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L7; Plate Tectonics

Chap. 2 p. 27-53;

CANADIAN HISTORY

J. Tuzo Wilson (19081993): Canadian geophysicist, father of "platetectonics", visionary scientist and educator

DIFFERENTIATION OF THE EARTH

Earth is differentiated into layers based on density and physical properties &strength

- Crust: 5 to 30km

- Mantle: 2900 km- Core: 3461 km

DENSITY MODEL

Thin crust rich in silicon and oxygen

Magnesium- and iron-rich mantle Iron-rich metallic core

ISOSTASY: Buoyancy principle

Low-density continents float on top of the denser mantle

PLATE TECTONICS MODEL Rigid lithosphere

- Crust and upper mantle

Plastic asthenosphere- Upper mantles

- Convection currents

- More viscous than asthenosphere- Slower convection

Liquid outer core

Solid inner core

CONVECTION: Heat transfer by fluid flow

LITHOSPHERE AND ASTHENOSPHERELow-density and rigid lithosphere floats on top of the dense and plastic asthenosphere

- Boundary at a depth of 100 kmHISTORY OF PLATE TECTONICS - Early hypotheses

Aristotle (384-322 BC)

- Earthquakes created by underground air escaping explosively after being heated by central fires

Kircher, Athanasius (German scholar, 1602-1680)- Had witnessed active volcanoes in Italy

- Drew an early cross-section of the Earth

- Subterranean reservoirs of fire inflame volcanoes around the globe

CATASTROPHISM

17th-18th centuriesTheory stating that the Earths landscape is shaped primarily by great catastrophes

- Based on Christian religious teaching- Earth has been created in 4004 BC

Required violent change to produce topographic features in only 6000 years

UNIFORMITARIANISMJames Hutton (1726 1797)

The Earths slow natural processes operate with the same intensity and under the same set of physical

constraints now as in the geologic past- What happened in the past will happen in the future

- The present is the key to the past (Sir Archibald Geike 1835-1924)

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AT THE END OF THE 19TH CENTURY

Earths internal structure understood- The Earth radiates heat

- Lord Kelvin estimated the cooling rate and calculated that the Earth is 20-100 Ma old

Basic geology of continents known

- Continents assumed to be fixed- Earth assumed to be as rigid as steel

Radioactivity discovered but not geochronology

ROCK CYCLEThe rock cycle describes processes by which older rocks are made into new rocks

Theoretical concept predating plate tectonics

The rock cycle fails to explain the processes on the surface of the earth- Why are continents and oceans different?

- How do mountains form?

- Why do you find same fossil plants on several continents?

ALFRED WEGENER (1880-1930)German meteorologist and Arctic explorer

Put forward the theory of "continental drift"

- Published "The Origin of the Continents and the Oceans" (1915, five editions to 1929)

- Hypothesis largely correct but with incorrect details- Rejected by the European scientific community

- Embraced by Southern hemisphere geologists

WEGENERS CONTINENTAL DRIFT1.Longitudinal shift between 2 positions on 2 continents over time

2.Jigsaw fit of continental margins

3.Same fossils found on widely separated continents4.Geological features on fitted continents match

5.Ancient glacial events found in tropical continents

CONTINENTAL DRIFT: MAIN PROBLEM

Inability to provide a mechanism capable of moving continents across the globe

- Wegener suggested that the low-density continents had moved laterally through the denser mantle- Continents plowing through oceans? Mechanically impossibility!

Oceanic plates not accounted forBreakthrough came from ocean floor studies following WWII technological advances: sonar, magnetometer,

Gravimeter

PLATE TECTONICS: WILSON

Many geologists have maintained that movements of the Earths crust are concentrated in mobile belts, whichmay take the form of mountains, mid-ocean ridges or major faults this article suggests that these features are

not isolated, that few come to dead ends, but that they are connected into a continuous network of mobile belts

about the Earth which divide the surface into several large rigid plates.

Other major contributions:

- "Hot spot" hypothesis (1963)The Hawaiian and other volcanic island chains may have formed due to the movement of a plate

over a stationary "hotspot" in the mantle- Transform faults (1965)

Transform faults offset the crust horizontally, without creating or destroying crust

PLATE TECTONICSContinents move laterally, as part of thicker, rigid lithospheric slabs that slide along a plastic asthenosphere

- Lithosphere: continental and oceanic crust and rigid upper mantle

- Asthenosphere: convecting upper mantleConvection in the asthenosphere is the basic driving force

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PLATE TECTONICS

Evidence- Ocean studies: magnetization patterns on the sea floors; age of ocean basins; bathymetry, study of

ocean floors

- Earthquakes

- Continental driftEARTHS INTERNAL MAGNETIC FIELD

Magnetic lines of force: theoretical representation of the force

acting on a small magnetEarths internal magnetic field resembles that of a bar magnet

- Equator: horizontal lines of force; parallel to surface

- Magnetic poles: vertical lines of force; pointing in at theNorth magnetic pole

GEOMAGNETISM

Earths internal magnetic field not constant through time

- Polar wanderingThe poles reverse polarity every few 0.1 Ma!!

