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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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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.

Geology in the Early 1900’s

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Geologic Time Scale

4.5 Billion

540 Million (= 0.540 Billion)

540 Million

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Age of the EarthOldest Rocks on Earth(3.9-4.0)

4.6BillionYears

20 million years

6,000 years1,000

10 billion

10,000

100,000

1,000,000

10,000,000

100 million

1 billion

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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.

Continental Drift ??

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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.

Until the second half of the 20th century, most models of the evolution of the Earth involved:

Vertical Tectonics

Continental Drift ??

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Rock Cycle

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Mountain Building

and Erosion

01:36 AM Alfred Wegener

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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

Stages in the Development of Plate Tectonics

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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

Alfred Wegener and Continental Drift

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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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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

Stages in the Development of Plate Tectonics

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Reconstruction of ancient climates

Similar fossils on widely separated

continents

Matching rock structures across ocean

basins

Geometrical fit of continental margins

Wegener’s Evidence for Continental Drift

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Glacial Ice

Today

18,000 years ago

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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

W. W. Norton. Modified from Hurley.

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W. W. Norton

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Reconstruction of Pangaea

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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

Stages in the Development of Plate Tectonics

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Fossil Distribution

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Wegener’s Mechanism forContinental 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°

Boston, Massachusetts - First Wave Arrives 4:04 (13 minutes)

Magnitude 7.6 PAKISTAN, October 08, 2005 at 03:51 UTC

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Distanced measured in degrees from epicenter.

Travel-time versus Distance

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M=6.7, ∆=25°

Magnitude 6.8 CAYMAN ISLANDS, December 14, 2004 at 23:20 UTC

Boston, Massachusetts - First Wave Arrives 23:26 (6 minutes)

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M=7.8, ∆=62°

Magnitude 7.8 TARAPACA, CHILE, June 13, 2005 22:45 UTC

Boston, Massachusetts - First Wave Arrives 22:55 (10 minutes)

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6371 km

1

2

3

Path Distance (km) Time (sec) Velocity

1 2,758 360 7.7 km/sec 2 6,563 600 10.9 km/sec 3 9,543 780 12.2 km/sec

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Crust6-7 km/sec

Mantle8-13 km/sec

Outer Core8-10 km/sec

Inner Core10-11 km/sec

P-wave Velocitie

s

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Crust6-7 km/sec

Mantle8-13 km/sec

Outer Core8-10 km/sec

Inner Core10-11 km/sec

P-wave Velocities

Air: 0.3Water: 1.4Concrete: 3.6Granite: 5.5-6.0Iron: 5.8Aluminum: 6.6

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Crust:Granite/Basalt

Mantle:Peridotite

Outer Core:Liquid Iron

Inner Core:Solid Iron

Composition of Earth’s Interior

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1909 - Andrija Mohorovicic analysed records of an earthquake in Croatia - discovered the existence of the “mantle” underneath the continental “crust”.

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GraniteBasaltPeridotite

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Wegener’s Mechanism forContinental Drift

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Mechanism of Plate Tectonics

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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

Stages in the Development of Plate Tectonics

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