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Transcript
Geology 12
Presents

Unit 3
 Chp
 Chp
 Chp
 Chp
 Chp
10 Earth’s Interior
11 Ocean Floor
12 Plate Tectonics
9 Seismic (EQ)
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 Movement
Evidence of Plate Tectonics
• 1. Continental fit/jig-saw puzzle pieces
QuickTime™ and a
Sorenson Video 3 decompressor
are 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 long
volcanic 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
young
old
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
Seamount
Abyssal
hill
Abyssal
plain
• d. Thickness of Sediments on Oceanic plates
e. Heat Flow Highest at Ridge: b/c
i)
Oceanic crust is thinnest at ridge = less insulation from
hot interior
ii)
Oceanic crust is newly formed from molten rock = hot
4
3
Oceanic ridge
Island arc
(volcanoes)
2
World average
1
0
trench
old crust
new crust
e. Heat Flow Highest at Ridge
Plate Boundaries

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 Margin
Cont’l Shelf
Abyssal
Plain
Oceanic Plate
Cont’l Plate
fused
Cont’l Margin
Oceanic Plate
Cont’l Plate
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.
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.
rift
mantle
• 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
Accretionary
Volcanic isld’ arc
wedge
Fore arc
trench
Back arc
basin
basin
c u.m.
asthenosphere
crust
Upper mantle
• 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 Fore arc
basin
Accretionary
Complex
Back arc
basin
Volcanic arc
• 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.
“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
Volcanic arc
Accretionary
wedge Fore arc
basin
trench
O.C.
U.M.
asthenosphere
Folded mtn’s
Back arc
basin
Cont’l crust
Upper mantle
• 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
Folded
Fore arc Mountains
basin
Back arc
basin
Accretionary
Volcanoes
Complex
• If an oceanic – continental subduction
continues … it will result in:
Passive margin
Active margin
O.C.
U.M.
asthenosphere
Cont’l crust
Cont’l crust
Upper
mantle
Cont’l crust
Upper
mantle
asthenosphere
Deformed &
metamorphosed
accretionary wedge
c) continental - continental
Mtn’ range
Cont’l crust
Cont’l crust
Upper mantle
U.M.
asthenosphere
Ex: Himalayas, Alps, Urals
c) Continental-continental
2. Convergent Boundaries
c) Continental-continental
3. Transform Boundary
RH
LH
• Where plates slide past each other
• Mainly associated with divergent
boundaries
Transform boundary
RH
•Shallow EQs <30 km
• 3. Transform
Boundary
LH
Transform Faults
LH
BC Coast Tectonic
Scenario
Juan de
Fuca plate
Pacific
plate
North
American
plate
Gorda
Plate
Note helper ends
 Please use your note book now.

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 my
Rodinia: 1000 my
Columbia: 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