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Chapter 2

Plate Tectonics:

A Unifying Theory

• A unifying theory is one that helps – explain a broad range of diverse observations – interpret many aspects of a science – on a grand scale– and relate many seemingly unrelated

phenomena• Plate tectonics is a unifying theory for

geology.

Unifying TheoryUnifying Theory

2.Plate Tectonics2.Plate Tectonics

Unifying theory Unifying theory –– plate tectonics describe plate tectonics describe movement of continental and oceanic movement of continental and oceanic

plates and forces driving them. It plates and forces driving them. It explains locations of mountain chains, explains locations of mountain chains, earthquakes, rock assemblages, and earthquakes, rock assemblages, and

structures on sea floor. structures on sea floor.

For geology Plate Tectonics is similar in For geology Plate Tectonics is similar in importance as the Discovery of DNA is to importance as the Discovery of DNA is to

biologybiology

• Plate tectonics helps explain – earthquakes– volcanic eruptions– formation of

mountains– location of

continents – location of ocean

basins

Plate TectonicsPlate Tectonics

• It influences– atmospheric and oceanic

circulation, – and climate– geographic distribution, – evolution and extinction

of organisms– distribution and formation

of resources

Introduction

Why should you know about plate tectonics?

Plate tectonics affects all of us, whether in relation to the destruction caused by volcanic eruptions and earthquakes, or politically and economically due to the formation and distribution of valuable natural resources. Plate tectonics tyies together many seemingly unrelated geologic phenomena and illustrating why Earth is a dynamic planet of interacting subsystems and cycles.

2.Plate Tectonics2.Plate Tectonics

Earlier related theories:Earlier related theories:Over the past 200 years (up to late 1960s) Over the past 200 years (up to late 1960s)

geologists had developed many theories geologists had developed many theories about the characteristics & location of about the characteristics & location of

EarthEarth’’s tectonic features.s tectonic features.Mountains, volcanoes, earthquakes, etc: Mountains, volcanoes, earthquakes, etc:

Understood Understood •• generally how are they formed/occur,generally how are they formed/occur,

•• but not but not -- why here, not there, relation to why here, not there, relation to one and otherone and other

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Early Ideas About Continental Drift

Alfred Wegener and the Continental Drift Hypothesis.

The idea that continents have moved in the past is not new.The concept of continental movement was first suggested when it was noticed that Africa and South America had coastlines which appeared to be counterparts of one anotherThis suggested they may once have been joined and drifted apart.

Fig. 2.3, p. 32

• Edward Suess• Australian, late 1800s

– noted similarities between – the Late Paleozoic plant fossils

• Glossopteris flora

Early Ideas Early Ideas about Continental Driftabout Continental Drift

– and evidence for glaciation

– in rock sequences of • India• Australia• South Africa • South America

• He proposed the name Gondwanaland– (or Gondwana) for a

supercontinent – composed of these

continents

• Frank Taylor (American, 1910) – presented a hypothesis of continental drift

with these features:• lateral movement of continents formed mountain

ranges• a continent broke apart at the Mid-Atlantic Ridge

to form the Atlantic Ocean• supposedly, tidal forces pulled formerly polar

continents toward the equator, • when Earth captured the Moon about 100 million

years ago

Early Ideas Early Ideas about Continental Driftabout Continental Drift

• German meteorologist

• Credited with hypothesis “1912 continental drift.”

• all landmasses were originally united into a supercontinent named Pangaea.

Alfred WegenerAlfred Wegener and the and the Continental Drift HypothesisContinental Drift Hypothesis

Early Ideas About Continental Drift

Alfred Wegenerand the Continental Drift Hypothesis.

Pangaea consisted of a northern landmass called Laurasia and a southern landmass called Gondwana.

As Pangaea broke up, the various continents moved to their present-day locations.

Fig. 2.1, p. 31

The Glossopteris fern, also known as the “Pangaea plant”

What is the Evidence for Continental Drift?

