6Cutaway Diagram of the Earth Inner CoreRadius ~1255 kmSolid Iron~ 4100˚CRotates W to E
7Cutaway Diagram of the Earth Outer Core~ 2,220 km thickLiquid Iron-Nickel~ 4100˚CRotates E to WRotation generates earth’s magnetic field
8Cutaway Diagram of the Earth Mantle~2,800 km thickMostly solid (“silly putty”)Mg/Fe/SiOx (Olivine)~1000-3,500˚CHeat generated by high pressure and radioactive decay (U, Th, K)
9Cutaway Diagram of the Earth Upper MantleOuter Mantle~ 30 to 70 km deepSolid rockAsthenosphere~70 to 300 km deepsoft - flows slowly
10Cutaway Diagram of the Earth Crust~ 5-50 km thickSolid, brittle rock
11Two Types of Crust: Continental crust Oceanic crust Ocean
12Continental Crust: Forms the continents km thick (average ~ 30 km)Granite (Al / SiOx) = metamorphic rockRelatively low density (~2.7 g/cc) = buoyantSurface averages ~ 125 m above sea levelOld (up to 3.8 billion years old)Covers ~ 35% of earth’s surfaceContinental crustOceanOceanic crust
13Oceanic Crust: Forms the deep sea floor km thick (average ~ 7 km)Basalt (Fe / Mg / Al / Na / Ca / SiOx) = igneous rockRelatively dense (~ 3 g/cc) = negatively buoyantSurface averages ~ 4 km below sea levelYoung ( ≤ million years old)Covers ~ 65% of earth’s surfaceContinental crustOceanOceanic crust
14Lithosphere = Crust + Solid Outer Mantle (from Greek: Lithos = rocky) km thickThicker under continentsThinner under oceansBroken into many platesLithospheric plates “float” on soft asthenosphere**Asthenosphere: From the Greek, asthenes = weak
17First proposed by German astronomer / meteorologist Continental Drift: Continents have moved over the earth’s surface during geological time.First proposed by German astronomer / meteorologistAlfred Wegener circaHighly controversial; ridiculed, esp. in U.S.Finally accepted by mainstream geology in 1960s.Alfred Wegener
18Continental drift incorporated into modern theory of Plate Tectonics*: *From the Greek: τεκτονικός "pertaining to building”Scientific theory describing large scale movementsof the Earth’s lithospheric platesDrifting continents have had a major impact on the distribution and evolution of animals and plants over the past 200+ million years.
19Plate Tectonics and Oceanic Island Formation (Highly simplified!)
20Convection Currents in Mantle Bring Molten Rock (Magma) Toward Lithosphere.
21Divergent Plate Boundary Magma pushes up from mantle throughlithospheric plateForms new oceanic crustPushes plates apart (~5 cm / yr)= Sea Floor Spreading CenterFormation of Oceanic Crust Animation
22Mid-ocean ridge system develops where sea-floor spreading occurs.
23Volcanic activity at mid-ocean ridge can form ocean islands (e. g Volcanic activity at mid-ocean ridge can form ocean islands (e.g., Iceland).
24Movement of lithospheric plate that includes continental crust results in continental drift. Click Here to Play Seafloor Spreading Animation
25Movement of lithospheric plates caused breakup of Pangea Super-continent ~300 million years ago Click to play Animation
26Convergent Plate Boundary Convergence of two oceanic plates: Denser plate sinks under lighter plate = subduction zone.Source: Wikipedia
33Hot spots under oceanic crust can form oceanic islands
34ReviewMost oceanic islands formed by volcanic activity:1. along mid-ocean ridge2. along subduction zone at convergent boundary of two crustal plates3. at “hot spot” in middle of crustal plate
35Eventually, 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 only5-10 million years.
