Download Sea Floor Spreading

Oceanography
CRN # 10053
Lecture 5a
Debbie Reynolds Lecture Certification:
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Lectures 1
a)
b)
Properties of sea water, ocean physics, ocean basins, explorers
Meteorology, heat balance, geostrophic circulation
Lectures 2
a)
b)
Surface ocean winds, currents, temperature and salinity
Deep circulation, instruments
Lectures 3
a)
b)
El Niño
Climate change in the oceans, including sea level rise
Lectures 4
a)
b)
Wind waves, rogue waves and tsunami
Tides
Lectures 5
a)
b)
Sea-floor spreading, continental drift − Plate Tectonics
Ocean sediments, coasts and estuaries
Lectures 6
a)
b)
Food web, plankton, invertebrate animals
Vertebrate animals, communities tidal to abyssal
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Introduction
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Ocean crust is denser than the continental crust
Ocean crust is much younger than the
continental crust
Thermal convection from the mantel drives
continental drift and sea floor spreading
− Plate Tectonics
New ocean crust formed in rifts (for example the
Mid-Atlantic Ridge) as the sea floor is pulled apart
− Sea floor spreading
Old ocean crust is forced down and under the
continental crust to balance the spreading
− Subduction
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Global Bathymetry
● Abyssal Plains ● Trenches ● Ridges
● Continental Margins (Shelves & Rises)
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Mariana
Trench
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Discovered by
Challenger
Expedition (1872-76)
Deepest part of the
world's oceans
In western Pacific
east of the Mariana
Islands
Long and narrow
Reaches a maximum
depth of 10,900 m
(35,800 ft or 6.78 mi)
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Earth Structure
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In 1692 Edmund
Halley thought the
Earth consisting of a
hollow shells
Two inner
concentric shells
around an innermost
core
The shells and the
core corresponded
to the diameters of
the planets:
Mercury, Mars and
Venus
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Earth
Structure
60 mi
120 mi
Core Temperature
5,500°C
9,900°F
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60 mi
120 mi
Lithosphere
Lithos – rock
Asthenosphere
Asthenes - weak
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Earth Structure
• Seismic Waves
• P−wave (Primary)
 Compression
waves
• S−wave (Secondary)
 Transverse wave
 Can’t travel
through liquid
 Slower than
P−wave
• Speed of wave
affected by density
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Earth Density
Density (g/cc)
Continental Crust (granite) ~ 2.7 g/cc
Oceanic Crust (basalt)
~ 2.9 g/cc
Water = 1.0 g/cc
Iron
= 7.8 g/cc
Nickel = 8.9 g/cc
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Earth model
Density
decreases
from center
Density jumps
at layer
boundaries
Moho (1909) − named for
Andrija Mohorovičić
Radius (km)
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Erosion &
Isostatic
Balance
Isostatic →
Gravitation equilibrium
Half Dome
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Alfred Wegener
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German
Meteorologist
Father of
“Continental Drift”
theory
1912: Theory
rejected at
symposium of the
American
Association of
Petroleum
Geologists
1915: Book “The
Origins of
Continents and
Oceans”
(1880 –1930)
Winter in Greenland 1912-13
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Continental Drift
• Wegener noticed
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How continents
fit together
How geological
and fossil
features
matched
He could not
explain how the
continents could
move
All continents
once part of
super continent:
“Pangaea”
Fit of continents also noted by Leonardo
da Vinci (1500s) & Francis Bacon (1620s)
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Sea Floor
Spreading
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In 1957 a map by
Heezen and Thorp
showed symmetry
in Mid Atlantic
Ridge
In 1960 geologists
Harry Hess and
Robert Diaz
speculated that
new sea floor was
being created at
the mid-Atlantic
Ridge and
destroyed
elsewhere
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Magnetic Evidence
• Earth’s magnetic poles irregularly reverse ~100,000 years
• Magnetite orients its own magnetic field with the earth when a
fluid and than holds that orientation once cooled
• Magnetometers towed behind ships can measure the local
magnetic changes due to magnetite
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Magnetic Evidence
Vine and Mathews (1963)
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Radioactive Decay
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Parent atoms radioactively decay to daughter atoms
Decay rate is defined by the half life; the time it takes half of the
parent atoms to decay
Begin with 100 parent atoms; at the end of 1 half life, 50 parent
atoms have decayed to 50 daughter atoms
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Dating Evidence
Using radioactive dating:
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Age of Earth 4.5 billion years
Oldest rocks ~ 4 billion years
Found in the Canadian Shield
and in Australia, Africa
Without radioactive heating
Earth’s core would be cold
• Uranium-238 (U238) decays with a half life of 4.5 billion years
• The final product is lead-206 (Pb206), Half life 1017 years
• There are 18 intermediate daughter products on path from
uranium to lead with much shorter half lives
• Measuring the ratio of U238 / Pb206 can determine the age
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Dating Evidence
Age of Ocean Basin in Millions of Years
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Deep Sea Vent
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Ocean water
circulates from
ocean into sea
floor and back
out at vent
Water becomes
heated to
~350°C (660°F)
Water picks up
chemicals
along path
through rock
Chemicals react
with each other
and some may
be precipitated
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Black & White
Smokers
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Mid-Atlantic
Ridge:
Iceland
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Earthquake Evidence
Pacific Ring of Fire
?
