Op ch04 lecture_earth3 Subduction

Chapter 4: The Way the Earth Works: Plate Tectonics Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak

Chapter 4The Way the Earth Works: Plate Tectonics

©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earthearth

LECTURE OUTLINE

The Way the Earth Works:Plate TectonicsThe Way the Earth Works:Plate Tectonics

Prepared by

Ron Parker, Earlham College Department of Geosciences

Richmond, Indiana

Prepared by

Ron Parker, Earlham College Department of Geosciences

Richmond, Indiana


Plate TectonicsPlate Tectonics The paradigm of “How the Earth Works.”The paradigm of “How the Earth Works.”

Earth’s outer shell is broken into rigid plates that move.Earth’s outer shell is broken into rigid plates that move. Moving plates change the face of planet Earth.Moving plates change the face of planet Earth.

A case study of a scientific revolution. A case study of a scientific revolution. A powerful idea based on multiple lines of evidence.A powerful idea based on multiple lines of evidence.

It allows scientists to predict geologic events, and...It allows scientists to predict geologic events, and...Reconstruct the geologic past.Reconstruct the geologic past.


Plate TectonicsPlate Tectonics Tectonic theory evolved in the 1960s.Tectonic theory evolved in the 1960s. Previous research provided a strong foundation.Previous research provided a strong foundation.

Wegener (1915) – Evidence supporting continental drift. Wegener (1915) – Evidence supporting continental drift. Hess / Dietz (1960) – The sea-floor spreading hypothesis.Hess / Dietz (1960) – The sea-floor spreading hypothesis.

By 1968, evidence for tectonics was overwhelming. By 1968, evidence for tectonics was overwhelming. This evidence changed the view of most This evidence changed the view of most

geologists.geologists.


Plate TectonicsPlate Tectonics Plate tectonic theory is powerful. Plate tectonic theory is powerful. It provides a unified mechanism explaining: It provides a unified mechanism explaining:

Igneous, sedimentary, and metamorphic rocks. Igneous, sedimentary, and metamorphic rocks. The distribution of earthquakes and volcanoes.The distribution of earthquakes and volcanoes. The origin of continents and ocean basins.The origin of continents and ocean basins. The distribution of fossil plants and animals.The distribution of fossil plants and animals. The genesis and destruction of mountain chains. The genesis and destruction of mountain chains. Continental drift. Continental drift.


LithosphereLithosphere Tectonic plates are fragments of lithosphere.Tectonic plates are fragments of lithosphere.

Lithosphere is made of Lithosphere is made of bothboth crust and the upper mantle. crust and the upper mantle. The lithosphere is in motion over the asthenosphere. The lithosphere is in motion over the asthenosphere.

Lithosphere bends elastically when loaded.Lithosphere bends elastically when loaded. Asthenosphere flows plastically when loaded. Asthenosphere flows plastically when loaded.


BuoyancyBuoyancy First described by Archimedes more than 2.2 ka.First described by Archimedes more than 2.2 ka. Floating solids displace water equal to their mass.Floating solids displace water equal to their mass. An iceberg “sinks” until the mass of water it An iceberg “sinks” until the mass of water it

displaces is equal to the total mass of the iceberg.displaces is equal to the total mass of the iceberg. This concept applies to lithospheric plates.This concept applies to lithospheric plates.

Continental – Floats higher.Continental – Floats higher. Oceanic – Sinks lower.Oceanic – Sinks lower.


2 Types of Lithosphere2 Types of Lithosphere Continental ~ 150 km thick.Continental ~ 150 km thick.

Granitic crust.Granitic crust.35-40 km thick.35-40 km thick.Lighter (less dense) .Lighter (less dense) .More buoyant – Floats higher.More buoyant – Floats higher.

Oceanic ~ 7 to 100 km thick.Oceanic ~ 7 to 100 km thick. Basaltic crust. Basaltic crust.

7-10 km thick.7-10 km thick.Heavier (more dense).Heavier (more dense).Less buoyant – Sinks lower.Less buoyant – Sinks lower.


