PLATE TECTONICS: A SCIENTIFIC REVOLUTION UNFOLDS

CHAPTER 7

FOCUS ON CONCEPTS What evidence was used to support the

continental drift hypothesis? What was one of the main objections to

the continental drift hypothesis? What is the theory of plate tectonics? In what major way does the plate

tectonics theory depart from the continental drift hypothesis?

FOCUS ON CONCEPTS What are the three types of plate

boundaries? Where does new lithosphere form? How do mountain systems such as the

Himalayas form? What type of plate motion occurs along

a transform fault boundary?

FOCUS ON CONCEPTS What evidence is used to support the

plate tectonics theory? What are the major driving forces for

plate tectonics? What models have been proposed to

explain the driving mechanism for plate motion?

7.1FROM CONTINENTAL DRIFT TO

PLATE TECTONICS

THE COLLISION OF INDIA AND ASIA PRODUCED THE HIMALAYAS

K7 IN PAKISTAN’S KARAKORAM (HIMALAYAS)

7.2CONTINENTAL DRIFT: AN IDEA

BEFORE ITS TIME

ALFRED WEGENER• Pangaea• Continental Drift Hypothesis

EDUARD SUESSGondwana

CONTINENTAL DRIFT HYPOTHESIS Began in early Mesozoic (~200 mya) Evidence

Continental jigsaw puzzle Fossil distribution Rock types & geologic features Ancient climates

CONTINENTAL JIGSAW PUZZLE

CONTINENTAL JIGSAW PUZZLE

EVIDENCE

FOSSIL DISTRIBUTION

FOSSIL DISTRIBUTION

BEARDMORE GLACIER

Location of the fossil site at the Oliver Bluffs on the Beardmore Glacier

ROCK TYPES & GEOLOGIC FEATURES There are rock formations (such as

mountain ranges) on different continents that match up beautifully when the continents are put back together.

ROCK TYPES & GEOLOGIC FEATURES

ANCIENT CLIMATES Paleoclimatic data Extreme global cooling? Wegener’s explanation

GLACIAL STRIATIONS

POST-DRIFT ICE SHEETS

PRE-DRIFT ICE SHEET

7.3THE GREAT DEBATE

OBJECTIONS TO THE CONTINENTAL DRIFT HYPOTHESIS

What mechanism allows movement? Not all evidence supported the

hypothesis.

Some thought the idea was intriguing or an answer to a previously unexplained phenomena.

OBJECTIONS TO THE CONTINENTAL DRIFT HYPOTHESIS

7.4PLATE TECTONICS

PLATE TECTONICS THEORY 1968; more encompassing than CDH Lithosphere broken into plates, slide

over asthenosphere

EARTH’S MAJOR TECTONIC PLATES

EARTH’S MAJOR TECTONIC PLATES LARGE – 94% of Earth’s S.A.

North American South American Pacific African Eurasian Australian-Indian Antarctic

EARTH’S MAJOR TECTONIC PLATES INTERMEDIATE – mostly oceanic

Caribbean Nazca Phillipine Arabian Cocos Scotia Juan de Fuca

PLATE BOUNDARIES Divergent (constructive) Convergent (destructive) Transform (conservative)

PLATE BOUNDARIES

PLATE BOUNDARIES

7.5DIVERGENT BOUNDARIES

PLATE TECTONICS: THE NEW PARADIGM

Plate boundaries Types of plate boundaries

Transform fault boundaries Plates slide past one another

No new crust is created or destroyed Transform faults

Most join two segments of a mid-ocean ridge Aid the movement of oceanic crustal material

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 HAVE FORMED OVER A HOT SPOT

TESTING THE PLATE TECTONICS MODEL

Evidence for the plate tectonics model Paleomagnetism

Probably the most persuasive evidence Ancient magnetism preserved in rocks Paleomagnetic records show

Polar wandering (evidence that continents moved) Earth’s magnetic field reversals

Recorded in rocks as they form at oceanic ridges

POLAR WANDERING PATHS FOR EURASIA AND NORTH AMERICA

PALEOMAGNETIC REVERSALS RECORDED BY BASALT FLOWS

MEASURING PLATE MOTION Measuring plate motion

By using hot spot “tracks” like those of the Hawaiian Island – Emperor Seamount chain

Using space-age technology to directly measure the relative motion of plates

Very Long Baseline Interferometry (VLBI) Global Positioning System (GPS)

DIRECTIONS AND RATES OF PLATE MOTIONS

WHAT DRIVES PLATE MOTION Driving mechanism of plate tectonics

No one model explains all facets of plate tectonics

Earth’s heat is the driving force Several models have been proposed

Slab-pull and ridge-push model Descending oceanic crust pulls the plate Elevated ridge system pushes the plate

SEVERAL MECHANISMS CONTRIBUTE TO PLATE MOTION

WHAT DRIVES PLATE MOTION Several models have been proposed

Plate-mantle convection Mantle plumes extend from mantle-core

boundary and cause convection within the mantle

Models Layering at 660 kilometers Whole-mantle convection

LAYERING AT 660 KM

WHOLE-MANTLE CONVECTION

PLATE TECTONICS INTO THE FUTURE Present-day motions have been

extrapolated into the future some 50 million years Areas west of the San Andreas Fault slide

northward past the North American plate Africa collides with Eurasia, closing the

Mediterranean and initiating mountain building

Australia and New Guinea are on a collision course with Asia

A POSSIBLE VIEW OF THE WORLD 50 MILLION YEARS FROM NOW

End of Chapter 7