Transcript Geology 12

Geology 12
Presents
UNIT 3
•
•
•
•
•
Chp 10 Earth’s Interior and Isostacy
Chp 11 Ocean Basin
Chp 12 Plate Tectonics
Chp 9 Seismology
Chp 13 Structure
Handout WS 10.1 Note Helper
Earth’s Interior and Isostacy
Outer Lower
core mantle
Inner
core
Lithosphere
Chapter 10
atmosphere
continental
crust
oceanic
crust
Moho
Upper
mantle
Earth’s Interior
Layer
Vol
%
Density
gms/cm3
Oceanic Crust
0.16
3.0
Continental Crust 0.44
2.7
Composition
upper: basalt
lower: gabbro
granodiorite
Mantle
83
3.3 – 5.7
Outer Core
15.7
9.9 – 12.2
Inner Core
0.7
12.6 – 13.0 80 - 90% Fe, +Ni
peridotite
88%Fe, +S,O2,Ni, K
How Layers of Earth was
Determined
1.
2.
3.
4.
Seismic
Heat Flow
Gravity
Magnetic Field
1. Seismic
•
Seismic waves that travel through the
Earth
• Caused by earthquakes or underground
nuclear tests
• Two types
1. Surface waves
2. Body waves
1. Surface Waves
– Travel on the surface
– 2-3 km/sec
• Rayleigh and Love waves (more on this in
Chp 9)
Surface
waves
EQ
Body
waves
1. Surface Waves
– Travel on the surface
– 2-3 km/sec
• Rayleigh and Love waves (more on this in
Chp 9)
2. Body Waves
– Travel inside the Earth
– a) P-waves/Primary waves
• Fastest (7-13 km/sec)
• A compressional/pull-push
wave like sound
• Travel through everything (rock,
magma, water, air ( can
sometimes hear EQ!)
– B) S-waves/Secondary waves
• slower (3 - 7 km/sec)
• Are shear waves because they move
perpendicular to direction of travel
• Travel only through solids (not fluids)
• Wave velocity affected by:
i) rock density: higher density speeds waves
(ex: Earth’s density increase towards the core:
S-waves travel faster at the bottom of the
mantle)
ii) Rock elasticity = tendency to return to original
shape
higher elasticity increases wave speed
(ex: P-waves travel slow through liquid outer
core than through solid inner core)
Discontinuity = marked change in wave
velocity
• Basis for dividing Earth into its layers.
• Please now refer to WS 10.1 top of page 2
of note helper
Gutenburg Discontinuity
Low velocity zone = asthenophere
crust
• Upon EQ/nuclear detonation, waves travel
outward and inward in every direction (like
ripples on a pond)
• Waves’ direction of travel:
– Refracted (bent) away from more dense/more
elastic rocks back to surface
• Most energy is refracted
– Reflected (change direction) at major rock
boundaries (discontinuities)
• Reflected waves (wave velocity) + time to travel =
depth to layer) gives us depth to layers (d = vt)
• GEOPHYSICS
EQ
Wave Reflection
Wave Refraction
Layers of the Earth Found
• 1. Core
– a) Outer Core & S-waves
Liquid
Outer
b) Inner Core & P-waves
•Inner core is
solid because
higher pressure
increases melting
temperature
•Inner core
rotates 20 km/yr
faster than outer
core
2. Mantle
•1909 Andrija Mohorovicic detected a
seismic disontinuity (Moho) at about 30
km.
• P-waves travel faster (8 km/sec) in mantle
than in crust (6.75 km.sec)
• Moho – bottom of crust
– Below continents: 2- 90 km (aver’ 35 km)
– Below seafloor: 5 -10 km.
Please refer to WS 10.1, p.2 of notehelper,
bottom diagram.
Upper mantle
M
.
Transition Zone
Lower Mantle
C
.
Low velocity zone =
Asthenosphere
Upper
Mantle
Seismic Tomography
• Numerous seismic waves are analyzed to
give a “CAT-scan” of the Earth
• Hot and cold areas of the mantle have
been detected by measuring wave
velocities (hot rock is less elastic = lower
velocity)
• Depressions and rises have been detected
where rising and sinking mantle deforms
the mantle and core.
Seismic Tomography
3. Crust
• a) Continental Crust:
– mainly granodiorite (P-waves 8 km/sec)
– 35 km aver’ thickness
– 20 km at rifts
–
90 km at Himalayas
• b) Oceanic Crust
– Top 1/2 : basalt
– Bottom ½: gabbro
– 5 km thick at spreading ridges
–
10 km thick where it is oldest
3. Crust
How Layers of Earth was
Determined
1.
2.
3.
4.
Seismic
Heat Flow
Gravity
Magnetic Field
Hand out WS 10.2 Note Helper
2: Heat Flow
• Is very low (Earth is a
good insulator/poor
conductor)
• Is measured with very
sensitive instruments
Heat flow Cal/cm2/sec
• Highest at spreading centres and areas of
recent volcanism; lowest at trenches
4
3
Oceanic ridge
Island arc
(volcanoes)
2
World average
1
0
trench
old crust
new crust
• 70% of heat is lost through oceans
• Earth has been cooling off (less and less
radioactive fuel) since its beginning.
• Most heat created by radioactive decay of
U, Th and K
• Geothermal Gradient = rate of increase in
temperature with depth Remember Chp 7!
– Highest at spreading ridge; lowest in mtns’
Region
Temperature
Geothermal Gradient
Crust base: 800- 1200
Mantle base: 3500-5000
Core
base: 6500
25
1
<1
How Layers of Earth was
Determined
•
•
•
•
Seismic
Heat Flow
Gravity
Magnetic Field
3: Gravity
• Exists because mass attracts mass
• We are attracted towards the centre of the
Earth
m1 x m2
•F= G
d2
Double the distance = ¼ the gravity
•
Gravity varies by:
1. Distance between masses (greater
distance = less gravity
a) Earth is not round: less gravity at
equator
6357 km
6378 km
b) less gravity at high altitudes
2. Centrifugal force at equator “throws” you
away from Earth = less gravity
•Generally. Gravity varies with latitude
and altitude
3. Density of rock below
• Gravity is measured with a gravimeters
(Positive gravity anomaly = more gravity)
• Important for Olympic records
• If you want to weigh less:
– Drive fast east at the equator on top of a
mountain over top of low density rock.
Centrifugal Force is seen here with
indirect high tides

