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CHAPTER 2 Plate Tectonics and the Ocean Floor

• Much evidence supports plate tectonic theory.

• Different plate boundaries have different features.

• Tectonic plates continue to move today.

• Lava Lamp!

Alfred Wegener: in 1912, he was the first person to present evidence other than continental margin fithttp://en.wikipedia.org/wiki/Pangaea

• 200 m.y. ago, the supercontinent Pangaea (all Earth) began to split apart.

• Geologic evidence• Fossil evidence• Paleoclimate evidence

In 1858, geographer Antonio Snider-Pellegrinimade these two maps showing his version of how the American and African continents may once have fit together, then later separated.

Figure 2.2

Evidence for Continental Drift

• Wegener proposed Pangaea – one large continent existed 200 million years ago

• Panthalassa – one large ocean

• Noted puzzle-like fit of modern continents

Figure 2.3

http://www.youtube.com/watch?v=uGcDed4xVD4&feature=related

Pangea

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Evidence for continental drift:• Geological evidence:

1) fold belts2) age provinces3) igneous provinces4) stratigraphic sections5) metallogenic provinces

Figure 2.4

• Matching sequences of rocks and mountain chains

• Similar rocks on different continents

Evidence for Continental Drift

Evidence for continental drift:

Paleontological evidence

1) distribution of tetrapods-early distribution - easy communication in Pangaea

2) early Permian reptile Mesosaurus -found in S Africa and Brazil

3) marine invertebrates-distribution of continents and oceans different from today

4) Cambrian trilobites

5) ammonites (shallow seas between India, Madagascar and Africa in J)

6) Glossopteris and Gangamopterisfauna in Gondwana (cold climate), tropical flora in Laurasia

7) diversity of species (increases towards Equator = drifting N-S controls the diversity)

Evidence for continental drift:

Paleoclimatic evidence:Some lithologies are indicative of particular climates and hence global position:

1) carbonates and reef deposits-warm water, 30 deg from equator

2) evaporates-hot, arid conditions

3) red beds-hot climate for form of hematite

4) coal and oil-warm, humid climate

5) phosphorites-within 45 deg of equator

6) bauxite and laterite-tropical, subtropical weathering

7) desert deposits-both warm and cold conditions, direction of wind, continental rotation

8) glacial deposits-30 deg from poles

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Figure 2.5

Paleoclimatic evidence:

• Glacial ages and other climate evidence

• Evidence of glaciation in now tropical regions

• Direction of glacial flow and rock scouring

• Plant and animal fossils indicate different climate than today

Evidence for Continental Drift

Alfred Wegener: He did not have a successful explanation for a mechanism to drive continental drift.

1. Gravitational forces: continental crust plows through oceanic crust like an icebreaker through ice (pole-fleeing force-movement of continents toward Equator) -however: continental crust is too weak to do this (2.7 g/cc vs. 3.3 g/cc); the force is several millions times smaller than force of gravity)

2. Centrifugal (rotational) and tidal forces are responsible for movement of land masses (westward movement of continents)- however: these forces are too weak to do this

http://www.suu.edu/faculty/colberg/hazards/platetectonics/18_Pangaea.html

• The hypothesis was widely rejected for many decades, elaborate but incorrect schemes were concocted to explain mountain ranges and large synclines.

In the 1950’s a wealth of new evidence emerged to revive the debate about Wegener's ideas:

(1) confirmation of repeated reversals of the Earths magnetic field in the geologic past;

(2) demonstration of the ruggedness and youth of the ocean floor;

(3) emergence of the seafloor-spreading hypothesis and associated recycling of oceanic crust; and

(4) precise documentation that the world's earthquake and volcanic activity is concentrated along oceanic trenches and submarine mountain ranges.

Evidence for Plate Tectonics• Paleomagnetism – study of Earth’s ancient

magnetic field– Interprets where rocks first formed (rocks

record magnetic field when they are formed)

– Magnetic dip (see dip angle)

• 1955 – deep water rock mapping

• Magnetic anomalies – regular pattern of north-south magnetism “stripes”

• Stripes were symmetrical about long underwater mountain range

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Figure 2.7a

Earth’s Magnetic Field

• Approximates a dipole (like a bar magnet), so it has polarity

• Orientation wanders over time

Figure 2.7b

Figure 2.7c

Dip Angle: the angle between the line of magnetic force and a plumb line.<<<<<<<<

Pole Wander: the magnetic pole has wandered through time >>>>>>>>

The curves on this map trace the apparent path followed by the north magnetic pole through the past 600 million years. The apparent polar wandering path for Europe is different from the path determined from measurements made in North America. If the continents are reassembled into a single supercontinent, the twopaths coincide, indicating that Europe and North America moved as one continent during this period. >>>

• Paleomagnetism –study of Earth’s ancient

magnetic field

Magnetic field linesNormal

transition

reversed

- The total time span of a reversal is up to 10.000 years- The reversal sequence has been calibrated for the last 5 million

years by dating basalts of known polarity.

