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continents are mobile. Proposed by Alfred Wegener (Pangea). Holmes (earths mantle moved by convection)
continental- granitic crust. lighter (less dense) more buoyant- floats higher.
oceanic- basaltic crust. heavier (more dense) less buoyant-floats lower
1) initial uplift from rising mantle
2) stretching (thinning) and faulting (breaking) form rift
3) melting forms magma
4) can lead to seafloor spreading and new ocean basin
5) ocean widens with spreading
continental crust is uplifted in rift settings
increased heat flow expands and uplifts rocks
rifting- linear fault block mountains and linear basins
old oceanic lithosphere is more dense. plate divides into the mantle (subduction)
recycles oceanic lithosphere. subduction is balanced by sea floor spreading. subducting plate descends.
volcanic belt on an overriding plate. descending plate partially melts, magmas fuel volcanic eruption. volcanic belts are curved.
arc type depends on overriding plate.
continental crust= continental arc
oceanic- island arc
1) oceanic plate subducted beneath continent.
2) overlying mantle melted
3) squeezing form mountain belt
4) volcanoes form
5) trenches form
6) accretionary prisms form
slab releases water
water causes melting of mantle
eruptions from volcanic island arc
trenches and accretionary prisms form
oceanic lithosphere complete subducted- subduction stops
closes the pre-existing ocean basin
brings blocks of continental crust together
continental crust will not subduct
1)subduction of oceanic part of plate
2)subduction brings continents closer
3) continents collide
other forces like convection in mantle
tectonic plates slide sideways. characterized by earthquakes and absence of volcanism.
Lithosphere is not created or destroyed. offset spreading ridge segments. cut through crust.
plate boundaries change over geologic time.
ocean plates- created at MOR/spreading centers. destroyed at subduction zones
continental plates- rifted apart. sutured together.
unpredictable, dangerous. build large mountains, blow it to bits.
hazardous- affect climate, kill people
positives? produce good soil, higher geothermal gradient
the products of volcanic eruption:
lava flow- molten rock on the surface
pyroclastic debris- fragments blown out
volcanic gases- vapor and aerosols exciting a volcano
lava: thin and runny or thick and sticky
flow type depends on viscosity via composition
mafic (basaltic): low silica/ high fe and mg
intermediate (andesitic): moderate silica, fe and mg
felsic (rhyolitic): high silica/ low fe and mg
mafic lava- very hot, low viscosity
thin and fluid. flow rapidly and for long distances
basalt with a jagged, sharp, angular texture
hot flowing basalt cools and thickens
lava crumbles into shards and fragments
a cooled crust forms on top of a basalt flow
a conduit develops- lava tube
tubes prevent cooling, facilitating lava flow
underwater, basalt cools instantly
cools forms a rounded blob-pillow
repeated to form a mound of pillow basalts.
common on mid ocean ridges
solidified flows may contract with cooling.
vertical fractures- hexagonal in cross-section
apple to refrigerator sized
blocks- pre-existing rock
bombs- ejected lava
deposits of pyroclastic debris.
tuff- lithified ash (with or without lapilli)
tephra moved by water
often deadly to people in volcanic valleys. moves rapidly. like wet cement
more viscous- difficult to flow and traps gas
less viscous- flows easier and gas can escape
vent and fissures
craters and calderas
located in the upper crust.
open cavity or area of highly fractured rock.
magma rises via a conduit
lava outlet on a volcano
summit vent- top
flank vent- side
magma erupting along a linear tear
may display a curtain of fire
evolve into a discrete vent
bowl shaped depression atop a volcano.
forms as lava piles up around the vent.
accentuated by summit collapse into conduit
a gigantic volcanic depression
magma chamber empties and volcano collapses into it
conical piles of tephra
the smallest type of volcano.
ejected lapilli-sized fragments piled up at a vent.
symmetrical with a deep summit crater
slopes are at the angle of repose
large cone-shaped volcano. often symmetric.
alternating layers of lava and tephra
broad, slightly domed shaped (inverted shield) largest type of volcano
shallow slopes cover large geographic areas.