Please refer to WS 10.2 Note Helper,
bottom of page 1.
Positive
gravity
anomaly
Negative
gravity
anomaly
Mountain
Ore
deposit
Low density
sedments Salt dome
(low density
• Principle of Isostacy is Earth’s crust is
“floating” on denser mantle.
– Like an iceberg, only 10% is above the
water no matter the size of the iceberg
Mountain
Crust
Mantle
root
Isostacy
• Crust floats like a raft:
– It sinks if:
a) glaciers/ice age
b) sediments accumulates in a delta or basin
(Evidence: sunken cities, docks, forests,
beaches)
– If rises = isostatic rebound if:
a) glaciers melt/recede
b) erosion of a mountain
(Evidence: above sea-level: docks, beaches,
marine fossils)
1.
2.
3.
4.
Glacial ice
Ice melts (i.e. Hudson’s Bay
Mtn’
Low density root
erosion
deposition
deposition
erosion
deposition
deposition
See question 11, WS 12.2
See question 16, UNIT 3 Review WS
How Layers of Earth was
Determined
•
•
•
•
Seismic
Heat Flow
Gravity
Magnetic Field
4. Magnetic Field
• Earth is dipolar = N & S pole
• Magnetic Filed is generated by electric
currents in outer core
• Rocks with magnetic substances (iron,
nickel) that cool below the Curie Point
(580’C for magnetite) “lock-in” the present
magnetic field.
4. Magnetic Field
• Magnetic Inclination = deviation in
magnetic field from horizontal
• Compass wants to point down at north
end; up at south end.
True north Magnetic
pole
pole
Lines of
magnetic
force
Inclination/tilt
of compass
needle
• Inclination can tell us the latitude of the
continents/terranes at the time of Curie.
– i.e. many of the BC terranes formed about 30’
latitude.
Magnetic Inclination
• Magnetic Declination = angle between
true north pole and magnetic north pole
True north pole
(Santa’s home)
Important for
surveying…more
on this in Chp 13
Chilliwack
magnetic north
pole
19’ East
compass
Magnetic Declination
The Magnetic Pole
moves clockwise
(precesses) CW
around and
generally near the
True Pole. The
large scale
movement of the
magnetic
pole/magnetic field
indicates plate
movement
• Magnetic Anomalies = variations in normal
strength of magnetic field
– Measured with magnetometer.
– Positive anomaly = stronger magnetism
• Iron ore area (Great Lake region)
• Lava flows (mafic rx) (Columbia basalts)
• submarine, ship
• Diamond mine/volcanic pipe
– Negative anomaly
• Sedimentary rock
• Salt dome, coral reef
See WS 10.2 Note helper, top of page 2.
+ mag’ anom’
- mag’ anom’
Sed’ rx
Lava flow/
diamond
pipe
Fe ore
Salt dome/
reef
• Magnetic Reversals: the magnetic field
periodically weakens and reverses N to S
and S to N. This is recorded in igneous rx
that cooled below the Curie Point
• The magnetic field has weakened 5% in
the last 100 yrs (15% in last 200), and is
predicted to be nil in a few 1000 years,
then will reverse, and build up again.
– Last switched 70,000 yrs ago.
• Paleomagnetism: study of ancient
magnetism.
Reversal of Earth’s
Magnetic Field

Animation
Do WS 10.3
 Look at Unit 3 Review WS. There is a
“bottleneck” at the end of Unit 3, so do
NOT leave the Unit 3 Review WS to the
last few days. You have been warned!