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• Early in the 20th century, paleomagnetists -- such as Bernard Brunhes in France (in 1906) and Motonari Matuyama in Japan (in the 1920s) -- recognized that rocks generally belongto two groups according to their magnetic properties:

(1) normal polarity and (2) reversed polarity• ocean floor shows a zebra-like pattern of alternating stripes ofmagnetically different rock creating “magnetic striping.”

• Several lines of evidence:(1) at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest;

(2) the youngest rocks at the ridge crest always have present-day (normal) polarity; and

(3) stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has flip-flopped many times.

The shape and intensity of magnetic anomalies depends on:

(1) the segmentation of the mid-ocean ridge by fracture zones (i.e. length of magnetized blocks along-axis),(2) spreading velocity (length of blocks across-axis). Fast spreading causes relatively longer blocks to form than slow spreading.(3) frequency of polarity reversals (length of blocks across-axis),(4) the direction of magnetization in a given block.

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Marine Record: oldest oceanic crust ~180 MaContinental Record: oldest continental crust ~2.1 Ga

Figure 2.10

What Drives Plate Motions:(1) Density vs. Gravity: causes oceanic crust to sink in

subduction zones, causes crust to extend at spreading ridges (called ridge push, but the ridge is not pushing, the crust is pulling as it sinks into subduction zones)…

(2) Thermal Convection: exerts drag force to base of crust, circulates heat and mantle material...

Thermal Convection: Thermal Convection:

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http://www.earthbyte.org/people/dietmar/Pdf/Muller_etal_digital_isochrons_jgr1997.pdf http://www.ngdc.noaa.gov/mgg/fliers/96mgg04.html

A published reference for the age grid is: Mueller, R.D., Roest, W.R., Royer, J.-Y., Gahagan, L.M., and Sclater, J.G., A digital age map of the ocean floor. SIO Reference Series 93-30, Scripps Institution of Oceanography.

Bathymetry and Topography Crustal Age Bathymetry

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Depth = 2.5 km + 0.33 x (sq root age in Ma)

• Average depth ocean ridges = 2.5 km• As plates cool, they get more dense and sink

Thermal Control on Crustal Depth Bathymetry and Topography

De Mets, et.al. 2010

Where (Why) Are There Earthquakes?

Earthquake epicenters (mostly along plate boundaries)

• magnitude >3 in black and >5.5 in red De Mets, et.al. 2010

Plate Boundaries

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Figure 2.13b

Plate Boundaries and Their Motions

Figure 2.14

Types of Plate

Boundaries

Divergent

Convergent

Transform

Table 2.1

Types of Plate

Boundaries

• Divergent• Convergent• Transform

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Figure 2.17

Evolution of Divergent Margins

Shallow heat source develops under continent

Movement apart creates a rift valley

Continued spreading creates linear sea

Ocean basin is created over millions of years

Figure 2.18a

Evolution of Divergent Margins

East Africa rift system with Red Sea and Gulf of Aden linear seas.• Divergent

boundary = red• Transform

boundary = green• Volcano = black

triangle• Where is oldest

divergent margin on map?

A view down the axis of the divergent plate boundary immediately south of the Red SeaA view down the axis of the divergent plate boundary immediately south of the Red Sea

Figure 2.18c

Rift formed in 2005 following earthquakes and volcanic eruption in Ethiopia

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Figure 2.19

Types of Spreading Centers

Fast and Slow Rates• Mid-Atlantic Ridge =

~3.5 cm/yr• East Pacific Rise =

~16.5 cm/yr

Figure 2.19a

Mid-Atlantic Ridge = ~3.5 cm/yr

Figure 2.19b

East Pacific Rise = ~16.5 cm/yr

Transform Margins• Connect spreading ridges• “Strike” slip

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Transform Margins• Connect spreading ridges• “Strike” slip

http://www.geocaching.com/seek/cache_details.aspx?guid=f6a2bf15-d163-4f3f-a383-0f647bf374a4

