Understanding Earth PLATE TECTONICS: The Unifying Theory Grotzinger • Jordan Understanding Earth Seventh Edition Chapter 2: PLATE TECTONICS: The Unifying Theory © 2014 by W. H. Freeman and Company

Chapter 2 Plate Tectonics: The Unifying Theory

About Plate Tectonics It is the movement of plates and the forces acting on them. It explains the distribution of volcanoes, earthquakes, mountain chains, rock assemblages, and seafloor structures. The forces that drive plate motions arise from the mantle convection system.

Lecture Outline The discovery of plate tectonics 2. Plates and their boundaries 3. Rates and history of plate motion 4. The grand reconstruction 5. Mantle convection: the engine of plate tectonics

Lecture Outline 6. Theory of plate tectonics and the scientific method

1. Discovery of Plate Tectonics Continental drift: “jig-saw puzzle” fit of continents

1. Discovery of Plate Tectonics Continental drift: similarity of rock assemblages and ages across oceans

1. Discovery of Plate Tectonics Continental drift: distribution of certain fossils

1. Discovery of Plate Tectonics Seafloor spreading: geological activity in mid-ocean ridges

1. Discovery of Plate Tectonics Seafloor spreading: new crust formed there

Thought questions for this chapter What mistakes did Wegener make in formulating his theory of continental drift? Do you think the geologists of his era were justified in rejecting his theory?

2. The Plates and Their Boundaries ● mosaic of rigid plates

2. The Plates and Their Boundaries ● three types of boundaries

2. The Plates and Their Boundaries ● divergent, convergent, transform

2. The Plates and Their Boundaries ● next: a detailed look at the above

rifting, volcanoes, and earthquakes 1. Divergent Boundaries (a) Oceanic plate separation rifting, volcanoes, and earthquakes Mid- Atlantic Ridge North American Plate Eurasian Plate

rift valleys, volcanoes, and earthquakes 1. Divergent Boundaries (b) Continental plate separation rift valleys, volcanoes, and earthquakes East African Rift Valley Somali Subplate African Plate

oceanic trench, volcanic island arc, and deep earthquakes 2. Convergent Boundaries (a) Ocean-ocean convergence oceanic trench, volcanic island arc, and deep earthquakes Mariana Islands Marianas Trench Philippine Plate Pacific Plate

volcanic mountain chain, folded mountains, and deep earthquakes 2. Convergent Boundaries (b) Ocean-continent convergence volcanic mountain chain, folded mountains, and deep earthquakes Andes Mountains Peru-Chile Trench South American Plate Nazca Plate

crustal thickening, folded mountains, and earthquakes 2. Convergent Boundaries (c) Continent-continent convergence crustal thickening, folded mountains, and earthquakes Himalaya Mountains Tibetan Plateau subduction Indian-Australian Plate Eurasian Plate

lateral (transform) fault and earthquakes 3. Transform-Fault Boundaries (a) Continental transform fault lateral (transform) fault and earthquakes Pacific Plate North American Plate

lateral (transform) faults and earthquakes 3. Transform-Fault Boundaries (b) Mid-ocean ridge transform fault lateral (transform) faults and earthquakes Eurasian Plate North American Plate

Thought questions for this chapter Why are there active volcanoes along the Pacific coast in Washington and Oregon but not along the east coast of the United States? How do the differences between continental and oceanic crust affect the way lithospheric plates interact?

3. Rates and History of Plate Motion Ship towing a sensitive magnetometer Magnetic anomalies: seafloor areas of high and low magnetic values Mid-Atlantic Ridge high intensity low intensity

3. Rates and History of Plate Motion Iceland Mid-Atlantic Ridge Mid-Atlantic Ridge high intensity low intensity ● seafloor as a magnetic tape recorder

