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Chapter 2

Internal Structure of Earth and Plate Tectonics

Dr. Joao Santos

Essentials of Geology, 3rd edition, by Stephen MarshakEssentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate TectonicsChapter 2: The Way the Earth Works: Plate Tectonics

A Layered Earth � We live on the thin outer skin of Earth.

� Early perceptions about Earth’s interior were wrong.

� Open caverns filled with magma, water, and air.

� Furnaces and flames.

� We now know that Earth

is comprised of layers.

� The Crust.

� The Mantle.

� The Core.

�Outer Core.

�Inner Core.

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Earth’s Interior Layers

� Crust

� Continental

� Oceanic

� Mantle

� Upper

� Lower

� Core

� Outer - Liquid

� Inner – Solid

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Two Types of Crust � Continental crust – Granitic, underlies the continents.

� Average rock density about 2.7 g/cm3.

� Average thickness 35–40 km.

� Oceanic crust – Basaltic, underlies the ocean basins.

� Density about 3.0 g/cm3.

� Avgerage thickness 7–10 km.

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Earth’s Mantle � Solid rock layer between the crust and the core.

� 2,885 km thick, the mantle is 82% of Earth’s volume.

� Mantle composition is the ultramafic rock peridotite.

� Below ~100–150 km, the rock is hot enough to flow.

� It convects: Hot mantle rises, cold mantle sinks.

� Three subdivisions: Upper, transitional, and lower.

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The Core

� Outer core

�Liquid iron-nickel-sulfur

�2255 km thick.

�Density – 10-12 g/cm3

� Inner core

�Solid iron-nickel alloy.

�Radius of 1220 km.

�Density – 13 g/cm3.

� An iron-rich sphere with a radius of 3,471 km.

� 2 components with differing seismic wave behavior.

� Flow in the outer core

generates the magnetic field.

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Lithosphere–Asthenosphere � Lithosphere – The outermost 100–150 km of Earth.

� Nonflowing, rigid material that moves as tectonic plates.

� Made of 2 components: Crust and upper mantle.

� Asthenosphere – Upper mantle below lithosphere.

� Shallow under oceans; deep under continents.

� Flows as a soft solid.

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� The hypothesis that continents are mobile.

� Proposed by German meteorologist Alfred Wegener.

� The Origins of Oceans and Continents published in 1915.

� Wegener hypothesized a former supercontinent, Pangaea.

� Idea was founded on strong evidence.

�“Fit” of continents.

�Location of glaciations.

�Fossil organisms.

�Rock type and structural similarities.

�Paleoclimates preserved in rocks.

Continental Drift

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Continental Drift � Wegener’s idea was debated and ridiculed.

� Most scientists didn’t believe him.

� Lack of a mechanism for drift a major criticism.

� Wegener died in 1930 at the age of 40.

� Lacking an advocate, the drift

hypothesis faded.

� His idea was revived in the 1950s.

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Continental Drift � Wegener was right!

� Sea-floor spreading.

� Subduction.

� Plate Tectonics.

� Why did scientists dismiss Wegener’s model?

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Continental “Fit” � Wegener noted that continents seem to fit together.

� He argued that the fit could not be coincidental.

� Present shorelines make a rough fit.

� The continental shelf edges make a better fit.

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Glacial Evidence � Permian glacial till is found on four continents.

� The tills in Africa and India are now near the equator.

� A cooler earth? No. Permian tropical plants are known.

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Paleoclimatic Evidence � Placing Pangaea over the Permian South Pole…

� He correctly predicted…

� Tropical coals.

� Tropical reefs.

� Subtropical deserts.

� Subtropical evaporites.

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Fossil Evidence � Identical fossils found on widely separated landmasses.

� Mesosaurus – A freshwater reptile.

� Glossopteris – A subpolar plant with heavy seeds.

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Fossil Evidence � Identical fossils found on widely separated land.

� Lystrosaurus – A non-swimming, land-dwelling reptile.

� Cynognathus – A non-swimming, land-dwelling mammal- like reptile.

� These organisms could not

have crossed an ocean.

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� Geologic phenomenan match across the Atlantic.

� Geologic structures.

� Rock types.

� Rock ages.

Matching Geology

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� Geologic phenomenan match across the Atlantic.

� Mountain belts.

�The Appalachians.

�The Caledonides.

Matching Geology

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Criticisms of Drift � Why wasn’t the continental drift hypothesis accepted?

� There were no mechanisms for moving continents.

� When Wegener died, the debate did too.

� The drift hypothesis needed new and different evidence.

� This was provided by paleomagnetism.

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The Earth’s Magnetic Field � Earth’s magnetic field acts like a giant bar magnet.

� It’s N and S ends are tilted ~11° from the axis of rotation.

� Measured everywhere on Earth, it extends out into space.

� Some iron minerals in rocks align to the magnetic field.