A. Continental Fit

Wegener and others amassed a large amount of evidence in support of continental drift.

There is a close fit between the continents off the coast at a depth of about 2000 m.

Fig. 2.3, p. 32

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Evidence for Continental Drift Evidence for Continental Drift

A. Jigsaw puzzle fit of continentsA. Jigsaw puzzle fit of continents

Evidence for Continental DriftEvidence for Continental Drift

•• Continental fit Continental fit best at continental best at continental

shelfshelf

Evidence for Continental Drift

B. Similarity of Rock Sequences and Mountain Ranges

Marine, nonmarine, and glacial rock sequences of Pennsylvanian to Jurassic age are nearly identical on all the Gondwana continents

Fig. 2.4, p. 32


Continental Continental driftdrift

B. similarity of rock B. similarity of rock assemblages & assemblages &

C. ages across C. ages across oceansoceans


D. Similarity of Rock Sequences and Mountain Ranges

The trend of several major mountain ranges produces a continuous mountain range when the continents are positioned next to each other as they were during the formation of Pangea.

Fig. 3-3, p. 35D. Correlation of Mountain ranges


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E. Glacial Evidence

Glacial tills and striations on the bedrock beneath the till provide evidence of glaciation at the same time on all the Gondwana continents, with South Africa located at the South Pole.

Fig. 2.5, p. 33 Fig. 3-3, p. 35E. Correlation of Glaciation



F. Fossil evidence F. Fossil evidence (distribution of (distribution of certain fossils) certain fossils)

– One of the strongest examples is the Mesosaurus, a fresh water reptile.


F. Fossil Evidence

• Also compelling evidence comes from fossils like the Glossopteris fern.

Evidence for Continental DriftEvidence for Continental Drift** support of theory by otherssupport of theory by others

• Alexander du Toit (South African geologist, 1937) – Proposed that a northern landmass called

Laurasia consisted of present-day • North America• Greenland• Europe • and Asia (except India).

– Provided additional fossil evidence for Continental drift

Evidence for Continental Drift* Problems with the theory

Wegener could not provide a convincing mechanism to demonstrate ‘how’ the continents could have moved.His ideas were largely ignored.

Fig. 2.2, p. 31

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Evidence for Continental DriftEvidence for Continental Drift* Problems with the theory* Problems with the theory

• Most geologists did not accept the idea of moving continents– because no one could provide – a suitable mechanism to explain – how continents could move over Earth’s

surface• Interest in continental drift revived when

– new evidence of Earth’s magnetic field – and oceanographic research – showed that the ocean basins were

geologically young

Paleomagnetism Paleomagnetism * Earth* Earth’’s Magnetic Fields Magnetic Field

• Similar to a giant dipole magnet– magnetic

poles essentially coincide

– with the geographic poles

– and may result from different rotation

– of outer core and mantle

((PaleoPaleo) Magnetism) MagnetismEarthEarth’’s Magnetic Fields Magnetic Field • Strength and orientation of the magnetic

field varies– weak and horizontal at the equator– strong and vertical at the poles

PaleomagnetismPaleomagnetism

– inclination and strength

– increase from the equator

– to the poles

• Paleomagnetism is – a remanent magnetism – in ancient rocks – recording the direction of Earth’s magnetic

poles – at the time of the rock’s formation

• Paleomagnetism in rocks – records the direction – and strength of Earth’s magnetic field

• When magma cools – below the Curie point temperature– magnetic iron-bearing minerals align – with Earth’s magnetic field

PaleomagnetismPaleomagnetism Paleomagnetism & Polar WanderingPaleomagnetism & Polar Wandering

• Magnetic poles apparently moved.– Their trails were called

polar wandering paths.– Different continents

had different paths.

