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

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;


A. What is an oceanic island?

B. Lithosphere and Plate Tectonics

Cutaway Diagram of the Earth


Inner Core • Radius ~1255 km• Solid Iron• ~ 4100˚C• Rotates W to E


Outer Core • ~ 2,220 km thick• Liquid Iron-Nickel• ~ 4100˚C• Rotates E to W•Rotation generates earth’s magnetic field


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)


Upper Mantle Outer Mantle • ~ 30 to 70 km deep• Solid rock

Asthenosphere• ~70 to 300 km deep• soft - flows slowly


Crust• ~ 5-50 km thick• Solid, brittle rock

Two Types of Crust:

• Continental crust

• Oceanic crust

Continental crust Oceanic crust

Ocean

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


Ocean

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


Ocean

• 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

Lithosphere = Crust + Solid Outer Mantle(from Greek: Lithos = rocky)

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.

• 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 Wegener1880-1930

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.

Plate Tectonics and Oceanic Island Formation

(Highly simplified!)

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

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


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

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

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

Click Here to Play Seafloor Spreading Animation



Movement of lithospheric plates caused breakup of Pangea Super-continent

~300 million years ago

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

Click to play Animation

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

Convergent Plate Boundary

Convergence of two oceanic plates: Denser plate sinks under lighter plate

= subduction zone.


Click Here to Play Subduction Animation




Convergence of Crustal Plates withSubduction zone results in earthquake and volcanic

activity (e.g., Pacific Rim of Fire).


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).

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

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

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

Hot spots under oceanic crust can form oceanic islands

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

• 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.


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

Hawaiian Islands


Northwest Movement of Pacific Plate Over Fixed Hawaiian Hot Spot


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

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

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


A. What is an oceanic island?B. Lithosphere and Plate Tectonics C. Formation of the Hawaiian Island ChainD. Formation of Bermuda

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?);


• 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);

Bermuda Sea Mount

San SalvadorBahama Banks

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Bermuda Rise

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


• 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

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


•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

http://www.gov.bm/portal/server.pt/gateway/PTARGS_0_2_11280_207_227543_43/http;/ptpublisher.gov.bm;7087/publishedcontent/publish/new_min_of_environment/environmental_protection___project_nature_fact_sheets/the_geology_of_bermuda_0.pdf




A. What is an oceanic island?B. Lithosphere and Plate Tectonics C. Formation of the Hawaiian Island ChainD. Formation of BermudaE. Formation of the Bahamas

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

Gulf of Mexico

Caribbean Sea

South America

Africa

Tethys Trench

Mediterranean

N

Mid

-Atla

ntic

Rid

ge

fault

North America

Bahamas Built on Limestone Platform

• 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

PeriodAge

present

35 my50 my

65 my

100 my

140 my

200 my

recent

EocenePalaeocene

LateCretaceous

EarlyCretaceous

Jurassic

Florida StraitsOf

Florida

CaySal Santeren

Channel

Andros

Tongue of the Ocean

Eleuthera

Atlantic Ocean

5000ft=1525m

10000ft=3050m

15000ft=4575m

20000ft=6100m

Pre-Jurassic

Crust?

Bahamian Banks = Tops of Limestone Platform

• 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

PeriodAge

present

35 my50 my

65 my

100 my

140 my

200 my

recent

EocenePalaeocene

LateCretaceous

EarlyCretaceous

Jurassic

Florida StraitsOf

Florida

CaySal Santeren

Channel

Andros

Tongue of the Ocean

Eleuthera

Atlantic Ocean

5000ft=1525m

10000ft=3050m

15000ft=4575m

20000ft=6100m

Pre-Jurassic

Crust?

Bahamas Banks

Bucket Theory for Formation of Bahamian Bank

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).

San 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 Island San Salvador Bank

Bermuda and San Salvador:Similar processes at ocean surface

Very different geological origins

• Bermuda • San Salvador

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 ShowMarch 28, 2011

Next Week:Corals and Coral Reefs

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

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