Download Recall Hypsometric Curve?

Structure and Composition of the Earth
(Why?)
Recall Hypsometric Curve?
Continental crust is
very different from
oceanic crust.
To understand this, we
need to know more
about the structure &
composition of Earth.
But how do you look
into the Earth to see its
structure?
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Earthquakes
provide key information on structure of Earth
Consider how Earthquakes work: (break pencils)
Types of Seismic Waves
• Two types of (body) Seismic Waves
• Primary Waves
– Slinky
• Secondary Waves
– Whip
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Primary Waves: (play w/slinky)
• Compression
waves
– Particles move
parallel to
propagation
• Fast: 6 km/s
• Travel through all
media (gas, liquid
or solid)
• Sometimes called
P-waves
Secondary Waves: (play w/rope)
• Shear waves
– Particles move
perpendicular
to propagation
• Slow: 3.5 km/s
• Only travel
through solids!
• Called S-Waves
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Characteristics of seismic waves
are key to understanding earth’s
structure.
• Difference in Velocity
allows geologist to
locate earthquakes
(not for us).
• Particle motion
provides important
information about
structure
If Earth was homogenous :
• Waves would travel ‘straight’
straight’ through
without ‘bending’
bending’ (refracting)
• If Density increased with depth, waves would
refract away from center (Snell’
(Snell’s Law).
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Draw what happens according to Snell’s
Law in Homogenous Earth as velocity
increases with depth (density).
Snell’s Law: sinq1/sinq2 = V1/V2
Or
sinq1/ V1 = sinq2/V2
What we observe: S-Waves and the outer core
• In outer portion, P-waves
refract as predicted by Snell’
Snell’s
Law. (mantle)
• Rays that intersect ‘central
region’
region’ stop propagating!
– S-wave shadow zone
– 105º
105º to 105º
105º
• Why?
Because the Earth has a liquid
core!
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What we observe: P-waves & The Liquid
Outer Core
• In outer portion, Pwaves refract as
predicted by Snell’
Snell’s
Law. (mantle)
• Rays that intersect
‘central region’
region’
refract ‘strangely’
strangely’.
– Result in P-shadow
zone (no P-waves
from 103º
103º to 143º
143º
– Evidence for liquid
core
• More…
More…
What we observe: P-Waves & the
Solid inner core
• ‘Complex’
Complex’ refraction of
Ray B (and those traveling
closer to center of earth)
indicates additional
structure.
• Direct wave (180º
(180º) arrives
anomalously fast
compared to whole liquid
core model.
• Thus, evidence for solid
inner core.
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Summarizing structure and
dimensions of the Core
• S-wave shadow indicates the outer core is liquid
(s-waves don’
don’t travel through liquids).
– From geometry we can calculate depth to outer core
(~3000 km).
– This is also, therefore, an estimate of the thickness of
the mantle.
• P-wave shadow zone and early arrival of direct
wave indicate the solid inner core
– From geometry we can calculate the depth to the inner
solid core (~5000 km).
– Also support the s-wave evidence for liquid outer core.
Seismic waves and the Mantle
• Mantle is complex…
complex…
• Notice the upper 250 km
– 10-35 km: Sharp increase
in velocity marks the
Crust-mantle boundary
– Decrease in velocity at
~100-150 km marks base
of lithosphere…
lithosphere…
• Crust vs.
vs. Lithosphere?
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Crust vs.
vs. Lithosphere
or
Composition vs.
.
vs Rheologic layers
• Composition layers on left:
• Ocean Crust: 0-10 km thick
– Si - Mg - Fe (Dense)
• Continental Crust: 35-75 km
thick
– Si - Al - Ca - Mg - Fe (light)
• Mantle: ~3000 km thick
– Si - Mg - Fe (Dense)
• Core: ~3500 km thick
– Fe (Ni-S) (very dense)
Crust vs.
vs. Lithosphere
or
Composition vs.
vs. Rheologic layers
• Rheologic layers on left:
• Lithosphere: 100-150 km thick
– Includes crust and rigid upper mantle
– This is the rigid plate in plate
tectonics
– Base is defined by 1200º
1200ºC boundary
• Asthenosphere:
– Top Marked by decrease in seismic
velocity
– no defined base (here it is 700 km or
base of transition zone)
– This is plastic region that lithosphere
plates ride on
• Mesosphere extends to ~3000 km
depth - we will call this Lower
Mantle
• Outer core and inner core - discussed
previously
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Now we can understand hypsometric curve
• Ocean floor is ‘deeper’
deeper’ than
continents because
– Ocean lithosphere is thinner
than continental lithosphere
– Oceanic lithosphere is denser
than continental lithosphere
• This is the concept of
isostacy…
isostacy…
– Continental lithosphere sinks
deeper into the asthenosphere
and rides higher than oceanic
lithosphere due to it’
it’s
thickness
Isostacy:
• concept of isostacy…
isostacy…
– Continental lithosphere
sinks deeper and rides
higher than oceanic
lithosphere due to it’
it’s
thickness.
– In this picture the thick
board represents the
continental lithosphere.
– The thinner boards
represent the oceanic
lithosphere.
– Note, if you remove the
top of a continent, it
‘bubs’
bubs’ up above the
oceanic lithosphere.
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More Examples of Isostacy:
Thick ice sinks deeper
and rides higher than
thin ice
More Isostacy:
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Now we understand this graph!
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