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Geology Exam 4


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vibration of earth produced by rapid release of energy

*most produced by rapid release of elastic energy stored in rock that's been subjected to great stress.
-when strength of rock is exceeded it ruptures causing earthquake vibrations

*often caused by slippage along fault in earth's crust

*energy released radiates in all directions from the source [focus]

*energy moves in waves

*recorded by seismographs
the source of energy of earthquake

most earthquakes occur:::
along faults associated w/ plate boundaries

-as soon as one is over continuous plate motion adds strain to rocks until they fail again
Elastic Rebound
-explains earthquakes

1)rocks on sides of fault deform by techtonic forces
2)rocks bend and store elastic energy (buildup of strain)
3)frictional resistence holding rocks together is overcome by techtonic forces
4)slippage occurs at weakest pt (focus)
5)displacement exerts stress farther along fault (additional slippage releases built up strain)
6)deformed rock "SNAPS BACK"
--->rock returns to original shape (& vibrations occur as rock elastically returns to original shape)
"springing back"
aka elastic rebound
smaller earthquakes caused by adjustments following a major earthquake (mainshock)

-illuminate the fault that ruptured in the mainshock
small earthquakes that precede a major earthquake
-by days or sometimes years
--used as means of predicting earthquakes
San Andres Fault
-active earthquake zone

-most studied fault system in the world

-displacement occurs along discrete segments 100 - 200 km long

--most segments slip every 100-200 years producing large earthquake
--[stick-slip behavior... alternating peiod of locked behavior followed by sudden slippage & release of strain]

-some portions exibit "fault creep"

-GPS (global positioning systems) measure motion between opposite sides of fault
"fault creep"
slow, gradual displacement

-occurs relatively smoothly
--little noticeable seismic activity
rocks at depth not capable of producing earthquake
at depth temp & confining pressure = HIGH

---rocks display ductile deformation
(strength of rock well exceeded)

----rock deforms by various flow mechangisms that produce slow slippage WITHOUT storing elastic strain
the study of earthquakes

--looks at seismic waves w/ seismographs


instruments that record seismic waves

-records movement of earth in relation to stationary mass
on rotating drum or magnetic tape
earthquakes cause
earthquakes cause

[type of seismic waves]
waves generated by slippage of rock along outer part of earth

-Complex Motion

-great destruction

-high amplitude

-low velocity

-longest periods (intervals between crest)

-called "long waves" / "L waves"

-up & down motion AND side to side motion (damaging to structures)

[type of seismic wave]
-waves that travel thru earth's interior

1)Primary ("P") Waves
2)Secondary ("S") Waves
Primary "P" Waves

(Type of Body Wave)
-push-pull motion
--(compress & expand rocks in direction the wave is traveling)
-----(temporarily changes the VOLUME of material by alternately compressing & expanding it)

-travel thru solids, liquids, gasses

location on surface directly above focus (where originates)

-located using difference in velocities of "P" & "S" waves
"P" "S" & Surface Wave

AMPLITUDE (height)
-fastest velocity
-lowest amplitude

-medium velocity
-medium amplitude

-lowest velocity
-greatest amplitude
--(greatest destruction)
---bc they're confined to narrow region near surface (not spread thruout earth like P & S)
---->retain max amplitude longer (called Long "L" waves)
Earthquake Belts
95% of energy released by earthquakes originates in narrow zones winding around globe

--Zones mark edges of tectonic plates

-greatest energy released is belt around Pacific Ocean
Earthquake Depths
-originate at depths of 5 - 700 km

-Shallow < 70 km
-Intermediate: 70-300 km
-Deep > 300 km

*most earthquakes occur at shallow depths
*all damaging earthquakes occur at shallow depths
*shallow EQ's have higher richter magnitudes

**foci depths increase w/ increasing destance from deep ocean trenches***
Depths of
Oceanic ridge system
circum-pacific belt
Oceanic Ridge System:
-shallow focus
-none very strong

Circum-Pacific Belt:
deep focus

**foci depths increase w/ increasing destance from deep ocean trenches***
foci depths increase w/ increasing distance from deep-ocean trenches
due to SUBDUCTION zones

--one plate under the other
2 measurements
describing earthquake size

*both measurements still used*
estimates amt of energy released by EQ


-Richter Magnitude
-Richter Scale
Modified Mercalli Intensity Scale
-measures EQ intensity

