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Rock and the Rock Cycle
California Rock StoriesLinking tectonics to rock formation
Ellen Metzger
BAESI – November 19, 2011
Rocks: Aggregates of Minerals
http://www.usoe.k12.ut.us/CURR/Science/core/8thgrd/sciber8/geology/images/GRANITE.jpg
Beyond ID:
Every Rock Tells a Story
How do geologists describe rocks?
How can you tell if it’s igneous, metamorphic, or sedimentary?
http://stone-network.com/images/classification.gif
Igneous Rocks
Gneiss
Granite(USGS)
Andesite with porphyritic texture
Igneous Rocks
Form as magma or lava cools and crystallizesTwo Types:
1) 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
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).
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
Sedimentary rocks
Economic importance • Coal• Petroleum and natural gas • Sources of iron and aluminum
Sedimentary rocks: Two groups Detrital (clastic) rocks
Material is solid particlesClassified by particle size
Shale (most abundant) SandstoneConglomerate
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: SiO2Limestone – made of the mineral calcite
(CaCO3)
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
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.
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
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?
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
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
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
California Tectonics: Past
Rocks of the Franciscan ComplexAn accretionary wedgeForms mélange
http://www.nps.gov/prsf/naturescience/images/Subduction-animation_1.gif
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?