Geology 12
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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. Chapter 10 Earth’s Interior and Isostacy. atmosphere. hydrosphere. continental crust.

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Geology 12

Geology 12

Presents


Unit 3

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


Chapter 10 earth s interior and isostacy

Chapter 10Earth’s Interior and Isostacy

atmosphere

hydrosphere

continental crust

oceanic crust

Lithosphere

Moho

Asthenosphere

Upper mantle

Outer core

Lower mantle

Inner core


Earth s interior

Earth’s Interior


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Layer Vol Density Composition

% gms/cm3

Oceanic Crust 0.16 3.0upper: basalt

lower: gabbro

Continental Crust 0.44 2.7granodiorite

Mantle 833.3 – 5.7peridotite

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

How Layers of Earth was Determined

  • Seismic

  • Heat Flow

  • Gravity

  • Magnetic Field


1 seismic

1. Seismic

  • Seismic waves that travel through the Earth

  • Caused by earthquakes or underground nuclear tests

  • Two types

    • Surface waves

    • Body waves


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1. Surface Waves

  • Travel on the surface

  • 2-3 km/sec

  • Rayleigh and Love waves (more on this in Chp 9)

  • Surface waves

    Body waves

    EQ


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    1. Surface Waves

    • Travel on the surface

    • 2-3 km/sec

  • Rayleigh and Love waves (more on this in Chp 9)


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    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!)


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    • 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)


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    • 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.


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    • Please now refer to WS 10.1 top of page 2 of note helper


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    Gutenburg Discontinuity

    Low velocity zone = asthenophere

    crust


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    • 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


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    EQ

    Wave Reflection

    Wave Refraction


    Layers of the earth found

    Layers of the Earth Found

    • 1. Core

      • a) Outer Core & S-waves

    Liquid

    Outer


    B inner core p waves

    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

    2. Mantle

    • 1909 Andrija Mohorovicic detected a seismic disontinuity (Moho) at about 30 km.


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    • 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.


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    Upper mantle

    M .

    Transition Zone

    Lower Mantle

    Low velocity zone = Asthenosphere

    C .


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    Upper Mantle


    Seismic tomography

    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 tomography1

    Seismic Tomography


    3 crust

    3. Crust

    • a) Continental Crust:

      • mainly granodiorite (P-waves 8 km/sec)

      • 35 km aver’ thickness

      • 20 km at rifts

      • 90 km at Himalayas


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    • b) Oceanic Crust

      • Top 1/2 : basalt

      • Bottom ½: gabbro

      • 5 km thick at spreading ridges

      • 10 km thick where it is oldest


    3 crust1

    3. Crust


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    How Layers of Earth was Determined

    • Seismic

    • Heat Flow

    • Gravity

    • Magnetic Field

      Hand out WS 10.2 Note Helper


    2 heat flow

    2: Heat Flow

    • Is very low (Earth is a good insulator/poor conductor)

    • Is measured with very sensitive instruments


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

    4

    Oceanic ridge

    Island arc (volcanoes)

    3

    Heat flow Cal/cm2/sec

    2

    World average

    1

    new crust

    old crust

    trench

    0


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    • 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


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    Remember Chp 7!

    • Geothermal Gradient = rate of increase in temperature with depth

      • Highest at spreading ridge; lowest in mtns’

        Region Temperature Geothermal Gradient

        Crustbase: 800- 120025

        Mantle base: 3500-5000 1

        Corebase: 6500<1


    How layers of earth was determined1

    How Layers of Earth was Determined

    • Seismic

    • Heat Flow

    • Gravity

    • Magnetic Field


    3 gravity

    3: Gravity

    • Exists because mass attracts mass

    • We are attracted towards the centre of the Earth

      • F = G

    m1 x m2

    d2

    Double the distance = ¼ the gravity


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    • Gravity varies by:

      • Distance between masses (greater distance = less gravity

        a) Earth is not round: less gravity at equator

    6357 km

    6378 km


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    b) less gravity at high altitudes


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    2. Centrifugal force at equator “throws” you away from Earth = less gravity

    • Generally. Gravity varies with latitude and altitude


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    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.


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    Centrifugal Force is seen here with indirect high tides


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


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    Positive gravity anomaly

    Negative gravity anomaly

    Mountain

    Ore deposit

    Low density sedments

    Salt dome (low density


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    • 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

    Isostacy


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    • 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)


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    1.

    2.

    Glacial ice

    3.

    Ice melts (i.e. Hudson’s Bay

    4.


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    Mtn’

    Low density root

    erosion

    deposition

    deposition

    erosion

    deposition

    deposition


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    See question 11, WS 12.2


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    See question 16, UNIT 3 Review WS


    How layers of earth was determined2

    How Layers of Earth was Determined

    • Seismic

    • Heat Flow

    • Gravity

    • Magnetic Field


    4 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 field1

    4. Magnetic Field


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    • Magnetic Inclination = deviation in magnetic field from horizontal

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

    True north pole

    Magnetic pole

    Inclination/tilt of compass needle

    Lines of magnetic force


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    • 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

    MagneticInclination


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

    True north pole (Santa’s home)

    magnetic north pole

    1290 km

    19’ East

    Important for surveying…more on this in Chp 13

    compass

    Chilliwack


    Magnetic declination

    Magnetic Declination


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    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


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    • 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.


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    + mag’ anom’

    - mag’ anom’

    Lava flow/ diamond pipe

    Sed’ rx

    Fe ore

    Salt dome/ reef


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    • 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


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    • 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

    Reversal of Earth’s Magnetic Field

    • Animation


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    • 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!


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