-developed using cali buildings as standard

*wood structures collapse v masonry collapse*
Richter Magnitude
-developed by richter in 1935

-first magnitude scale using seismic records to estimate EQ size

-uses Richter Scale
Richter Scale
-based on amplitude of largest seismic wave (P, S, or Surface)

-each unit of richter magnitude corresponds to 10x increase in wave amplitude
& 32x increase in energy

-10 greatest - 0 smallest

*takes distance into account bc seismic waves weaken as get farther from foci

-largest recorded = 8.9
Moment Magnitude
-developed bc richter can't closely estimate size of really large earthquakes

-derived from amt of displacement that occured along fault
& the area of the fault that slips

1)only magnitude scale that adequately estimates size of large earthquakes
2)can be derived mathematically from size of rupture surface & amt of displacement
[better reflects total energy released]
3)can be verified by 2 independent methods
[field studies based on displacement -&-
seismographic methods using long period waves]
Amt of Structural Damage by Earthquakes
Depends On:::
1)Intensity & Duration of Vibrations

2)Nature of material structure is built on
-soft sediments amplify vibrations more than solid bedrock

3)Design of Structure
-wood-frame fair well
-unreinfoced masonry MOST DAMAGED (most serious safety threat)
During EQ,
Region W/I _____
of epicenter experiences same amt of ground shaking
20 - 50 km

(12.5 - 30 miles)

{however destruction varies}
in areas where materials are saturated with water
what had been a stable soil turns into mobile fluid

-can't support buildings or other structures
-underground objects (sewer lines) can float to surface
rhythmic sloshing of water in lakes, reservoirs, & enclosed basins

-can weaken dams endangering lives downstream
seismic sea waves

-destructive waves called "tidal waves" (tho incorrect term- not caused by tides)

-result from push of fault block on ocean floor
OR from underwater landslide triggered by EQ

--open ocean: > 1 m
--shallow coast h2o: > 30 m
*bc water slows down & piles up [shallow h2o ~ 50 km/hr]*

-high speeds 500 - 950 km/hr

-can pass undetected in open ocean

-warning: rapid withdrawal of h2o from beach
Short-Range EQ Predictions
-goal: provide warning of location & magnitude of earthquake w/i narrow time frame

-concentrate on possible precursers:
--uplift --subsidence (sink) --strain in rocks

Long-Range EQ Forecasts
-calculates probability of certain magnitude EQ occuring over given period of time
earthquakes provide evidence for

EQ distribution along plate boundaries
measurement of ocean depths
& charting of shape / topography of ocean floor
mapping the ocean floor
-depth originally measured by lowering weighted lines overboard

-Echo Sounder [sonar]
Echo Sounder

-invented in 1920s

-main instrument for measuring depth

-reflects soundwave (ping) from ocean floor
--knowning speed of soundwave & time required to reach floor & back == can calculate depth

*gives data back from a pt on sea floor
Multibeam Sonar
sends out a fan of sound
-possible to survey floor in strips rather than a pt
-can collect bathymetric data of high resolutions
--(can distinguish depths of less than a meter)

-greater efficiency & enhanced detail
--(so only 5% of seafloor mapped in great detail)
3 major topographic units of ocean floor
1)Continental Margins

2)Deep-Ocean Basins

3)Mid-Ocean Ridges

of ocean floor
1) Passive Margins

2) Active Margins
Passive Continental Margins

of ocean floor
-found along coastal areas surrounding oceans

-NOT associated w/active plate boundaries

-little volcanism, few earthquakes

1)Continental Shelf
2)Continental Slope
3)Continental Rise

of Passive Continental Margin
-gently sloping submerged surface extending from shoreline to deep-ocean basin

-underlain by continental crust (flooded extension of continents)

-slopes gently seaward

-often contains petroleum deposits (economic importance)

-covered by thick accumulation of sediment

of Passive Continental Margins
-steep seaward edge of continental slope

-boundary between continental & oceanic crust

-onlapped by deep-sea fans [turbidites]

of Passive Continental Margins
-steep continental slope merges into gradual continental rise

*occurs when trenches don't exist*

(2/2 continental margin
of ocean floor)
-continental slope descends abruptly into deep ocean trench