Plate BoundariesPlate Boundaries Lithosphere is fragmented into ~ 20 tectonic plates. Lithosphere is fragmented into ~ 20 tectonic plates.

Plates move continuously at a rate of 1 to 15 cm/yr.Plates move continuously at a rate of 1 to 15 cm/yr.

This motion appears to be slow on a human time scale.This motion appears to be slow on a human time scale. Yet it is extremely rapid on a geologic time scale.Yet it is extremely rapid on a geologic time scale.

Plates interact along their boundaries. Plates interact along their boundaries.


Locations on Earth where tectonic plates meet.Locations on Earth where tectonic plates meet.

Identified by concentrations of earthquakes.Identified by concentrations of earthquakes. Associated with many other dynamic phenomena.Associated with many other dynamic phenomena.

Plate interiors are almost earthquake-free. Plate interiors are almost earthquake-free.

Plate BoundariesPlate Boundaries


Continental MarginsContinental Margins Where land meets the ocean. Where land meets the ocean.

Margins near plate boundaries are “active.”Margins near plate boundaries are “active.” Margins far from plate boundaries are “passive.”Margins far from plate boundaries are “passive.”

Passive margin continental crust thins seaward.Passive margin continental crust thins seaward. Transitions into oceanic crust.Transitions into oceanic crust. Traps eroded sediment.Traps eroded sediment. Develops into the Develops into the

continental shelf.continental shelf.


Plate Boundaries: 3 Types Plate Boundaries: 3 Types Divergent – Tectonic plates move apart. Divergent – Tectonic plates move apart.


Plate Boundaries: 3 TypesPlate Boundaries: 3 Types Convergent – Tectonic plates move together.Convergent – Tectonic plates move together.


Plate Boundaries: 3 TypesPlate Boundaries: 3 Types Transform – Tectonic plates slide sideways.Transform – Tectonic plates slide sideways.


Divergent BoundariesDivergent Boundaries Sea-floor spreading causes plates to move apart.Sea-floor spreading causes plates to move apart. Magma wells up to fill the gap. Magma wells up to fill the gap. Magma cools, adding material to each plate. Magma cools, adding material to each plate.


Divergent BoundariesDivergent Boundaries Sea-floor spreading progression.Sea-floor spreading progression.

Early stageEarly stageRifting has progressed to mid-ocean ridge formation.Rifting has progressed to mid-ocean ridge formation.Before substantial widening of the ocean.Before substantial widening of the ocean.Forms a long, thin ocean basin with young oceanic crust.Forms a long, thin ocean basin with young oceanic crust.

Example: The Red SeaExample: The Red Sea

Time 1

Note: This diagram only depicts the crust, not the entire lithosphere.



Mid-stageMid-stageOcean begins to widen.Ocean begins to widen.New seafloor is added at the mid-ocean ridge.New seafloor is added at the mid-ocean ridge.Continents move farther apart.Continents move farther apart.

Example: Greenland and the North AtlanticExample: Greenland and the North Atlantic

Time 2




Late stageLate stageMature, wide ocean basin.Mature, wide ocean basin.Linear increase in age with distance from central ridge.Linear increase in age with distance from central ridge.Edge of ocean basin - oldest; ridge proximal - youngest.Edge of ocean basin - oldest; ridge proximal - youngest.

Example: The Atlantic OceanExample: The Atlantic Ocean

Time 3



Mid-Ocean RidgesMid-Ocean Ridges Linear mountain ranges in Earth’s ocean basins.Linear mountain ranges in Earth’s ocean basins. Example: The Mid-Atlantic RidgeExample: The Mid-Atlantic Ridge

Snakes N-S through the entire Atlantic Ocean.Snakes N-S through the entire Atlantic Ocean. Elevated ridge (1,500 km wide) 2 km above abyssal plains.Elevated ridge (1,500 km wide) 2 km above abyssal plains. Axial rift valley.Axial rift valley.

500 m deep.500 m deep.10 km wide.10 km wide.Symmetric.Symmetric.Site of eruptions.Site of eruptions.


