Geology 12
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Geology 12. Presents. Unit 3 Chp 10 Earth’s Interior Chp 11 Ocean Floor Chp 12 Plate Tectonics Chp 9 Seismic (EQ) Chp 13 Structure. Chp 12 Plate Tectonics.

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Geology 12

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Geology 12

Geology 12

Presents


Geology 12

  • Unit 3

    • Chp 10 Earth’s Interior

    • Chp 11 Ocean Floor

    • Chp 12 Plate Tectonics

    • Chp 9 Seismic (EQ)

    • Chp 13 Structure


Chp 12 plate tectonics

Chp 12 Plate Tectonics

  • Theory is that Earth consists of about 18-20 rigid lithospheric plates that move about the Earth’s surface on a plastic asthenosphere and mantle.

  • Lithosphere = crust + upper mantle (UM)

  • Lithospheric plates:

    • Cont’l: up to 250 km thick (crust 90 + UM 160)

    • Oceanic: up to 100 km thick (crust 10 + UM 90)

  • Move 2 – 20 cm/yr but average is 2-3 cm/yr


Chp 12 plate tectonics1

Chp 12 Plate Tectonics


Major plate boundaries

Major Plate Boundaries


Lithospheric plates crust upper mantle

Lithospheric Plates = crust + upper Mantle

Up to 100 km thick

Up to 250 km thick


Plates move 2 20 cm yr but average is 2 3 cm yr

Plates move 2 – 20 cm/yr but average is 2 – 3 cm/yr


Rate of plate movement

Rate of Plate Movement


Evidence of plate tectonics

Evidence of Plate Tectonics

  • 1. Continental fit/jig-saw puzzle pieces


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  • 2. Similarity of Rocks and Mountains


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  • 3. Glacial Evidence: Glacial striations indicate movement of ice away from the pole


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  • 4. Fossil Evidence: same fresh water land fossils found on different continents


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  • 5. Paleomagnetism and Polar Wandering: plates moved N/S as given by magnetic inclination.


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volcanic mountain chain (ridge) in the oceans are where the sea floor splits and spreads apart.

  • 6. Seafloor Spreading: a 65,000 km long

5 pieces of evidence to support seafloor spreading to come


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lithosphere

mantle

  • As oceanic plates are driven apart by thermal convection cells/currents in the mantle, new oceanic crust forms in the rift.

  • New oceanic crust is created at the ridge; old oceanic crust is destroyed as it plunges down the trenches.


6 evidence of seafloor spreading

6. Evidence of Seafloor Spreading

  • a) GPS = Global Positioning Satellites in space give exact positions of continents; they tells us exactly how the plates are moving.


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  • b. Reversal of Earth’s Magnetic Field is recorded on the seafloor as iron-rich magma cools below the Curie Point to form pillow lavas and gabbro recording the Earth’s present magnetic field.

animation


Geology 12

  • b. Reversal of Earth’s Magnetic Field is recorded on the seafloor as iron-rich magma cools below the Curie Point to form pillow lavas and gabbro recording the Earth’s present magnetic field.


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Q 60, p.18 WS 12.2


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To find the middle of oceanic ridge, use the “dirty diaper” model

Lab 12.1 is next…it covers magnetic striping


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young

old

old

  • c. Radiometric Dating of Oceanic Plate: youngest at ridge; older as you move away

Oldest oceanic crust is 180 ma

Oldest continental crust is

4,000 ma (4 ba)


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  • c. Radiometric Dating of Oceanic Plate


C radiometric dating of oceanic plate

c. Radiometric Dating of Oceanic Plate


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d. Thickness of Sediments on Oceanic plates

  • Thinnest near the ridge; thicker as you move away

Abyssal hill

Seamount

Abyssal plain


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  • d. Thickness of Sediments on Oceanic plates


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e. Heat Flow Highest at Ridge: b/c

  • Oceanic crust is thinnest at ridge = less insulation from hot interior

  • Oceanic crust is newly formed from molten rock = hot

4

Oceanic ridge

Island arc (volcanoes)

3

2

World average

1

new crust

old crust

trench

0


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e. Heat Flow Highest at Ridge


Plate boundaries

Plate Boundaries


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  • Please hand out WS 12.1 Note helper.


Plate boundaries1

Plate Boundaries

  • A. Passive Margins: where oceanic and cont’l plates are fused and larges amount of sediment is deposited.

Cont’l Margin

Cont’l Shelf

Cont’l Slope

Abyssal Plain

Cont’l Rise

Oceanic Plate

Cont’l Plate

fused


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Cont’l Margin

  • As oceanic plate becomes thicker, it becomes heavier, plus it gets pushed down with sediment. If/when this boundary becomes active, the sediment will be pushed into mtn’s.

Oceanic Plate

Cont’l Plate

fused

i.e. like the Rockies


Plate boundaries2

Plate Boundaries

  • A. Passive Margins


Plate boundaries3

Plate Boundaries

  • B. Active Margins: where plates are moving away (#1: plate is being created), towards (#2: plate is being destroyed), or past each other (#3)


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Crust is pulled apart by convecting mantle, thins, breaks open, and magma (lower pressure lower melting temp’) wells up to form sheeted dikes of gabbro, basalt and pillow lava.