flow of low viscosity basaltic lava
basalt- low viscosity flows away from vent, allows gas to release
felsic- high viscosity builds up at the vent, prevents gas release
earthquakes- moving magma causes earthquakes.
tsunamis- water explosions create giant waves
clouds of rolling ash and gas racing downslope over a carpet of superheated air.
active- erupting, recently erupted or likely to
dormant- hasn't erupted in awhile but still could
extinct- no longer capable of erupting.
oceanic hot spot. plume under an oceanic plate.
basalt erupts at the seafloor
grows above sea level to form an island
occurs due to..
motion on a fault
inflation/deflation of a magma chamber
most earthquakes occur along faults.
crustal fractures that move
amount of movement is slip
faults- breaks in blocks of crust
on a sloping fault, crustal blocks are
footwall- block below the fault
hanging wall- block above the fault
angle of surface down from the horizontal
perpendicular to strike, measured downslope
angle of surface down from the horizontal
hanging wall goes up relative to footwall
due to crustal shortening
slope(dip) of fault is steep
special kind of reverse fault. low angle
hanging wall goes up relative to footwall, slope is not steep
vertical fault plane.
left lateral you move right
right lateral you move left
combination of dip slip and strike slip.
hanging wall slips diagonally
varies from small to large. maxima near hypocenter/epicenter
diminishes with distance.
faulting may result in uplift or subsidence
tectonic forces stress unbrocken rocks.
continued stress causes growth of cracks
cracks grow to the point of failure.
elastic strain transforms into brittle deformation. release of earthquake energy
fault motion starts and stops quickly due to friction
strain will build up again, causing failure
this is stick slip behavior.
stick- friction prevents motion
slip- friction briefly overwhelmed by motion
push-pull (compress and expand motion)
travel through solids, liquids, and gasses
shear or secondary waves
travel through only solids, not liquids
slower than P waves
produce back and forth motion. stronger than p waves
travel along the earths surface
slowest and most destructive
Love waves- s waves intersecting the surface back and forth
Rayleigh waves- p waves intersecting the surface, ripples on a pond
study of earthquake waves
seismographs- instruments that record seismicity
the amount of energy released.
richter and moment scale
depends on magnitude of earthquake
distance from the hypocenter
intensity and duration of the vibrations
the nature of the subsurface material
shaking causes slopes to fail
rockslides or snow avalances
water-saturated sediments turn into a mobile fluid. sand become quick sand
land will slump and flow
may be enormous
faulting drops or raises the seabed
displaces overlying water
a giant mound forms on the sea surface
wind- breaks in shallow water and expend all stored energy.
tsunami- entire water depth, come ashore as raised plateau of water that pours onto land
shallow- divergent and transform boundaries
intermediate and deep- convergent boundaries
upper part of trenches
continental transform faults
not near plate boundaries
remnant crustal weakness, stress transmitted inboard?
shape, a rotating sphere requires centered mass. solid interior- surface doesn't change
density- plumb bob deflection estimates density
seismic wave radiates in all directions
at a boundary, wave will reflect or refract
from faster to slower material
from slower to faster material
rising wave from faster to slower
size of core indicated by location of Pwave shadow zone
s-waves do not pass through outer core
crust- continental and oceanic
mantle- upper and lower
core- outter-liquid inner- solid
the outermost skin of the earth
thickest under mountain ranges
thinnest under mid ocean ridges
boundary between the crust from the upper mantle
solid rock layer between the crust and the core
composition- periodite (ultramafic rock)
three subdivisions- upper, transitional, and lower
iron rich sphere
outer core- liquid iron-nickel sulfur
inner core- solid nickel alloy
flow in the outer core generates the magnetic field
mountains occur in elongate, linear belts
constructed by tectonic plate interactions
translation-change in location
rotation- change in orientation
distortion- change in shape
stress: force applied across an area
large force per area- much deformation
strain: result of deformation caused by stress
tends to thicken material
differential stress- greater in one direction
tends to thin material
blocks of rock sliding past one another
crust is neither thickened or thinned
changes in shape caused by deformation.
