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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

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
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




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



Evidence of plate tectonics
Evidence of Plate Tectonics

  • 1. Continental fit/jig-saw puzzle pieces






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


lithosphere sea floor splits and spreads apart.

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 sea floor splits and spreads apart.

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


animation



Q 60, p.18 WS 12.2 seafloor as iron-rich magma cools below the Curie Point to form pillow lavas and gabbro recording the Earth’s present magnetic field.


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

Lab 12.1 is next…it covers magnetic striping


young diaper” model

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)




d. Thickness of Sediments on Oceanic plates diaper” model

  • Thinnest near the ridge; thicker as you move away

Abyssal hill

Seamount

Abyssal plain



e. Heat Flow Highest at Ridge: b/c diaper” model

  • 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



Plate boundaries
Plate Boundaries diaper” model



Plate boundaries1
Plate Boundaries diaper” model

  • 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


Cont’l Margin diaper” model

  • 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 diaper” model

  • A. Passive Margins


Plate boundaries3
Plate Boundaries diaper” model

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


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


  • Also: open, and magma (lower pressure lower melting temp’) wells up to form sheeted dikes of gabbro, basalt and pillow lava.

    • 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 open, and magma (lower pressure lower melting temp’) wells up to form sheeted dikes of gabbro, basalt and pillow lava.

  • B. Active Margins

    • 1. Divergent Boundaries


Triple junctions
Triple Junctions open, and magma (lower pressure lower melting temp’) wells up to form sheeted dikes of gabbro, basalt and pillow lava.


c open, and magma (lower pressure lower melting temp’) wells up to form sheeted dikes of gabbro, basalt and pillow lava.

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


  • Magma melting temperature lowered by water open, and magma (lower pressure lower melting temp’) wells up to form sheeted dikes of gabbro, basalt and pillow lava.

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

  • Andestic magma


Fore arc basin open, and magma (lower pressure lower melting temp’) wells up to form sheeted dikes of gabbro, basalt and pillow lava.

Back arc basin

  • 2. Convergent Boundaries

    • a) Oceanic-oceanic

Accretionary Complex

Volcanic arc


  • Driving Force on oceanic plate is: open, and magma (lower pressure lower melting temp’) wells up to form sheeted dikes of gabbro, basalt and pillow lava.

    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.


Back arc basin open, and magma (lower pressure lower melting temp’) wells up to form sheeted dikes of gabbro, basalt and pillow lava.

Volcanicarc

Fore arc basin


Ridge push slab pull
“Ridge Push – Slab Pull” open, and magma (lower pressure lower melting temp’) wells up to form sheeted dikes of gabbro, basalt and pillow lava.


WA

OR

CA


Melange accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)

Fore arc basin

Volcanic arc


  • b) Oceanic-continental accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)

Volcanic arc

Folded mtn’s

Accretionary wedge

Fore arc basin

Back arc basin

trench

Cont’l crust

O.C.

U.M.

Upper mantle

asthenosphere


  • Magma melting temperature lowered by water accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)

  • 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


  • b) Oceanic-continental accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)


Folded Mountains accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)

Fore arc basin

b) Oceanic-continental

Back arc basin

Accretionary Complex

Volcanoes


Active margin accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)

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


Deformed & metamorphosed accretionary wedge accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)

c) continental - continental

Mtn’ range

Cont’l crust

Cont’l crust

Upper mantle

U.M.

oceanic crust

asthenosphere

Ex: Himalayas, Alps, Urals


c) Continental-continental accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)


2 convergent boundaries c continental continental
2. Convergent Boundaries accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas) c) Continental-continental


RH accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)

LH

3. Transform Boundary

  • Where plates slide past each other

  • Mainly associated with divergent boundaries

Transform boundary

RH

  • Shallow EQs <30 km


  • 3. Transform Boundary accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)

LH


Transform Faults accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)


LH accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)


Bc coast tectonic scenario
BC Coast Tectonic Scenario accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)


Juan de Fuca plate accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)

North American plate

Pacific plate

Gorda Plate


  • Note helper ends accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)

  • Please use your note book now.


Interplate setting
Interplate setting: accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)

  • 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 accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)


Paleozoic accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)300 my

North America


  • Mesozoic 100 my accretionary wedge = melange = subduction complex (mainly deep sea sediments/shale + pillow lavas)

  • North America


  • 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.


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 my Rodinia: 1000 my Columbia: 1800 my


Pangea 275 my
Pangea: 275 my opens and closes, and continents split apart and collide to form supercontinents, over and over again.


Rodinia 1000 my
Rodinia: 1000 my opens and closes, and continents split apart and collide to form supercontinents, over and over again.


Columbia 1800 my
Columbia: 1800 my opens and closes, and continents split apart and collide to form supercontinents, over and over again.


  • 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.


  • Do WS 12.2 builds creating diverging convection cells.


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