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Chapter 4
The Way the Earth Works: Plate Tectonics
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
The Way the Earth Works:
Plate Tectonics
Prepared by
Ron Parker,
Earlham College Department of Geosciences
Richmond, Indiana
Plate Tectonics

The paradigm of “How the Earth Works.”
Earth’s outer shell is broken into rigid plates that move.
 Moving plates change the face of planet Earth.


A case study of a scientific revolution.

A powerful idea based on multiple lines of evidence.
It allows scientists to predict geologic events, and...
Reconstruct the geologic past.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Plate Tectonics
Tectonic theory evolved in the 1960s.
 Previous research provided a strong foundation.

Wegener (1915) – Evidence supporting continental drift.
 Hess / Dietz (1960) – The sea-floor spreading hypothesis.

By 1968, evidence for tectonics was overwhelming.
 This evidence changed the view of most geologists.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Plate Tectonics
Plate tectonic theory is powerful.
 It provides a unified mechanism explaining:

Igneous, sedimentary, and metamorphic rocks.
 The distribution of earthquakes and volcanoes.
 The origin of continents and ocean basins.
 The distribution of fossil plants and animals.
 The genesis and destruction of mountain chains.
 Continental drift.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Lithosphere

Tectonic plates are fragments of lithosphere.
Lithosphere is made of both crust and the upper mantle.
 The lithosphere is in motion over the asthenosphere.

Lithosphere bends elastically when loaded.
 Asthenosphere flows plastically when loaded.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Buoyancy
First described by Archimedes more than 2.2 ka.
 Floating solids displace water equal to their mass.
 An iceberg “sinks” until the mass of water it
displaces is equal to the total mass of the iceberg.
 This concept applies to lithospheric plates.

Continental – Floats higher.
 Oceanic – Sinks lower.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
2 Types of Lithosphere

Continental ~ 150 km thick.

Granitic crust.
35-40 km thick.
Lighter (less dense) .
More buoyant – Floats higher.

Oceanic ~ 7 to 100 km thick.

Basaltic crust.
7-10 km thick.
Heavier (more dense).
Less buoyant – Sinks lower.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Plate Boundaries
Lithosphere is fragmented into ~ 20 tectonic plates.
 Plates move continuously at a rate of 1 to 15 cm/yr.

This motion appears to be slow on a human time scale.
 Yet it is extremely rapid on a geologic time scale.


Plates interact along their boundaries.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Plate Boundaries

Locations on Earth where tectonic plates meet.
Identified by concentrations of earthquakes.
 Associated with many other dynamic phenomena.


Plate interiors are almost earthquake-free.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Continental Margins

Where land meets the ocean.
Margins near plate boundaries are “active.”
 Margins far from plate boundaries are “passive.”


Passive margin continental crust thins seaward.
Transitions into oceanic crust.
 Traps eroded sediment.
 Develops into the
continental shelf.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Plate Boundaries: 3 Types

Divergent – Tectonic plates move apart.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Plate Boundaries: 3 Types

Convergent – Tectonic plates move together.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Plate Boundaries: 3 Types

Transform – Tectonic plates slide sideways.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Divergent Boundaries
Sea-floor spreading causes plates to move apart.
 Magma wells up to fill the gap.
 Magma cools, adding material to each plate.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Divergent Boundaries

Sea-floor spreading progression.

Early stage
Rifting has progressed to mid-ocean ridge formation.
Before substantial widening of the ocean.
Forms a long, thin ocean basin with young oceanic crust.

Example: The Red Sea
Note: This diagram only depicts the crust, not the entire lithosphere.
Time 1
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Divergent Boundaries

Sea-floor spreading progression.

Mid-stage
Ocean begins to widen.
New seafloor is added at the mid-ocean ridge.
Continents move farther apart.

Example: Greenland and the North Atlantic
Note: This diagram only depicts the crust, not the entire lithosphere.
Time 2
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Divergent Boundaries

Sea-floor spreading progression.

Late stage
Mature, wide ocean basin.
Linear increase in age with distance from central ridge.
Edge of ocean basin - oldest; ridge proximal - youngest.

Example: The Atlantic Ocean
Note: This diagram only depicts the crust, not the entire lithosphere.
Time 3
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Mid-Ocean Ridges
Linear mountain ranges in Earth’s ocean basins.
 Example: The Mid-Atlantic Ridge

Snakes N-S through the entire Atlantic Ocean.
 Elevated ridge (1,500 km wide) 2 km above abyssal plains.
 Axial rift valley.

500 m deep.
10 km wide.
Symmetric.
Site of eruptions.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Mid-Ocean Ridges
Sea-floor spreading opens the axial rift valley.
 Rising asthenosphere melts, forming mafic magma.
 Pooled magma solidifies into oceanic crustal rock.

basalt – Magma quenched at the sea floor.
 Dikes – Preserved magma conduits.
 Gabbro – Deeper magma.
 Pillow
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Mid-Ocean Ridges

“Black smokers” are found at some MORs.
Water entering fractured rock is heated by magma.
 Hot water dissolves minerals and cycles back out of rock.
 When water reaches the sea, minerals precipitate quickly.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Ocean Crustal Age

Oceanic crust spreads away from the ridge axis.
New crust is closer to the ridge; older crust farther away.
 Oldest oceanic crust is found at the far edge of the basin.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Oceanic Lithosphere
The hot asthenosphere is at the base of the MOR.
 Aging ocean crust moves away from this heat…

 Cooling,
increasing in density, and sinking.
 Accumulating increasing thicknesses of sediment.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Convergent Boundaries
Lithospheric plates move toward one another.
 One plate dives back into the mantle (subduction).
 Subduction recycles oceanic lithosphere.