36Geological Formation of Oceanic Islands A. What is an oceanic island?B. Lithosphere and Plate TectonicsC. Formation of the Hawaiian Island Chain
40Hawaiian Island -Emperor Seamount Chain Emperor Seamount chain extends north from Hawaiian islands
41Conventional plate tectonic theory assumes that lithospheric plates move, while hotspots are stationary; as plate moves over hotspot, volcano goes inactive.
42However, recent evidence suggests that hotspots can move However, recent evidence suggests that hotspots can move. Emperor Seamount chain may have formed by hotspot that moved south as Pacific plate moved northwest.
43Geological Formation of Oceanic Islands A. What is an oceanic island?B. Lithosphere and Plate TectonicsC. Formation of the Hawaiian Island ChainD. Formation of Bermuda
44Geological 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?);
45Geological 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 coastBermuda Rise comprises three seamounts (relicts of volcanic cones): Argus Bank, Challenger Bank, and Bermuda Seamount (= Bermuda Pedestal);
46Bermuda Sea Mount Mid-Atlantic Ridge San Salvador Bahama Banks Mid-Atlantic RidgeSan SalvadorBahama Banks
48Geological 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 withconcentric layers; mm in diameter)Ooids
50Geological 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)
51Geological Formation of Oceanic Islands A. What is an oceanic island?B. Lithosphere and Plate TectonicsC. Formation of the Hawaiian Island ChainD. Formation of BermudaE. Formation of the Bahamas
52200 MYA: Pangea Pulls Apart Atlantic Ocean formsStretches margin of continental crustWarm, shallow seas form over crustal platformCaCO3 precipitates – forms ooidsSediments accumulate at ~ 5 cm / 1000 yearsOoids cemented together to form oolitic limestoneTethys TrenchMediterraneanNorth AmericaMid-Atlantic RidgeNAfricaGulf of MexicoCaribbean SeafaultSouth America
53Bahamas Built on Limestone Platform CaySalStraitsOfFloridaAgePeriodFloridaAndrosEleutheraSanterenChannelpresentrecentTongue of the OceanAtlantic Ocean35 myEocene50 myPalaeoceneLateCretaceous5000ft=1525m65 myEarlyCretaceous10000ft=3050m100 my15000ft=4575m140 myJurassic20000ft=6100mPre-Jurassic200 myCrust?Formed by precipitation of CaCO3 in warm, shallow seas over 120 MYOoids cemented together to form oolitic limestoneContinental crust subsided under weight of limestoneCores to 6,100 meters (20,000 feet) are surface-cemented limestone!!Crust NOT found in any cores to date
54Bahamian Banks = Tops of Limestone Platform CaySalStraitsOfFloridaAgePeriodFloridaAndrosEleutheraSanterenChannelpresentrecentTongue of the OceanAtlantic Ocean35 myEocene50 myPalaeoceneLateCretaceous5000ft=1525m65 myEarlyCretaceous10000ft=3050m100 my15000ft=4575m140 myJurassic20000ft=6100mPre-Jurassic200 myCrust?Channels cut through limestone platform (erosion; geological faults);Deepest channel = Tongue of the Ocean (~ 3000 m deep)Coral reefs formed around edges and on tops of platformInner lagoons accumulated sediments that formed banks and islands
57LandSat Image of San Salvador Island San Salvador sits on isolated portion of Bahamas PlatformNear-vertical wall of the platform drops off to depths of meters (west) to 4000 meters (east).
58San Salvador Bank is rimmed by coral reef = “bucket” walls Much of San Salvador’s terrestrial rock is “fossilized” sanddunes (aeolian* limestone) rising above limestone platform;Some rock is ancient coral reef formed when sea level was higher.San Salvador BankSan Salvador Island
59Bermuda and San Salvador: Similar processes at ocean surface Very different geological origins
60Is San Salvador an oceanic island? No evidence of direct, terrestrial connection to continent (now or in the past);Separated from continent by deep ocean.
61Next Week: Corals and Coral Reefs End of Slide Show March 28, 2011Next Week: Corals and Coral Reefs