-located around Pacific Ocean

-accretionary wedges formed
(sediments scraped from descending oceanic plate and plastered against edge of overriding continent

of ocean floor
~ 30 % of Earth's surface
--comparable to % of land above water

1)abyssal plains
4)deep ocean trenches
5)oceanic plateaus
Deep Ocean Trench

of Deep-ocean basin
-long narrow features

-deepest parts of ocean

-most located in Pacific Ocean

-occur where plates plunge into mantle

-associated w/volcanic activity
Abyssal Plains

of deep ocean basins
-likely the most level places on earth

-can be sites of thick sediment accumulation (account for flatness)
-most likely fine sediments precipitated out of h2o

-found in all oceans

of deep ocean basin
shield volcanoes

of deep ocean basin
submerged flat-topped seamounts

(islands that have sunk)
oceanic plateaus

of deep ocean basin
form from vast outpourings of lava onto ocean floor
Sources of Seafloor Sediment
-Turbidity Currents on continental margins

-Sediment slowly settles to bottom from above

(sediment layer is above hard rock layer)
Thickest seafloor sediment?

can exceed 9 km
types of seafloor sediment
1)biogenous sediment

(chert, diatoms [silicious ooze])
biogenous sediment
-shells / skeletons of marine animals & plants

-calcareous oozes from microscopic organisms
(ONLY in shallow water)

-siliceous oozes composed of opaline skeletons of diatoms & radiolaians
(ONLY in deep water)

-function of carbonate compensation depth
Mid-Ocean Ridges
along well developed divergent plate boundaries seafloor is elevated forming swell
--ideally run along middle of oceans
--not narrow or steep

-made up of faulted & uplifted basaltic rocks

-little deep water ooze (young crust)

-sediment gets thicker further from axis

-rift valleys can form along axis

-interconnected ridge system

--longest feature on earth's surface

-over 70,000 km long

-20% of earth's surface

-winds thru ALL major Oceans

-where new oceanic lithosphere originates
--greatest volume of magma produced here
Mid-Ocean Ridges

-elevated ridge
-closely spaced normal faulting
-volcanic structures developed on newly formed crust
rift valleys
deep down faulted structures along axis of some ridge systems

-width can exceed 50 km
-depth can exeed 2000 m

-rugged topography due to faulted / tilted crust & volcanic cones
Seafloor Spreading
plates diverge &
fractures immediately filled w/molten rock
-cools = new seafloor

-concept in 60's by Harry Hess

-occurs in narrow rift zones at crests of oceanic ridges

--new lithosphere moves from ridge crest in conveyer belt fashion

--new crust is elevated bc its hot & occupies more volume than cooler rocks in deep ocean basin
[[[rock rising has decreasing confining pressure & can undergo melting w/o addition of heat
structure of Oceanic Crust
(the sequence of rocks composing oceanic crust)

1)Upper Layer
-pillow lavas

2)Middle Layer
-sheeted dikes

3)Lower Layer

*above all layers is unconsolidated sediments*
pillow lavas


pillow basalts
basaltic lavas in pillowlike structures

*lava accumulates behind congealed margin & breaks thru
[occurs over & over, squeezed out like toothpaste]

(in Upper Layer of Oceanic Crust)
Sheeted Dikes
numerous interconnected dikes

--vertical orientation

*new dikes intrude old dikes that are still warm & weak

(in Middle Layer of Oceanic Crust)

in oceanic crust
couarse grained equivalent of basalt

-crystallized in magma chamber below ridge axis

**Formed in basaltic magma chambers**
magma that creates new ocean floor
originates from
partially melted peridotite in the asthenosphere


*peridotite = basaltic magma that migrates upward from partially melted mantle rock*
--bc partially molten & less dense than surrounding solid rock --> magma gradually moves upward
proposed theory of continental drift
Alfred Wegener
Alfred Wegener
proposed theory of continentatl drift
in 1915

-published The Origin of Continents & Oceans
Continental Drift Hypothesis
supercontinent, Pangea
began breaking apart
200 million yrs ago

(coastlines similar on sides of atlantic ocean)

*oceans opened between rigid plates

*continents drifted to current positions
how long ago did pangea began breaking apart?
200 million years ago
evidence supporting continental drift
-fit of continents

-fossil evidence
[identical fossils seperated by ocean]--refuted by idea of land bridges

-rock type & mountain belts

-paleoclimatic evidence
objections to continental drift
no mechanism provided capable of moving continents across globe

-wegener suggested continents broke thru oceanic crust (like ice breakers cut thru ice)
mountain range

evidence of continental drift
north american Appalacians
fit with British Isles
of Europe
paleoclimatic (ancient climate)

evidence for continental drift
wegener found evidence for dramatic global climate changes during geologic past