• In 1950s, research revealed – that paleomagnetism – of ancient rocks

showed – orientations different – from the present

magnetic field

• The best explanation – is stationary poles and – moving continents

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• Earth’s present magnetic field is called normal, – with magnetic north near the north geographic pole – and magnetic south near the south geographic pole

• At various times in the past, – Earth’s magnetic field has completely reversed, – with magnetic south near the north geographic pole – and magnetic north near the south geographic pole

• The condition for which Earth’s magnetic field – is in this orientation is called a magnetic reversal

Paleomagnetism & Magnetic ReversalsPaleomagnetism & Magnetic Reversals

• Measuring paleomagnetism and dating continental lava flows lead to – the realization that magnetic

reversals existed– the establishment of a

magnetic reversal time scale

Paleomagnetism & Magnetic ReversalsPaleomagnetism & Magnetic Reversals

Paleomagnetism

Paleomagnetic studies during the 1950's revived interest in continental drift

They indicated that either the magnetic poles had wandered and each continent had its own pole (an impossibility), or The continents had moved over time. If the continents were moved into different positions relative to each other, the separate poles could be resolved into one.

Fig. 2.9, p. 36

Paleomagnetism

Paleomagnetism is the remnant magnetism in ancient rocks recording the direction and intensity of Earth’s magnetic field at the time of the rock’s formation.

The Curie point is the temperature at which iron-bearing minerals gain their magnetism. It is important because as long as the rock is not subsequently heated above the Curie point, it will preserve that remnant magnetism.

Fig. 2.8, p. 35

Paleomagnetism & Polar Wandering

How can the apparent wandering of the magnetic poles be best explained?

The magnetic poles have remained near their present locations at the geologic north and south pole and the continents have moved. Evidence: When the continents are fitted together, the paleomagnetic data point to only one magnetic pole.

Fig. 2.9, p. 36

Paleomagnetism and Seafloor Spreading

Earth’s present magnetic field is considered normal

Normal - with the north and south magnetic poles located approximately at the north and south geographic poles.At various times in the geologic past, Earth’s magnetic field has completely reversed.

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Paleomagnetism and Seafloor Spreading

Earth’s present magnetic field is considered normal

The existence of such magnetic reversals was discovered in continental lava flows by

A) age dating B) determining the orientation of the remnant magnetism.

Fig. 2.10, p. 37 Fig. 2-10, p. 37Stepped Art

North magneticpole (reversed)

North magneticpole (reversed)

South magneticpole (normalposition)

South magneticpole (normal)

South magneticpole (reversed)

South magneticpole (reversed)

North magneticpole (normal)

North magneticpole (normalposition)

Magnetic Reversals and Seafloor Spreading

Harry Hess proposed the theory of seafloor spreading in 1962.

He suggested that the seafloor separates at oceanic ridges, where new crust is formed by upwelling magma. As the magma cools, the newly formed oceanic crust moves laterally away from the ridge.

Fig. 2.11, p. 38

Continental Drift, Seafloor Spreading, Continental Drift, Seafloor Spreading, and Plate Tectonicsand Plate TectonicsEarly explanation of motion/drift Early explanation of motion/drift

Wegener (1915) suggested continents Wegener (1915) suggested continents drifted (floated) on oceanic crust due to drifted (floated) on oceanic crust due to sun/moon tidal forces (too weak) and he sun/moon tidal forces (too weak) and he was meteorologist so mostly ignoredwas meteorologist so mostly ignored

Sea floor spreadingSea floor spreading (1928, Arthur Homes, (1928, Arthur Homes, British) first to speculate that convection British) first to speculate that convection currents within mantle moved continents currents within mantle moved continents apart (little evidence or details) apart (little evidence or details)

Seafloor Spreading, Continental Seafloor Spreading, Continental Drift and Plate TectonicsDrift and Plate Tectonics

PostPost--WWII WWII (1950s)(1950s)

• new techniques help map seafloor

•• geological activity geological activity in midin mid--ocean ocean ridges ridges