� This permits some rocks to preserve magnetic information.

� Preserved magnetism can be read from these rocks.

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Magnetic Poles � The “bar magnet” intersects Earth’s surface.

� Magnetic North Pole; magnetic south pole.

� Differs from geographic north pole (rotational axis).

� The magnetic poles move constantly, but stay in the vicinity of the N and S geographic poles.

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� Above 350-550°C.

� Thermal energy of atoms high.

� Magnetic dipoles randomly oriented.

� No magnetic signature.

� Below 350-550°C.

� Thermal energy slows atoms.

� Dipoles align with Earth’s field.

� Material permanently magnetized.

� Fe-minerals can lock in the Earth’s

magnetic signal at the time formed.

� Can be used to determine lat / long.

Magnetic Overprinting

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Paleomagnetism � Rock magnetism can be measured in the laboratory.

� Study of fossil magnetism is called paleomagnetism.

� Ancient rocks reveal latitudes / longitudes unlike today.

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Polar Wander

� Paleomagnetism from ancient lavas didn’t align with the present magnetic field.

� This lack of alignment indicates past magnetic polar wandering.

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Polar Wander � Each continent had a separate polar-wander path.

� Either the location of the magnetic pole is not fixed, or…

� The lava flows themselves have moved.

� These curves align when continents are assembled.

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Apparent Polar Wander � Polar wander is now known to be an artifact.

� Not the signature of a wandering pole on a fixed continent.

� The signature of a fixed pole on a wandering continent.

� Apparent polar wander is strong evidence for drift.

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� Sonar was used to map the ocean bathymetry.

� Oceanographers were surprised to discover that…

� The deepest parts of the ocean occur near land.

� A mountain range runs through every ocean basin.

� Submarine volcanoes form lines across ocean floors.

The Ocean Floor

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The Ocean Floor � Modern views of the ocean floor reveal…

� Mid-ocean ridges.

� Trenches.

� Fracture zones.

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New Observations: Oceanic Crust � By 1950, we had learned much about oceanic crust.

� Oceanic crust is covered by sediment. It is…

� Thickest near the continents.

� Thinnest (or absent) at the mid-ocean ridge.

� Oceanic crust is mafic (basalt and gabbro).

� No granitic rocks.

� No metamorphic rocks.

� High heat flow characterizes the mid-ocean ridge.

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New Observations: Oceanic Crust � Belts of concentrated subsea earthquakes were found.

� The earthquakes were surprising. They were limited to…

� Parts of oceanic fracture zones.

� Mid-ocean ridge axes.

� Deep ocean trenches.

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Sea-Floor Spreading � In 1960, Harry Hess published his “Essay in Geopoetry.”

� He called his theory “sea-floor spreading”.

� Upwelling mantle erupts at the mid-ocean ridges.

� New crust moves away from ridges, gathering sediment.

� At trenches, the sea-floor dives back into the mantle.

� Provided a potential mechanism for continental drift.

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Sea-Floor Spreading

� Drilling in the late 1960s recovered crust samples.

� Ages increase away from the mid-ocean ridge.

� Ages are “mirror images” across the mid-ocean ridge.

� Strong supporting evidence for sea-floor spreading.

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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.

� Allows scientists to predict events and rebuild the past.

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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.

� Even reluctant scientists were eventually won over.

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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.

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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.

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Two 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.

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

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

� Slow on a human time scale; extremely rapid geologically.

� Plates interact along their boundaries.

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� Locations on Earth where tectonic plates meet.

� Identified by concentrations of earthquakes.

� Associated with many other dynamic phenomena.

� Plate interiors are almost earthquake free.

Plate Boundaries

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Continental Margins � Where land meets the ocean.

� Margins near plate boundaries are “active.”

� Margins far from a plate boundaries are “passive.”

� Passive margin continental crust thins seaward.

� Transitions into oceanic crust.

� Traps eroded sediment.

� Develops into the

continental shelf.

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Plate Boundaries: Three Types � Divergent – Tectonic plates move apart.

� Lithosphere thickens away from the ridge axis.

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Plate Boundaries: Three Types � Convergent – Tectonic plates move together.

� The process of plate consumption is called subduction.

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Plate Boundaries: Three Types � Transform – Tectonic plates slide sideways.

� Plate material is neither created, nor destroyed.

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Divergent Boundaries � Sea-floor spreading causes plates to move apart.

� Magma wells up to fill the gap.

� Magma cools, adding material to each plate.

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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.

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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.

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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.

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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 (1500 km wide) 2 km above abyssal plains.

� Axial rift valley.

�500 m deep.

�10 km wide.

�Symmetric.

�Site of eruptions.

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Mid-Ocean Ridges � 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.

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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.

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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.

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� The hot asthenosphere is at the base of the MOR.

