Download S 15 Formation of Islands (new)

Geological Formation of
Oceanic Islands
Start March 11, 2013
Island
In very general terms:
An island is an area of suitable habitat
surrounded by an expanse of unsuitable
habitat.
An “island” can be a…
•  land mass surrounded by water (stream, lake
or ocean);
•  coral reef surrounded by expanse of sandy
bottom or grass beds without reefs;
•  Isolated terrestrial habitat amid a larger land
mass, such as a
- mountain top;
- forest remnant surrounded by cleared land;
- water-filled tree hole in forest
Islands are important natural laboratories for the
study of biogeography, ecology, population
genetics, evolutionary biology, etc.
Types of Islands
A.  Continental Islands: Formed on
continent; may have formerly been
connected to mainland by land bridge:
Current Sea Level
Continent
Island
Former
Sea Level
Submerged Land Bridge
Continental Shelf
Examples of Continental Islands
1.  Greenland
2.  New Guinea
3.  British Isles
4.  California Channel Islands
5.  Block Island, Nantucket, Martha’s
Vineyard
Continental Islands:
Two Special Cases
1.  Eroded limestone islands of the Bahamas and
Bermuda:
Rising sea level and wind eroded parts of San
Salvador and Bermuda, leaving many small,
erosion-resistant (“satellite”) islands
(= cays* in the Bahamas).
* Pronounced “keys”
Satellite cays on the horizon (arrows) were once
part of San Salvador; still part of San Salvador
“continent” (San Salvador Bank).
Gerace Research Centre
San Salvador’s Satellite Cays
Several of these cays have relict populations of
an endangered lizard (San Salvador Rock
Iguana, Cyclura rileyi) that is now rare on San
Salvador itself.
Bermuda is a collection of ~180 limestone islands,
many formed by erosion of a larger aeolian
limestone mass (“continent”).
Continental Islands:
Two Special Cases
1. Eroded limestone islands of the Bahamas and Bermuda
2. Barro Colorado Island, Panama Canal
Barro Colorado Island (BCI):
- A 1500 hectare remnant of lowland moist
forest in the middle of the Panama Canal;
- managed by the Smithsonian Institution as a
tropical research site.
B. Oceanic Islands: Never connected to
continent; often formed by volcanic activity
and isolated from continent by deep
ocean.
Oceanic
Island
Current Sea Level
Continental
Shelf
Former Sea
Level
Sea Floor
Undersea
Volcano
Examples of Oceanic Islands
Volcanic Origin
•  Iceland
•  Aleutians
•  Hawaiian Islands
•  Antilles
•  Bermuda
•  Western Pacific
atolls
Non-Volcanic Origin
•  Bahamas
•  Belize atolls and
Barrier Reef cays
•  Great Barrier Reef
Islands
•  New Zealand
•  Madagascar
Lithosphere and Plate
Tectonics
Cutaway Diagram of the Earth
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
•  Solid outer mantle and crust “float” on
asthenosphere
*Asthenosphere: From the Greek, asthenes = weak
Cutaway Diagram of the Earth
Crust
•  ~ 5-50 km thick
•  Solid, brittle rock
•  Two types of crust
•  - continental
•  - oceanic
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
Ocean
Continental crust
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*
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:*
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.
*From the Greek: τεκτονικός "pertaining to building”
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 boundary
•  Forms new oceanic crust
•  Pushes plates apart (~5 cm / yr)
= Sea Floor Spreading Center
•  Movement of lithospheric plate with continental
crust à continental drift.
Sea Floor Spreading Animation
http://www.wwnorton.com/college/geo/animations/sea_floor_spreading.htm
Formation of Oceanic Crust Animation
http://www.wwnorton.com/college/geo/animations/formation_of_ocean_crust.htm
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 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.
Click Here to Play Subduction Animation
http://www.wwnorton.com/college/geo/animations/the_process_of_subduction.htm
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: Aleutian Arc formed where Pacific Plate is
sliding under N. American Plate
Sources: http://www.borough.kenai.ak.us/emergency/Volcano/active.gif
http://www.nationalatlas.gov/articles/geology/features/aleutians.html
The Lesser Antilles (Eastern Caribbean) formed
where the Caribbean Plate is sliding under the
North American Plate
http://www.caribbeanvolcanoes.com
In areas where lithospheric plate is thin,
magma plume from mantle can push up through plate,
forming a “hot spot.”
Hotspot Volcano Animation
http://www.wwnorton.com/college/geo/animations/hot_spot_volcanoes.htm
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
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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,
•  volcanic island erodes
•  aging oceanic crust becomes more dense
•  volcanic cone submerges,
•  forms undersea mountain
= seamount (rounded top) or
= guyot (flat top);
•  Oceanic islands estimated to last only
5-10 million years.