•• ring of firering of fire•• seafloor mapsseafloor maps

• Ocean mapping revealed– a ridge system– 65,000 km long,– the most extensive mountain range in the

world• The Mid-Atlantic Ridge

– is the best known– and divides Atlantic Ocean basin – in two nearly equal parts

Seafloor Spreading &Seafloor Spreading &Mapping Ocean BasinsMapping Ocean Basins

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Atlantic Ocean BasinAtlantic Ocean Basin

Mid-Atlantic Ridge

Seafloor Spreading Seafloor Spreading

Seafloor SpreadingSeafloor Spreading

Theories on Seafloor spreadingTheories on Seafloor spreadingNo accepted theory until 60No accepted theory until 60’’s. s. Harry Hess, submariner & professor, Princeton, Harry Hess, submariner & professor, Princeton, showed (postshowed (post--WWII) sonar & other geophysical WWII) sonar & other geophysical maps of sea floor (early maps of sea floor (early ‘‘60s). 60s).

Plate Tectonics TheoriesPlate Tectonics Theories (1960(1960--68)68)Mid-Atlantic sea floor ridge formed by upwelling crust due plate divergent motion (1962, H. Hess, Princeton Univ. & Bob Dietz, Scripps)

Seafloor Spreading or Seafloor Spreading or Plate Tectonics?Plate Tectonics?


Seafloor Seafloor spreadingspreading

new crust new crust formed due formed due expanding expanding earth?earth?

Seafloor Spreading or Seafloor Spreading or Plate TectonicsPlate Tectonics

Plate Tectonics Theories (1960Plate Tectonics Theories (1960--68)68)1. Mid-Atlantic sea floor ridge formed by

upwelling crust due plate divergent motion (1962, H. Hess, Princeton Univ. & Bob Dietz, Scripps) – possible expanding earth

2. Ring of Fire - mapping & correlation of volcanoes & (more important) the deep sea trenches along Pacific margins (1962-64)

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Plate Tectonics Evidence Plate Tectonics Evidence Ring of Fire Ring of Fire

Plate Tectonics Evidence: ModelPlate Tectonics Evidence: Model

Convection?Convection?

Plate Tectonics Evidence: ModelPlate Tectonics Evidence: Model Plate Tectonics: research drilling Plate Tectonics: research drilling beneath sea: layers of earth beneath sea: layers of earth

Density, Crust, Lithosphere (Crust & upper mantle, to 100km), Asthenosphere (mantle below lithosphere)


Deep-Sea Drilling and the Confirmation of Seafloor Spreading

Seafloor spreading was confirmed by the discovery of magnetic anomalies in the ocean crust that were both parallel to and symmetric around the ocean ridges. This indicates that new oceanic crust must be formed along the spreading ridges.

Fig. 2.12, p. 39

• Harry Hess, in 1962, proposed the hypothesis of seafloor spreading– Continents and oceanic crust move together– Seafloor separates at oceanic ridges

• where new crust forms from upwelling & cooling magma

• the new crust moves laterally away from the ridge– the mechanism to drive seafloor spreading was

thermal convection cells in the mantle• hot magma rises from mantle to form new crust• cold crust subducts into the mantle at oceanic

trenches, where it is heated and recycled


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• In addition to mid-ocean ridges, – ocean research revealed – magnetic anomalies on the sea floor

• A magnetic anomaly is a deviation – from the average strength – of Earth’s Magnetic field

Confirmation of HessConfirmation of Hess’’s s HypothesisHypothesis

• The magnetic anomalies were discovered to be

Confirmation of HessConfirmation of Hess’’s s HypothesisHypothesis

parallel to the oceanic ridgesstriped, and symmetrical with the ridges

How Do Magnetic Reversals How Do Magnetic Reversals Relate to Seafloor Spreading?Relate to Seafloor Spreading?