Mid-Ocean RidgesMid-Ocean Ridges Sea-floor spreading opens the axial rift valley. Sea-floor spreading opens the axial rift valley. Rising asthenosphere melts, forming mafic magma. Rising asthenosphere melts, forming mafic magma.

Pooled magma solidifies into oceanic crustal rock.Pooled magma solidifies into oceanic crustal rock.

Pillow basalt – Magma quenched at the sea floor. Pillow basalt – Magma quenched at the sea floor. Dikes – Preserved magma conduits.Dikes – Preserved magma conduits. Gabbro – Deeper magma.Gabbro – Deeper magma.


Mid-Ocean RidgesMid-Ocean Ridges ““Black smokers” are found at some MORs.Black smokers” are found at some MORs.

Water entering fractured rock is heated by magma.Water entering fractured rock is heated by magma. Hot water dissolves minerals and cycles back out of rock. Hot water dissolves minerals and cycles back out of rock. When water reaches the sea, minerals precipitate quickly. When water reaches the sea, minerals precipitate quickly.


Ocean Crustal AgeOcean Crustal Age Oceanic crust spreads away from the ridge axis. Oceanic crust spreads away from the ridge axis.

New crust is closer to the ridge; older crust farther away.New crust is closer to the ridge; older crust farther away. Oldest oceanic crust is found at the far edge of the basin.Oldest oceanic crust is found at the far edge of the basin.


The hot asthenosphere is at the base of the MOR.The hot asthenosphere is at the base of the MOR. Aging ocean crust moves away from this heat…Aging ocean crust moves away from this heat…

Cooling, increasing in density, and sinking. Cooling, increasing in density, and sinking. Accumulating increasing thicknesses of sediment. Accumulating increasing thicknesses of sediment.

Oceanic LithosphereOceanic Lithosphere


Convergent BoundariesConvergent Boundaries Lithospheric plates move toward one another. Lithospheric plates move toward one another. One plate dives back into the mantle (subduction). One plate dives back into the mantle (subduction). Subduction recycles oceanic lithosphere. Subduction recycles oceanic lithosphere.

Subduction is balanced by sea-floor spreading.Subduction is balanced by sea-floor spreading. Earth maintains a constant circumference.Earth maintains a constant circumference.


SubductionSubduction Old oceanic lithosphere is more dense than mantle.Old oceanic lithosphere is more dense than mantle. A flat-lying oceanic plate won’t subduct.A flat-lying oceanic plate won’t subduct. Bent down, the leading edge sinks like an anchor.Bent down, the leading edge sinks like an anchor.


Convergent BoundariesConvergent Boundaries The subducting plate descends at an average of 45The subducting plate descends at an average of 45..

Plate descent is revealed by Wadati-Benioff earthquakes.Plate descent is revealed by Wadati-Benioff earthquakes.Mark frictional contact and mineral transformations. Mark frictional contact and mineral transformations. Earthquakes deepen away from trench.Earthquakes deepen away from trench.

Quakes cease below 660 km. Quakes cease below 660 km.


Fate of Subducted Plates?Fate of Subducted Plates? Plate descent continues past the earthquake limit. Plate descent continues past the earthquake limit. The lower mantle may be a “plate graveyard.” The lower mantle may be a “plate graveyard.”


Subduction FeaturesSubduction Features Subduction is associated with unique features. Subduction is associated with unique features.

Accretionary prisms.Accretionary prisms. Volcanic arcs.Volcanic arcs. Back-arc basins.Back-arc basins.


Convergent BoundariesConvergent Boundaries Accretionary prisms – Deformed sediment wedges.Accretionary prisms – Deformed sediment wedges.

Sediments scraped off subducting plates are smeared Sediments scraped off subducting plates are smeared and welded onto the overriding plates. and welded onto the overriding plates.

These contorted sediments can be pushed above sea.These contorted sediments can be pushed above sea.Washington’s Olympic Peninsula.Washington’s Olympic Peninsula.Taiwan.Taiwan.


Convergent BoundariesConvergent Boundaries Volcanic arc – Volcanic belt on an overriding plate. Volcanic arc – Volcanic belt on an overriding plate.