Proof of magnetic field reversal

As lava cools, magnetic minerals align in the direction of the Earths magnetic field

- Record magnetic polarity at the time of coolingSEAFLOOR SPREADING

Magnetic records of the sea floor are studied

- Seafloor surveys (19501960)- revealed patterns of normal and reversely magnetized crust

Symmetrical stripes interpreted to result from the creation of new ocean floor at mid-ocean ridges

BATHYMETRYBathymetry: study of underwater depth in a body of water

Detection of underwater artifacts

Mid-ocean ridges

- Shallow above mid-ocean ridges

- Asthenospheric budgeGradually deeper away from ridges

- Rocks increase in density as they cool- Isostatic down warping due to sediment accumulation

Question

We now know new crust/lithosphere is constantly being created on the seafloor

To maintain equilibrium, what does this mean?Answer

The crust/lithosphere must also be destroyed to maintain equilibrium

Lithosphere recycled into mantle at convergent margins

OCEANIC SUBDUCTION ZONES

Oceanic lithosphere gets older, colder, denser with time, eventually dense enough to sink into mantleEARTHQUAKES SUPPORT SUBDUCTION: Hypocenter distribution

Hypocenter: point of origin of an earthquake in the subsurface- Relation between hypocenter location and the subduction process

- Shallow earthquakes in the overriding and subducting plates

- Intermediate and deep earthquakes in the subducting plate-No earthquake below 600 km

- Material not rigid enough

EARTHQUAKESEpicenter: point on the Earths surface directly above the hypocenter

Epicenter locations outline plate boundaries

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SUBDUCTION ZONES

Subduction: process of one lithospheric plate descending beneath anotherLight plate: overriding plate

Denser plate: subducting plate

- Pulled down by gravity

- Average angle at which oceanic lithosphere descends into the mantle is 45- Surface expression is an ocean trench

- Gradually melts into the asthenosphere

CONTINENTAL CONVERGENCECompression

Low-density continental crust cannot subduct

- Thickening of the crust, mountain building

TRANSFORM MARGINSCrustal blocks slide past each other

No crust created and no crust destroyed

SUMMARY OF PLATE BOUNDARIES

CURRENT THEORY: TECTONIC CYCLE

Tectonic: related to the deformation forces acting on the Earths lithosphere and responsible for the creation of

mountain ranges and faults

- In Ancient Greece, a builder was called a "tekton"

A tectonic cycle is an episode of large-scale deformation of the Earths surfaceOne cycle 250 Ma

Conveyor belt model- Melted asthenosphere

Flows upwards by convection

Cools to form new lithosphere along mid-ocean ridgesNew oceanic crust created at a rate of 2-10 cm/year

New lithosphere moves away laterally

- Lighter continental crust moves as on a conveyor belt

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OUR TECTONIC WORLD

Circum-Pacific belt- Largest plate: Pacific plate

- 80% of earthquakes

- 75% of active and dormant volcanoes

Mediterranean Sea and trans-Asiatic belt- 15% of earthquakes

Largest continent-continent collision: Himalayas

NORTH AMERICAN PLATEEastern boundary:

- Sea floor spreading along the mid-Atlantic ridge

Western boundary:-North: Subduction of the Juan de Fuca plate under the North American plate

- South: North American plate and Pacific plate moving along side each other

San Andrea transform faultTECTONIC ENVIRONMENTS

Four tectonic environments:

1.Spreading centers

2.Transform faults3.Convergent zones

- Subduction zones

- Continent-continent collision zones

4.Hot spotsSPREADING CENTERS

Spreading center: An elongated region where two plates are being pulled away from each other

- Stress regime: tensionNew crust is formed as molten rock is forced upward into the gap

TRANSFORM FAULTS

Transform fault: Fault that accommodates horizontal movement of tectonic plates against each other (J. TuzoWilson, 1965)

- No material created nor consumed

- Stress regime: shear

CONVERGENT ZONES

Convergent zone: Region where two tectonic plates collideStress regime: compression

Type depends on the type of lithosphere in the plates that are colliding-Where a dense oceanic plate collides with a less-dense oceanic or continental plate subduction zone

-Where two continental plates collide, plates crumple and compress continent-continent collision

zone

HOT SPOTSHot spots: plumes of slowly rising mantle rocks that create volcanism on the Earths surface

Deep seated

- Originate in the mesosphere

- Pass through the lithosphere as magma

- Supply an active volcanoSource? Need a zone with temperature contrast to produce upwelling plume:

1.Core-mantle boundary2.Other density boundaries in upper mantle that might also be convection boundaries

Question

What are the important differences between Wegeners Theory of Continental Drift and the modern Theory ofPlate Tectonics?

1.Wegener: Continents are drifting (Plate tectonics: Continental and oceanic plates are in motion )

2.Wegener: Could not identify driving force (Plate tectonics: Convection in mantel brings new crust to theseafloor )

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OTHER DRIVING MECHANISMS

Slab Pull (primary)- Descending slab old, cold, dense, pulls rest of lithospheric plate

behind it

Opposed by viscous drag in mantle

Clear relationship between rate of plate motion and length ofsubduction margin

Ridge Push (secondary)

Lithosphere thickens away from ridgeRidge topography supported by low-density asthenosphere, pushesout on lithosphere

Gravity pulls young lithosphere down at ridge