Origin of the Oceans II. Earth A Living Planet. Earthquakes and Volcanoes. Plate Tectonics II

Transcription

1 Origin of the Oceans II Plate Tectonics II Earth A Living Planet Heat of formation of the planet is trapped at center, gradually escaping Center is nickel and iron Earthquakes and Volcanoes 1

2 Tracing the Plates Earthquake and volcano activity follows distinct patterns Surface manifestations of the living planet Plate Tectonics A great discovery of 20 th 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. 2

3 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 3

4 Glacial Striations Common Fossils Common Ages between Continents 4

5 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?! 5

6 Sea-Floor Spreading Geopoetry Harry Hess Theory of Sea Floor Spreading Paleomagnetism Harry Hess Based on mapping of mid-ocean ridges 6

7 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 7

8 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 midocean ridge spreading centers Dense, thin Continental crust Thick, less dense 8

9 Structure of Earth (Lithological) Structure of Earth (Mechanical) 9

10 Earthquakes Seismic Waves Explosions Conventional Nuclear Wave Types S-Wave Cannot pass through liquid P-Wave Transmitted through liquids and solids Seismic Waves Refraction and Reflection Changes in angles reflect different densities Absence of S-waves on receiving end implies liquid 10

11 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 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 11

12 Asthenosphere Convection Plate Boundaries Plate Boundaries 12

13 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 13

14 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 Center Sheet 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) 14

15 Water and the Ridge 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 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 bitats-environment/oceans/hydrothermal-vents.html Ocean Features Why are the oceans on oceanic crust? 15

16 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 Spots Examples: Hawaii Sao Tome and Principe Galapagos Samoa Marquesas Canary Islands 16

17 Continental Hot Spots Central 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 17

18 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 18

19 Seamounts and Guyots Island-Arc Volcanoes Subducted plate melts Pressure builds Magma erupts Underwater volcano builds, emerges, and continues to build Montserrat Lesser Antilles 19

20 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 20

21 Terrane Formation Break-up of Pangaea

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31 547 Non-Tectonic Plate Movement Isostatic rebound Non-Tectonic Plate Movement Isostatic rebound 31

32 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 32