3. Rates and History of Plate Motion ● magnetic time scale developed

3. Rates and History of Plate Motion ● velocity of seafloor spreading = d / t

3. Rates and History of Plate Motion ● example area: mid-ocean ridge, south of Iceland

3. Rates and History of Plate Motion ● Velocity = 60 km / 3.3 mil. yr. = 18 km / mil. yr. (or 18 mm / yr)

3. Rates and History of Plate Motion Example relative plate velocities: East Pacific Rise (Pacific and Nazca plates) – 138 to 150 mm/yr South Atlantic (Mid-Atlantic Ridge) – 34 to 35 mm/yr Southern Ocean, south of Australia – 70 to 75 mm/yr Southern Ocean, south of Africa – 14 mm/yr

Thought questions for this chapter In Figure 2.15, the isochrons are symmetrically distributed in the Atlantic Ocean, but not in the Pacific Ocean. For example, seafloor as much as 180 million years old (in darkest blue) is found in the western Pacific, but not in the eastern Pacific. Why?

4. The Grand Reconstruction Reconstructing the history of plate motions: Assembly and breakup of the supercontinent Rodinia Assembly and breakup of the supercontinent Pangaea

4. The Grand Reconstruction Earth’s geography 1 billion years ago. Let’s see continental motion!

ASSEMBLY OF RODINIA Late Proterozoic (750 Ma) Formed about 1.1 billion years ago; began to break up about 750 million years ago

ASSEMBLY OF PANGAEA Late Proterozoic (650 Ma) The distribution of continents and oceans between Rodinia and the assembly of Pangaea

The distribution of continents and oceans about 458 million years ago ASSEMBLY OF PANGAEA Middle Ordovician (458 Ma) The distribution of continents and oceans about 458 million years ago

The distribution of continents and oceans about 390 million years ago ASSEMBLY OF PANGAEA Early Devonian (390 Ma) The distribution of continents and oceans about 390 million years ago

ASSEMBLY OF PANGAEA Early Triassic (237 Ma) The distribution of continents and oceans about 237 million years ago; Pangaea is formed

BREAKUP OF PANGAEA Early Jurassic (195 Ma) The breakup of the super-continent about 195 million years ago; Pangaea is being rifted

The distribution of continents and oceans about 152 million years ago BREAKUP OF PANGAEA Late Jurassic (152 Ma) The distribution of continents and oceans about 152 million years ago

BREAKUP OF PANGAEA Late Cretaceous-Early Tertiary (66 Ma) The distribution of continents and oceans about 66 million years ago; much like today in some ways

The distribution of continents and oceans as we know them today PRESENT DAY The distribution of continents and oceans as we know them today

5. Mantle Convection: The Engine of Plate Tectonics Upper mantle Theory 1: whole mantle convection 700 km Lower mantle Plate recycling extends to the core-mantle boundary. 2900 km Outer core

5. Mantle Convection: The Engine of Plate Tectonics Theory 2: stratified convection Boundary near 700 km separates the two different convection systems. The lower mantle convects more sluggishly than the upper mantle.

5. Mantle Convection: The Engine of Plate Tectonics spreading centers and hot spots

6. Theory of Plate Tectonics and the Scientific Method Plate tectonics is not a dogma, but a confirmed theory whose strength lies in its simplicity, its generality, and its consistency with many types of observations. This theory has survived so many attempts to prove it wrong and has been so important in explaining and predicting so many phenomena that geologists treat the theory as fact. Reasons why proof and acceptance took so long: very cautious approach of many scientists studying this issue; global scale of the problem; and specialized technology required to gain data took time to develop.

Thought questions for this chapter Would you characterize plate tectonics as a hypothesis, a theory, or a fact? Why? The theory of plate tectonics was not widely accepted until the banded patterns of magnetization on the ocean floor were discovered. In light of earlier observations – the jigsaw-puzzle fit of the continents, the occurrence of fossils of the same life-forms on both sides of the Atlantic, and the reconstruction of ancient climate conditions – why are these banded patterns of magnetism such key pieces of evidence?

Key terms and concepts Convergent boundary Divergent boundary Geodesy Continental drift Convergent boundary Divergent boundary Geodesy Island arc Isochron Magnetic anomaly Magnetic time scale Mantle plume Mid-ocean ridge Pangaea Plate tectonics Relative plate velocity Rodinia

Key terms and concepts Spreading center Subduction Transform fault Seafloor spreading Spreading center Subduction Transform fault