Transcript Origin of Oceanic Islands

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;
I. Geological Formation of
Oceanic Islands
A. What is an oceanic island?
B. Lithosphere and Plate Tectonics
Cutaway Diagram of the Earth
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 earth’s
magnetic field
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 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
Ocean
Continental crust
Oceanic 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
Ocean
Continental 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)
• 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
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 Wegener
1880-1930
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
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
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.
Source: Wikipedia http://en.wikipedia.org/wiki/Plate_tectonics
Click Here to Play Subduction Animation
http://www.wwnorton.com/college/geo/egeo/flash/2_9.swf
Convergence of Crustal Plates with
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
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.
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 http://pubs.usgs.gov/gip/hawaii/page05.html
Northwest Movement of Pacific Plate Over
Fixed Hawaiian Hot Spot
Source: USGS http://pubs.usgs.gov/gip/hawaii/page12.html
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.
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;
• 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)
• 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);
San Salvador
Bahama Banks
http://topex.ucsd.edu/marine_topo/gif_topo_track/topo8.gif
Bermuda Sea Mount
Bermuda Rise
http://hoopermuseum.earthsci.carleton.ca/Bermuda/Geology/BERM5-1A.HTML
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
Source: http://earthobservatory.nasa.gov/images/imagerecords/7000/7397/bermuda_l7_1999226_lrg.jpg
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%3
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
•
•
North America
•
N
•
Africa
•
Gulf of Mexico
•
Caribbean Sea
South America
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
Bahamas Built on Limestone Platform
Age Period
present recent
Florida
Straits Cay
Of Sal Santeren
Channel
Florida
35 my Eocene
50 myPalaeocene
Late
65 my
Cretaceous
Early
100 my
Cretaceous
Eleuthera
Andros
Tongue
of the
Ocean
Atlantic Ocean
5000ft=1525m
10000ft=3050m
15000ft=4575m
140 myJurassic
20000ft=6100m
Pre200 myJurassic
•
•
•
•
•
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
Bahamian Banks = Tops of Limestone
Platform
Age Period
present recent
Florida
Straits Cay
Of Sal Santeren
Channel
Florida
35 my Eocene
50 myPalaeocene
Late
65 my
Cretaceous
Early
100 my
Cretaceous
Eleuthera
Andros
Tongue
of the
Ocean
Atlantic Ocean
5000ft=1525m
10000ft=3050m
15000ft=4575m
140 myJurassic
20000ft=6100m
Pre200 myJurassic
•
•
•
•
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
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” 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
• 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