Transcript Document

Plate Tectonics
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Plate Tectonics
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Mechanism of Plate Tectonics
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Cross-section of Atlantic Ocean
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Directions and Rates of Plate Motion
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Geology in the Early 1900’s
By about 1900, advances in geology, biology and
geochemistry (primarily radioactivity)
 Earth is billions of years old.
 Earth is dynamic - The rocks and landforms that we see
today evolved over a very long history, including:
• mountain building
• erosion
• sedimentation
• metamorphism
• etc.
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540 Million
Geologic Time
Scale
4.5 Billion
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540 Million
(= 0.540 Billion)
Age of the Earth
Oldest
Rocks on
Earth
(3.9-4.0)
10 billion
1 billion
100 million
10,000,000
1,000,000
100,000
10,000
6,000 years
1,000
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20 million years
4.6
Billion
Years
Continental Drift ??
By about 1900, advances in geology, biology and geochemistry (primarily
radioactivity)
 Earth is billions of years old.
 Earth is dynamic - The rocks and landforms that we see today evolved over
a very long history, including:
• mountain building
• erosion
• sedimentation
• metamorphism
• etc.
However, until the second half of the 20th century, most models of the
evolution of the Earth involved:
Vertical Tectonics
Very few geologists believed that large scale horizontal motions could occur.
It was thought that the physical properties of Earth materials could not
permit such motions.
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Continental Drift ??
Until the second half of the 20th century,
most models of the evolution of the Earth
involved:
Vertical Tectonics
Very few geologists believed that large
scale horizontal motions could occur.
It was thought that the physical properties
of Earth materials could not permit such
motions.
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Rock
Cycle
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Mountain
Building
and
Erosion
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Alfred Wegener
Stages in the Development of Plate Tectonics
1912-1915: Continental drift proposed by Alfred Wegener
1915-1930: Continental drift debated
1930-1950: Stalemate
1950-1960: Revival of interest in continental drift
• Paleomagnetism
• Ocean Floor Exploration (Mid-ocean ridges/young age of oceanic crust)
1962: High heat flow over mid-ocean ridges/convection (H. Hess)
1963: Magnetic anomalies parallel to mid-ocean ridges (F. Vine and D.
Matthews)
1965: Transform faults and earthquake locations in ocean basins (J.T. Wilson)
1968: Earthquake locations and direction of earthquake motion consistent
with plate motion (J. Oliver, B. Isacks and L. Sykes)
1970-?: Plate tectonics accepted by most geoscientists
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Alfred Wegener and Continental Drift
Although several people had proposed continental drift as far back as
the 1600’s, such an hypothesis was not generally accepted.
In 1912, a German climatologist named Afred Wegener published a
book entitled Origin of Continents and Oceans in which he proposed
an hypothesis of continental drift and listed supporting evidence for
it.
Wegener’s evidence came from:
Reconstruction of ancient climates
Similar fossils on widely separated continents
Matching rock structures across ocean basins
Geometrical fit of continental margins
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Directions and Rates of Plate Motion
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Wegener proposed that an original
super-continent that he called Pangaea
(“all land”) existed before continental
drift began about 180 million years ago.
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According to Wegener’s hypothesis,
South America and Africa began to drift
apart about 70 million years ago.
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Breakup of Pangaea
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Geophysicists countered Wegener’s
argument: They argued that physical
properties of Earth materials would not
permit that much horizontal motion.
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Stages in the Development of Plate Tectonics
1912-1915: Continental drift proposed by Alfred Wegener
1915-1930: Continental drift debated
1930-1950: Stalemate
1950-1960: Revival of interest in continental drift
• Paleomagnetism
• Ocean Floor Exploration (Mid-ocean ridges/young age of oceanic crust)
1962: High heat flow over mid-ocean ridges/convection (H. Hess)
1963: Magnetic anomalies parallel to mid-ocean ridges (F. Vine and D.
