drift wegener move gaea · c:\new folder june 2018 cdrive tue 27\bc science 10 new\unit 4\ch...
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C:\New folder june 2018 cdrive tue 27\BC Science 10 New\Unit 4\CH 12\Notes\ch 12 tectonics.docx Page 1 of 12 Last printed 25/07/2018 12:28 PM
Sec 12.1 p. 506 Evidence for Continental Drift
Alfred Wegener: continental drift theory
Continents move around
200 mya Pangaea (pan = all, gaea= earth)
fg 12.8 p. 508
Evidence: http://www.youtube.com/watch?v=RQm6N60bneo vid vid 2
-world map fig: fg 12.1 p. 506
Jigsaw puzzle Fit: continents on either sides of
the oceans seem to fit together
best fit underwater with continental shelves
-Similar geology:
mountain: ranges,
folds, rock age,
-Similar fossils: fg
12.4 p. 508 6000 km
apart across salt water
ocean?
Mesosaurus (fresh
water)
Cynognathus &
Lystsrosaurus (land
animals)
Glossopteris (mild
climate plant fern)
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Climate: fg 12.5 p. 509 patterns of
paleoglaciation
Coal in Antarctica:
formed by decomposition of living things
(tropical swamp material)
see science watch p. 500
Wegener: no hypothesis: How can continents
move? died 1930
Possible mechanism: plate tectonics
Surface of earth is broken into movable slabs of
rock = rigid plates slide over partly molten rock
Evidence:
a) Fg 12.6 p. 510 Location of earthquakes and volcanoes → pattern → boundaries between tectonic plates
ex: “ring of fire” in pacific ocean vid4 vid4
b) fg 12.7 p. 510 underwater mountain
ranges: ex: Mid-Atlantic Ridge (MAR)
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c) age of ocean floor: fg 12.8 p. 511 changes from new on either side of Ridge to old, farther away from the
Ridge
vid3 d) thickness of sediment: fg 12.8 p. 511 thin near the ridge, thick away from the ridge
e) magnetism: i) fg 12.9 p. 511 earth acts like a giant bar magnet (magnetic N pole, magnetic S pole)
undergoes magnetic reversal every 250 000 y:
(S pole → N pole)
***** geographic poles: axis of rotation of the earth, do not change ****
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ii) paleomagnetism: fg 12.10 p. 512
study of the magnetic properties of ancient rocks
magnetic striping: rock on either side of the MAR changes its magnetic field pattern as you move
away from the ridge due to magnetic reversals
Explanation for observations:
1960 Harry Hess: sea floor spreading
Fg 12.11 p. 512 Less dense magma (liquid rock) rises at a spreading ridge and hardens forming new
ocean floor
→ earth’s magnetic field magnetizes the new ocean floor as it hardens
New magma keeps rising at spreading ridge
Older sea floor is pushed aside by newer sea floor
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1960: Canadian J.Tuzo Wilson → plate tectonic theory
Combined the following concepts:
➢ sea floor spreading (ocean floor creation)
➢ ocean floor destruction
➢ mountain formation
➢ earthquakes
➢ volcanoes
➢ island arcs from hot spots (Hawaii)
➢ continent movements
read 12.1 Wb 12.1
Sec. 12.2 Features of Plate Tectonics p. 518
4.5 billion years ago (bya): liquid earth cooled → density differences → least dense elements (Si, O) at
surface of crust → silicates
layers of the earth: fg 12.14 p. 520
1) crust: solid: thin 2 types
a) continental (granite) 70 km thick (less dense)
b) oceanic (basaltic) 10 km thick (more dense)
2) mantle (2): p. 519 fg 12.13
2900 km thick ,70% of volume
a) upper: partly molten rock: 660 km, flows
toothpaste
b) lower: solid (2225 km)
3) lithosphere: p. 520 fg 12.15
tectonic plates: crust and top of upper mantle
part that move across surface of earth
12 plates (continental & oceanic)
4) outer core: p. 519 fg 12.13
2300 km
liquid (Fe, Ni)
5) inner core: 1200 km
5000°C
solid
cores rotate at different speeds → earth’s protective magnetic fields = magnetosphere
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asthenosphere: p. 520 fg 12.14
-below lithosphere
-part of upper mantle where convection currents occur
p. 520 fg 12.15
source of plate movements
spreading ridges (2) p. 522 fg 12.16
ridge push: rising convection currents bring magma to crust surface becoming new crust which
pushes the older crust over
a) oceanic ridge (ocean) p. 522 fg 12.16
b) rift valley (land)p. 523 fg 12.18
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Process for spreading ridges is subduction: p. 522 fg 12.15
one plate (more dense) will go underneath the other less dense colliding plate
→ subduction zones → earthquakes & volcanoes p. 525 fg 12.19A
slab pull: p. 522 fg 12.16
gravity pulls old plate near subduction zone downward HW: quiz 12, Cyu 12.1.