-lands containing evidence of prior glaciation now lie near equator
wegener refuted
wegener's hypo correct in principle but had incorrect details

-no supporting evidence (which is needed for view to be widely accepted)

-tidal energy is too weak to cause continents to move
[if it were strong enough it would bring earth's rotation to a halt]
-continents don't break thru the ocean floor
[there was no evidence to suggest the ocean floor was weak enough to permit passage of continents]

--theory didn't stand up to critical testing from all areas of science
rocks that formed thousands / millions of yrs ago contain a "record" of direction of magnetic poles at the time of their formation

-provides a means of determining the latitude at the time they became magnetized

-provides a record of the direction / distance to the magnetic poles at time rock was magnetized

Curie Point
iron-rich minerals when heated above Curie point lose their magnetism

cooled below curie point become magnetized in the direction of the existing magnetic lines of force
--"point" toward position of magnetic poles at time of their formation
"Polar Wandering"
view that magnetic poles had migrated thru time

bc magnetic alignment of iron-rich inerals of different ages indicated many different poles existed
continental drift & poles
continental drift explains why it appears that different poles existed

--more acceptable than "polar wandering"

-studies show that poles remain relatively stationary around geographic poles

-continental drift explains apparent movement of poles
evidence for continental drift bc of paleomagnetism
-europe was covered in coal producing swamps 300 m years ago
---paleometric evidence shows europe was near the equator at that time (which works for coal swamps)

-polar-wandering path for N America & Europe are the same
(but seperated by about 24* longitude)
--bc two magnetic north poles didn't migrate parallel to eachother (very unlikely)
----->the differences in paths make sense if the 2 continents are placed next to eachother
technologic strides in 50s & 60s
provided extensive mapping of ocean floor


-found mid-atlantic ridge where crust was being pulled apart
seafloor spreading hypo
harry hess in 60s

-oceanic ridges are located above zones of upwelling in the mantle
--rising material spreads laterally and seafloor is carried away from ridge crest
-older crust is drawn back into interior as new crust is formed

--hess proposed that convective flow in mantle caused earth's entire outer shell to move
---accounting for continental drift
"geomagnetic reversal"
earth's magnetic field periodically reverses polarity
-magnetic N pole becomes S pole & vice versa

"normal polarity" - when rocks exhibit same magnetism as present magnetic field

"reverse polarity" - where rocks exhibit opposite magnetism to present magnetic field

-dates when magnetic field changed determined from lava flows
magnetic time scale
major divisions = CHRONS
(last for ~ 1 million years

most recent
1)Brunches Normal Chron
2)Matuyama Reversed Chron
-includes jarmillo normal subchron
-includes olduvai normal subchron
3)Gauss Normal Chron
-includes mammoth reversed subchron
4)Gilbert Reversed Chron
(oldest- 4 million yrs ago)
magnetic stripes in ocean crust
as evidence for seafloor spreading
-Vine & Matthews 1963

-as magma solidifies along narrow rifts on oceanic ridge
--its magnetized w/polarity of existing magnetic field
-bc of seafloor spreadingi stripe increases in width
--when earth's magnetic field reverses polarity newly formed seafloor (having opp polarity) forms in the middle of the old stripe)
--2 parts of old stripe carried in opp directions away from ridge
-patterns of stripes (size & polarity = mirror images on either side of rift.
most convincing evidence of
continental drift &
seafloor spreading?
Plate Techtonics
more encompassing theory than continental drift

explains motion of earth's lithosphere by subduction & seafloor spreading

(in turn generated continents, mtns, & ocean basins)
plate tectonics explained by lithosphere & asthenosphere
brittle lithosphere overlies
weaker asthenosphere

bc rocks at top of asthenosphere are near melting point it creates a weak zone
--ALLOWING the l ithosphere to be detached from the layers below
---& thus allowing it to MOVE independently of layers below
uppermost mantle & overlying crust

-strong rigid layer

--broken into pieces called PLATES
(thinnest in ocean)

-overlies asthenosphere
weaker region in mantle

-temp of upper asthenosphere such that rocks near melting pt & weak
peices of lithosphere

-in motion in respect to one another
-continually changing in shape & size

-**7 major plates**
-largest = pacific
-several plates include an entire continent + portion of seafloor
plate boundaries
-interactions amoung ind plates occur along their boundaries