Subduction is balanced by sea-floor spreading.
 Earth maintains a constant circumference.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Subduction
Old oceanic lithosphere is more dense than mantle.
 A flat-lying oceanic plate won’t subduct.
 Bent down, the leading edge sinks like an anchor.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Convergent Boundaries

The subducting plate descends at an average of 45.

Plate descent is revealed by Wadati-Benioff earthquakes.
Mark frictional contact and mineral transformations.
Earthquakes deepen away from trench.

Quakes cease below 660 km.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Fate of Subducted Plates?
Plate descent continues past the earthquake limit.
 The lower mantle may be a “plate graveyard.”

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Subduction Features

Subduction is associated with unique features.
Accretionary prisms.
 Volcanic arcs.
 Back-arc basins.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Convergent Boundaries

Accretionary prisms – Deformed sediment wedges.
Sediments scraped off subducting plates are smeared
and welded onto the overriding plates.
 These contorted sediments can be pushed above sea.

Washington’s Olympic Peninsula.
Taiwan.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Convergent Boundaries

Volcanic arc – Volcanic belt on an overriding plate.
The descending plate partially melts at ~ 150 km depth.
 Magmas burn upward, fueling volcanic eruptions.
 A curved Earth dictates that volcanic belts are curved.


Arc type depends on overriding plate.
Continental crust – Continental arc.
 Oceanic – Island arc.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Convergent Boundaries

Back arc basin – A marginal sea behind an arc.
Forms between an island arc and a continent.
 Offshore subduction traps a piece of oceanic crust, or…
 Stretching creates a new spreading ridge.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Transform Boundaries

Lithosphere slides past; not created or destroyed.
Many transforms offset spreading ridge segments.
 Some transforms cut through continental crust.


Characterized by…
Earthquakes.
 Absence of volcanism.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Oceanic Transforms

The MOR axis is offset by transform faults.
Offset of linear MOR is geometric necessity on a sphere.
 Transforms bear strong evidence of sea-floor spreading.

Abundant earthquakes common between ridge segments.
Earthquakes vanish past ridge segment overlaps.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Transform Boundaries

Oceanic transforms – Offsets along the MOR.

Older interpretation – Faulting occurs after MOR forms.

Modern interpretation – Faulting occurs with the MOR.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Transform Boundaries

Continental transforms – Chop continental crust.

Example: The San Andreas Fault.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Triple Junctions
Places where 3 plate boundaries coincide.
 Multiple boundary combinations occur.
 Triple junctions migrate and change across time.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Hot Spots

Volcanic plumes independent of tectonic plates.
Mafic magmas derived from the lower mantle.
 Tattoo overriding plates with volcanoes.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Hot Spots
Volcanoes perforate overriding plates.
 Make volcanoes that drift off-plume.

Volcano goes extinct and erodes.
 Subsidence creates a guyot.
 Supports sea-floor spreading.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Hot Spots

Hot spot volcanoes create seamounts.
Seamounts age away from originating hot spot.
 Age change marks direction of overriding plate motion.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Tectonic Boundaries Evolve
Plate boundaries change over geologic time.
 Oceanic plates.

Created at MOR spreading centers.
 Destroyed at subduction zones.


Continental plates.
Torn apart at rifts.
 Joined during collision.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Continental Rifting

Continental lithosphere can break apart.
Lithosphere stretches and thins.
 Brittle upper crust faults.
 Ductile lower crust flows.
 Asthenosphere melts.
 Melts erupt, leading to…


Sea-floor spreading.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Continental Rifting

Example: East Africa.
The Arabian plate is rifting from the African plate.
 Rifting has progressed to sea-floor spreading in…

The Red Sea
The Gulf of Aden

East African Rift – On-going rift.
Thinned crust.
Elongate trough.
Volcanoes.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Plate Collision
Subduction consumes ocean basins.
 Ocean closure ends in continental collision.

 Buoyant
continental crust will not subduct.
 Subduction ends; mountains are smashed upward.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Plate Collision

Plate tectonic collision may involve…
Two continents.
 A continent and an island arc.


Collision “sutures” the convergent plate boundary.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Driving Mechanisms

Old: Plates are dragged atop a convecting mantle.
Plate motions are much too complex.
 Convection does occur.

It is not the prime
driving mechanism.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Driving Mechanisms

Modern: 2 other forces drive plate motions.
Ridge-push – Elevated MOR pushes adjacent lithosphere.
 Slab-pull – Gravity pulls a subducting plate downward.
 Convection in the asthenosphere adds or subtracts.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Plate Velocities

Absolute plate velocities may be mapped by…
Plotting plate motion relative to a fixed spot in the mantle.
 Measuring volcano ages / distance along a hot spot track.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
Plate Velocities

Plate vectors are determined GPS measurements.
Global Positioning System (GPS) uses satellites.
 Knowledge of plate motion is now accurate and precise.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
The Dynamic Planet

Plate tectonics: The key to understanding geology.
Mantle is transferred to the surface and back down again.
 The interior and surface of Earth are in constant motion.
 Explains earthquakes, volcanoes, and continental drift.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
The Dynamic Planet

Earth’s surface changes continuously.
These changes appear slow to us.
 Geologically, change is rapid.

Earth looked different in the past.
 Earth will look different in the future.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
The Dynamic Planet

Plate Tectonics Summary: Ocean floor created at mid-ocean
ridges is consumed at oceanic trenches.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
The Dynamic Planet

Plate Tectonics Summary: Ocean floor created at mid-ocean
ridges is consumed at oceanic trenches.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics
This concludes the
Chapter 4
The Way the Earth Works: Plate Tectonics
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics