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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 l.jpg

I. Geological Formation of Oceanic Islands

A. What is an oceanic island?


Oceanic island l.jpg
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 l.jpg

I. Geological Formation of Oceanic Islands

What is an oceanic island?

Lithosphere and Plate Tectonics



Cutaway diagram of the earth6 l.jpg
Cutaway Diagram of the Earth

  • Inner Core

  • Radius ~1255 km

  • Solid Iron

  • ~ 4100˚C

  • Rotates W to E


Cutaway diagram of the earth7 l.jpg
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 l.jpg
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 l.jpg
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


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Cutaway Diagram of the Earth

  • Crust

  • ~ 5-50 km thick

  • Solid, brittle rock


Two types of crust l.jpg
Two Types of Crust:

  • Continental crust

  • Oceanic crust

Continental crust

Ocean

Oceanic crust


Continental crust l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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.


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Plate Tectonics and Oceanic Island Formation Tectonics*:

(Highly simplified!)



Divergent plate boundary l.jpg
Divergent Plate Boundary Toward Lithosphere.

  • 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


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


Volcanic activity at mid ocean ridge can form ocean islands e g iceland l.jpg
Volcanic activity at mid-ocean ridge can form Toward Lithosphere.ocean islands (e.g., Iceland).


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


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Convergent Plate Boundary Super-continent Convergence of two oceanic plates: Denser plate sinks under lighter plate = subduction zone.

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


Slide27 l.jpg

Click Here to Play Subduction Animation Super-continent

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


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Convergence of Crustal Plates with Super-continent Subduction 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 l.jpg
Volcanic Activity at Tectonic Plate Boundaries Super-continent

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


Volcanic activity at subduction zone can form oceanic islands e g aleutians lesser antilles l.jpg
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


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

Hotspot Volcano Animation


Map of hot spots l.jpg
Map of hot spots islands

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


Hot spots under oceanic crust can form oceanic islands l.jpg
Hot spots under oceanic crust can islandsform oceanic islands


Slide34 l.jpg

Review islands

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 l.jpg


Slide36 l.jpg

Geological Formation of Oceanic Islands crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

A. What is an oceanic island?

B. Lithosphere and Plate Tectonics

C. Formation of the Hawaiian Island Chain


Hawaiian islands l.jpg
Hawaiian Islands crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

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


Northwest movement of pacific plate over fixed hawaiian hot spot l.jpg
Northwest Movement of Pacific Plate Over crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top); Fixed Hawaiian Hot Spot

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


Ages of hawaiian islands l.jpg
Ages of Hawaiian Islands crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

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


Hawaiian island emperor seamount chain emperor seamount chain extends north from hawaiian islands l.jpg
Hawaiian Island -Emperor Seamount Chain crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top); Emperor Seamount chain extends north from Hawaiian islands


Slide41 l.jpg

Conventional plate tectonic theory assumes that crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top); lithospheric plates move, while hotspots are stationary; as plate moves over hotspot, volcano goes inactive.


Slide42 l.jpg

However, recent evidence suggests that hotspots can move. crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top); Emperor Seamount chain may have formed by hotspot that moved south as Pacific plate moved northwest.


Slide43 l.jpg

Geological Formation of Oceanic Islands crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

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 l.jpg

Geological Formation of Bermuda (1) crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

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 l.jpg

Geological Formation of Bermuda (2) crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

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 l.jpg

Bermuda Sea Mount crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

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

Mid-Atlantic Ridge

San Salvador

Bahama Banks


Bermuda rise l.jpg
Bermuda Rise crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

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


Geological formation of bermuda 3 l.jpg
Geological Formation of Bermuda (3) crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

  • 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 l.jpg
Satellite Image of Bermuda crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

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


Geological formation of bermuda 4 l.jpg

Geological Formation of Bermuda (4) crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

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 l.jpg

Geological Formation of Oceanic Islands crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

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 l.jpg
200 MYA crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top); : 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 l.jpg
Bahamas Built on Limestone Platform crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

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 l.jpg
Bahamian Banks = Tops of Limestone Platform crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

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


Slide55 l.jpg

Bahamas Banks crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);


Bucket theory for formation of bahamian bank l.jpg
Bucket Theory for Formation of Bahamian Bank crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);


Slide57 l.jpg

LandSat Image of San Salvador Island crust becomes more dense, volcanic cone submerges to form undersea mountain = seamount (rounded top) or guyot (flat top);

  • 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 l.jpg

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 l.jpg
Bermuda and San Salvador walls:Similar processes at ocean surfaceVery different geological origins

  • Bermuda

  • San Salvador


Is san salvador an oceanic island l.jpg

Is San Salvador an oceanic island? walls

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 l.jpg

End of Slide Show wallsMarch 28, 2011

Next Week:Corals and Coral Reefs


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