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I. Geological Formation of Oceanic Islands

I. Geological Formation of Oceanic Islands

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I. Geological Formation of Oceanic Islands

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  1. I. Geological Formation of Oceanic Islands

  2. I. Geological Formation of Oceanic Islands A. What is an oceanic island?

  3. Oceanic Island: • No direct, terrestrial connection to continent (now or in the past); • Usually separated from continent by deep ocean. • Usually formed by volcanic activity;

  4. I. Geological Formation of Oceanic Islands What is an oceanic island? Lithosphere and Plate Tectonics

  5. Cutaway Diagram of the Earth

  6. Cutaway Diagram of the Earth • Inner Core • Radius ~1255 km • Solid Iron • ~ 4100˚C • Rotates W to E

  7. Cutaway Diagram of the Earth • Outer Core • ~ 2,220 km thick • Liquid Iron-Nickel • ~ 4100˚C • Rotates E to W • Rotation generates earth’s magnetic field

  8. Cutaway Diagram of the Earth • Mantle • ~2,800 km thick • Mostly solid (“silly putty”) • Mg/Fe/SiOx (Olivine) • ~1000-3,500˚C • Heat generated by high pressure and radioactive decay (U, Th, K)

  9. Cutaway Diagram of the Earth • Upper Mantle • Outer Mantle • ~ 30 to 70 km deep • Solid rock • Asthenosphere • ~70 to 300 km deep • soft - flows slowly

  10. Cutaway Diagram of the Earth • Crust • ~ 5-50 km thick • Solid, brittle rock

  11. Two Types of Crust: • Continental crust • Oceanic crust Continental crust Ocean Oceanic crust

  12. Continental Crust: • Forms the continents • 20 - 70 km thick (average ~ 30 km) • Granite (Al / SiOx) = metamorphic rock • Relatively low density (~2.7 g/cc) = buoyant • Surface averages ~ 125 m above sea level • Old (up to 3.8 billion years old) • Covers ~ 35% of earth’s surface Continental crust Ocean Oceanic crust

  13. Oceanic Crust: • Forms the deep sea floor • 5 - 10 km thick (average ~ 7 km) • Basalt (Fe / Mg / Al / Na / Ca / SiOx) = igneous rock • Relatively dense (~ 3 g/cc) = negatively buoyant • Surface averages ~ 4 km below sea level • Young ( ≤ 160 - 190 million years old) • Covers ~ 65% of earth’s surface Continental crust Ocean Oceanic crust

  14. Lithosphere = Crust + Solid Outer Mantle(from Greek: Lithos = rocky) • 70-250 km thick • Thicker under continents • Thinner under oceans • Broken into many plates • Lithospheric plates “float” on soft asthenosphere* • *Asthenosphere: From the Greek, asthenes = weak

  15. Tectonic Plates of the World Source: Wikipedia

  16. Continental Drift: Continents have moved over the earth’s surface during geological time. • First proposed by German astronomer / meteorologist • Alfred Wegener circa 1910-12. • Highly controversial; ridiculed, esp. in U.S. • Finally accepted by mainstream geology in 1960s. Alfred Wegener 1880-1930

  17. Continental drift incorporated into modern theory of Plate Tectonics*: *From the Greek: τεκτονικός "pertaining to building” Scientific theory describing large scale movements of the Earth’s lithospheric plates Drifting continents have had a major impact on the distribution and evolution of animals and plants over the past 200+ million years.

  18. Plate Tectonics and Oceanic Island Formation (Highly simplified!)

  19. Convection Currents in Mantle Bring Molten Rock (Magma) Toward Lithosphere.

  20. Divergent Plate Boundary • Magma pushes up from mantle through • lithospheric plate • Forms new oceanic crust • Pushes plates apart (~5 cm / yr) • = Sea Floor Spreading Center Formation of Oceanic Crust Animation

  21. Mid-ocean ridge system develops where sea-floor spreading occurs.

  22. Volcanic activity at mid-ocean ridge can form ocean islands (e.g., Iceland).

  23. Movement of lithospheric plate that includes continental crust results in continental drift. Click Here to Play Seafloor Spreading Animation