Matthews)
1965: Transform faults and earthquake locations in ocean basins (J.T. Wilson)
1968: Earthquake locations and direction of earthquake motion consistent
with plate motion (J. Oliver, B. Isacks and L. Sykes)
1970-?: Plate tectonics accepted by most geoscientists
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Wegener’s Evidence for
Continental Drift
Reconstruction of ancient climates
Similar fossils on widely separated continents
Matching rock structures across ocean basins
Geometrical fit of continental margins
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Glacial Ice
18,000 years ago
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Today
Pangaea - Glacial Ice
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Reconstruction of Ancient Climates
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Climate Zones
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Similar Fossils
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Fossil Distribution
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Explanation of Fossil Evidence
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Similar Rock Structures
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Fig. 3.06a
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W. W. Norton. Modified from Hurley.
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W. W. Norton
Reconstruction of Pangaea
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Stages in the Development of Plate Tectonics
1912-1915: Continental drift proposed by Alfred Wegener
1915-1930: Continental drift debated
1930-1950: Stalemate
1950-1960: Revival of interest in continental drift
• Paleomagnetism
• Ocean Floor Exploration (Mid-ocean ridges/young age of oceanic crust)
1962: High heat flow over mid-ocean ridges/convection (H. Hess)
1963: Magnetic anomalies parallel to mid-ocean ridges (F. Vine and D.
Matthews)
1965: Transform faults and earthquake locations in ocean basins (J.T. Wilson)
1968: Earthquake locations and direction of earthquake motion consistent
with plate motion (J. Oliver, B. Isacks and L. Sykes)
1970-?: Plate tectonics accepted by most geoscientists
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Fossil Distribution
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Wegener’s Mechanism for
Continental Drift
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The Earth’s Crust
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By the early
1900’s,
seismologists had
already discerned
the general
structure and
physical properties
of the Earth’s
interior.
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P and S Wave Paths
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M=7.6, ∆=97°
Magnitude 7.6 PAKISTAN, October 08, 2005 at 03:51 UTC
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Boston, Massachusetts - First Wave Arrives 4:04 (13 minutes)
Travel-time versus Distance
D
Distanced measured in degrees from epicenter.
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M=6.7, ∆=25°
Magnitude 6.8 CAYMAN ISLANDS, December 14, 2004 at 23:20 UTC
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Boston, Massachusetts - First Wave Arrives 23:26 (6 minutes)
M=7.8, ∆=62°
Magnitude 7.8 TARAPACA, CHILE, June 13, 2005 22:45 UTC
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Boston, Massachusetts - First Wave Arrives 22:55 (10 minutes)
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1
Path
2
6371 km
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3
1
2
3
Distance (km)
2,758
6,563
9,543
Time (sec)
360
600
780
Velocity
7.7 km/sec
10.9 km/sec
12.2 km/sec
P-wave
Velocities
Mantle
8-13 km/sec
Outer Core
8-10 km/sec
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Crust
6-7 km/sec
Inner Core
10-11 km/sec
P-wave Velocities
Air: 0.3
Water: 1.4
Concrete: 3.6
Granite: 5.5-6.0
Iron: 5.8
Aluminum: 6.6
Mantle
8-13 km/sec
Outer Core
8-10 km/sec
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Crust
6-7 km/sec
Inner Core
10-11 km/sec
Composition
of Earth’s
Interior
Crust:
Granite/Basalt
Mantle:
Peridotite
Outer Core:
Liquid Iron
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Inner Core:
Solid Iron
1909 - Andrija Mohorovicic analysed records of an
earthquake in Croatia - discovered the existence of the
“mantle” underneath the continental “crust”.
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Basalt
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Peridotite
Granite
Wegener’s Mechanism for
Continental Drift
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Mechanism of Plate Tectonics
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Stages in the Development of Plate Tectonics
1912-1915: Continental drift proposed by Alfred Wegener
1915-1930: Continental drift debated
1930-1950: Stalemate
1950-1960: Revival of interest in continental drift
• Paleomagnetism
• Ocean Floor Exploration (Mid-ocean ridges/young age of oceanic crust)
1962: High heat flow over mid-ocean ridges/convection (H. Hess)
1963: Magnetic anomalies parallel to mid-ocean ridges (F. Vine and D.
Matthews)
1965: Transform faults and earthquake locations in ocean basins (J.T. Wilson)
1968: Earthquake locations and direction of earthquake motion consistent
with plate motion (J. Oliver, B. Isacks and L. Sykes)
1970-?: Plate tectonics accepted by most geoscientists
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