Geology 12Geology 12

PresentsPresents

UNIT 3UNIT 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

Chapter 10

Earth’s Interior and Isostacy

atmosphere

Lower mantle

hydrospherecontinental

crust oceanic crust

Lith

osph

ere

Upper mantle

MohoAsthenosphere

Outer coreInner

core

Earth’s InteriorEarth’s Interior

Layer Vol Density Composition

% gms/cm3

Oceanic Crust 0.16 3.0 upper: basalt

lower: gabbro

Continental Crust 0.44 2.7 granodiorite

Mantle 83 3.3 – 5.7 peridotite

Outer Core 15.7 9.9 – 12.2 88%Fe, +S,O2,Ni, K

Inner Core 0.7 12.6 – 13.0 80 - 90% Fe, +Ni

How Layers of Earth was Determined

1. Seismic

2. Heat Flow

3. Gravity

4. 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)

EQ

Surface waves 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

Low

vel

ocity

zon

e =

ast

heno

pher

e

crust

Gut

enbu

rg D

isco

ntin

uity

• 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

Wave Refraction

EQ

Wave Reflection

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.

M .

C .

Low velocity zone = Asthenosphere

Upper mantle

Transition Zone

Lower Mantle

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 How Layers of Earth was DeterminedDetermined

1. Seismic

2. Heat Flow

3. Gravity

4. 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

• Highest at spreading centres and areas of recent volcanism; lowest at trenches

Hea

t flo

w C

al/c

m2 /

sec 4

3

2

1

0

Island arc (volcanoes)

World average

Oceanic ridge

trenchnew crustold 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– Highest at spreading ridge; lowest in mtns’

Region Temperature Geothermal Gradient

Crust base: 800- 1200 25

Mantle base: 3500-5000 1

Core base: 6500 <1

Remember Chp 7!

How Layers of Earth was How Layers of Earth was DeterminedDetermined

• Seismic

• Heat Flow

• Gravity

• Magnetic Field

3: Gravity

• Exists because mass attracts mass

• We are attracted towards the centre of the Earth

• F = Gm1 x m2

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

6378 km

6357 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, Please refer to WS 10.2 Note Helper, bottom of page 1.bottom of page 1.

Ore deposit

Low density sedments Salt dome

(low density

Mountain

Positive gravity

anomaly

Negative gravity

anomaly

• 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

Crust

root

Mountain

Mantle

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’

erosion

Low density root

deposition deposition

erosiondeposition deposition

See question 11, WS 12.2

See question 16, UNIT 3 Review WS

How Layers of Earth was How Layers of Earth was DeterminedDetermined

• 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 Field4. Magnetic Field

• Magnetic Inclination = deviation in magnetic field from horizontal

• Compass wants to point down at north end; up at south end.

Magnetic pole

True north 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.

MagneticMagnetic InclinationInclination

• Magnetic Declination = angle between true north pole and magnetic north pole

True north pole (Santa’s home) magnetic north

pole

Chilliwack

19’ EastImportant for surveying…more on this in Chp 13

1290 km

compass

Magnetic DeclinationMagnetic Declination

The Magnetic The Magnetic Pole moves Pole moves clockwise clockwise

(precesses) CW (precesses) CW around and around and

generally near generally near the True Pole. the True Pole. The large scale The large scale movement of movement of the magnetic the magnetic pole/magnetic pole/magnetic field indicates field indicates

plate movementplate 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

pipeSalt dome/

reef

Fe ore

• 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 Reversal of Earth’s

Magnetic FieldMagnetic Field AnimationAnimation

Do WS 10.3Do WS 10.3Look at Unit 3 Review WS. There is a Look at Unit 3 Review WS. There is a

“bottleneck” at the end of Unit 3, so “bottleneck” at the end of Unit 3, so do NOT leave the Unit 3 Review WS do NOT leave the Unit 3 Review WS to the last few days. You have been to the last few days. You have been warned!warned!