  • Divergent Boundaries/Spreading Ridge

rift

basalt

mantle

gabbro


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  • Also:

    • High heat flow

    • Basaltic/mafic lava

    • Shallow (& mild) EQs (<30 km)

    • Rugged topography (seamounts, basalt floods, pillow lava)

    • Starts off as

      • i) doming/crustal unwrap

      • ii) rift valley & basalt floods

      • iii) narrow sea (i.e. Red, Dead) as continents split up

      • iv) spreading ocean (i.e. Atlantic)


Plate boundaries4

Plate Boundaries

  • B. Active Margins

    • 1. Divergent Boundaries


Triple junctions

Triple Junctions


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c

crust

u.m.

Upper mantle

asthenosphere

over

  • 2. Convergent Boundaries = where 2 plates collide

    a) oceanic-oceanic

under

Accretionary wedge

Volcanic isld’ arc

Fore arc basin

trench

Back arc basin


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  • Magma melting temperature lowered by water

  • Deepest trenches (11 km) because both plates are heavy (3.0 gm/cm3)

  • Andestic magma


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Fore arc basin

Back arc basin

  • 2. Convergent Boundaries

    • a) Oceanic-oceanic

Accretionary Complex

Volcanic arc


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  • Driving Force on oceanic plate is:

    i) pushed/dragged by convecting mantle = “ridge push”:

    ii) Pulled by sinking oceanic slab in mantle = “slab-pull”:

  • Deep EQs (100 - 700 km)

  • Ex: Aleutian Islds, Japan, Taiwan, Philippines, New Zealand, Caribbean Islds.


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Back arc basin

Volcanicarc

Fore arc basin


Ridge push slab pull

“Ridge Push – Slab Pull”


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  • Sediment is scraped off descending ocean floor to form: accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)

WA

OR

CA


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Melange

Fore arc basin

Volcanic arc


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  • b) Oceanic-continental

Volcanic arc

Folded mtn’s

Accretionary wedge

Fore arc basin

Back arc basin

trench

Cont’l crust

O.C.

U.M.

Upper mantle

asthenosphere


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  • Magma melting temperature lowered by water

  • Andestic magma

  • Driving force on oceanic plate is:

    • i) pushed/dragged by convecting mantle

    • ii) pulled by sinking oceanic slab in mantle

  • Deep EQs: up top 700 km

  • Ex: Nazca and S. American Plates


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  • b) Oceanic-continental


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Folded Mountains

Fore arc basin

b) Oceanic-continental

Back arc basin

Accretionary Complex

Volcanoes


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Active margin

Passive margin

  • If an oceanic – continental subduction continues … it will result in:

Cont’l crust

O.C.

U.M.

Upper mantle

asthenosphere

Cont’l crust

Cont’l crust

Upper mantle

O.C.

U.M.

asthenosphere


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Deformed & metamorphosed accretionary wedge

c) continental - continental

Mtn’ range

Cont’l crust

Cont’l crust

Upper mantle

U.M.

oceanic crust

asthenosphere

Ex: Himalayas, Alps, Urals


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c) Continental-continental


2 convergent boundaries c continental continental

2. Convergent Boundaries c) Continental-continental


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RH

LH

3. Transform Boundary

  • Where plates slide past each other

  • Mainly associated with divergent boundaries

Transform boundary

RH

  • Shallow EQs <30 km


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  • 3. Transform Boundary

LH


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Transform Faults


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LH


Bc coast tectonic scenario

BC Coast Tectonic Scenario


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Juan de Fuca plate

North American plate

Pacific plate

Gorda Plate


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  • Note helper ends

  • Please use your note book now.


Interplate setting

Interplate setting:

  • Continental: during the Paleozoic (570 – 245 ma) and Mesozoic (245 – 66 ma), inland seas covered most of the continents, except mountains, so it ranged from swampy (i.e. ferns – coal at the edges of the seas in W. Alberta & Pennsylvannia, Kentucky) to inland shallow marine seas (Devonian reefs from Alberta to Texas)


Interplate setting1

Interplate Setting


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Paleozoic 300 my

North America


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  • Mesozoic 100 my

  • North America


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  • Cenozoic (66 ma) to present, it has been mainly erosion of the continents and sedimentation on the margins.

  • Oceanic setting: plates are very new, largely 2 major events occuring in the middle of the plates:

    • i) sedimentation (clays and ooze)

    • ii) hot spot volcanism (Hawaii-Emperior chain) give absolute plate velocity.


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Wilson Cycle is 500 ma period where the Atlantic Ocean opens and closes, and continents split apart and collide to form supercontinents, over and over again.3 times at least:Pangea: 275 myRodinia: 1000 myColumbia: 1800 my


Pangea 275 my

Pangea: 275 my


Rodinia 1000 my

Rodinia: 1000 my


Columbia 1800 my

Columbia: 1800 my


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  • 0 – 100 ma: “supercontinent” insulates mantle; heat builds creating diverging convection cells.

  • 100 – 300 ma: rifting and creation of new ocean basin. New continents separated by widening ocean basin.

  • 300 – 500 ma: oceanic crust becomes thicker, heavier, & sinks at passive margin becoming an active margin – subduction bdy’; continents come back together, collide and create high mtn’ chain.


Geology 12

  • Do WS 12.2


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