Geo Quiz 5: metamorphic rocks, rock deformation, earthquakes

metamorphic rocks, rock deformation, earthquakes

The three metamorphic processes that change the minerals in rocks during metamorphism

recrystallization, neocrystallization, metasomatism

The minerals stay the same but the crystals grow larger and fuse together

recrystallization

Hot fluid (usually water) flows through the rock, removing some material and bringing new material in, thus changing the composition of the rock

metasomatism

old minerals become unstable under the heat and pressure and break down, and the atoms in the rock rearrange into new minerals, but the composition of the rock stays the same

neocrystallization

 

layering in a rock that formed through metamorphic processes

Foliation

What does foliation tell us about the grade of metamorphism?

more pronounced foliation - flakier rocks: higher grade

less pronounced foliation - blockier rocks: lowergrade

What can you look for in a metamorphic rock to determine the grade of metamorphism?

foliation,  

size of the mineral crystals 

what minerals are in the rock

burial metamorphism occurs

passive margin basins, metamorphosed by weight of material above.

contact metamorphism occurs

subduction zones, divergent plate boundaries, mantle plums; rock is a good insulator, so metamorphism only occurs very close to the magma, metamorphic shell forms around the magma. more foliation larger crystals and more new minerals closer to magma

regional metamorphism occurs

at subduction zones and continental collisions; rocks metamorphosed in mountain ranges that form at subjection zones/continental collisions. rocks at the core of the mountain rage exposed to the most heat and pressure, rocks in the foothills exposed to least, thus wide range

What is a protolith?

 

what the rock was before any metamorphism occured

protolith for quartzite

quartz sandstone

protolith for slate

mudstone

protolith for marble

limestone

protolith for schist

mudstone

protolith for gneiss

mudstone or granite

protolith for greenschist/amphibolite/granulite

mafic rock

protolith for metaconglomerate

conglomerate

protolith for phylite

mudstone

protolith for serpentinite

peridotite (mantle rock)

which process changes sandstone to quartzite?

recrystallization

Neocrystallization

minerals break down under heat pressure. the elements must end up somewhere, so they recombine to form new minerals that didn’t exist before, such as mica and garnet(in the case of mudstone). The “neo” in “neocrystallization” refers to new minerals. Although new minerals form, the composition of the rock does not change with neocrystallization.

metasomatism

hydrothemral fluid(hot water) takes carries and deposits chemical elements, changing the chemical composition of the rock. Since the composition changes, new minerals must grow. Hydrothermal fluids can accelerate other processes.

skarn

silica + carbonate

actual changes to in the minerals in each process?

Recrystallization

 – minerals don’t change but the crystals grow in size

Neocrystallization

 – new minerals form but the rock composition doesn’t change

Metasomatism

 – the composition of the rock changes as hot fluids bring new material in and take old material out

name all the processes of foliated rocks in slide

mudstone or Shale -> Slate -> Phyllite -> Schist -> Gneiss

shale

sub microscopic clays bedding plane cleavage

slate

Slate is low-grade so the foliation is hard to see and the mica crystals are microscopic, but the rock breaks into sheets.

phyllite

in medium grade phyllite, the mica are still mostly microscopic but all the shiny cleavage planes give the rock itself a sheen. The rock is starting to look flaky and the foliation is visible

schist

in high-grade schist, the mica crystals are visible, the rock is very flaky and has well-developed foliation. Neocrystallization continues as the mineral garnet starts to grow. Thus, the presence of garnet in a rock is a clue that it is high-grade.

gneiss

At very high levels of metamorphism, felsic and mafic minerals separate out into distinct layers, called gneissic banding.

Migmatite

At very high heat, felsic minerals start to melt and drain out of the rock, freezing later in separate layers. This rock is called migmatite and is right on the boundary between metamorphic and igneous.

Mylonite

forms from shear stress (smeared out between two pieces of moving rock)

nonfoliated rocks

nonfoliated rocks form under heat/no pressure, or when made of minerals that form equant (chunky) grains. Recrystallization since no new crystals formed.

Which of the following are aspects of stick-slip behavior on faults?

 

• faults are irregular and thus have high friction 

• the rocks along a fault get stuck due to friction as the plate moves 

• stress builds up over time as rocks stretch 

• eventually the built up stress overcomes friction and the rocks move suddenly to "catch up" with the rest of the plate

seimic waves and properties

P-wave: fastest and weakest

S-wave:med speed, med strength

Surface waves (l&S) - slowest and strongest

Which type of seismic wave is most damaging, and why?

 Surface waves (L&S), because they are the strongest (highest amplitude ie greatest ground movement) and they move the ground horizontally, which buildings are not built to withstand.

How is the strength of an earthquake determined?

Comparing the amplitude of the waves to the distance from the epicenter

How is the epicenter of an earthquake located?

Comparing the arrival time of P- and S-waves at three different locations

Which tectonic settings can produce earthquakes? Which produce the strongest earthquakes?

 

all plate boundaries; subduction zones

strike-slips at which boundaries?

transform / transpressional range

normal faults at which boundaries?

divergent boundaries/ rift valleys

thrust faults at which boundaries?

subduction zones and continental collisions/ foreland basins

stick-slip motion

• fault is irregular and thus has high friction

• rocks along the fault get stuck on each other as the underlying plates move

• straing builds up over time as rocks stretch until friction is overcome

• fault slips suddenly and all the stored energy is released at once, rocks move to relieve stress “catching up” with the rest of the plate

what affects the severity of and damage from ground shaking? (slide)

• earthquake strength (magnitude?)