Transform Boundaries

PACIFIC

NORTH AMERICA

San Andreas Fault, Carrizo Plain

36 mm/yr

Loma Prieta

1989

Ms 7.1

Davidson et al., 2002

Transform Margins• “Strike” slip

Figure 2.20

Convergent Margins

• Ocean Continent Subduction

• Ocean-Ocean Subduction

• Continent-Continent Collision

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Figure 2.20a Figure 2.20b

Figure 2.20c

Convergent Margins

• Ocean Continent Subduction• Ocean plate is

subducted• Continental arcs

generated• Explosive andesitic

volcanic eruptions• Cascadia subduction

zone• Juan de Fuca and Gorda

plates subduct northeastwardly beneath the North America plate

• Subduction zone earthquakes can cause tsunami

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Atwater, 1970

• Ocean Continent Subduction• Cascadia subduction zone• Magnetic anomalies

USGS

Ocean Continent Subduction

• Cascadia subduction zone Cross Section

• Water content fluxes melt to generate volcanism

USGS

Ocean Continent Subduction

• Subduction zone earthquakes can generate tsunami

Mt St Helens, WA March 1980

Mt St Helens, WA May 18 ,1980

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Oceanic-Oceanic Convergent Plate Boundary

Convergent Margins

• Ocean-Ocean Subduction• Denser plate is subducted• Deep trenches generated• Volcanic island arcs generated• Marianas Trench (video)

marianas_hdv.mov

Figure 2.22

Convergent Margins

• Continent-Continent Collision• No subduction• India-Asia collision ~55 Ma• Created/Creating the

Himalayas

Figure 2.22a

• Continent-Continent Collision• Initially oceanic-continental subduction• Denser oceanic crust (~3.3 g/cc)

subducts beneath less dense continental crust (~2.7 g/cc)

• Marine sediments deposited with marine fossils

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Figure 2.22b

• Continent-Continent Collision• Continental crust collides

with continental crust, creating mountain uplift

• Marine sediments uplifted with mountain growth

Tapponier and PeltzerPlasticene experiment.

• Continent-Continent Collision• Indenter Tectonics

• Continent-Continent Collision• Indenter Tectonics

Figure 2.23b

Plate Boundary Motion

• Relative Motion• Geologic Offsets• Magnetic Anomalies

• Absolute Motion• GPS

• Reference Frames• Plates defined

geodetically (“stable North America”)

• Hot Spots (record ancient plate motions)

• Nematath – hotspot track

Applications of Plate Tectonics

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http://maps.unomaha.edu/maher/GEOL1010/lecture15/USGSSanAndreasGPS.gif

Plate Boundary Motion

• San Andreas transform fault system• GPS sites• Relative to stable

North America• Many faults interact• Pacific plate moves

relative to North America plate

http://geology.gsapubs.org/content/37/4/359/F2.large.jpg

Plate Boundary Motion

• San Andreas transform fault system• San Andreas

fault, Eastern California shear zone, Walker Lane, Central Nevada seismic belt, Wasatch fault (Utah).

• Fault slip rate (mm/yr)

• GPS Velocity A-A’ (mm/yr)

Figure 2.24

Plate Boundary Motion

• Reference Frames• Hot Spots• They move, but at rates

less than the plate rates

Figure 2.25

Hawaiian Island – Emperor Seamount Nematath

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Age relations of the Emperor and Hawai’ian Island Chains

Figure 2.26

• Seamounts – Rounded tops

• Tablemounts or guyots– Flattened tops

• Subsidence of flanks of mid-ocean ridge

• Wave erosion may flatten seamount

Figure 2.27

Coral Reef Development

Figure 2.29

Coral Reef Development

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Figure 2.30

Plate Boundary Motion• Absolute Motion

• GPS• Reference Frames

• Global Reference Frame

Figure 2.32

World Map 50 million Years in Future

Plate Boundary Motion• Ocean Basin Structure

• Bathymetry• Topography• Plate Boundaries

http://www.youtube.com/watch?v=JmC-vjQGSNM&feature=relatedPlate Tectonics Video

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http://www.futura-sciences.com/fileadmin/Fichiers/images/Terre/cyclewilson.jpg

John Tuzo Wilson

Life cycle of ocean basinsFormation

Growth

Destruction

Figure 2.33

Wilson cycle

Wilson cycle Wilson cycle