Sea floor spreading is confirmed by

the ages of fossils in sediments overlying oceanic crustradiometric dating of rocks on oceanic islands.

These indicate that oceanic crust is youngest at the spreading ridges and oldest at the farthest points from the ridges.

Fig. 2.13, p. 40

• Seafloor spreading theory indicates that – oceanic crust is geologically young because – it forms during spreading – and is destroyed during subduction

• Radiometric dating confirms the youth – of the oceanic crust– and reveals that the youngest oceanic crust – occurs at mid-ocean ridges – and the oldest oceanic crust – is less than 180 million years old

• whereas oldest continental crust – is 3.96 billion yeas old

Oceanic Crust Is YoungOceanic Crust Is Young Magnetic Reversals and Seafloor Spreading


Further evidence confirming seafloor spreading came from the Deep Sea Drilling Project are the age and thickness of the sediments overlying the oceanic crust.

Fig. 2.14, p. 41

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Plate Tectonics: A Unifying Theory

Overwhelming evidence in support of plate tectonics led to its rapid acceptance and elaboration since the early 1970's.

The theory is widely accepted because it explains so many geologic phenomena, including volcanism, seismicity, mountain building, climatic changes, animal and plant distributions in the past and present, and the distributions of natural resources.

For these reasons, it is known as a unifying theory.

• Plate tectonic theory is based on the simple model that– the lithosphere is rigid & consists of both

• oceanic crust with upper mantle• continental crust with upper mantle

– variable-sized pieces called plates• that move as a unit• that can be continental, oceanic or both

– some regions containing continental crust • are up to 250 km thick

– whereas regions containing oceanic crust • are up to 100 km thick max


Plate Tectonics MapPlate Tectonics Map

Numbers represent average rates of relative movement, cm/yr

Plate Tectonics MapPlate Tectonics Map


Mosaic of PlatesMosaic of Plates1.1. 12 major plates with numerous smaller plates 12 major plates with numerous smaller plates

with three main boundarieswith three main boundaries2.2. Divergent boundaries: where plates moved Divergent boundaries: where plates moved

apart and new lithosphere is createdapart and new lithosphere is created3.3. Convergent boundaries: where plates come Convergent boundaries: where plates come

together and plates are recycledtogether and plates are recycled4.4. Transform boundaries: plates slide past each Transform boundaries: plates slide past each

otherother

Plate Tectonics: A Unifying TheoryThe lithosphere overlies the asthenosphere, and through some type of heat-transfer system within the asthenosphere, moves the plates.

As the plates move over the asthenosphere, they separate mostly at oceanic ridges and collide and are subducted into Earth’s interior at oceanic trenches.

Fig. 14 p. 41

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The Three Types of Plate BoundariesPlate tectonics has operated since at least the Proterozoic. It is important to understand how the plates move and interact with one another.

Types of Plate Boundaries

Divergent

Convergent

Transform

The Three Types of Plate BoundariesDivergent Boundaries - Divergent boundaries form when plates move away from one another.

New oceanic lithosphere is forms at the opening rift.Most divergent boundaries occur along the crests of oceanic ridgesThey are also present under continents during the early stages of continental breakup.

Fig. 2.16, p. 45

Fig. 2-16, p. 45Stepped Art

Volcanic activity

MagmaContinental crust

Rift valley

Coastal mountainrange

Narrow fault-bounded sea

Continental “seaboard”(coastal mountains gone)

Wide ocean

Divergent plate boundaries – or spreading ridges occur – where plates are separating – and new oceanic lithosphere is forming.

