Answer four Earth Science Essays with a minimum of 200 words in APA Format

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Foundations of Earth

Science, 6e

Lutgens, Tarbuck, & Tasa

Plate Tectonics: A

Scientific Theory

Unfolds Foundations, 6e - Chapter 5

Stan Hatfield

Southwestern Illinois College

Continental Drift: An idea

before its time

• Alfred Wegener

• First proposed his continental drift hypothesis in 1915

• Published The Origin of Continents and Oceans

• Continental drift hypothesis

• Supercontinent called Pangaea began breaking apart about 200 million years ago

Pangaea approximately 200

million years ago

Continental Drift: An idea

before its time

• Continental drift hypothesis

• Continents “drifted” to present positions

• Evidence used in support of continental drift hypothesis

• Fit of the continents

• Fossil evidence

• Rock type and structural similarities

• Paleoclimatic evidence

Fossil evidence supporting

continental drift

Matching mountain ranges

The great debate

• Objections to the continental drift hypothesis

• Lack of a mechanism for moving continents

• Wegener incorrectly suggested that continents broke through the ocean crust, much like ice breakers cut through ice

• Strong opposition to the hypothesis from the scientific community

The great debate

• Continental drift and the scientific

method

• Wegener’s hypothesis was correct in

principle, but contained incorrect

details

• A few scientists considered Wegener’s

ideas plausible and continued the

Plate Tectonics: A modern

version of an old idea

• Earth’s major plates

• Associated with Earth’s strong, rigid

outer layer

• Known as the lithosphere

• Consists of uppermost mantle and

overlying crust

• Overlies a weaker region in the mantle

called the asthenosphere

Plate Tectonics: A modern

version of an old idea

• Earth’s major plates

• Seven major lithospheric plates

• Plates are in motion and continually

changing in shape and size

• Largest plate is the Pacific plate

• Several plates include an entire

continent plus a large area of seafloor

Earth’s tectonic plates

Plate Tectonics: A modern

version of an old idea

• Earth’s major plates

• Plates move relative to each other at a

very slow but continuous rate

• About 5 centimeters (2 inches) per year

• Cooler, denser slabs of oceanic lithosphere

descend into the mantle

Plate Tectonics: A modern

version of an old idea

• Plate boundaries

• Interactions among individual plates occur along their boundaries

• Types of plate boundaries • Divergent plate boundaries

(constructive margins)

• Convergent plate boundaries (destructive margins)

• Transform fault boundaries (conservative margins)

Divergent plate boundaries

• Most are located along the crests of

oceanic ridges

• Oceanic ridges and seafloor spreading

• Along well-developed divergent plate

boundaries, the seafloor is elevated

forming oceanic ridges

Divergent plate boundaries

• Oceanic ridges and seafloor spreading

• Seafloor spreading occurs along the oceanic ridge system

• Spreading rates and ridge topography

• Ridge systems exhibit topographic differences

• These differences are controlled by spreading rates

Divergent plate boundary

Divergent plate boundaries

• Continental rifting

• Splits landmasses into two or more

smaller segments along a

continental rift

• Examples include the East African rift

valleys and the Rhine Valley in

northern Europe

• Produced by extensional forces

acting on lithospheric plates

Continental

rifting

Convergent plate boundaries

• Older portions of oceanic plates are

returned to the mantle in these

destructive plate margins

• Surface expression of the descending

plate is an ocean trench

• Also called subduction zones

• Average angle of subduction =

45 degrees

World’s oceanic trenches

and ridge system

Convergent plate boundaries

• Types of convergent boundaries

• Oceanic-continental convergence

• Denser oceanic slab sinks into the

asthenosphere

• Along the descending plate partial melting of

mantle rock generates magma

• Resulting volcanic mountain chain is called

a continental volcanic arc (Andes and

Cascades)

Oceanic-continental convergence

Convergent plate boundaries

• Types of convergent boundaries

• Oceanic-oceanic convergence

• When two oceanic slabs converge, one

descends beneath the other

• Often forms volcanoes on the ocean floor

• If the volcanoes emerge as islands, a

volcanic island arc is formed (Japan,

Aleutian islands, and Tonga islands)

Oceanic-oceanic

convergence

Convergent plate boundaries

• Types of convergent boundaries

• Continental-continental convergence

• Less dense, buoyant continental

lithosphere does not subduct

• Resulting collision between two continental

blocks produces mountains (Himalayas,

Alps, and Appalachians)

Continental-continental

convergence

Transform fault boundaries

• Plates slide past one another and no

new lithosphere is created or destroyed

• Transform faults

• Most join two segments of a mid-ocean

ridge along breaks in the oceanic crust

known as fracture zones

Transform fault boundaries

• Transform faults

• A few (the San Andreas Fault and the

Alpine Fault of New Zealand) cut

through continental crust

Transform faults

San Andreas Fault near

Taft, California

Testing the plate

tectonics model

• Evidence from ocean drilling

• Some of the most convincing

evidence confirming seafloor

spreading has come from drilling

directly into ocean-floor sediment

• Age of deepest sediments

• Thickness of ocean-floor sediments

verifies seafloor spreading

Testing the plate

tectonics model

• Hot spots and mantle plumes

• Caused by rising plumes of mantle

material

• Volcanoes can form over them

(Hawaiian Island chain)

• Mantle plumes

• Long-lived structures

• Some originate at great depth, perhaps at

the mantle-core boundary

The Hawaiian Islands

Testing the plate

tectonics model

• Paleomagnetism

• Iron-rich minerals become magnetized

in the existing magnetic field as they

crystallize

• Rocks that formed millions of years

ago contain a “record” of the direction

of the magnetic poles at the time of

their formation

Testing the plate

tectonics model

• Apparent polar wandering

• Lava flows of different ages indicated

several different magnetic poles

• Polar wandering paths are more readily

explained by the theory of plate

tectonics

Polar Wandering paths for

Eurasia and North America

Testing the plate

tectonics model

• Geomagnetic reversals

• Earth’s magnetic field periodically

reverses polarity—the north magnetic

pole becomes the south magnetic pole,

and vice versa

• Dates when the polarity of Earth’s

magnetism changed were determined

from lava flows

Testing the plate

tectonics model

• Geomagnetic reversals

• Geomagnetic reversals are recorded in

the ocean crust

• In 1963 Vine and Matthews tied the

discovery of magnetic stripes in the

ocean crust near ridges to Hess’s

concept of seafloor spreading

Paleomagnetic reversals

recorded in oceanic crust

What drives plate motions?

• Researchers agree that convective

flow in the mantle is the basic driving

force of plate tectonics

• Forces that drive plate motion

• Slab-pull

• Ridge-push

Some of the forces

that act on plates

What drives plate motions?

• Models of plate-mantle convection

• Any model must be consistent with

observed physical and chemical

properties of the mantle

• Models

• Layering at 660 kilometers

• Whole-mantle convection

End of Chapter 5