Geology 101 Quiz 8 Flashcards
metamorphic rocks definition
rocks that have been altered in composition or texture by heat or pressure
high-grade rocks form at
high temperature and/or pressure
low-grade rocks form at
lower temperature and/or pressure
if the temperature rises still higher,
the rock melts
regional metamorphism
occurs over a very large area
regional metamorphism is typically associated with
convergent plate boundaries
examples of regional metamorphism
subduction zones (high temp from friction) and continental collisions (high pressures from colliding plates)
contact metamorphism
high temperature metamorphism where magma heats up but does not melt adjacent rock
aureole
the halo of altered rock around the igneous intrusion formed by metamorphism
right next to the igneous intrusion will typically be
high grade metamorphic rock
rock farther away from the igneous intrusion that heats up will be
lower grade
cataclastic metamorphism
crushing and smearing of rock along fault zones
examples of cataclastic metamorphism
subduction zones, transform fault boundaries
hydrothermal metamorphism
metamorphism caused by interaction with hot water (usually involves exchange of some ions)
examples of hydrothermal metamorphism
mid-ocean ridges (Mid-Atlantic Ridge, East Pacific Rise)
what happens during hydrothermal metamorphism?
water in the pores in the ridge gets heated, the hot water thermally alters the rock and dissolves some of the minerals, the heated water is less dense and rises
when the hot water hits the cooler ocean water, some of the dissolved minerals precipitate (creates “black smokers”), cooler seawater enters from the sides to replace the rising hot water, continuing the cycle
burial metamorphism
heat and pressure caused by deep burial
example of burial metamorphism
sedimentary basins
what do sedimentary basins do?
collect eroded sediment from nearby mountains - as the sediment layers accumulate, the weight causes the crust to sink deeper in the mantle; over time, deposits can become buried under thousands of feet of sediment
the earth gets ? with depth by how much
hotter; about 30*C per km
shock metamorphism
caused by large meteorite impact
example of shock metamorphism
Meteor Crater, Arizona
at low temperature and high pressure, expect
cataclastic and regional metamorphism
at localized high temperature and pressure, expect
impact metamorphism
at intermediate temperature and pressure, expect
burial metamorphism
at high temperature and low pressure, expect
contact and hydrothermal metamorphism
metamorphism can produce both ? and ? alterations
mineral and textural
mineral alteration is achieved by
changing either the arrangement of atoms, or swapping some atoms for others
structural alteration
the chemical formula is the same, but the arrangement of atoms changes (usually more compact arrangement due to high pressure)
graphite turned to diamond is an example of
structural alteration (both are pure C, but the C atoms in diamond are much more closely arranged)
element substitution
one atom is exchanged for another resulting in a new mineral
exchange an Fe atom for a Mg atom is an example of
element substitution
textural alteration is caused by
either recrystallizing or by stretching or smearing
recrystallize
under high temperature and pressure, atoms within minerals can migrate to make larger crystals, or crystals oriented due to high differential pressures (i.e. more pressure in one direction)
when the pressure is the same in all directions, crystals will have a(n) ? alignment
random
if the pressure is greater in one direction (like from two plates colliding), crystals tend to align ? to the greater force
perpendicular
if differential pressure results in movement, minerals may be
smeared, stretched, or folded
alteration, both mineral and textural, can cause ?
foliation
foliation
flat or wavy lines created by oriented mineral grains
foliated rocks are metamorphic rocks with
oriented mineral grains
low grade foliated rocks will often exhibit
cleavage
cleavage
tendency to break along parallel planes (into flat slabs)
fine grained rocks and coarse grained rocks - which way does not work?
fine grained CAN become coarse grained, but coarse grained CANNOT become fine grained
non-foliated rocks
no oriented minerals; often products of burial, contact, and hydrothermal metamorphism (uniform pressure in all directions)
examples of foliated rocks
slate, phyllite, schist, gneiss
examples of non-foliated rocks
quartzite, marble, greenstone
quartz rich sandstone with sand grains fused together
quartzite
metamorphosed limestone
marble
basalt plus hot seawater (hydrothermal alteration)
greenstone
greenstone sometimes looks green, but sometimes it doesn’t - the green color comes from
chlorite
porphryoblasts
large crystals developed in a fine grained matrix
example of porphryoblast
garnet schist
deformational texture
stretched rocks in which the original texture is still recognizable
example of rock with deformational texture
stretched pebble conglomerate
folding (aka ?)
bending without breaking (aka plastic deformation)
faulting
breaking and moving rock on at least one side of the rupture
a break without any movement is referred to as a
joint
folding or faulting happen when
forces acting on the rock are greater in one direction than another, and the primary force exceeds the rock’s ability to resist deformation
brittle
breaks rather than bend (like a stick of chalk)
ductile
bends rather than break (like a copper sheet)
confining pressure means
the pressure is high in all directions - if a rock is put in a vertical press, the slightest crack formed in a mineral will tend to propagate and the rock will abruptly break
rock is more likely to fold under high ? pressure
confining
raising the temperature will make a rock
softer and more easily folded
strain rate: if rock movement is initiated rapidly, ? is likely; if the rock is allowed to move slowly, ? is more likely
faulting; folding
three types of stress
compression, tension, shear
compression
forces push toward the middle ->
tension
forces pull apart
shear
forces push one side parallel but opposite the other ^ v
compression: starting with horizontal layers, compression results in ?
shortening by folding
compression is common at
convergent boundaries
tensional forces pull apart, resulting in
stretching and thinning (like plastic stretched beyond its elastic capacity)
tension is common at
divergent boundaries
shear: lateral forces ?
stretch and smear
shear is common at
transform fault boundaries
limbs
the layers pointing upward or downward from the bottom or top of a fold
syncline
bottom of a fold (limbs pointing upward) - memory tip: we fall into sin
anticline
top of a fold (limbs pointing downward)
the youngest layers are exposed in the center of
eroded synclines (or in the center of a basin/outside of a dome)
the oldest layers are exposed at the center of
an anticline (or in the center of a dome/outside of a basin)
compressional forces can also cause rock layers to form ? instead of simple folds
domes and basins
dome
limbs point in all directions (analogous to anticline)
basin
limbs point upward in all directions (analagous to syncline)
footwall vs hanging wall
hanging wall: could hang a lantern from - on the outside
footwall: could stand on (or at least put your foot on) - closer to the break
reverse fault
hanging wall moves upward relative to footwall
if the rising block ends up entirely on top of the underlying block, a reverse fault is called a
thrust fault
normal fault
hanging wall moves downward relative to footwall
normal fault is caused by
tension
reverse fault and thrust fault are caused by
compression
strike slip fault is caused by
strike slip
forced apart
normal fault
forced to overlap
reverse fault
how can the knowledge of faulting help us deduce tectonic history
the type of faulting can tell us if a plate was compressed, stretched, or split laterally
how can the knowledge of faulting help us map hazards
earthquake awareness - where there are faults, there have been earthquakes
a normal or reverse fault underwater can generate tsunamis
how can the knowledge of faulting help us drill for oil
thrusted fault block doesn’t mean there’s no more oil - you can find sedimentary rocks under that igneous rock
