Dynamic Earth Exam 4

Metamorphic Rock

any rock that has been changed from its original condition by heat, pressure, and the chemical activity of fluids

Metamorphism

the phenomenon of changing rocks subjected to heat, pressure, and fluids so that they are in equilibrium with a new set of environmental conditions

Another Important Factor of Metamorphism

time for the agents of metamorphism to chemically alter rocks

Heat and pressure increases…

rate of chemical reactions that may produce minerals different from those in the original rock

Pressure increases with…

depth

Lithostatic Pressure

pressure extended on rocks

Differential Pressure

pressure is not applied equally to all sides of a body (typically when two plates collide)

3 Primary Sources of Chemically Active Fluids

1. water trapped in pore spaces of sedimentary rock as they form

2. volatile fluid within magma

3. dehydration of water-bearing minerals

Example of fluid activity

seawater moving through hot basaltic rock converts olivine into metamorphic mineral serpentine

Contact Metamorphism

metamorphism of country rock by magma, intruding magma raises temps of surrounding rock, causing thermal alteration, release of hot fluids into country rock by cooling intrusion to aid in formation of new minerals

Important Factors of Contact Metamorphism

initial temp, size of intrusion, fluid content of magma, nature of country rock

Aureole

zone of metamorphism surrounding an intrusion, also occurs with lava flows, degree of metamoprhic change decreases with distance from intrusion

Dynamic Metamorphism

occurs in fault zones where rocks are under high degrees of differential pressure

Mylonites

frequently created by dynamic metamorphism, hard, dense, fine-grained rocks, characterized by thin laminations

Regional Metamorphism

occurs over a large area, caused by simultaneous high temps, pressures, and deformation, most obvious at convergent plate boundaries

Gradation of metamorphism is based on…

the severity of conditions

Index Mineral

a mineral that forms within specific temp and pressure ranges during metamorphism

Metamorphic Grade

the degree to which a rock has undergone metamorphic changes

Index Minerals help detirmine…

metamorphic grade

Different rock compositions have…

different sets of index minerals

Levels of Metamorphic Grade

unaltered → low (200°c) → intermediate → high (800°c)

Classifications of Metamorphic Rock

foliated and non-foliated textures

Foliated Texture

minerals aligned in a parallel fashion, platy and elongated minerals, subjected to heat and differential pressure

What determines whether foliation is fine or coarse?

size and shape of mineral grains

Non-Foliated Texture

no discernible preferred orientation of minerals

Slate

low-grade metamorphism, finely foliated texture, minerals can only be seen with magnification, exhibits slaty cleavage, result of regional metamorphism of shale

Phyllite

coarser-grained than slate but similar in composition, glossy or lusterous sheen, needs to be magnified to be seen, intermediate grain size between slate and schist

Schist

result of regional metamorphism, type depends on intensity of metamorphism and character of original rock, minerals are clearly visible, schistose foliation, each type is known by its most conspicuous mineral

Many rock types yield…

schist

Gneiss

high-grade metamorphic rock, segregated bands of light and dark minerals, forms from recrystallization of clay-rich sedimentary rocks during regional metamorphism, or igneous, or metamorphic rock

Light Colored Bands of Gneiss

quartz and feldspar

Dark Colored Bands of Gneiss

biotite and hornblende

Other Foliated Rocks

ampibolite and migmatites

Non-Foliated Metamorphic Rocks

non-foliated texture, mosaic or roughly equidimensional minerals, indication of a granular rock’s metamorphism is the large grain size resulting from recrystallization

Marble

predominantly calcite or dolomite, grains range from fine to coarsely granular, result of contact or regional metamorphism of limestone or dolostone, color varieties come from impurities from parent rock

Quartzite

formed from quartz sandstone, intermediate to high-grade metamorphism, complete recrystallization results in uniform strength, color varieties come from inpurities

Other Nonfoliated Rocks

greenstone, hornsfels, anthracite

Metamorphic Zones

the regions between isogrades

Isogrades

lines of equal metamorphic intensity

Metamorphic rocks are divided into zones based on…

the presence of distinctive silicate mineral assemblages

Successive appearance of index minerals shows…

gradually increasing or decreasing intensity of metamorphism

First appearance of an index mineral indicates…

the location of miminum temperature and pressure conditions

Layers of Metamorphic Zones

nonmetamorphosed rocks → chlorite → biotite → garnet → starolite → silimanite

Metamorphic Facies

a group of metamorphic rocks characterized by particular mineral assemblages formed under the same broad temperature and pressure conditions, named after their most characteristic rock or mineral

The concept of metamorphic facies is not applicable when…

original rocks were pure quartz sandstones or pure limestones or dolostones

A pressure is generated when…

an oceanic plate collides with a continental plate, oceanic plate subducts and heats slowly, metamorphism is caused with increasing pressure with depth, which produces rocks typical of with blueschist facies

