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

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i geological formation of oceanic islands2

I. Geological Formation of Oceanic Islands

A. What is an oceanic island?

oceanic island
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;
i geological formation of oceanic islands4

I. Geological Formation of Oceanic Islands

What is an oceanic island?

Lithosphere and Plate Tectonics

cutaway diagram of the earth6
Cutaway Diagram of the Earth
  • Inner Core
  • Radius ~1255 km
  • Solid Iron
  • ~ 4100˚C
  • Rotates W to E
cutaway diagram of the earth7
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
cutaway diagram of the earth8
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)
cutaway diagram of the earth9
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
cutaway diagram of the earth10
Cutaway Diagram of the Earth
  • Crust
  • ~ 5-50 km thick
  • Solid, brittle rock
two types of crust
Two Types of Crust:
  • Continental crust
  • Oceanic crust

Continental crust

Ocean

Oceanic crust

continental crust
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

oceanic crust
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

lithosphere crust solid outer mantle from greek lithos rocky
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
tectonic plates of the world
Tectonic Plates of the World

Source: Wikipedia http://en.wikipedia.org/wiki/Plate_tectonics

continental drift continents have moved over the earth s surface during geological time
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

continental drift incorporated into modern theory of plate tectonics
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.

divergent plate boundary
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

http://www.wwnorton.com/college/geo/egeo/flash/2_7.swf

movement of lithospheric plate that includes continental crust results in continental drift
Movement of lithospheric plate that includes continental crust results in continental drift.

Click Here to Play Seafloor Spreading Animation

http://www.wwnorton.com/college/geo/egeo/flash/2_5.swf

movement of lithospheric plates caused breakup of pangea super continent 300 million years ago
Movement of lithospheric plates caused breakup of Pangea Super-continent ~300 million years ago

Click to play Animation

http://sos.noaa.gov/videos/Scotese.mov

slide26
Convergent Plate BoundaryConvergence of two oceanic plates: Denser plate sinks under lighter plate = subduction zone.

Source: Wikipedia http://en.wikipedia.org/wiki/Plate_tectonics

slide27

Click Here to Play Subduction Animation

http://www.wwnorton.com/college/geo/egeo/flash/2_9.swf

slide28
Convergence of Crustal Plates withSubduction zone results in earthquake and volcanic activity (e.g., Pacific Rim of Fire).

Source: Wikipedia http://en.wikipedia.org/wiki/Plate_tectonics

volcanic activity at tectonic plate boundaries
Volcanic Activity at Tectonic Plate Boundaries

Source: USGS http://pubs.usgs.gov/gip/hawaii/page10.html

volcanic activity at subduction zone can form oceanic islands e g aleutians lesser antilles
Volcanic activity at subduction zone can form oceanic islands(e.g., Aleutians; Lesser Antilles).

Source: Wikipedia http://en.wikipedia.org/wiki/File:Japan_separation.png

slide31
In areas where lithospheric plate is thin, magma plume from mantle can push up through plate, forming a “hot spot.”

Hotspot Volcano Animation

map of hot spots
Map of hot spots

http://www.math.montana.edu/~nmp/materials/ess/geosphere/advanced/activities/hotspots/index.html

slide34

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

slide35

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.
slide36

Geological Formation of Oceanic Islands

A. What is an oceanic island?

B. Lithosphere and Plate Tectonics

C. Formation of the Hawaiian Island Chain

hawaiian islands
Hawaiian Islands

Source: USGS http://pubs.usgs.gov/gip/hawaii/page05.html

northwest movement of pacific plate over fixed hawaiian hot spot
Northwest Movement of Pacific Plate Over Fixed Hawaiian Hot Spot

Source: USGS http://pubs.usgs.gov/gip/hawaii/page12.html

ages of hawaiian islands
Ages of Hawaiian Islands

Source: http://volcano.und.edu/vwdocs/vwlessons/hotspots.html

hawaiian island emperor seamount chain emperor seamount chain extends north from hawaiian islands
Hawaiian Island -Emperor Seamount ChainEmperor Seamount chain extends north from Hawaiian islands
slide41

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

slide42

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

slide43

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

geological formation of bermuda 1

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?);

geological formation of bermuda 2

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);

slide46

Bermuda Sea Mount

http://topex.ucsd.edu/marine_topo/gif_topo_track/topo8.gif

Mid-Atlantic Ridge

San Salvador

Bahama Banks

bermuda rise
Bermuda Rise

http://hoopermuseum.earthsci.carleton.ca/Bermuda/Geology/BERM5-1A.HTML

geological formation of bermuda 3
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

satellite image of bermuda
Satellite Image of Bermuda

Source: http://earthobservatory.nasa.gov/images/imagerecords/7000/7397/bermuda_l7_1999226_lrg.jpg

geological formation of bermuda 4

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) http://www.gov.bm/portal/server.pt/gateway/PTARGS_0_2_11280_207_227543_43/http%3B/ptpublisher.gov.bm%3B7087/publishedcontent/publish/new_min_of_environment/environmental_protection___project_nature_fact_sheets/the_geology_of_bermuda_0.pdf

slide51

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

E. Formation of the Bahamas

200 mya pangea pulls apart
200 MYA: Pangea Pulls Apart

Tethys Trench

Mediterranean

  • 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

North America

N

Mid-Atlantic Ridge

Africa

Gulf of Mexico

Caribbean Sea

fault

South America

bahamas built on limestone platform
Bahamas Built on Limestone Platform

Cay

Sal

Straits

Of

Florida

Eleuthera

Age

Period

Florida

Andros

Santeren

Channel

present

recent

Tongue of the Ocean

Atlantic Ocean

35 my

Eocene

50 my

Palaeocene

  • 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

5000ft=1525m

Late

Cretaceous

65 my

Early

Cretaceous

10000ft=3050m

100 my

15000ft=4575m

140 my

Jurassic

20000ft=6100m

Pre-

Jurassic

Crust?

200 my

bahamian banks tops of limestone platform
Bahamian Banks = Tops of Limestone Platform

Cay

Sal

Straits

Of

Florida

Eleuthera

Age

Period

Florida

Andros

Santeren

Channel

present

recent

Tongue of the Ocean

Atlantic Ocean

35 my

Eocene

50 my

Palaeocene

  • 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

5000ft=1525m

Late

Cretaceous

65 my

Early

Cretaceous

10000ft=3050m

100 my

15000ft=4575m

140 my

Jurassic

20000ft=6100m

Pre-

Jurassic

Crust?

200 my

slide57

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 2000-3000 meters (west) to 4000 meters (east).
slide58

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

bermuda and san salvador similar processes at ocean surface very different geological origins
Bermuda and San Salvador:Similar processes at ocean surfaceVery different geological origins
  • Bermuda
  • San Salvador
is san salvador an oceanic island

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.

end of slide show march 28 2011

End of Slide ShowMarch 28, 2011

Next Week:Corals and Coral Reefs

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