The descending plate partially melts at ~ 150 km depth.The descending plate partially melts at ~ 150 km depth. Magmas burn upward, fueling volcanic eruptions. Magmas burn upward, fueling volcanic eruptions. A curved Earth dictates that volcanic belts are curved. A curved Earth dictates that volcanic belts are curved.

Arc type depends on overriding plate.Arc type depends on overriding plate. Continental crust – Continental arc.Continental crust – Continental arc. Oceanic – Island arc. Oceanic – Island arc.


Convergent BoundariesConvergent Boundaries Back arc basin – A marginal sea behind an arc.Back arc basin – A marginal sea behind an arc.

Forms between an island arc and a continent.Forms between an island arc and a continent. Offshore subduction traps a piece of oceanic crust, or…Offshore subduction traps a piece of oceanic crust, or… Stretching creates a new spreading ridge.Stretching creates a new spreading ridge.


Transform BoundariesTransform Boundaries Lithosphere slides past; not created or destroyed. Lithosphere slides past; not created or destroyed.

Many transforms offset spreading ridge segments.Many transforms offset spreading ridge segments. Some transforms cut through continental crust.Some transforms cut through continental crust.

Characterized by…Characterized by… Earthquakes.Earthquakes. Absence of volcanism.Absence of volcanism.


Oceanic TransformsOceanic Transforms The MOR axis is offset by transform faults.The MOR axis is offset by transform faults.

Offset of linear MOR is geometric necessity on a sphere.Offset of linear MOR is geometric necessity on a sphere. Transforms bear strong evidence of sea-floor spreading.Transforms bear strong evidence of sea-floor spreading.

Abundant earthquakes common between ridge segments.Abundant earthquakes common between ridge segments.Earthquakes vanish past ridge segment overlaps.Earthquakes vanish past ridge segment overlaps.


Transform BoundariesTransform Boundaries Oceanic transforms – Offsets along the MOR. Oceanic transforms – Offsets along the MOR.

Older interpretation – Faulting occurs after MOR forms.Older interpretation – Faulting occurs after MOR forms.

Modern interpretation – Faulting occurs with the MOR.Modern interpretation – Faulting occurs with the MOR.


Transform BoundariesTransform Boundaries Continental transforms – Chop continental crust. Continental transforms – Chop continental crust.

Example: The San Andreas Fault. Example: The San Andreas Fault.


Triple JunctionsTriple Junctions Places where 3 plate boundaries coincide.Places where 3 plate boundaries coincide. Multiple boundary combinations occur. Multiple boundary combinations occur. Triple junctions migrate and change across time.Triple junctions migrate and change across time.


Hot SpotsHot Spots Volcanic plumes independent of tectonic plates. Volcanic plumes independent of tectonic plates.

Mafic magmas derived from the lower mantle.Mafic magmas derived from the lower mantle. Tattoo overriding plates with volcanoes.Tattoo overriding plates with volcanoes.


Hot SpotsHot Spots Volcanoes perforate overriding plates. Volcanoes perforate overriding plates. Make volcanoes that drift off-plume. Make volcanoes that drift off-plume.

Volcano goes extinct and erodes.Volcano goes extinct and erodes. Subsidence creates a guyot.Subsidence creates a guyot. Supports sea-floor spreading.Supports sea-floor spreading.


Hot SpotsHot Spots Hot spot volcanoes create seamounts.Hot spot volcanoes create seamounts.

Seamounts age away from originating hot spot.Seamounts age away from originating hot spot. Age change marks direction of overriding plate motion. Age change marks direction of overriding plate motion.


Tectonic Boundaries EvolveTectonic Boundaries Evolve Plate boundaries change over geologic time.Plate boundaries change over geologic time. Oceanic plates.Oceanic plates.

Created at MOR spreading centers.Created at MOR spreading centers. Destroyed at subduction zones.Destroyed at subduction zones.

Continental plates.Continental plates. Torn apart at rifts.Torn apart at rifts. Joined during collision. Joined during collision.


