I. Geological Formation of Oceanic Islands

Transcription

1 I. Geological Formation of Oceanic Islands Start March 11, 2013

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 A. What is an oceanic island? B. 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/SiO x (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 Outer mantle and crust float on asthenosphere *Asthenosphere: From the Greek, asthenes = weak

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

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

12 Continental Crust: Forms the continents km thick (average ~ 30 km) Granite (Al / SiO x ) = 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 Ocean Continental crust Oceanic crust

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

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

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16 Tectonic Plates of the World Source: Wikipedia

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

18 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.

19 Plate Tectonics and Oceanic Island Formation (Highly simplified!)

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

21 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

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

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

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

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

26 Convergent Plate Boundary Convergence of two oceanic plates: Denser plate sinks under lighter plate = subduction zone. Source: Wikipedia

27 Click Here to Play Subduction Animation

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

29 Volcanic Activity at Tectonic Plate Boundaries Source: USGS

30 Volcanic activity at subduction zone can form oceanic islands: Aleutian Arc formed where Pacific Plate is sliding under N. American Plate Sources:

31 The Lesser Antilles (Eastern Caribbean) formed where the Caribbean Plate is sliding under the North American Plate

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

33 Map of hot spots

34 Hot spots under oceanic crust can form oceanic islands The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location.

35 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

36 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.

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

38 Hawaiian Islands Source: USGS

39 Age of Hawaiian Islands decreases from west to east Source:

40 Hawaiian islands: Volcanoes formed as Pacific Plate moved northwest over hot spot. Source: USGS

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

42 Hawaiian Island - Emperor Seamount Chain Emperor Seamount chain extends north from Hawaiian islands Sharp, W.D. and D.A. Clague (2006) Science Vol. 313 no pp

43 Hawaiian Island -Emperor Seamount Chain Emperor Seamount chain extends north from Hawaiian islands

44 Hawaiian Island-Emperor Seamount Bend has been attributed to a change in the direction of plate movement from north to northwest about million years ago.

45 However, recent evidence suggests that hotspots can move. Emperor Seamount chain may have formed by hotspot that moved south (~ 40 mm/yr) as Pacific plate moved northwest. Refs: Tarduno, J.A. et al., Science 22 August 2003: vol. 301 no Tarduno, J.A. et al., Science 3 April 2009: vol. 324 no pp

46 About million years ago, hot spot stopped migrating. Hawaiian Island chain formed once hotspot became stationary. Refs: Tarduno, J.A. et al., Science 22 August 2003: vol. 301 no Tarduno, J.A. et al., Science 3 April 2009: vol. 324 no pp

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

48 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; MYA: Second phase of volcanic activity probably not 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?);

49 Geological Formation of Bermuda (2) 30 MY to present: Bermuda Rise continued moving to present location, 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 Bermuda Seamount (= Bermuda Pedestal)

50 San Salvador Bahama Banks Bermuda Sea Mount

51 Bermuda Rise

52 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; mm in diameter) Ooids

53 Satellite Image of Bermuda Source:

54 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) %3B/ptpublisher.gov.bm%3B7087/publishedcontent/publish/new_min_of_environment/ environmental_protection project_nature_fact_sheets/the_geology_of_bermuda_0.pdf

55 I. Geological Formation of Oceanic Islands A. What is an oceanic island? B. Lithosphere and Plate Tectonics C. Formation of the Hawaiian Island Chain D. Formation of Bermuda E. Formation of the Bahamas

56 200 MYA: Pangea Pulls Apart North America Gulf of Mexico Caribbean Sea fault Africa Tethys Trench Mediterranean N Atlantic Ocean forms Stretches margin of continental crust Warm, shallow seas form over crustal platform CaCO3 precipitates forms ooids Sediments accumulate at ~ 5 cm / 1000 years Ooids cemented together to form oolitic limestone South America

57 Bahamas Built on Limestone Platform Age Period Florida present recent 35 my Eocene 50 my Palaeocene Late 65 my Cretaceous Straits Cay Of Sal Santeren Florida Channel Andros Tongue of the Ocean Eleuthera Atlantic Ocean 5000ft=1525m Early 100 my Cretaceous 10000ft=3050m 140 my Jurassic 15000ft=4575m 20000ft=6100m Pre- 200 my Jurassic Crust? Formed by precipitation of CaCO3 in warm, shallow seas over 120 MY Ooids cemented together to form oolitic limestone Continental crust subsided under weight of limestone Cores to 6,100 meters (20,000 feet) are surface-cemented limestone!! Crust NOT found in any cores to date

58 Bahamian Banks = Tops of Limestone Platform Age Period present recent 35 my Eocene 50 my Palaeocene Late 65 my Cretaceous Florida Straits Cay Of Sal Santeren Florida Channel Andros Tongue of the Ocean Eleuthera Atlantic Ocean 5000ft=1525m Early 100 my Cretaceous 10000ft=3050m 140 my Jurassic 15000ft=4575m 20000ft=6100m Pre- 200 my Jurassic Crust? 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 platform Inner lagoons accumulated sediments that formed banks and islands

59 Bahamas Banks

60 Bucket Theory for Formation of Bahamian Bank

61 LandSat Image of San Salvador Island San Salvador sits on isolated portion of Bahamas Platform Near-vertical wall of the platform drops off to depths of meters (west) to 4000 meters (east).

62 San Salvador Bank is rimmed by coral reef = bucket walls Much of San Salvador s terrestrial rock is fossilized sand dunes (aeolian* limestone) rising above limestone platform; Some rock is ancient coral reef formed when sea level was higher. San Salvador Bank San Salvador Island

63 Bermuda and San Salvador: Similar processes at ocean surface Very different geological origins Bermuda San Salvador

64 Is San Salvador an oceanic island? No evidence of direct, terrestrial connection to continent (now or in the past); Separated from continent by deep ocean.

65 End of Slide Show March 11, 2013 Next Week: Island Biogeography