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STAG1032: PHYSICAL GEOLOGY (Geologi Fizik)

Teori Tektonik Keping(Plate Tectonic Theory)

Dr. Wan Zuhairi Wan YaacobProgram Geologi, UKM

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continent

Mechanism of continental drift(Wegener, 1915)

Oceanic crust

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Earth’s Internal Structure

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Earth’s Major Plates

• Lithosphere– Crust and Uppermost

mantle– Strong & Rigid layer– Plates

• Asthenosphere– Weaker layer– Mantle

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Lithosphere• Solid & rigid• Responds to stress by bending and

breaking• 100-150kms (below continents)• 70-80kms (beneath oceans)• Light (granitic) continental crust – SIAL

(2.7g/cm3)• Denser (basaltic) oceanic crust – SIMA

(3.0 g/cm3)• Crust & uppermost mantle

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Asthenosphere

• Greek “weak rock”• More plastic response to stress

(semiplastic)• Squeeze and flow• Beneath the lithosphere• 250kms thick• Less dense compared to oceanic crust

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PLATES• Broken segments in lithosphere• 7 major plates : North American, South

American, Pacific (the largest), African, Eurasian, Australian-Indian, Antarctic Plates

• Move slow but continuous (slow but steady !!):5cm/50mm/2inches per year

• Movement creates : Earthquakes; volcanoes; mountains deformation

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PLATE TECTONICS THEORY

• Lithosphere (oceanic, continental crust, upper mantle) – called “plates” fragmentation of lithosphere

• Lithospheric plates move over the hotter/weaker semiplastic/plastic asthenosphere.

• Move-separate – mid oceanic ridge

• Plates collide– i. One plate dives underneath the other– ii. Slide past one another

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Plates and plate boundaries• Three types of plate

boundaries:-

• (1) Divergent Plate Boundaries– Tensional stress

• (2) Convergent Plate Boundaries – Compressional stress

• (3) Transform Faults– Shear stress

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(1) Divergent Plate Boundaries

• Tensional stress • Divergent : spread out (mencapah)• Constructive margins (sempadan membina)• Spreading centers; mid oceanic ridges

– High heat flow; mild volcanic activity; shallow earthquakes (<40km deep)

• Theory of Sea Floor Spreading (SFS)– Teori Pemuaian Dasar Lautan– 2-5-15 cm per year

• New oceans Form

Di = apart; vergere = to move

MOVIE

The mechanism that operates along the oceanic ridge systemto create new seafloor.

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OCEANIC RIDGE

1000-4000 kms

2-3 kms

70,000 kms

Rift Valley(deep downfaultedvalley)

30-50kms

<200m – 3km

The longest topographic feature on Earth’s surface

(1) Divergent Plate Boundaries

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Mid Atlantic ridge

trench

“The ring of fire”

Mid Indian ridge

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(1) Divergent Plate Boundaries

Continental Rifts

• Spreading centers• Continent • East African Rift • Initial stage of

breakup• Extensive volcanic

activity• Red sea (formed

about 20 m.a.)

Tarbuck & Lutgens, 2003. Earth Sc.

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(2) Convergent Plate Boundaries

con = together; vergere = to move

• Compression stress• Subduction (sub=under; duct =lead) (subduksi)• Destructive margins (memusnah)• Convergent (towards the same point)

• Oceanic-oceanic • Continental-oceanic• Continental-continental

• Ocean Trench (parit lautan)

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Anda di sini

Subduction zones-lithosphere is descending (being subducted) into the astenosphere

A

B

Long: thousands of KmsDeep: 8-12 kmsWide:50-100 kms

(2) Convergent Plate Boundaries

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Plat A Plat B

A B

Lithosphere (oceanic)- High density

Lithosphere (continental)- Less dense

AsthenosphereAsthenosphere(less dense)

Direction of plate movementDeep ocean trench

Destructive plate margin(subduction zone)

(2) Convergent Plate Boundaries

Angle of descent(ب )

SFS

The leading edge

Partial melting(peleburan separa)- Generate 10% molten material;intermixed with unmelted mantle rockTeardrop-

shaped structure

100km

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(2) Convergent Plate Boundaries

(AVERAGE angle)

45o

A few degrees

90o

Angle of descent depends on DENSITY

Close from spreading centre-Young lithosphere-Warm and buoyant

Far from spreading centre-Ages (old) lithosphere-Cool, thicken and more dense

oceaniccontinent

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A. Oceanic – continentaleg. Andes; Western USA

B. Oceanic – Oceaniceg. Japan ; Indonesia

C. Continental – continentaleg. Himalaya

(2) Convergent Plate Boundaries Continental volcanic arch

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(2) Convergent Plate Boundaries

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(3) Transform Faults

• Shear stress• Two plates grind past each other• Only earthquakes; no volcanic• Ocean basin : cut the spreading

centers• Continental crust:

– i. San Andreas Fault of California– Ii. Alpine fault of New Zealand

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(3) Transform Faults

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Conservative: San Andreas Fault(3) Transform Faults

movie

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EVIDENCE OF PLATE TECTONICS MODEL

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1. PALEOMAGNETISM

• “FOSSIL” magnetic field– Study of ancient earth’s magnetic field– Iron bearing minerals align themselves with

earth magnetic field• Magnetometer [World War II (1941-45)]

– sensitive devices for detecting magnetic fields (measure intensity of magnetic field).

• i. Polar wandering (continental drift hypo)

• ii. Magnetic reversals and sea floor spreading

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i. POLAR WANDERING

• The migration of magnetic pole through time

• 500 m.y. : wandering of magnetic north pole from Hawaii – Siberia – present spot.

• Why ? (i) Plate tectonic or (ii) Polar wandering

• “If the magnetic poles remain stationary, their apparent movement was produced by the drifting of the continents”.

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i. POLAR WANDERING

North America

North America

EurasiaEurasia

AfricaAfrica

The positions of wandering paths when the landmasses are reassembled in their predriftlocations

Polar Wandering of the earth magnetic pole. Same age with different pole positions (???)

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ii.Magnetic reversals and sea floor spreading

Evidence that support SEA FLOOR SPREADING THEORY

Earth’s magnetic field periodically reverses polarity; i,e, the north magnetic pole becomes the south magnetic pole, and vice versa

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2. EARTHQUAKE PATTERNS

• Global distribution of earthquakes and their close association with plate boundaries

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3. OCEAN DRILLING

• Drilling into ocean-floor sediment• “Deep Sea Drilling Project” – using

drilling ship• Taking sediment samples and determine

their age (fosil / radiometric dating).

• Result: – (1) the age of sediment increased with the

increasing distance from the ridge.– (2) the thickness of sediments increased

from the ridge

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3i. Crustal Age

younger older

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4. HOTSPOTS

• Hotspot: a location on the earth’s surface that has experienced active volcanism for a long period of time.

• Age Radiometric dating increases with increasing distance from Hawaii Rising plume of mantle is located below Hawaii (i.e. HOTSPOT)

• 40-50 hotspots

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Hawaiian chain of islands

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Fixed / stationary hotspot !!)

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THE DRIVING MECHANISM

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The Driving Mechanisms

• 1. Thermal convection– Rising convection

current– Descending

convection current

• 2. Gravity driven “slab-pull” or “ ridge-push”

Arus Olakan Haba

Analogy for slab-pullPage 148:Dutch et al

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Thermal convection current

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THE END