stretching- pulling apart
shortening- compressing, squeezing
shear- sliding oast
rocks break by fracturing
in the shallow crust
rocks deform by flow and folding
in the deeper crust
bearing (compass direction)
plunge - angle from the horizontal
planar rock fractures without offset
systematic joints occur in parallel sets
minerals can fill joints to form veins
joints control weathering of rock
planar fractures offset by movement
faults vary by type of stress
faults vary by position in crust
layered rocks deformed into curves- folds
often occur in a series
orogenic settings- large volumes of folded rock
may be complex
shattered and crushed rock
fault breccia- broken fragments of rock
slickensides- surface polished by fault motion
plastically deformed rocks
rocks do not break, instead they are intensely sheared
dip slip- blocks move parallel to the dip of the fault plane
strike slip- blocks move parallel to fault plane strike
oblique- combination of dip slip and strike slip
bring old rocks up and over younger rocks
shorten crust and thicken mountain belts
commonly occur in groups- fault systems
thrust fault systems- stack fault blocks shorten and thicken the crust.
fault blocks slide away from one another.
fault join a detachment
blocks rotate creating half-graben basins
arch like fold
limbs dip away from the hinge
appears as an overturned bowl
erosion exposes old rocks in center
fold shaped like a bowl
erosion exposes young rocks in the center
mountain building is driven by plate tectonics
convergent plate boundaries
mountains are steep and jagged from erosion
rock structures can affect erosion
resistant layers form cliffs
soft rocks form slopes
igneous rocks= intrusive and extrusive
metamorphic rock= regional and contact
sedimentary= terrestrial and marine
compression stacks thrust faults
volcanic arcs- and overriding plate
accretionary prisms grow upward
fragments of continental lithosphere
geologic history differs from surroundings
added to the overriding plate- wont subduct
marine fossils high in the Apennines
ancient marine animals in loose sediment
lithification and uplift suggested long periods of time
father of modern geology.
incomprehensible time- enormous time-significance of angular unconformities.
mountains created, erased and new sediment deposited
based upon order of formation
actual number of years (absolute)
method developed in the early 1900s
age is given a number
the present is key to the past
modern processes help us understand ancient events
strata often form laterally extensive horizontal sheets
flat-lying rock layers are unlikely to have been disturbed
erosion dissects once-continuous layer
in an undeformed sequence of layered rocks
younger strata are on top; older strata below
each bed is older that the one about and younger than the one below
younger features truncate (cut across) older features
faults, dikes, erosion etc must be younger than the material that is faulted, intruded, or eroded
thermal metamorphism- when country rock is intruded by a magma
the rock must have been there before the magma
earth history is recorded in strata
missing strata- missing history
time gap in the rock record
metamorphic or igneous rocks overlain by sedimentary strata
horizontal sediments deformed by orogenesis
erode away mountains
deposit horizontally sediments on the erosion surface
fossils remains or traces of once living organisms
are often preserved in sedimentary rocks
species evolve, exist for a time and then go extinct
succeed one another in a known order
a time period is recognized by its fossil content
first and last appearance
each fossil has a unique range
overlapping ranges provide distinctive time markers
permit correlation of strata locally, regionally, and globally
similar rock types in a similar order
could be matched across distances
correlation- based on rock type
sequence- relative order
limited to correlation between nearby regions
correlation based on fossils
stratigraphic columns depict strata in a region
drawn to scale- accurately show relative thickness
a composite stratigraphic column
constructed from incomplete sections world wide
it brackets almost the entirety of earths history
sediments can be bracketed by absolute dates
based on radioactive decay of atoms in minderals
radioactive decay- a know, fixed rate
atoms with the same number of protons, different # of neutrons
different atomic mass numbers
time for half unstable nuclei to decay
as the parent disappears the daughter appears
the age of a mineral can be determined by measuring the ratio of parent to daughter isotopes
geochronology requires analytical precision
must pick the right isotope
cannot be directly dated
bracketed by absolute dates
defines major boundaries in the geologic column
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