Rock and the Rock Cycle California Rock Stories Linking 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?
Rock Description Color Texture Weathering/resistance to erosion
The Rock Cycle Source: USGS
Igneous Rocks http://stone-network.com/images/classification.gif Gneiss Andesite with porphyritic texture Granite (USGS)
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
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 http://stone-network.com/images/classification.gif Sandstone (USGS) Chert (NPS)
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 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)
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 http://stone-network.com/images/classification.gif Gneiss Marble
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 group • Chrysotile 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 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
Bay Area Rocks • Do your students bring you grungy, fine-grained, black and green rocks?
Bay Area Rocks • Young sedimentary and volcanic rocks • Mesozoic rocks • Franciscan Complex • Coast Range Ophiolite • Great Valley Group • Salinian basement
California Tectonics: Present Source: USGS
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
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 lavas Greenstone (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 • 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?
Formation of Blueschist in a Subduction Zone http://openlearn.open.ac.uk/file.php/3512/SXR260_1_007i.jpg Unique conditions: High pressure combined with low temperature Note depressed isotherms. Due to slow heating of cold, down-going oceanic plate