Download The Floors of the Oceans

The Floors of the Oceans:
Concealed Revealers of Change
Earth2Class Workshops for
Teachers
Originally presented 20 Sep 2003
with Guest Scientists
Dr. Gerard Bond
Doherty Senior Scholar, Biology and
Paleoenvironment Division
Ramona (Rusty) Lotti
Curator, L-DEO Deep Sea Sample
Repository
World Ocean Facts
• Oceans cover 71% of
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•
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Earth’s surface
Northern Hemisphere
is 39% land, 61% sea
Southern Hemisphere
is 19% land, 81% sea
Pacific is largest basin,
followed by Atlantic,
Indian, Southern, and
Arctic
• Southern is defined as
•
waters between 60o S
and Antarctica, where
the Antarctic
Circumpolar Current
flows
Numerous seas and
gulfs lie adjacent to
the major ocean
basins
Earth’s Crust
Continental crust
• granitic
• thicker
• less dense
• Average elevation 838 m (2,750 ft.)
Oceanic Crust
• basaltic
• thinner
• Denser
• Average depth 3800 m (12,500 ft)
Continental Margins
• Continental shelf
gentle slope out to average depth of 130
m (430 ft.)
variations in width, materials, origins,
etc.
coral reefs
• Continental slope
more steeply sloping edge of continent
• Continental rise
apron of sediments
Submarine Canyons
• Cut through continental slope and shelf
• Probably created through erosion by
turbidity currents
• Also connected to Pleistocene river
drainage; e.g., Hudson Canyon
• Differences between East and West Coast
canyons
• Submarine fans at base of many canyons
Ocean Basins
Abyssal Plains
• vast, flat basins
• abyssal hills generally less than 100 m
• sediment blankets from terrestrial and
pelagic sources
Ocean Ridges
• volcanic mountain range with rift valley
• about 23% of sea floor
Other Ocean Basin Features
• Trench and island arc systems
• Volcano and seamount chains
• Atolls
• Guyots
• Hydrothermal vents
“Heezen-Tharp Maps”
Perhaps the most famous version of what the
floors of the oceans look like were the series
of maps created beginning in the 1950s by
Bruce Heezen and Marie Tharp of LamontDoherty. Using thousands of sonar tracks,
they produced physiographic charts of the
ocean basins. These were later published in
enhanced versions by the National
Geographic Society, and are still widely used.
Online Bathymetry Maps
http://www.ngdc.noaa.gov/mgg/image/2minrelief.html
Ocean Basins and Plate Tectonics
• Francis Bacon (1561 – 1626) noted “fit” of
South America and Africa
• Alfred Wegener (1880 – 1930) and Frank
Taylor (1860 – 1939) independently
proposed continental drift
• Challenged existing theories, could not
provide satisfactory mechanism
Supporting Evidence
• Earthquake and volcano patterns
• SONAR and other observation techniques
• Geomagnetic patterns
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•
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-- “normal” and “reverse” polarity
-- “mirror” images on opposite side of MOR
-- 170 reversals over past 76 million years
Increasing age away from MOR rift valley
Greatest heat flow along MOR
Increasing thickness of sediments away from MOR
Modern Exploration Technologies
Acoustic Monitoring of
Ocean Seismicity
The hydroacoustic method
allows detection of lowmagnitude seismicity and
volcanic activity and
provides more accurate
source locations than land
seismic networks.
http://www.pmel.noaa.gov/vents/acoustics/seismicity/seismicity.html
Evolution of a theory
During the past century, understanding
of the ocean floors changed dramatically.
• “Continental Drift”
• “Convection” in the upper mantle
• “Sea-floor Spreading”
• Plate Tectonics
3 Types of Plate Boundaries
• Divergent
• Convergent
• Transform
http://www.cotf.edu/ete/modules/msese/earthsysflr/plates4.html
Spreading and Closing Cycles
J. Tuzo Wilson (U. Toronto) proposed that oceans
open and close over cycles lasting 500 – 600
million years.
An excellent explanation has been created by
Prof. Lynn Fichter of James Madison University
http://csmres.jmu.edu/geollab/Fichter/Wilson/Wilson.html
The cycle starts with a stable craton (core of
a continent.)
“Embryonic” stage
• Supercontinents remain in one spot for hundred
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•
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of millions of years, acting like a blanket
Mantle convedction warms the overlying
supercontinent, causing expansion and
stretching
Fracturing forms a rift valley
Examples: East African Rift Valley
“Juvenile” stage
• Rift valleys gradually widen and connect to
•
ocean
Examples: Red Sea, Gulf of California
“Mature” stage
• Lateral spreading of rift valley leads to
•
development of a divergent plate boundary and
new oceanic crust
Atlantic is a mature ocean with passive edges
“Declining” stage
• Subduction becomes more widespread
• Convection in mantle drags lithosphere plates
•
•
into subduction zones
Examples: Scotia Arc in South Atlantic, West
Indies near Barbados
Explains why there is little oceanic crust more
than 200 my
• Ocean basin begins to close as subduction rates
•
•
exceed spreading rates
Sediment deposits are deformed and uplifted,
creating mountain ranges in new supercontinent
on site of former ocean
Example: Appalachians (formed at end of
previous Wilson cycle)
• Wilson cycles influence sea level relative to
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•
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continents of 105 years
When supercontinents dominate, land stands
higher as mantle warms them
As spreading begins, land is lowered and seas
encroach, flooding low-lying continental areas
As basin widens and crust ages, it deepens and
sea level falls
During periods of higher sea level, oceans
covered about 80% of surface
During periods of lower sea level, oceans
covered only about 65% of surface
Sea Floor Sediments
• Terrestrial and pelagic origins
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•
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(important types in next slide)
Provide evidence of processes and
environmental conditions at time of origin
Relatively protected from erosion, so more
complete “record” than continental deposits
Connection with rocks now forming continents
(ex., limestones, shales)
Types of Deep Sea Sediments
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Terrigenous muds
Turbidites
Glacial (“ice-rafted”)
Brown/red clays
Authigenic (hydrogenous) deposits
Manganese nodules
Pelagic oozes—diatoms, “forams,” “rads”
Volcanic
Ocean Sediment Distribution
Sediment thickness is influenced by:
• Age of the underlying crust
• Tectonic history of the underlying crust
• Structural trends in the basement rocks
• Location and nature of sediment sources
• Nature of depositional processes delivering the
sediments
You can see a digital image of worldwide sediment distribution
through the DataStreme Ocean web page:
www.ametsoc.org/amsedu/DS-Ocean/home.html
LDEO Deep Sea Sample Repository
• Largest collection in
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•
the world
E2C has visited many
times – see archived
series for related
PowerPoints
Web site provides
many useful starting
points for education
http://www.ldeo.columbia.edu/CORE_REPOSITORY/RHP1.html
Sediments and Climate Change
Dr. Gerard Bond will discuss some of his
research about climate change based on
interpretation of deep-sea sediments. Of
particular interest for much of his study
are ice-rafted materials collected in the
North Atlantic. You can see versions of
some of his work in the Core Repository
pages.
Resources for Educational Projects
In addition to what is available through the
L-DEO and other university-related web
sites, one of the best sources for
information about this topic may be found
in the USGS Coast & Marine Geology
Program web pages
http://walrus.wr.usgs.gov/infobank/
Special Reference
“DataStreme Ocean” Preview Package
Ch. 2: Ocean Basins and Plate Tectonics
Authors: M. Grant Gross and Elizabeth
Gross
Sponsor: AMS/NOAA Cooperative Program
for Earth System Education