10/11/2009

1

Plate Tectonic Theory

• Not continental ‘drift’ !!

• Importance:

– The unifying theory for all other theories in the Earth sciences

– Has resulted in a more detailed understanding of Earth history

– Has enabled geologists to more precisely (and cheaply) discover new oil, natural gas, and other subsurface natural resource deposits

Lithosphere and Asthenosphere

Important Definitions

• Lithospheric Plate – a large segment of Earth’s crust and upper parts of mantle; extends to top of the low velocity zone

• Lithospheric Plate Boundary (Margin)

– Divergent - plates move away from one another

– Convergent – plates move toward one another

– Transform – plates move horizontally past one another

Part I

Development of Plate Tectonic

Theory

Alfred Lothar Wegener (1880-1930)

10/11/2009

2

Pangaea (Greek, meaning ‘all lands’)

Positions of the continents approximately 200 million years ago

Wegener’s Evidence for Pangaea:

• ‘Fit of the Continents’

– Continental margins fit together like pieces of a jigsaw puzzle

• Paleontological

– Similar fossils on opposite sides of ocean basins

• Geological

– Similar rock types on opposite sides of ocean basins

• Climatological

– Evidence of glaciation in tropical climates

– Fossils of tropical plant species in polar climates

Fit of the Continents Paleontological Evidence

Locations of Mesosaurus fossils

Glossopteris

10/11/2009

3

Glossopteris

Geological Evidence

Similarity of rock types and ages in the

Appalachian, British Isles and Caledonian

mountain systems

Geological Evidence

Reconstruction of the Appalachian mountain

system at the time it formed, approximately

300 million years ago

Scottish Highlands

Highlands, North CarolinaClimatological Evidence

10/11/2009

4

Glacial GroovesWhy was Wegener’s idea rejected?

Wegener (left) and an Innuit guide on his last expedition in Greenland.

This is one of the last photographs taken of Wegener, taken shortly

before his death.

Discovery of The Mid-Ocean Ridge System

Mid-Ocean Ridge System

Paleomagnetism – the study of Earth’s

magnetic field as recorded in ancient rocks

Magnetic minerals ‘point’ in the direction of

the magnetic north pole.

10/11/2009

5

Apparent Polar Wandering: Two North Poles? Apparent Polar Wandering: Single North Pole

Use of Magnetic Minerals to Determine Paleolatitude

Step 1:

Determine Magnetic Dip:

a) 90o (NH)

b) 63o (NH)

c) 37o (NH)

d) 60o (SH)

e) 6o (SH)

Step #2: Determine Paleolatitude

From Magnetic Dip

a)90o N (North Pole)

b)42o N

c)18o N

d)38o S

e)3o S

Cox and Dallrymple: Magnetic Reversals

from all continents record changes in Earth’s

magnetic field at the same time

Cox and Dallrymple: The Magnetic Time

Scale

10/11/2009

6

Harry Hess: The mid-ocean ridge is a site of

sea floor spreading

Vine and Matthews: Magnetic sea floor

stripes record magnetic reversals

Normal Polarity – exhibited by rocks with

magnetic minerals which ‘point’ towards

today’s position of the magnetic north pole

Reversed Polarity – exhibited by rocks with

minerals which ‘point’ towards today’s

position of magnetic south

As the sea floor spreads, repeated ‘stripes’

of normal and reversed polarity ocean floor

basalts are erupted along the MOR Problem:

What did the theory of sea

floor spreading imply about

the size of the Earth?

10/11/2009

7

Solution to Problem:

Benioff Zones

Distribution of shallow-, intermediate-, and

deep-foci earthquakes:

Benioff Zones

Part II

The Theory

Types of Lithospheric Plate Boundaries

• Convergent Plate Boundaries

– Characterized by compressive tectonic stress

– Oceanic – continental convent boundary

– Oceanic-oceanic convergent boundary

– Continental-continental convergent boundary

• Divergent Plate Boundaries

– Characterized by extensional tectonic stress

– Oceanic-oceanic divergent boundary

– Continental-continental divergent boundary

Types of Lithospheric Plate Boundaries

• Transform Plate Boundaries

– Characterized by shear tectonic stress

– Continental-continental transform plate

boundary

– Oceanic-oceanic transform plate boundary

10/11/2009

8

Oceanic-Continental Convergent Plate Boundary

• Subduction of higher density (mafic)