• distance from epicenter

• ground composition (loose sediment in valleys vs solid rock in mountains)

• depth of earthquake

• quality of construction

What affects the intensity and damage from ground shaking during an earthquake? (quiz)

• distance to the fault or epicenter

• ground composition

• depth of earthquake 

• quality of building and infrastructure construction

Where does ground displacement occur during an earthquake?

 

along the fault

What are the conditions necessary for soil liquefaction to occur?

•  ground shaking

•  ground water near the surface/wet ground

•  ground made of loose material

Which of the following are true for tsunami?

 

• tsunami form due to vertical displacement of crust underwater during an earthquake

• the best protection against tsunami is to immediately run to high ground after an earthquake on the coast

• tsunami behave like the ocean running rapidly onto the land

• sea withdrawal is a dangerous sign that a tsunami has been produced

Which of the following are secondary disasters caused by earthquakes?

• ruptured gas lines

• broken transport lines 

• damage to health care facilities 

• disruption of water supplies 

• loss of power and communication lines

ground displacement

• ground moves along fault during earthquake

• This thrust fault pushed the hanging wall over the footwall, disrupting the surfare

• Normal faults dropped the hanging wall down off the footwall, disrupting the surface

• The ground moved horizontally along strike-slips faults

earthquake prediction map

• based on past earthquakes

• stick-slip behavior tells us that the most dangerous faults are where no earthquakes have been in a long time - ie stress build up in areas marked with lowest seismeic hazard

landslides

• shaking triggers landslides on weak rock layers

Tsunami aspects (slides)

• vertical displacement of crust underwater causes waves

• wave intensifies when it hits shallow water

• they have up and down phases and strike multiple times like ripples in a pond

• sea withdrawal - down phase of tsunami, watch out

In general terms, deformation refers to

 

a change in shape, size, or position of a material

If a material deforms by breaking, this is called ? deformation.

If a material deforms without breaking, this is called ? deformation.

brittle, ductile

What affects whether a material deforms in a brittle or ductile manner?

temperature 

composition 

rate of deformation 

pressure

rocks will move toward and grow in the direction of

minimum stress

rocks will move away from and shrink in the direction of

maximum stress

Joints, fractures, and veins open up in the direction of ?? stress but don't tell you the direction of ?? stress.

minimum, maximum

how to identify parts of fault

Imagine a person standing so the fault passes through them. Their feet are on the footwall, the other side is the hanging wall

normal faults characteristics

hanging wall moves down relative to footwall

associated with tensional stress

found at divergent plate boundaries

strike-slip fault characteristics

found at transform plate boundaries

associated with shear stress

motion on the fault is horizontal

reverse and thrust faults

found at convergent plate boundaries

form from compression

hanging wall moves up and over footwall

Which of the following are features found at fault zones that can help identify faults?

 

fault breccia/fault gouge

slickensides

scarps

Foliation forms __________ to the maximum stress. Cleavage forms __________ to the maximum stress.

 

perpendicular; perpendicular

Folds are always produced by __________ stress.

compressional

anticline characteristics

oldest rocks are at the axis

"up" fold

rock layers come up out of the ground toward the axis

layers on the limb are lower than layers at the axis

syncline

layers on the limb are higher than layers at the axis

"down" fold

youngest rocks are at the axis

rock layers come up out of the ground away from the axis

monocline characteristics

no symmetry across the axis

older rocks on one side of the axis, younger rocks on the other

rock layers are horizontal on either side of the axis but highly tilted at the axis

features both associated with convergent plate boundaries. Fold and thrust belts at ocean-continent subduction zones are so called because they are made up of crust that has been _____? -

folded and broken by thrust faults

stress vs strain vs distortion

stress= pressure on the rock

strain = amount of deformation

distortion - the size an shape of the rock body is changed

vein

a joint that has been filled by growing minerals crystals. joints will always open in the direction of minimum stress. Can’t tell maximum.

Faults vs Joints

faults are where displacement occurs

strike of fault

is the line along which it intersects the surface

the clift left when the hanging wall moves either up or down is called

fault scarp

the extreme friction between the two sides of the fault polishes the fault surface and leaves long grooves. is called? Similar to striations left be glaciers so be careful

slickenslide

the friction along a fault can also shatter the rocks caught right along the fault. This creates a zone of fractured, powered rock called

fault breccia or fault gouge

metamorphic feature that indicates strain

foliation, remember foliation is perpendicular to the maximum stress. cleavage is a type of foliation

axis and limb

axis - plane or line running along the center or apex of fold

limb- sides of the fold

symmetrical, asymmetrical, and overturned folds

sym - axial plane is vertical

asym - beds in one limb dip more steeply that those in others

over- both limbs dip in the same direction but one limb has been tilted beyond vertical

basin and dome

these are basically just anticlines and synclines but with four limbs and a round shape instead of two limbs and linear.

dome - round anticline

basin - round syncline

thin skinned deformation

forms anticlines and synclines. upper sedimentary layers of the crust are thrusted and folded by compressive forces. a mountain range made of folds and thrusts - this is how a fold-thrust belt at o-c subduction zone forms

thick skinned deformation

forms monoclines, folds develop as upper sedimentary layers drape over deep crustal faults

rock beds slides past each other, bed thickness remains constant

flexural slip, slip on the bedding planes in the limbs

more ductile beds flow into the hinge as more brittle beds separate, ductile beds change thickness

flexural flow