Origin of the Oceans II · Continental Drift: The Evidence • Geologic Evidence – If continents...

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Origin of the Oceans II

Plate Tectonics II

Professor Rosenheim EENS/EBIO 223

Introductory Oceanography

Earth – A Living Planet

• Heat of formation of the planet is trapped at center, gradually escaping

• Center is nickel and iron



Earthquakes and Volcanoes

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Tracing the Plates

• Earthquake and volcano activity follows distinct patterns

• Surface manifestations of the living planet



Plate Tectonics

• A great discovery of 20th century geologists• One of the basic tenets of Earth Sciences• Explains why

– Oceans are located where they are– Oceans are as deep as they are– Oceans have geographic features (ridges, trenches,

etc.)– Earthquakes and volcanoes favor specific zones



Plate Tectonics

Rigid plates (crust) “float” on plastic layer of Earth’s interior and interact, driven by interior heat, to form mountains, oceans, and various other geographic features of the planet.

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Continental Drift: The Evidence• Sir Francis Bacon (1620’s)

– Continents seem to fit together• Alfred Wegener (1912)

– Continents are mobile, “Continental Drift”– Enabled by accurate world maps



Continental Drift: The Evidence• Wegener

– Used continental shorelines– Large gaps, and some continental overlap– How could the continents move?!



Continental Drift: The Evidence• Geologic Evidence

– If continents were once attached, rock types and fossils must be the same

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Glacial Striations



Common Fossils



Common Ages between Continents

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Continental Drift: The Evidence

• Evidence was piling up by the 1960’s• Still, how would one propose that the

continents move?!• Many objections to Wegener’s theory:

– Wegener’s theory (1930) was that continents plowed through the oceans and built mountains as a result of drag

– Gravitational attraction of continents to Earth’s equatorial bulge



Continental Drift to Plate Tectonics

• Wegener failed to convince scientific community of his theory

• Technology allowed further tests of the theory after his death



Continental Drift: The Evidence• Sir Edward Bullard

– Used depth of 1000m (1000m isobath) to define continental boundaries

– Fit continents with computer– How could the continents move?!

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Sea-Floor Spreading – “Geopoetry”• Harry Hess

– Theory of Sea Floor Spreading



Paleomagnetism• Harry Hess

– Based on mapping of mid-ocean ridges

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Sea-floor Spreading• Formation of new oceanic crust

Great Rift Valley – Africa

Formation of a new ocean.

Mid-Atlantic ridge, Iceland

Continuation of mid-ocean ridge



Paleomagnetism

• Earth’s magnetic field influences magnetic particles in rocks

• N-S poles switch over time (polarity reversals)– Igneous rock– Sedimentary rock



Sea Floor Spreading and Paleomagnetism

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Steady State?

• New crust is formed in the oceans.– T/F – The Earth is growing.



Steady State? - yes

• New crust is formed in the oceans.– T/F – The Earth is growing.

– Subduction of crust counter-acts new crust



Oceanic vs. Continental Crust

• Oceanic crust (basalt) is formed at mid-ocean ridge spreading centers– Dense, thin

• Continental crust– Thick, less dense

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Structure of Earth (Lithological)



Structure of Earth (Mechanical)



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Seismic Waves

• Earthquakes

• Explosions– Conventional– Nuclear



Seismic Waves

• Wave Types– S-Wave

• Cannot pass through liquid

– P-Wave• Transmitted

through liquids and solids



Refraction and Reflection

• Changes in angles reflect different densities

• Absence of S-waves on receiving end implies liquid

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Earth’s Structure

• Inner heat drives plate tectonics

• Liquid outer core invokes magnetic field (and polarity reversals)

• Lithosphere is small, cool rigid crust in layer cake



Earth’s Structure• Lithosphere – Crust and rigid part of mantle• Asthenosphere – Plastic-part of mantle

• Solid layers– Crust (100%)– Mantle (~90%)– Inner Core (100%)

• Liquid layers– Mantle (~10%)– Outer Core (100%)



Asthenosphere Convection

Convection: Transfer of heat by circulation of a fluid

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Asthenosphere Convection



Plate Boundaries



Plate Boundaries

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Plate Boundaries – Transform Faults