  24. Movement of lithospheric plates caused breakup of Pangea Super-continent ~300 million years ago Click to play Animation

  25. Convergent Plate BoundaryConvergence of two oceanic plates: Denser plate sinks under lighter plate = subduction zone. Source: Wikipedia

  26. Click Here to Play Subduction Animation

  27. Convergence of Crustal Plates withSubduction zone results in earthquake and volcanic activity (e.g., Pacific Rim of Fire). Source: Wikipedia

  28. Volcanic Activity at Tectonic Plate Boundaries Source: USGS

  29. Volcanic activity at subduction zone can form oceanic islands(e.g., Aleutians; Lesser Antilles). Source: Wikipedia

  30. In areas where lithospheric plate is thin, magma plume from mantle can push up through plate, forming a “hot spot.” Hotspot Volcano Animation

  31. Map of hot spots

  32. Hot spots under oceanic crust can form oceanic islands

  33. Review Most oceanic islands formed by volcanic activity: 1. along mid-ocean ridge 2. along subduction zone at convergent boundary of two crustal plates 3. at “hot spot” in middle of crustal plate

  34. Eventually, as volcanic island erodes and aging oceanic crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top); • Oceanic islands estimated to last only • 5-10 million years.

  35. Geological Formation of Oceanic Islands A. What is an oceanic island? B. Lithosphere and Plate Tectonics C. Formation of the Hawaiian Island Chain

  36. Hawaiian Islands Source: USGS

  37. Northwest Movement of Pacific Plate Over Fixed Hawaiian Hot Spot Source: USGS

  38. Ages of Hawaiian Islands Source:

  39. Hawaiian Island -Emperor Seamount ChainEmperor Seamount chain extends north from Hawaiian islands

  40. Conventional plate tectonic theory assumes that lithospheric plates move, while hotspots are stationary; as plate moves over hotspot, volcano goes inactive.

  41. However, recent evidence suggests that hotspots can move.Emperor Seamount chain may have formed by hotspot that moved south as Pacific plate moved northwest.

  42. Geological Formation of Oceanic Islands A. What is an oceanic island? B. Lithosphere and Plate Tectonics C. Formation of the Hawaiian IslandChain D. Formation of Bermuda

  43. Geological Formation of Bermuda (1) 110 Million Years Ago (MYA): Volcanoes along Mid-Atlantic Ridge; Seafloor spreading moved volcanic cones NW at 2 cm/year; 30-50 MYA: Second phase of volcanic activity – probably due to hotspot -three volcanic cones formed Bermuda Rise. Bermuda Rise continued to migrate NW; One volcanic cone emerged above sea level (= 1,000 meter high mountain?);

  44. Geological Formation of Bermuda (2) 30 MY to present:Bermuda Rise continued moving to present location, 32° 10-30’N ~ 1000 km east-southeast of Cape Hatteras, NC ~ 1000 km southeast of Connecticut coast Bermuda Rise comprises three seamounts (relicts of volcanic cones): Argus Bank, Challenger Bank, and Bermuda Seamount (= Bermuda Pedestal);

  45. Bermuda Sea Mount Mid-Atlantic Ridge San Salvador Bahama Banks

  46. Bermuda Rise

  47. Geological Formation of Bermuda (3) • Top of Bermuda Seamount exposed (eroded) and submerged several times with rising and falling sea levels; • Seamount capped with limestone precipitated from seawater (oolitic* limestone) and laid down by corals and other marine organisms (biogenic limestone) while submerged. *Oolitic: “Egg-stone”- formed from ooids (spherical grains with concentric layers; 0.25-2mm in diameter) Ooids

  48. Satellite Image of Bermuda Source:

  49. Geological Formation of Bermuda (4) Coral reefs form rim around the Bermuda Platform. Islands of Bermuda are primarily “fossilized” sand dunes (aeolian* limestone) rising above limestone platform. *Aeolian: Wind-blown (From Aeolus, the Greek God of Wind) Reference: The Geology of Bermuda (Bermuda Zoological Society, GEO-01, 2006)