Plate Tectonic Presentation

8/6/2019 Plate Tectonic Presentation

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Plate Tectonics

RAKESH ROSHAN RANA


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Structure of the Presentation

Interior of EarthPlate

Evolution of Plate Tectonic Theory

Type of Plate MarginsEvidence

Cause

Post-Pangea TectonicsSupercontinent CycleDiscussions & Remarks


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Interior of the Earth


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One of the earliest discoveries of seismology was a discontinuity at a depth of 2900 km where the

velocity of P-waves suddenly decreases. This boundary is the boundary between the mantle and

the core and was discovered because of a zone on the opposite side of the Earth from an

Earthquake focus receives no direct P-waves because the P-waves are refracted inward as a result

of the sudden decrease in velocity at the boundary. This zone is called a P-wave shadow zone.


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This discovery was followed by the discovery of an S-wave shadow zone. The S-wave shadow zone occurs

because no S-waves reach the area on the opposite side of the Earth from the focus. Since no direct S-waves

arrive in this zone, it implies that no S-waves pass through the core. This further implies the velocity of S-wave

in the core is 0. In liquids m = 0, so S-wave velocity is also equal to 0. From this it is deduced that the core, or at

least part of the core is in the liquid state, since no S-waves are transmitted through liquids. Thus, the S-wave

shadow zone is best explained by a liquid outer core.


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Have a Look

Distribution of Continents

Mid-ocean Ridges

Trenches

Orogenic Belts

Deformation

Metamorphism

Volcanism

Earthquakes


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Plate


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Development of Continental Drift

Lots of people had noticed that the coastlines

of Africa and South America are similar

Frank Taylor (1910)

Alfred Wegener (1912) Die Entstehung Der

Kontinente Und Ozeane

1. How we know plate tectonics happens


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Fit ofContinents

Across the

Atlantic


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The PermianIce Age

Problem



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WegenersTheory



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Dating

the

Breakup



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Frank Taylor

In somerespects, Taylors

ideas were more

modern than

Wegeners

Taylor always

thought

Wegener hadstolen credit

from him



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Frank Taylor

Recognizedrole of Mid-

Atlantic Ridge

Never

reconstructed

the continents

like Wegener

did



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Confirmation of Continental Drift

World War II technology

International Geophysical Year (IGY) 1957-58

Worldwide Standardized Seismic Network1963-



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The Geomagnetic

Reversal Time

Scale



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Discovery of Sea-Floor Spreading



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Sea-Floor

Spreading



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Where Does Ocean Crust Go?

Hugo Benioff, 1954



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Benioffs Interpretation



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Benioffs Interpretation Updated



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PLATESPlate Area (km2) Plate Area (km2)

Pacific 103,300,000 Scotia 1,600,000

North American 75,900,000 Burma microplate 1,100,000

Eurasian 67,800,000 Fiji microplates 1,100,000

African 61,300,000 Tonga microplate 960,000

Antarctic 60,900,000 Mariana microplate 360,000

Australian 47,000,000 Bismark microplate 300,000

South American 43,600,000 Juan de Fuca 250,000

Somali 16,700,000 Solomon microplate 250,000

Nazca 15,600,000South Sandwichmicroplate

170,000

Indian 11,900,000 Easter microplate 130,000

Philippine 5,500,000Juan Fernandezmicroplate

96,000

Arabian 5,000,000 Rivera microplate 73,000

Caribbean 3,300,000 Gorda microplate 70,000

Cocos 2,900,000 Explorer microplate 18,000

Caroline microplate 1,700,000 Galapagos microplate 12,000


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What Drives It: Convection


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Pea-soup analogy


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Other Causes

Differential rate of motion

Earth Slope

Heat Gradient


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How Plates Move


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The Plate Tectonics Model


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The Plate Tectonics Model


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Plate motions


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Plate Motions


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Divergent boundary


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Magnetic

Stripes in

theFAMOUS

Area


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Anatomy of a Mid-Ocean Ridge

Con ergent bo ndar


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Convergent boundary

Subduction


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Terrane Accretion

Transform boundary


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Transform boundary


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Mass Construction and Destruction


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Relationship between plate tectonic

setting and structural style:

Tectonic SettingStressState

Types of Structures Examples

Divergent plates extension normal faults, roll overanticlines, tilted blocks

North Sea, Red Sea,Basin and Range

Convergent plates compression thrust faults, folds,faulted folds

Andes, Zagros Mts(Iran), Canadian

Rockies

Transform plate boundaries strike-slipstrike-slip faults,

compressional andextensional flower

structures

San Andreas fault,Alpine Fault (New

Zealand).


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Terranes in

Western NorthAmerica


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A h T i


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Archean Tectonics Archean Crust formed 4.4 to 2.5 billion years ago.

The formation of these cratonic nucleii marks thetransition from an early Earth that was so hot andenergetic that no remnants of crust were preserved, toa state where crustal preservation became possible.

Most of the cratons are attached to a high velocitymantle root that extends to depths of at least 200 km(King, 2005).

These cratonic roots are composed of stiff and

chemically buoyant mantle material (Section 11.3.1) whose resistant qualities have

contributed to the long-term survival of the Archeancontinental lithosphere (Carlson et al., 2005).


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The beginning of the Archean Eon approximately

coincides with the age of the oldest continental crust.A conventional view places this age at approximately

4.0 Ga, which coincides with the age of the oldest rocks found

so far on Earth: the Acasta gneisses of the Slave craton in

northwestern Canada (Bowring & Williams, 1999).However, >4.4 Ga detrital zircon minerals found in the Yilgarn

craton of Western Australia (Wilde et al., 2001) suggest that

some continental crust may have formed as early as 4.44.5

Ma, although this interpretation is controversial (Harrison et al.,

2005, 2006; Valley et al., 2006).


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PRECAMBRIAN HEAT FLOW

The majority of terrestrial heat production comes from thedecay of radioactive isotopes dispersed throughout the core,

mantle, and continental crust.

Heat flow in the past must have been considerably greater than

at present due to the exponential decay rates of these isotopes.For an Earth model with a K/U ratio derived from

measurements of crustal rocks, the heat flow in the crust at 4.0

Ga would have been three times greater than at the present day

and at 2.5 Ga about two times the present value (Mareschal &

Jaupart, 2006). For K/U ratios similar to those in chondritic meteorites, which

are higher than those in crustal rocks, the magnitude of the

decrease would have been greater.


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Heat Flow with TimeFig. Variation of

surface heat flowwith time. Solidline, based on achondritic model;

dashed line,based on a K/Uratio derived fromcrustal rocks

(McKenzie &Weiss, 1975).


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In Many of the cratons includes an abundance of high

temperature/low pressure metamorphic mineral assemblages and

the intrusion of large volumes of granitoids, suggestrelatively high (500700 or 800C) temperatures in

the crust during Archean times, roughly similar to those which

occur presently in regions of elevated geotherms.

By contrast, geophysical surveys and isotopic studies of mantle

nodules suggest that the cratonic mantle is strong and cool and

that the geotherm has been relatively low since the Archean Some

of the most compelling evidence of cool mantle lithosphere comes

from thermobarometric studies of silicate inclusions in Archean

diamonds, which suggest that temperatures at depths of 150200 km during the Late Archean were similar to the

present-day temperatures at those depths (Boyd et al.,

1985; Richardson et al., 2001).


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Proterozoic Tectonics


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Supercontinent Cycle

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Plate Tectonic Presentation