Chapter 4
The Way the Earth Works: Plate Tectonics
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak
Third Edition
©2008 W. W. Norton & Company, Inc.
Chapter 4: The Way the Earth Works: Plate Tectonics

Prepared by
Ron Parker,
Earlham College Department of Geosciences
Richmond, Indiana


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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 Divergent – Tectonic plates move apart.
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 Convergent – Tectonic plates move together.
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 Transform – Tectonic plates slide sideways.
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics


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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 Sea-floor spreading opens the axial rift valley.
 Rising asthenosphere melts, forming mafic magma.
 Pooled magma solidifies into oceanic crustal rock.
 Pillow basalt – Magma quenched at the sea floor.
 Dikes – Preserved magma conduits.
 Gabbro – Deeper magma.
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics


“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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 Old oceanic lithosphere is more dense than mantle.
 A flatlying oceanic plate won’t subduct.
 Bent down, the leading edge sinks like an anchor.
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 Plate descent continues past the earthquake limit.

The lower mantle may be a “plate graveyard.”
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 Subduction is associated with unique features.
 Accretionary prisms.
 Volcanic arcs.
 Back-arc basins.
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 Continental transforms – Chop continental crust.
 Example: The San Andreas Fault.
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 Places where 3 plate boundaries coincide.
 Multiple boundary combinations occur.
 Triple junctions migrate and change across time.
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 Volcanic plumes independent of tectonic plates.
 Mafic magmas derived from the lower mantle.
 Tattoo overriding plates with volcanoes.
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 Example: East Africa.
 The Arabian plate is rifting from the African plate.
 Rifting has progressed to seafloor 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics


Plate tectonic collision may involve…
 Two continents.
 A continent and an island arc.

Collision “sutures” the convergent plate boundary.
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics


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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics


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, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 Plate Tectonics Summary: Ocean floor created at mid-ocean ridges is consumed at oceanic trenches.
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 4: The Way the Earth Works: Plate Tectonics

 Plate Tectonics Summary: Ocean floor created at mid-ocean ridges is consumed at oceanic trenches.
Earth: Portrait of a Planet, 3 rd 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
Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak
Third Edition
©2008 W. W. Norton & Company, Inc.
Chapter 4: The Way the Earth Works: Plate Tectonics