oceanic lithosphere beneath lower density

(felsic) continental lithosphere

– Formation of magma in the mantle wedge

– Formation of a continental margin volcanic arc

on the overriding continental lithosphere

• Examples: Cascade Range, Andean Range

Formation of a subduction zone and continental

margin volcanic arc

Cascade Range Andes Range

Oceanic-Oceanic Convergent Plate Boundary

• Subduction of (mafic) oceanic lithosphere

beneath (mafic) oceanic lithosphere

– Formation of magma in the mantle wedge

– Formation of a volcanic island arc on the

overriding oceanic lithosphere

• Examples: Lesser Antilles, western Aleutians,

Japanese islands

Formation of a subduction zone and

volcanic island arc

10/11/2009

9

Lesser AntillesAleutian Islands

Japanese Islands Continental-Continental Convergent Plate

Boundary

• Low density (felsic) continental rocks

pushed upward by compressive tectonic

forces

– Medium to high grade metamorphism

– Earth’s highest mountain ranges

• Examples: Himalayas, Appalachians

HimalayasAppalachians

10/11/2009

10

Formation of a mountain range along a

continental-continental convergent plate boundaryThe On-Going Collision of India and Asia

Approximately 45 million years ago

Paleolatitudes of India (71 million years to present)

Modern-Day Tectonic Setting of India

Mount Everest, Himalayas Oceanic Lithosphere-Oceanic Lithosphere

Divergent Plate Boundary

• Results in partial melting of underlying ultramafic mantle rock, producing a mafic (basaltic) magma

• Mafic lavas are erupted on ocean floor, forming new ocean floor basalts

• Lithospheric plates on either side of the plate boundary mive in opposite directions

• Off-set segments of the ridge axis are connected by transform faults

10/11/2009

11

Oceanic Lithosphere-Oceanic Lithosphere

Divergent Plate Boundary

Continental Lithosphere-Continental

Lithosphere Divergent Plate Boundary

• Stretching, fracturing and sinking of continental lithosphere forms a continental rift (rift valley)

– Example: East African Rift

• Eruption of basaltic lavas and filling with seawater forms a linear sea

– Examples: Red Sea, Gulf of Aden

• Widening of linear sea forms an ocean basin

Continental Rifting

East African Rift East African Rift Volcanoes

10/11/2009

12

Transform Boundaries

• Continental lithospheric plates move

laterally past one another

– Transform fault – a fault which connects two

off-set segments of a mid-ocean ridge

• Characterized by frequent earthquake

activity

– Example: San Andreas Fault System

San Andreas Fault

Plate Tectonic Boundaries

Western North America

50 ma

10/11/2009

13

Present

Cascade Range Sierra Nevada Range and Coast Range

Basin and RangeRocky Mountains

10/11/2009

14

Terrain vs. Terrane

• Terrain – the natural features of an area of

land; the lay of the land’

• Terrane – A crustal fragment whose

geologic history is distinct from that of

adjoining terranes

– Accreted Terrane - a terrane accreted to

adjoining terranes by tectonic forces

Terrane Accretion – Formation of Coast Range

Accreted Terranes in Western N. AmericaFormation of Rocky Mountains and Basin

and Range Provinces

• Rocky Mountains

– Major intraplate mountain system

– Exhibits evidence of compressive stresses,

typical of a convergent plate margin

• Basin and Range

– Major intraplate mountain system

– Exhibits evidence of extensional stresses,

typical of a divergent plate margin

Rocky Mountains – formed due to ‘shallow’ subduction of

oceanic lithosphere beneath western N. America.

10/11/2009

15

Rocky MountainsBasin and Range – formed due to ‘sinking’ of the

subducted lithosphere, allowing for upwelling of hot mantle

rock beneath the crust.

Basin and RangeRio Grande Rift – part of the Basin and Range

Rio Grande RiftRio Grande Rift

10/11/2009

16

Mantle Plumes – Intraplate VolcanismPacific Ocean Hot Spot Tracks

The Driving Force of Plate Tectonics

• Mantle Convection

– Heat transfer from Earth’s core drives convection

currents within the mantle

• Slab-Pull/Slab-Push

– Gravity ‘pushes’ oceanic lithosphere (slabs) away

from mid-ocean ridge axes, and ‘pulls’ slabs into

subduction zones

• Hot Plumes

– Hot rising plumes of mantle rock drive mantle

convection currents

Mantle ConvectionSlab-Pull/Slab-Push

10/11/2009

17

Hot PlumesPangaea – 200 million years ago

The Breakup of Pangaea The Breakup of Pangaea

The Breakup of Pangaea The Breakup of Pangaea

10/11/2009

18

Present-day Tectonic Plate Rates and

Directions of Motion

Tectonic Setting of Northern

Sumatra