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 Ocean

12. Continental Crust: Forms the continents
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
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 ( ≤ 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

16. Tectonic Plates of the World
Source: Wikipedia

17. First proposed by German astronomer / meteorologist
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
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 (e. g
Volcanic activity at subduction zone can form oceanic islands (e.g., Aleutians; Lesser Antilles).
Source: Wikipedia

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

32. Map of hot spots

33. Hot spots under oceanic crust can form oceanic islands

34. 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

35. 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.

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

37. Hawaiian Islands
Source: USGS

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

39. Ages of Hawaiian Islands
Source:

40. Hawaiian Island -Emperor Seamount Chain Emperor Seamount chain extends north from Hawaiian islands

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

42. However, 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.

43. 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

44. 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?);

45. 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);

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

47. Bermuda Rise

48. 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

49. Satellite Image of Bermuda
Source:

50. 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)

51. 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

52. 200 MYA: Pangea Pulls Apart
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
Tethys Trench
Mediterranean
North America
Mid-Atlantic Ridge N
Africa
Gulf of Mexico
Caribbean Sea
fault
South America

53. Bahamas Built on Limestone Platform
Cay
Sal
Straits Of
Florida
Age
Period
Florida
Andros
Eleuthera
Santeren
Channel
present
recent
Tongue of the Ocean
Atlantic Ocean
35 my
Eocene
50 my
Palaeocene
Late
Cretaceous
5000ft=1525m
65 my
Early
Cretaceous
10000ft=3050m
100 my
15000ft=4575m
140 my
Jurassic
20000ft=6100m
Pre-
Jurassic
200 my
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

54. Bahamian Banks = Tops of Limestone Platform
Cay
Sal
Straits Of
Florida
Age
Period
Florida
Andros
Eleuthera
Santeren
Channel
present
recent
Tongue of the Ocean
Atlantic Ocean
35 my
Eocene
50 my
Palaeocene
Late
Cretaceous
5000ft=1525m
65 my
Early
Cretaceous
10000ft=3050m
100 my
15000ft=4575m
140 my
Jurassic
20000ft=6100m
Pre-
Jurassic
200 my
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

55. Bahamas Banks

56. Bucket Theory for Formation of Bahamian Bank

57. 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).

58. 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

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

60. 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.

61. Next Week: Corals and Coral Reefs
End of Slide Show March 28, 2011
Next Week: Corals and Coral Reefs