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Plate Tectonics
• Tectonics: from Latin “tectonicus” − pertaining to
building
 Powered by heat from radioactive decay
• Lithosphere: Crust plus rigid uppermost part of mantel
• Asthenosphere and lower mantel are plastic
• Lithosphere ‘floats’ on top of Asthenosphere
 Isostatic balance due to density differences
 Isostatic balance − gravitational equilibrium between the
lithosphere and asthenosphere
• Ocean Crust is more dense than Continental Crust
• Boundary Interactions
 Divergent plate boundaries
 Convergent plate boundaries (determined by density)
 Transverse plate boundaries (strike-slip fault)
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Convergent Plate Boundaries
• Oceanic crust slips below the Continental crust
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Subduction zone
Example: West Coast of South America
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Convergent Plate Boundaries
• Continental crusts
collide
 Example: Himalayas
• Oceanic crusts
collide
 Example: Japan
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Earth Convection Drives Plate
Tectonics
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Strike-Slip Fault
• Plates slide by each other; side by side
 Example: San Andreas Fault, California
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North Eastern Pacific
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Hot Spots
• Hot Spots: Volcanic regions underlying
mantle that are anomalously hot
• Tectonic Plates slide above the hot spots
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Example: Hawaii
Lo'ihi: will break the ocean surface
in 10,000 to 100,000 years
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Hot Spot
Emperor Seamounts: 70 − 30 million years
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Development of Atoll
Charles Darwin
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Submarine
Eruption
1.
2.
3.
4.
5.
6.
7.
8.
Water vapor cloud
Water
Stratum
Lava flow
Magma conduit
Magma chamber
Dike
Pillow lava
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Davidson
Seamount
• Located off the coast of
Central California, 80 mi (129
km) southwest of Monterey
• 26 mi (42 km) long and 8 mi
(13 km) wide
• One of the largest
seamounts
• 7,480 ft (2,280 m) tall
• Summit 4,101 ft (1,250 m)
below the sea surface
• Biologically diverse, with
237 species and 27 types of
deep-sea coral
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Transform Fault
Transform faults occur because sea
floor spreading cannot occur
uniformly on a spherical earth
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Major Tectonic Plates (~10)
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Motion of Tectonic Plates
• Maximum rates of spreading ~ 10 cm/yr (4 in/yr)
• Faster spreading rates at the East Pacific Rise compare to
the Mid-Atlantic Ridge
• Pressure from India Plate to increase heights of Himalayas
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Plate Tectonics Summary
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Summary
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Ocean crust is denser than the continental crust
Ocean crust is much younger (less than 280
million years old) than the continental crust (up
to 4 billion years old)
Thermal convection from the mantel drives
continental drift and sea floor spreading
− Plate Tectonics
New ocean crust formed in rifts (for example the
Mid-Atlantic Ridge) as the sea floor is pulled apart
− Sea floor spreading
Old ocean crust is forced down and under the
continental crust to balance the spreading
− Subduction
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