Plate Interactions: p. 523 fg 12.17
Plate boundaries: 3
1) Divergent: spreading ridges: p. 522 fg 12.16
ocean or rift valley (land) p. 523 fg 12.18
New crust is formed
2) Convergent: 3 types of plate collisions
a) oceanic-continental p. 525 fg 12.19A
more dense oceanic plates subduct → ocean trench (very deep) → mountains, volcanoes, earthquakes
ex: BC’s Coast Mountains
Cascade Mountain Range
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b) oceanic-oceanic p. 525 fg 12.19B
older more dense plate subducts
→ volcanoes → island arcs
ex: Japan, Indonesia, Philippines, Aleutians (Alaska)
c) continental-continental plate collisions p. 525 fg 12.19C
no subduction: equal density
→ massive mountains ex: Himalayas
3) Transform boundaries: p. 526 fg 12.20
plates slide past each other
most along spreading ridges
→ faults (breaks in rock layers) → earthquakes
ex: San Andreas Fault off California
Hotspots: place on ocean floor where magma rises and penetrates the
the surface of the passing plates
Islands are formed ex: fg 12.12 p. 513
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Earthquakes: major 200-800 y http://www.youtube.com/watch?v=oSSqaVJz4gc
Friction between tectonic plates (95%)
80% = ring of fire (Pacific Ocean)
p. 528 fg 12.22
Can not predict: Timing, size, location
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focus: origin of earthquake: underground
epicentre: location on surface above the focus
classification of earthquake based upon the
depth from focus / hypocenter
p. 528 table 12.22
shallow: < 70 km from surface
intermediate 70-300 km
deep > 300 km
90 % occur < 100 km focus
shallow = more damage
Seismic waves: earthquake vibrations (waves)
are used to determine earth’s
layers locations
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3 types of waves: table 12.3 p. 529
a) body waves (2 kinds): fastest
travel inside the earth
i) primary waves (P-waves): 6 km/s, fastest
move through all states of matter (solid, liquid, gas)
like a spring (compression : stretching)
ii) secondary waves (S-waves): shear waves: 3.5 km/s, second fastest
move through solids only but not liquids or gasses
travel like a snake: perpendicular to the direction of travel
larger, cause more damage
b) surface waves (L waves): travel along surface only
slowest ( 1km/s), ripples on a pond
Body waves travel at different speeds and directions within the earth due to density differences of the layers
Fg 12.23 p. 530
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Seismometers: fg 12.24 p. 530
measure degree of ground movement → seismogram (paper with squiggly lines)
shows: time, length, and magnitude
magnitude = strength
Richter scale: each number is 10 times more powerful then
the number before
2 = 10x more than magnitude 1can feel
3 = 100 x more
4 = 1000 x
5 = 10 000 x
6 = 100 000 x = building damage
Time-distance Graph for Seismic Waves: fg 12.25 p. 521
Tells: how fast different waves are traveling
You need 3 seismometers to determine the location of an earthquake