sandstone)
Radiolarian Chert
Radiolarians: Tiny ocean animals that make their skeletons of silica (SiO2)

27. Unique Bay Area Rocks Metamorphic
Glaucophane 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?

28. 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