Plate Boundaries – Transform Faults

• Fault – plane on which two plates have different velocities

• Transform Fault – lateral movement in horizontal plane (normal faulting)– Spawn shallow but strong earthquakes



Plate Collisions• Oceanic - Continental

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Plate Collisions• Oceanic – Continental

– Oceanic Crust more dense – subducts– Continental Crust more buoyant– Oceanic Crust melts as it subducts

• Volcanoes– Continental Crust

• compaction• mountain-building



Plate Collisions• Oceanic – Oceanic

– Oldest plate is subducted (cooler, denser)• Continental to Continental

– No subduction– Tall mountains result



The Ridge and Spreading CenterSheet Dikes – injected and cooled magma

Gabbro – similar to basalt, but larger crystals due to slower cooling

Peridotite – crystallized mantle

Ophiolite – ridge sequence on land (pushed above sea level by tectonics)

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Water and the Ridge

www.divediscover.whoi.edu/vents/ventchemistry.html

Hydrothermal Vents

1. Cold water sinks into cracks in basalt

2-3. Elements are removed from seawater (O, K, Mg, S, Ca) in interactions with basalt. Oxygen and potassium are first to react.

4-5. Na, Ca, and K enter the hotter fluid from the surrounding rocks. At the fluids’ hottest temperatures, Cu, Zn, Fe, and S enter the fluids.

6. Hot fluid rises.

7. Fluid is ejected into cold surrounding ocean and minerals (sulfides) precipitate rapidly. The event looks like smoke and builds a “chimney” of sorts.



Hydrothermal Vents• http://video.nationalgeographic.com/video/player/environment/ha

bitats-environment/oceans/hydrothermal-vents.html

• http://www.youtube.com/watch?v=HHlGlWyJ34I



Ocean Features

• Why are the oceans on oceanic crust?

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Ocean Features

• Why are the oceans on oceanic crust?– Dense crust with shallow “root,” low-lying

compared with high-floating cont. crust



Oceanic Crust Evolution

• Oceanic crust cools and becomes denser as it moves away from ridge



Hot SpotsExamples:

Hawaii

Sao Tome and Principe

Galapagos

Samoa

Marquesas

Canary Islands

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Continental Hot SpotsCentral West Africa

Continuation of Sao Tome and Principe Islands



Emperor Seamount Chain• Seamount – sunken hot spot island (or

any volcano, as long as it protruded in past)– Becomes more dense as cooling occurs with

surrounding crust



Emperor Seamount Chain

• Change in plate direction during life of hot spot

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Darwin’s Theory of Coral Reef Evolution

• Darwin noticed many islands of unique shape



Theory of Atoll Formation

• Darwin proposed sinking of volcanic islands led to stages of coral reef development in 1842!– His theory lacked a mechanism for their

sinking– Plate tectonics provided a mechanism for

Darwin’s theory



Island-Atoll-Seamount/Guyot

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Seamounts and Guyots



Island-Arc Volcanoes

• Subducted plate melts• Pressure builds• Magma erupts• Underwater volcano builds , emerges, and

continues to build



Montserrat – Lesser Antilles

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The Newest Antilles Island (soon?)



Arc-Island Volcanoes – Another Example

• Aleutians Islands– Pacific and N. American Plates



Trenches

• Normally associated with island-arcs• Seismically active• Deepest parts of the ocean

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Terrane Formation



Break-up of Pangaea



0.0

22



14



27.7



30.3

23



59.2



69.4



88.0

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94.0



100.0



118.7

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130.2



152.2



166.0

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195.0



216.0



237.0

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255.0



277.0



306.0

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342.0



363.0



377.0

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390.0



425



433

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458



497



514

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547



Non-Tectonic Plate Movement

• Isostatic rebound



Non-Tectonic Plate Movement

• Isostatic rebound

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Wilson Cycle and Supercontinents

• The life cycle of an ocean basin• 300 My between formation of

supercontinents



Key Terms

• Subduction-sinking of a plate underneath another

• Plate Boundaries-boundaries between lithospheric plates (oceanic or continental)

• Transform Faults-faults allowing lateral motion between rifts (spreading centers)

• Convergence and Divergence – plate boundaries moving in opposite directions

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Origin of the Oceans II · Continental Drift: The Evidence • Geologic Evidence – If continents...

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