• Crust is extended– thinned and fractured

• The magma– originates from partial melting of the mantle– is basaltic– intrudes into vertical fractures to form dikes– some rises to the surface and is extruded

as lava flows

Divergent BoundariesDivergent Boundaries

• Successive injections of magma – cool and solidify– form new oceanic crust– record the intensity and orientation – of Earth’s magnetic field

• Divergent boundaries most commonly – occur along the crests of oceanic ridges – such as the Mid-Atlantic Ridge

• Ridges have– rugged topography resulting from displacement – of rocks along large fractures– shallow earthquakes

Divergent BoundariesDivergent Boundaries• Ridges also have

– high heat flow– and basaltic flows or pillow lavas


• Pillow lavas that formed along the Mid-Atlantic Ridge– with a

distinctive bulbous shape resulting

– from underwater eruptions

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• Divergent boundaries are also present – under continents during the early stages – of continental breakup


• Beneath a continent – when

magma wells up

– the crust is initially

• elevated, • stretched • and thinned

• The stretching produces fractures and rift valleys.

Rift ValleyRift Valley

• During this stage, – magma typically– intrudes into the

fractures– and flows onto

the valley floor• Example: East

African rift valleys

Narrow SeaNarrow Sea• As spreading proceeds, some rift valleys

– will continue to lengthen and deepen until– the continental

crust eventually breaks

– a narrow linear sea is formed,

– separating two continental blocks

– Examples: • Red Sea • Gulf of

California

Modern DivergenceModern Divergence

View looking down the Great Rift Valley of Africa.

Little Magadisoda lake

The Three Types of Plate BoundariesDivergent Boundaries

An Example of Ancient Rifting

Characteristic features of ancient continental rifting include faulting, dikes, sills, lava flows, and thick sedimentary sequences within rift valleys.Pillow lavas and associated deep-sea sediments are evidence of ancient spreading ridges.

Fig. 2.17, p.46

OceanOcean• As a newly created narrow sea

– continues to spread, – it may eventually become – an expansive ocean basin– such as the

Atlantic Ocean basin is today,

• separating North and South America

• from Europe and Africa

• by thousands of kilometers

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Atlantic Ocean BasinAtlantic Ocean Basin

Europe

Africa

North America

South America

Atlantic Ocean basin

Thousands of kilometers

An Example of Ancient RiftingAn Example of Ancient Rifting• What features in the rock record

– can geologists use to recognize ancient rifting?• faults• dikes• sills• lava flows• thick sedimentary

sequences • within rift valleys

– Example:• Triassic age fault basins

in eastern US

Ancient RiftingAncient Rifting

Palisades of Hudson River

sill

• These Triassic fault basins – mark the zone of rifting – between North America

and Africa

– They contain thousands of meters of continental sediment

– and are riddled with dikes and sills

The Three Types of Plate Boundaries

Convergent Boundaries are places where two plates collide

There are three types of convergent boundaries.

An oceanic-oceanic boundary is where two oceanic plates collide, one ocean plate will subduct beneath the margin of the other plate.

An oceanic-continental boundary is where an oceanic plate and a continental plate collide, the oceanic plate will subduct.

A continental-continental boundary occurs when two continents collide


Convergent BoundariesOceanic-Oceanic Boundaries

One oceanic plate is subducted beneath the other and a volcanic island arc forms on the non-subducted plateAn oceanic trench forms parallel to the volcanic island arc where the subduction is taking place. The volcanoes result from rising magma produced by the partial melting of the subducting plate.

Fig. 2.18a, p. 47

The Three Types of Plate BoundariesConvergent Boundaries

Oceanic-Continental Boundaries

An oceanic plate and a continental plate converge, with the denser oceanic plate being subducted under the continental plate. Just as with an oceanic-oceanic boundary, a chain of volcanoes forms on the nonsubducted plate.

Fig. 2.18b, p. 47

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The Three Types of Plate BoundariesConvergent Boundaries

Continental - Continental Boundaries

Two continents converge and the ocean floor separating them is subducted, resulting in a collision between the two continents. Neither plate will subduct.When the two continents collide, they are welded together to form an interior mountain chain along a zone marking the former site of subduction. Fig. 2.18c, p. 47


Convergent BoundariesRecognizing Ancient Convergent Plate Boundaries

Intensely deformed rocks, andesite lavas, and ophiolites are all evidence of ancient subduction zones, marking former convergent plate boundaries.