Continental lithosphere can break apart. Continental lithosphere can break apart. Lithosphere stretches and thins. Lithosphere stretches and thins. Brittle upper crust faults.Brittle upper crust faults. Ductile lower crust flows.Ductile lower crust flows. Asthenosphere melts. Asthenosphere melts. Melts erupt, leading to…Melts erupt, leading to…

Sea-floor spreading. Sea-floor spreading.

Continental RiftingContinental Rifting


Continental RiftingContinental Rifting Example: East Africa.Example: East Africa.

The Arabian plate is rifting from the African plate.The Arabian plate is rifting from the African plate. Rifting has progressed to sea-floor spreading in…Rifting has progressed to sea-floor spreading in…

The Red SeaThe Red SeaThe Gulf of AdenThe Gulf of Aden

East African Rift – On-going rift.East African Rift – On-going rift.Thinned crust.Thinned crust.Elongate trough (channel).Elongate trough (channel).Volcanoes.Volcanoes.


Plate CollisionPlate Collision Subduction consumes ocean basins.Subduction consumes ocean basins. Ocean closure ends in continental collision.Ocean closure ends in continental collision.

Buoyant continental crust will not subduct.Buoyant continental crust will not subduct. Subduction ends; mountains are smashed upward.Subduction ends; mountains are smashed upward.


Plate CollisionPlate Collision Plate tectonic collision may involve…Plate tectonic collision may involve…

Two continents.Two continents. A continent and an island arc.A continent and an island arc.

Collision “sutures” the convergent plate boundary.Collision “sutures” the convergent plate boundary.


Driving MechanismsDriving Mechanisms Old: Plates are dragged atop a convecting mantle.Old: Plates are dragged atop a convecting mantle.

Plate motions are much too complex.Plate motions are much too complex. Convection Convection doesdoes occur. occur.

It is not the prime It is not the prime

driving mechanism.driving mechanism.


Driving MechanismsDriving Mechanisms Modern: 2 other forces drive plate motions. Modern: 2 other forces drive plate motions.

Ridge-push – Elevated MOR pushes adjacent lithosphere.Ridge-push – Elevated MOR pushes adjacent lithosphere. Slab-pull – Gravity pulls a subducting plate downward. Slab-pull – Gravity pulls a subducting plate downward. Convection in the asthenosphere adds or subtracts.Convection in the asthenosphere adds or subtracts.


Plate VelocitiesPlate Velocities Absolute plate velocities may be mapped by…Absolute plate velocities may be mapped by…

Plotting plate motion relative to a fixed spot in the mantle.Plotting plate motion relative to a fixed spot in the mantle. Measuring volcano ages / distance along a hot spot track. Measuring volcano ages / distance along a hot spot track.


Plate VelocitiesPlate Velocities Plate vectors are determined GPS measurements.Plate vectors are determined GPS measurements.

Global Positioning System (GPS) uses satellites.Global Positioning System (GPS) uses satellites. Knowledge of plate motion is now accurate and precise.Knowledge of plate motion is now accurate and precise.


The Dynamic PlanetThe Dynamic Planet Plate tectonics: The key to understanding geology.Plate tectonics: The key to understanding geology.

Mantle is transferred to the surface and back down again.Mantle is transferred to the surface and back down again. The interior and surface of Earth are in constant motion.The interior and surface of Earth are in constant motion. Explains earthquakes, volcanoes, and continental drift.Explains earthquakes, volcanoes, and continental drift.


The Dynamic PlanetThe Dynamic Planet Earth’s surface changes continuously. Earth’s surface changes continuously.

These changes appear slow to us.These changes appear slow to us. Geologically, change is rapid.Geologically, change is rapid.

Earth looked different in the past. Earth looked different in the past. Earth will look different in the future.Earth will look different in the future.


The Dynamic PlanetThe Dynamic Planet

Plate Tectonics Summary: Ocean floor created at mid-ocean Plate Tectonics Summary: Ocean floor created at mid-ocean ridges is consumed at oceanic trenches.ridges is consumed at oceanic trenches.


Chapter 4The Way the Earth Works: Plate Tectonics

©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earthearth

LECTURE OUTLINE

This concludes the

Op ch04 lecture_earth3 Subduction

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