3 TYPES of boundaries:

-each plate bounded by combination of 3 types of boundaries

-new boundaries created in response to changes in forces on ridge slabs
divergent plate boundaries

2 plates move apart resulting in upwelling of material from mantle to create new seafloor

-most located along crests of oceanic ridges

-seafloor is elevated forming oceanic ridges
spreading rates control topographic differences in ridges
1) SLOW RATES: (1-5 cm/yr)
-rift valleys develop on ridge crest
--30-50 km wide
--1500-3000 m deep

-rift valleys shallow
-subdued topography

3)RATES > 9cm/yr
-no rift valley develops
---or are narrow & extensively faulted
divergent boundaries in continents

splits landmass into 2+ smaller segments

*east african rift
(created red sea in between land masses)

-produced by extensional forces acting on lithospheric plates

**not all rift valleys develop into spreading centers (otherwise nevada would be an ocean)**
continental rift can develop into ocean basin
1)upwelling from mantle
2)splits crust into rift valley
3)creates linear sea & mid-ocean ridge
4)spreads more creates rift & oceanic crust

2 plates come together resulting in oceanic lithosphere descending beneath over-riding plate
--eventually to be reabsorbed into mantle
--or collision can create a mountain system

*Ocean trench (when plate descends)
*subduction zones
*avg angle ~45*

-all have same basic characteristic but can have highly variable features

1)oceanic-continental convergence
2)oceanic-oceanic convergence
3)continental-continental convergence
Oceanic-Continental Convergence

(type 1/3 of convergent plate boundary)
-denser oceanic slab sinks into asthenosphere
-bathymetry marked by TRENCH
-when plate descends partial melting creates basaltic or andesitic magmas
-can create volcanic arcs (andes & cascades)
"continental volcanic arcs"
mountains produced by volcanic activity associated w/subduction of oceanic lithosphere

Oceanic-Oceanic Convergenge

(type 2/3 of convergent plate boundary)
-2 ocean slabs converge - one descends beneath other
-can form volcanos on ocean floor
-volcanos can emerge as islands & form an island arc
"volcanic island arc"
newly formed land consisting of arc shaped chain of small volcanic islands

*Japan**Aleution Islands*
Continental-Continental Convergence

(type 3/3 of convergent plate boundary)
-continued subduction brings continents together
-when both plates are less dense buoyant lithosphere they don't subduct
--->results in collision between blocks
--->forms mountains


-plates slide past one another
-NO new lithosphere
-NO lithosphere created or destroyed

-most join 2 segments of mid-ocean ridge where there are breaks in oceanic crust called fracture zones
-accomodate simultaneous movement of offset ridges
Plate tectonics accounts for global distribution of earthquakes
-absence of deep focus EQ's along oceanic ridges
---consistent w/tectonic theory

-deep focus earthquakes associated w/subduction zones (of convergent boundaries

-pattern of EQ's along a trench tracks plate's descent
Evidence of plate tectonics from ocean drilling
-ocean drilling = best evidence for seafloor spreading

-found age of sediment increased with increasing distance from the ridge

-no sediment exceeding 180 million years in age was found
(continental crust > 4 billion yrs has been found)

-sediments almost entirely absent from ridge crest & thickens with increasing distance from ridge
--bc ridge is younger it has less sediments deposited (& is therefore thinner)
no one mechanism accounts for plate tectonics
main mechanism = convective flow of mantle

other mechanisms:
-ridge push
main mechanism of plate tectonics?
convective flow in mantle
(2,900km thick)
--by basal traction

*warm buoyant rock rises & cooler dense material sinks*

*subducting oceanic plates drive cold downward flow & rising mantle plumes drive upward flow*
[plate tectonics & convection drive one another]
the subduction of cold dense slabs = main driving force of plate motion

--as slabs sink into asthenosphere they pull the trailing plate along
gravity driven mechanism resulting from the elevated position of the oceanic ridge

--causes slabs of lithosphere to slide down flanks of ridge

*contributes less than slab-pull*
importance of plate tectonics
-provides explanation of earth's major processes (especially in oceans)

-explains distribution of EQ's volcanos & mountains

-explanation for distribution / evolution of
& Climate Record
mountains, continents, ocean basins
explained in 1850
thought they formed when earth cooled from initial formation & stayed in the same spot
plate tectonics
1)early history
2)new technology
1)early history:
-contraction theory
-continental drift

