Presentation on theme: "I. Geological Formation of Oceanic Islands. A. What is an oceanic island?"— Presentation transcript:
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;
I. Geological Formation of Oceanic Islands 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
Cutaway Diagram of the Earth Outer Core ~ 2,220 km thick Liquid Iron-Nickel ~ 4100˚C Rotates E to W Rotation generates earths magnetic field
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)
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 Earth 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 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 earths surface Continental crust Oceanic crust Ocean
Oceanic Crust: Forms the deep sea floor 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 earths surface Continental crust Oceanic crust Ocean
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
Continental Drift: Continents have moved over the earths 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
Continental drift incorporated into modern theory of Plate Tectonics*: *From the Greek: τεκτονικός "pertaining to building Scientific theory describing large scale movements of the Earths 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
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 Click to play Animation
Convergent Plate Boundary Convergence of two oceanic plates: Denser plate sinks under lighter plate = subduction zone. Source: Wikipedia
Click Here to Play Subduction Animation
Convergence of Crustal Plates with Subduction zone results in earthquake and volcanic activity (e.g., Pacific Rim of Fire). Source: Wikipedia
Volcanic Activity at Tectonic Plate Boundaries Source: USGS
Volcanic activity at subduction zone can form oceanic islands (e.g., Aleutians; Lesser Antilles). Source: Wikipedia
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
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.
I.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 Source: USGS
Northwest Movement of Pacific Plate Over Fixed Hawaiian Hot Spot Source: USGS
Ages of Hawaiian Islands Source:
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.
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
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 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) 30 MY to present: Bermuda Rise continued moving to present location, 32° 10-30N ~ 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 Salvador Bahama Banks Mid-Atlantic Ridge
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
Satellite Image of Bermuda Source:
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) B/ptpublisher.gov.bm%3B7087/publishedcontent/publish/new_min_of_environment/environ mental_protection___project_nature_fact_sheets/the_geology_of_bermuda_0.pdf B/ptpublisher.gov.bm%3B7087/publishedcontent/publish/new_min_of_environment/environ mental_protection___project_nature_fact_sheets/the_geology_of_bermuda_0.pdf
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
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-Atlantic Ridge 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 my 50 my 65 my 100 my 140 my 200 my recent Eocene Palaeocene Late Cretaceous Early Cretaceous Jurassic Florida Straits Of Florida Cay Sal 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 my 50 my 65 my 100 my 140 my 200 my recent Eocene Palaeocene Late Cretaceous Early Cretaceous Jurassic Florida Straits Of Florida Cay Sal Santeren Channel Andros Tongue of the Ocean Eleuthera Atlantic Ocean 5000ft=1525m 10000ft=3050m 15000ft=4575m 20000ft=6100m Pre- Jurassic Crust?
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 meters (west) to 4000 meters (east).
San Salvador Bank is rimmed by coral reef = bucket walls Much of San Salvadors 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 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 Show March 28, 2011 Next Week: Corals and Coral Reefs