Fig. 2.19, p. 48

The Three Types of Plate BoundariesTransform Boundaries

These are boundaries along which plates slide laterally past each other along transform faults

These boundaries change one type of motion between plates into another type of motion.

Fig. 2.20a, p. 49


Transform Boundaries

These are boundaries along which plates slide laterally past each other along transform faultsGenerally, no diagnostic features are left by transform faults.Example: San Andreas fault

Fig. 2.21, p. 50

Hot Spots and Mantle Plumes

What are hot spots?

A hot spot is the location on Earth’s surface where a stationary column of magma, originating deep within the earth (possible below the mantle), has slowly risen to the surface and formed a volcano.

Fig. 2.17, p. 46

Hot Spots and Mantle Plumes

Because mantle plumes apparently remain stationarywithin the mantle while plates move over them

the resulting hot spots leave a trail of extinct and progressively older volcanoes that record the movement of the plate.

Fig. 2.22, p. 51

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Plate Movement and Motion

Determining rate and direction of plate movement

Hot spots enable geologists to determine absolute motion because they provide an apparently fixed reference point from which the rate and direction of plate movement can be measured.

Fig. 2.23 a-b, p. 51

Plate Movement and Motion

Determining rate & direction of plate movement

Fig. 2.23a, p. 51

The average rate of plate movement is most commonly determined by dividing the distance from an oceanic ridge axis to any magnetic anomaly in the crust of the seafloor by the age of that anomaly. Satellite-laser ranging techniques are also used to determine the rate of movement and relative motion of one plate with respect to another.

The Driving Mechanism of Plate Tectonics

What drives the plates?

Most geologists agree that some type of convective heat system is the basic process responsible for plate motion.

Fig. 2.25, p. 52


How do thermal convection cells move plates?

Two models involving thermal convection cells have been proposed to explain plate movement. 1. thermal cells are restricted to the asthenosphere2. the entire mantle is involved.

Problems with both models involve the source of heatfor the convection cells and how heat is transferredfrom the outer core to the mantle. Fig. 2.25, p. 52


Gravity driven plate motion

Some geologists think a gravity-driven mechanismalso plays a major role.

“Slab-pull” involves pulling the plate behind a subducting cold slab of lithosphere“Ridge-push” involves gravity pushing the oceanic lithosphere away from the higher spreading ridges and toward the subduction trenches

Fig. 2.26, p. 53


The Supercontinent Cycle (Wilson cycle)

In the early 1970s J. Tuzo Wilson put forth the hypothesis of a large-scale global cycle of supercontinents.Supercontinents like Pangea form, break up, and re-form in a cycle spanning approximately 500 million years. The breakup forms rift valleys within the supercontinent that eventually becomes a long, linear ocean basins as the crust is depressed below sea level. As the width of the narrow sea continues to expand an open ocean develops. As the ocean basins close, another supercontinent forms.

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Plate Tectonics and the Distribution of Natural Resources

Mineral Deposits

Many metallic mineral deposits are related to igneous and associated hydrothermal activity, so it is not surprising that a close relationship exists between plate boundaries and the occurrence of these valuable deposits. Many of the world’s major metallic ore deposits are associated with convergent and divergent plate boundaries. Copper, iron, lead , zinc, gold and silver ore deposits are associated with plate boundaries.

Fig. 2.27, p. 54

Plate Tectonics and the Distribution of Life

Fossil evidence provided one of the first proofs for plate tectonics. Together, plate tectonics and evolution have changed the way we view our planet.

The world’s plants and animals occupy biotic provinces controlled mostly by:

ClimateGeographic barriers

The location of these provinces is mostly controlled by plate movement.

Fig. 2.28, p. 56

End of Chapter 2

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