As subduction continues…

temps and pressures increase, yields high-grade metamorphic rocks, plate melts and generates magma resulting in contact metamorphism

Divergent Boundaries

contact metamorphism with magma, metal-bearing hydrothermal solutions

Most common metamoprhic rocks are…

marble and slate

Metamorphism and Natural Resources

ore deposits from hot hydrothermal fluids, migrate into surrounding rock

Copper

bornite and chalcopyrite

Lead

galena

Zinc

sphalerite

Iron

hematite, magnetite, pyrite

Asbestos

talc, graphite, garnets, corundum

Earthquake

shaking or trembling of ground caused by sudden release of energy as a result of faulting involving the displacement of rocks along fractures

Aftershocks

series of earthquakes from continued adjustements along a fault, usually smaller than initial quake

Elastic Rebound Theory

rocks undergoing deformation bend and store energy, when strength of rock is exceeded they rupture and release energy as an earthquake, rock reboudn to their original undeformed shape

Seismology

the study of earthquakes

Siesmic Waves

energy from earthquakes

Seismographs

instruments that detect, record, and measure earthquakes

Seismogram

record made by a seismograph

Earthquakes occur along…

faults, where movement is stored as energy in rocks

Most faults are related to…

plate movements

Divergent and Transform Boundaries

always shallow focus

Convergent Boundaries

shallow, intermediate, and deep-focus earthquakes

Benioff-Wadati Zones

dipping seismic zones indicate the angle of plate descent along a convergent plate boundary

Major Earthquake Regions

80% in circum-pacific belt, 15% in mediterranean-asian belt, 5% in plate interiors along ocean spreading ridges

Intraplate Earthquakes

earthquakes that arise from localized stresses caused by the compression that most plates experience along their margins

Earthquakes result from…

convergence along plate margins

Seismic Waves

all waves generated by an earthquake - body waves (p + s) and surface waves (r + l)

Body Waves Travel…

faster through less dense rocks with greater elasticity

P-Waves

primary, fastest seismic wave, travels through solids, liquids, gases, push-pull, expand and compress material, like sound waves

S-Waves

secondary, slower than p-waves, travel only through solids, shear waves, move material perpendicular to the direction of wave movement, create shear stresses

Surface Waves

travel at or just below the surface, slower than body waves, rayleigh and love waves,

Rayleigh Waves

r-waves, particles move in eliptical path like water waves

Love Waves

L-waves, faster than R-waves, particles move back and forth in a horizontal plane perpendicular to the direction of wave travel

Locating an Earthquake

p and s wave average speeds are known, epicenter can be detirmined when the p-s time intervals of at least 3 seismic stations are known

Time-Distance Graphs

difference in the arrival time of the two waves as a function of the distance between a seismograph and the earthquake’s focus

P-S Time Interval

time difference between the arrival of p and s waves, the further the waves travel, the greater the p-s time interval

Earthquake Intensity

subjective or qualitative measure of earthquake damage and human reaction

Modified Mercalli Intensity Scale

ranges from 1-12, maps with intensity zones, used by insurance companies to classify earthquakes

Factors That Affect Earthquake Intensity

size, distance from epicenter, focal depth, population density, geology of area, building construction, duration of shaking

Quantitative Measure

amount of energy released

Richter Magnitude Scale

total amount of energy released by an earthquake at its source, measures amplitude of largest wave

Logarithmic Scale

each whole number increase is a 10-fold increase in amplitude but a 30-fold increase in energy

Richter Magnitude scale underestimates the…

energy of large quakes, only measures highest peak energy not complete duration

Siesmic Moment Magnitude Scale

strength of rocks, area of fault rupture, amount of movement of rocks adjacent to fault

Earthquake Destruction

death, injury, property damage

Destructive Effects

ground shaking, fire, seismic sea waves, landslides, disruption of services, panic and phsycological shock

Ground Shaking

depends on magnitude and distnace from epicenter, poorly consolidated and water-saturated materials experience stronger s waves, poor building materials, most common cause of injuries and fatalities

Liquefaction

water-saturated sediments behave as fluids

Fire

most common in urban areas, 1906 san francisco earthquake, 1923 tokyo earthquake

Tsunamis are caused by…

submarine earthquakes, volcanoes, and landslides, can travel across entire oceans

Tsunamis are…

seismic sea waves not tidal waves

Tsunami Facts

travel at hundreds of km per hour, wave height less than 1 m in open ocean, wave length of several hundred miles, in shallow water, waves slow down and get taller

Prior to a Tsunami

sea withdrawls, exposing the seafloor

Pacific Tsunami Early Warning System

seismographs, instruments detect seismic sea waves, there is no warning system in the indian ocean

Ground Failure

earthquake triggered landslides, very dangerous in moutain regions, causes many deaths and damage

Successful Earthquake Prediction Must Include…

time frame, location, strength, successful predictions are rare, seismic risk maps and paleoseismology help