� Aging ocean crust moves away from this heat source…

� Cooling, increasing in density and slowly sinking.

� Accumulating an increasing thickness of sediment.

Oceanic Lithosphere

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Convergent Boundaries � Lithospheric plates move toward one another.

� One plate dives back into the mantle (subduction).

� The subducting plate is always oceanic lithosphere.

� Subduction recycles oceanic lithosphere.

� Subduction is balanced by sea-floor spreading.

� Earth maintains a constant

circumference.

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Subduction � Old oceanic lithosphere is more dense than mantle.

� A flat-lying oceanic plate doesn’t subduct easily.

� Once bent downward, however, the leading edge sinks like an anchor rope.

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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.

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Fate of Subducted Plates? � Plate descent may continue past the earthquake limit.

� The lower mantle may be a “plate graveyard.”

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Subduction Features � Subduction is associated with unique features.

� Accretionary prisms.

� Volcanic arcs.

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� Accretionary Prism – A deformed sediment wedge.

� Sediments are scraped off of subducting plates.

� This thrusts them onto the overriding plate.

� Contorted prism sediments can be pushed above sea-level.

�Olympic Peninsula.

�Taiwan.

Convergent Boundaries

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Convergent Boundaries � Volcanic Arc – A chain of volcanoes on overriding plate.

� The descending plate partially melts at ~ 150 km depth.

� Magmas burn through overriding plate.

� Volcanic arcs are curved because the Earth is a sphere.

� Arc type depends upon the overriding plate.

� Continental crust – Continental Arc.

� Oceanic – Island Arc.

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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.

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Oceanic Transforms � The Mid-Ocean Ridge axis is offset by transform faults.

� A geometric necessity for a line spreading on a sphere.

� Transforms bear strong evidence of sea-floor spreading.

�Abundant earthquakes common when offsets are opposed.

�Earthquakes vanish when offsets are concurrent.

Figure 2.23a

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Transform Boundaries � Continental transforms – Chop continental crust.

� Example: The San Andreas Fault.

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Triple Junctions � Places where three plate boundaries coincide.

� Multiple boundary combinations occur.

� Triple junctions migrate and change across time.

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� Volcanic plumes independent of tectonic plates.

� Most are located far from plate boundaries.

� Comprised of mafic magmas from the lower mantle.

� Tattoo overriding plates with volcanoes.

Hot Spots

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Hot Spots � Hot spot perforates overriding plate.

� Volcano builds above sea level.

� Plate motion pulls volcano off plume.

� Volcano goes extinct and erodes.

� Subsidence creates a guyot.

� Hot spots reinforce

sea-floor spreading.

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Hot Spots � Hot-spot seamounts age away from originating hot spot.

� Age change marks rate of plate motion.

� Line of seamounts indicates direction of plate motion.

Figure 2.25a

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Tectonic Boundaries Evolve � Plate boundaries change over geologic time.

� Oceanic plates are...

� Created at mid-ocean ridge spreading centers.

� Destroyed at subduction zones.

� Continental plates are...

� Torn apart at rifts.

� Joined during collision.

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� Continental lithosphere can break apart.

� Lithosphere stretches and thins.

� Brittle upper-crust faults.

� Ductile lower-crust flows.

� Asthenosphere melts.

� Melt erupts.

� Continuation of this

process leads to full

sea-floor spreading.

Continental Rifting

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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

� Rifting continues along the

East African Rift.

�Thinned crust.

�Elongate trough.

�Volcanoes.

� The rift and 2 spreading ridges

comprise a triple junction.

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Plate Collision � Subduction consumes ocean basins.

� Ocean closure ends in continental collision.

� Buoyant continental crust will not subduct.

� Subduction ceases and mountains are uplifted.

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Plate Collision � Plate tectonic collision may involve…

� Two continents.

� A continent and an island arc.

� Collision “sutures” the convergent plate boundary.

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Driving Mechanisms � What drives plate motion?

� Old idea: Plates are dragged atop a convecting mantle.

�Plate motions are much too complex.

�Convection does occur.

�It is not the prime driving mechanism.

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Driving Mechanisms � Modern thinking: Two other forces drive plate motions.

� Ridge-push – Elevated MOR pushes lithosphere away.

� Slab-pull – Gravity pulls a subducting plate downward.

� Convection in the asthenosphere adds or subtracts.

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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.

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Plate Velocities � Plate vectors are determined GPS measurements.

� Global Positioning System (GPS) uses satellites.

� Knowledge of plate motion is now accurate and precise.

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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.

� PTs explains earthquakes, volcanoes and continental drift.

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The Dynamic Planet � Earth’s surface changes continuously.

� These changes appear slow to us.

� Geologically, change is rapid.

� Earth looked very different in the past.

� Earth will look very different in the future.

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The Dynamic Planet

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

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The Dynamic Planet

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

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End of Chapter 2