2)new technology:
-sea floor spreading
contraction theory
earth has been steadily cooling since beginning of geologic time

-most materials contract as they cool so earth did too
SUESS contraction theory
as earth contracted surface wrinkled to accomodate lessened surface area

-crust was initially continuous
-broke apart
-collapsed portions = oceans
-raised portions = continents
-continental wrinkles = mountains

-continents & ocean basins interchanged over time
DANA contraction theory
"permanence theory"

continents formed early in earth's history, oceans formed later

-continents = early formation from low-T minerals

-oceans = later formation from high-T minerals

-boundaries between continents & oceans took up pressure -- forming mtns
evidence challenging contraction theories
1)folds so extensive impossibly huge amts of contraction couldn't explain

2)gravity on continents::
-continents must be less dense than rock beneath them
--concluded continents floated & couldn't sink to become ocean basins

3)discovery of radioactivity in earth's interior
-NOT steadily cooling
state of equilibrium resembling floatation
-segments of earth's crust stand at levels determined by thickness & density

-isostatic equilibrium attained by flow of material in mantle

Observed: continents appear to have moved

Hypothesis: continental drift

Observed: fossil magnetism points the way

Hypothesis: still continental drift

Observed: new sea floor & magnetic reversals

New Hypothesis: sea floor spreading
1970s to Present
Observed: EQ locations, real-time GPS

THEORY: plate tectonics
mesosaurus fossils
found in south america & africa
(where they could fit)
& no where else


glossopteris (fossil fern)
large seeds - can't be blown very far

found widely dispered in late paleozoic

-grew in sub polar climate


paleoclimate evidence
alignment of distribution of ancient
-sand dunes
-salt deposits
-glacial deposits
-glacial grooves

wegener's theory hotly debated

1920s & 1930s

-wegener suggested continents broke thru oceanic crust
--refuted by rock strength issue
---->continental cruts & oceanic crust have similar strength
a compass tells more than just
"which was is north"
if free to swing in 3 dimensions
its inclination to earth's surface can also tell the latitude
why do rocks exhibit magnetism below curie point
& not above curie point?
Above curie point:
-dipoles are random & changing

Below Curie Point:
-dipoles lock into parallelism & allign w/ the earth's dipole

CURIE PT = 580* C
"thermoremanent magnetism"
when atoms of minerals allign in direction of current magnetic field

(aka paleomagnetism)
"magnetic stratigraphy"
gives a time scale of polarity reversals
recorded geomagnetic reversals in ocean crust
theory of plate tectonics
theories of continental drift + seafloor spreading
combine to become theory of plate tectonics

-explains motion of earth's lithosphere by seafloor spreading & subduction
continent-ocean convergence

continental arc

(by subduction)
ocean-ocean convergence

oceanic arc

(by subduction)
continent-continent convergence

continental collision

mechanism for plate motion

-mantle circulation pulls both subducting & overriding plates toward trench

---further creates flow in mantle
abrupt velocity changes in waves at particular depths
allow for
earth composed of distinct shells with varying compositions

earth divided into 5 layers based on physical properties
-curst & upper mantle
-cool rigid shell

-in upper mantle
-weak layer
-melting at top

-lower mantle
-increased pressure counteracts higher temp
--stronger rocks
-still hot -- capable of gradual flow

4)outer core
-iron-nickel alloy
-liquid layer
-convective flow generates earth's magnetic field

5)inner core
-iron-nickel alloy
-behaves like solid
-stronger than outer core bc of increased pressure
boundary seperating crustal rock from underlying mantle

-shallow waves arrive at closer seismic station location
[travel slower in crust]

-deep waves arrive at further seismic station location
[traveling faster --> travel faster thru mantle (deeper)

***EQ velocity was shown to be higher at stations further away
"p wave shadow zone"
=belt where p waves are absent (~35* wide)

-p waves die out 105* from EQ
-reappear 140* away
**earth must contain core different than mantle**
-shadow zone refracts p waves
-s waves don't travel thru core
-p wave velocities derease by 40% when enter core
**bc melting reduces elasticity of rock===
know liquid layer existed below mantle**
discovery of inner core
1936 lehmann discovered reflection & refraction w/i the core
Divergent & Transform fault boundaries

The only sites of deep focus EQ's??
Subduction zones
subducted plates are always

(bc continental lithosphere is less dense & more buoyant)

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