Geology 12Geology 12

PresentsPresents

Unit 3Unit 3Chp 10 Earth’s InteriorChp 10 Earth’s InteriorChp 11 Ocean FloorChp 11 Ocean FloorChp 12 Plate TectonicsChp 12 Plate TectonicsChp 9 Seismic (EQ)Chp 9 Seismic (EQ)Chp 13 StructureChp 13 Structure

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 Tectonics

Major Plate Boundaries

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

Rate of Plate MovementRate of Plate Movement

Evidence of Plate Tectonics• 1. Continental fit/jig-saw puzzle pieces

QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

• 2. Similarity of Rocks and Mountains

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

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

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

• 6. Seafloor Spreading: a 65,000 km longvolcanic mountain chain (ridge) in the oceans are where the sea floor splits and spreads apart.

5 pieces of evidence to support seafloor spreading to come

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

lithosphere

mantle

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

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.

• 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

• 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.

Q 60, p.18 WS 12.2

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

Lab 12.1 is next…it covers magnetic striping

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

youngold old

Oldest oceanic crust is 180 ma

Oldest continental crust is 4,000 ma (4 ba)

• c. Radiometric Dating of Oceanic Plate

c. Radiometric Dating of Oceanic Plate

d. Thickness of Sediments on Oceanic plates

• Thinnest near the ridge; thicker as you move away

Abyssal plain

Abyssal hill

Seamount

• d. Thickness of Sediments on Oceanic plates

e. Heat Flow Highest at Ridge: b/ci) Oceanic crust is thinnest at ridge = less insulation from

hot interior

ii) Oceanic crust is newly formed from molten rock = hot

4

3

2

1

0

Island arc (volcanoes)

World average

Oceanic ridge

trenchnew crustold crust

e. Heat Flow Highest at Ridge

Plate Boundaries

Please hand out WS 12.1 Note helper.Please hand out WS 12.1 Note helper.

Plate Boundaries• A. Passive Margins: where oceanic and cont’l

plates are fused and larges amount of sediment is deposited.

Cont’l PlateOceanic Plate

Cont’l Shelf Cont’l Slope Cont’l RiseAbyssal

Plain

Cont’l Margin

fused

• 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.

Cont’l PlateOceanic Plate

Cont’l Margin

fused

i.e. like the Rockies

Plate Boundaries

• A. Passive Margins

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)

1. Divergent Boundaries/Spreading Ridge

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.

basalt

gabbromantle

rift

• 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 Boundaries

• B. Active Margins– 1. Divergent Boundaries

Triple Junctions

– 2. Convergent Boundaries = where 2 plates collide

a) oceanic-oceanic

over

under

c u.m.Upper mantle

crust

asthenosphere

Accretionary wedge

trench Fore arc basin

Back arc basin

Volcanic isld’ arc

• Magma melting temperature lowered by water

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

• Andestic magma

• 2. Convergent Boundaries• a) Oceanic-oceanic

Accretionary Complex

Fore arc basin

Volcanic arc

Back arc basin

• 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.

For

e ar

c ba

sin

Vol

cani

c ar

c

Back arc

basin

“Ridge Push – Slab Pull”

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

WA

OR

CA

Melange

Fore arc basin

Volcanic arc

b) Oceanic-continental

O.C.U.M.

Cont’l crust

Upper mantle

asthenosphere

Folded mtn’s

Volcanic arc

Back arc basin

Fore arc basin

Accretionary wedge

trench

• 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

b) Oceanic-continental

b) Oceanic-continental

Fore arc basin

Accretionary Complex

Back arc basin

Folded Mountains

Volcanoes

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

O.C.U.M.

Cont’l crust

Upper mantle

asthenosphere

O.C.U.M.

Cont’l crust

Upper mantle

asthenosphere

Passive margin Active margin

Cont’l crust

c) continental - continental Deformed & metamorphosed

accretionary wedge

Mtn’ range

Upper mantle

asthenosphere

oceanic crust

Cont’l crustCont’l crust

U.M.

Ex: Himalayas, Alps, Urals

c) Continental-continental

2. Convergent Boundaries c) Continental-continental

3. Transform Boundary

• Where plates slide past each other

• Mainly associated with divergent boundaries

RH LH

Transform boundaryRH

•Shallow EQs <30 km

• 3. Transform Boundary

LH

Transform Faults

LH

BC Coast Tectonic Scenario

Pacific plate

North American

plate

Juan de Fuca plate

Gorda Plate

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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 Setting

Paleozoic 300 my

North America

• Mesozoic 100 my • 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 myRodinia: 1000 myColumbia: 1800 my

Pangea: 275 my

Rodinia: 1000 my

Columbia: 1800 my

• 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