crustal deformation and mountain building

Question 1

orogenesis

Answer 1

mountain building
mountains are born and have a finite life span

Question 2

young vs middle-aged vs old mountains

Answer 2

young: high, steep, still growing upward
middle-aged: lowered by erosion
old: deeply eroded remnants

Question 3

deformed vs undeformed

Answer 3

undeformed: horizontal beds with no folds or faults
deformed: tilted beds with folding and faulting

Question 4

what are the three types of deformation?

Answer 4

displacement: change in location by faulting
rotation: change in orientation
distortion: change in shape

Question 5

brittle deformation

Answer 5

rocks break by fracturing
occurs in the shallow crust

Question 6

ductile deformation

Answer 6

rocks deform by flowing and faulding
occurs at higher P and T in deeper crust
10-15 km depth

Question 7

strike and dip

Answer 7

strike: horizontal intersection with tilted surface
dip: angle of surface down from the horizontal

Question 8

joints

Answer 8

planar rock fractures without any offset
develop from tensile tectonic stress in brittle rock
occur in parallel sets
control weathering of rock they occur in

Question 9

veins

Answer 9

Veins are joints filled with dissolved minerals brought in by groundwater

Question 10

faults

Answer 10

planar fractures showing displacement
abundant in crust and occur at all scales
sudden movements along faults cause earthquakes
active or inactive

Question 11

what are the different types of motion of faults?

Answer 11

dip slip: blocks move parallel to dip of the fault
strike slip: blocks move parallel to fault plane strike
oblique slip: dip slip and strike slip

Question 12

normal faults

Answer 12

hanging wall moves DOWN relative to footwall
accomodate crustal extension (pulling apart)

Question 13

reverse and thrust faults

Answer 13

hanging wall moves UP relative to footwall
reverse: steeper than 35 degrees
thrust: less than 35 degrees
accomodate crustal shortening (compression)

Question 14

strike-slip faults

Answer 14

fault motion parallel to the strike of the fault
usually vertical
right lateral and left lateral
large ones may slice entire crust

Question 15

SAN ANDREAS FAULT

Question 16

fold geometry

Answer 16

anticline: arch
syncline: opens upward like a trough
monocline: fold like carpet draped over stair step (these do not cut through to the surface)

Question 17

folds are described by geometry of the hinge

Answer 17

plunging: hinge that is titled
non plunging: horizontal hinge

Question 18

circular folds

Answer 18

dome: overturned bowl
basin: upright bowl
dome: older rocks in center
basin: younger rocks in center

Question 19

flexural slip

Answer 19

layers slide past one another
think of bent deck of cards

Question 20

passive flow

Answer 20

folds form in hot soft ductile rock at high T

Question 21

forming folds

Answer 21

horizontal compression causes rocks to buckle. shear causes rocks to fold over on themselves
when layers move over step-shaped faults, they fold
deep faults cause monocline

Question 22

tectonic foliation

Answer 22

foliation develops via compressional deformation
flattening develops perpendicular to shortening strain
sand grains flatten and elongate, clays reorient
foliation parallel to axial planes of folds

Question 23

subduction

Answer 23

convergent boundaries create mountains
compression shortens and uplifts overriding plate
fold-thrust belt develops landward of the orogen
detachment at depth

Question 24

what happens to island fragments of continental crust at subduction margins?

Answer 24

too buoyant to subduct, sutured onto upper plate

Question 25

how does subduction lead to mountain building?

Answer 25

• Subduction where oceanic crust goes under
• Causes continental crusts to move closer together
• continental crusts collide

Question 26

crustal thickening

Answer 26

results from continental collisions
fold-thrust belts on margins of the orogen
center of belt consists of high-grade metamorphic rocks
thrusting brings metamorphic rocks up to shallow depths

Question 27

how does continental rifting create mountains?

Answer 27

As the plate stretches and thins, the underlying asthenosphere flows upward and expands like a hot-air balloon, lifting the region to higher elevations. The continental crust breaks along faults, forming long mountain ranges separated by rift valleys

Question 28

where are different types of rocks created near orogenous zones?

Answer 28

igneous: collisions and rift zones
sedimentary: basins and erosion of uplifted rocks
metamorphism: continental collisiosn

Question 29

isostasy

Answer 29

balance between forces
gravity pulls plates into mantle
buoyancy floats lithosphere on top of mantle
adding/removing weight resets isostatic equilibrium
change in lithosphere thickness or density also alters

Question 30

what does covergent margin horizontal compression do?

Answer 30

horizontal shortening
vertical thickening
double crust thickness
thick crustal root beneath mountain ranges

Question 32

how does lithosphere make mountains high?

Answer 32

removal of lithospheric mantle can cause uplift
thinning and heating lithosphere during rifting creates elevated MOR mountains

Question 33

why are himalayas at maximum height possible?

Answer 33

upper limit to mountain heights due to rock strength
orogenic collapse: deep hot rocks forced outward, mountains collapse downward

Question 34

craton

Answer 34

crust that hasn’t deformed in 1 Ga
cool, strong and stable crust
shield: ancient rocks exposed
platform: ancient rocks covered by younger rock