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?

Rock Description

• Color• Texture• Weathering/resistance to erosion

The Rock Cycle

Source: USGS

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

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 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

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

Development of foliation: directed pressure

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

• 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

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?

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