Origin of the Oceans II · Continental Drift: The Evidence • Geologic Evidence – If continents...
Transcript of 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
Professor Rosenheim EENS/EBIO 223
Introductory Oceanography
Earthquakes and Volcanoes
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Introductory Oceanography
Tracing the Plates
• Earthquake and volcano activity follows distinct patterns
• Surface manifestations of the living planet
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Introductory Oceanography
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
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Introductory Oceanography
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
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Introductory Oceanography
Continental Drift: The Evidence• Wegener
– Used continental shorelines– Large gaps, and some continental overlap– How could the continents move?!
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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
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Common Fossils
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Common Ages between Continents
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Introductory Oceanography
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
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Introductory Oceanography
Continental Drift to Plate Tectonics
• Wegener failed to convince scientific community of his theory
• Technology allowed further tests of the theory after his death
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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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Introductory Oceanography
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Introductory Oceanography
Sea-Floor Spreading – “Geopoetry”• Harry Hess
– Theory of Sea Floor Spreading
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Introductory Oceanography
Paleomagnetism• Harry Hess
– Based on mapping of mid-ocean ridges
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Introductory Oceanography
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
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Introductory Oceanography
Paleomagnetism
• Earth’s magnetic field influences magnetic particles in rocks
• N-S poles switch over time (polarity reversals)– Igneous rock– Sedimentary rock
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Introductory Oceanography
Sea Floor Spreading and Paleomagnetism
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Steady State?
• New crust is formed in the oceans.– T/F – The Earth is growing.
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Introductory Oceanography
Steady State? - yes
• New crust is formed in the oceans.– T/F – The Earth is growing.
– Subduction of crust counter-acts new crust
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Introductory Oceanography
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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Introductory Oceanography
Structure of Earth (Lithological)
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Introductory Oceanography
Structure of Earth (Mechanical)
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Seismic Waves
• Earthquakes
• Explosions– Conventional– Nuclear
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Introductory Oceanography
Seismic Waves
• Wave Types– S-Wave
• Cannot pass through liquid
– P-Wave• Transmitted
through liquids and solids
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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
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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%)
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Asthenosphere Convection
Convection: Transfer of heat by circulation of a fluid
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Asthenosphere Convection
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Plate Boundaries
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Plate Boundaries
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Plate Boundaries – Transform Faults
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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
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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
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Introductory Oceanography
Plate Collisions• Oceanic – Oceanic
– Oldest plate is subducted (cooler, denser)• Continental to Continental
– No subduction– Tall mountains result
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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.
Professor Rosenheim EENS/EBIO 223
Introductory Oceanography
Hydrothermal Vents• http://video.nationalgeographic.com/video/player/environment/ha
bitats-environment/oceans/hydrothermal-vents.html
• http://www.youtube.com/watch?v=HHlGlWyJ34I
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Introductory Oceanography
Ocean Features
• Why are the oceans on oceanic crust?
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Introductory Oceanography
Ocean Features
• Why are the oceans on oceanic crust?– Dense crust with shallow “root,” low-lying
compared with high-floating cont. crust
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Oceanic Crust Evolution
• Oceanic crust cools and becomes denser as it moves away from ridge
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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
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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
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Introductory Oceanography
Emperor Seamount Chain
• Change in plate direction during life of hot spot
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Introductory Oceanography
Darwin’s Theory of Coral Reef Evolution
• Darwin noticed many islands of unique shape
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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
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Introductory Oceanography
Island-Atoll-Seamount/Guyot
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Seamounts and Guyots
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Island-Arc Volcanoes
• Subducted plate melts• Pressure builds• Magma erupts• Underwater volcano builds , emerges, and
continues to build
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Montserrat – Lesser Antilles
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The Newest Antilles Island (soon?)
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Introductory Oceanography
Arc-Island Volcanoes – Another Example
• Aleutians Islands– Pacific and N. American Plates
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Trenches
• Normally associated with island-arcs• Seismically active• Deepest parts of the ocean
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Terrane Formation
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Introductory Oceanography
Break-up of Pangaea
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Introductory Oceanography
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Non-Tectonic Plate Movement
• Isostatic rebound
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Introductory Oceanography
Non-Tectonic Plate Movement
• Isostatic rebound
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Introductory Oceanography
Wilson Cycle and Supercontinents
• The life cycle of an ocean basin• 300 My between formation of
supercontinents
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Introductory Oceanography
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