Formation of the Hawaiian
Island Chain
Source: USGS http://pubs.usgs.gov/gip/hawaii/page05.html
Age of Hawaiian Islands decreases from west to east
Source: http://volcano.und.edu/vwdocs/vwlessons/hotspots.html
Volcanoes formed islands as Pacific Plate moved
northwest over hot spot.
Source: USGS http://pubs.usgs.gov/gip/hawaii/page12.html
Conventional plate tectonic theory assumes that
lithospheric plates move, while hotspots are stationary;
as plate moves over hotspot, volcano goes inactive.
http://geology.com/usgs/hawaiian-hot-spot/
Hawaiian Island - Emperor Seamount Chain
Emperor Seamount chain extends north
from Hawaiian islands
Sharp, W.D. and D.A. Clague (2006) Science Vol. 313 no. 5791 pp. 1281-1284
Hawaiian Island -Emperor Seamount Chain
Emperor Seamount chain extends north
from Hawaiian islands
Hawaiian Island-Emperor Seamount Bend has been attributed
to a change in the direction of plate movement from north to
northwest about 47-50 million years ago.
Recent studies: Hotspots can move
Emperor Seamount chain may have formed by hotspot that
moved south (~ 40 mm/yr) as Pacific plate moved northwest.
Refs: Tarduno, J.A. et al., Science 22 August 2003: vol. 301 no. 5636 1064-1069
Tarduno, J.A. et al., Science 3 April 2009: vol. 324 no. 5923 pp. 50-53
About 47-50 million years ago, hot spot stopped migrating.
Hawaiian Island chain formed once hotspot became stationary.
Refs: Tarduno, J.A. et al., Science 22 August 2003: vol. 301 no. 5636 1064-1069
Tarduno, J.A. et al., Science 3 April 2009: vol. 324 no. 5923 pp. 50-53
Bermuda Sea Mount
San Salvador
Bahama Banks
http://topex.ucsd.edu/marine_topo/gif_topo_track/topo8.gif
Geological 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
not 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?)
Seafloor spreading
moved volcanic cones
NW ~ 2 cm/year
http://topex.ucsd.edu/marine_topo/gif_topo_track/topo8.gif
110 Million Years Ago (MYA):
Volcanoes along
Mid-Atlantic Ridge
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
Bermuda Seamount (= Bermuda Pedestal)
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 = 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
Bermuda Sea Mount
San Salvador
Bahama Banks
http://topex.ucsd.edu/marine_topo/gif_topo_track/topo8.gif
Geological Formation of the Bahamas
200 MYA: Pangea Pulls Apart
ench
r
T
s
y
Teth
nean
a
r
r
e
t
i
Med
• 
• 
• 
North America
N
• 
• 
Africa
• 
Gulf of Mexico
Caribbean Sea
fault
South America
Atlantic Ocean forms
Margin of continental
crust stretched out
Warm, shallow seas
form over crustal
platform
CaCO3 precipitates
– forms ooids
Sediments
accumulate at rate of
~ 5 cm / 1000 years
Ooids cemented
together to form
oolitic limestone
Bahamas Built on Limestone Platform
Age
Period
present
recent
Cay
Florida
Straits
Sal
Santeren
of
Channel
Florida
35 my
Eocene
50 my
Palaeocene
Late
65 my
Cretaceous
Eleuthera
Andros
Tongue
of the
Ocean
Atlantic Ocean
5000ft = 1525m
Early
100 my
Cretaceous
10000ft = 3050m
15000ft = 4575m
140 my
Jurassic
20000ft = 6100m
Pre-
200 my
Jurassic
• 
• 
• 
• 
• 
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
Cay
Florida
Straits
Sal
Santeren
of
Channel
Florida
35 my
Eocene
50 my
Palaeocene
Late
65 my
Cretaceous
Eleuthera
Andros
Tongue
of the
Ocean
Atlantic Ocean
5000 ft = 1525m
Early
100 my
Cretaceous
10000 ft = 3050m
15000 ft = 4575m
140 my
Jurassic
20000 ft = 6100m
Pre-
200 my
Jurassic
• 
• 
• 
• 
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
oolitic
limestone
LandSat Image of San Salvador Island
•  San Salvador sits on
isolated portion of Bahamas
Platform
•  Near-vertical wall of
platform drops off to depth of
2000-3000 m (west)
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 9, 2015
Up Next:
March 23
Dr. Idjadi: Coral Reefs