Copyright (c) 2005 Pearson Education Canada, Inc.19-1 PowerPoint Presentation Stan Hatfield....

Copyright (c) 2005 Pearson Education Canada, Inc. 19-1

PowerPoint PresentationStan Hatfield . Southwestern Illinois College

Ken Pinzke . Southwestern Illinois College

Charles Henderson . University of Calgary

Tark Hamilton . Camosun College

Chapter 19 in 1st edition& 1 in 2nd edition

Plate Tectonics

Copyright (c) 2005 Pearson Education Canada Inc. 19-2


Continental Drift: An Idea Before Its Time

Alfred Wegener• First proposed his continental drift hypothesis in

1915 • Published The Origin of Continents and Oceans

Continental Drift Hypothesis • Supercontinent called Pangaea began breaking

apart about 200 million years ago


Pangaea approximately 200 million years ago.


Late Paleozoic Supercontinent, Equatorial Tethys Sea, Pan Thallassic Ocean


Continental Drift Hypothesis• Continents "drifted" to present positions

Evidence used in support of the continental drift hypothesis

• Fit of the continents • Fossil evidence• Rock type and structural similarities • Paleoclimatic evidence


Copyright (c) 2005 Pearson Education Canada Inc. 19-6Wegener’s matching of mountain ranges on different continents.


Copyright (c) 2005 Pearson Education Canada Inc. 19-7Permian Glaciation, Tillites, Pavements: Paleoclimatic evidence for continental drift.



The Great Debate: No Viable Mechanism

Objections to the Continental Drift Hypothesis • Inability to provide a mechanism capable of

moving continents across the globe • Wegener suggested that continents plowed through

the ocean crust, much like ice breakers cut through ice

• Proposed Tidal forcing, too weak, Rocks too strong


Continental Drift and the Scientific Method• Wegener’s hypothesis was correct in principle, but

contained incorrect details • For any scientific viewpoint to gain wide

acceptance, supporting evidence from all realms of science must be found

• A few scientists considered Wegener’s ideas plausible & continued the search a generation later

• Perseverance is important in Science, ideas must be supported and widely publicized

Lessons from Continental Drift’s Failure


Continental Drift and Paleomagnetism

Initial impetus for the renewed interest in continental drift came from rock paleomagnetism

• Keith Runcorn’s Lab at Newcastle & Ted Irving

Magnetized minerals in rocks • Record the direction of Earth’s magnetic poles • Provide a means of determining their latitude of

origin: tan ( Inclination ) = 2 tan ( Latitude )• Oriented strata of known age, yet from different

continents track relative motions• Polar Wander Paths


Polar Wandering• Rocks from successive ages give successive N-S

poles for their continents• The apparent movement of the magnetic poles

illustrated in magnetized rocks indicates that the continents have moved both absolutely and relative to each other

• Shows that Europe was much closer to the equator when coal-producing swamps existed

• Brazil, Africa & India were much closer to the S pole in Permian time

• Antarctica is the least travelled continent!


Copyright (c) 2005 Pearson Education Canada Inc. 19-12Apparent polar-wandering paths for Eurasia and North America.



Polar Wandering• Polar wandering curves for North America and

Europe have similar paths, but are separated by about 24 of longitude

– PW Paths for Cambrian through Permian strata are parallel for Europe and North America

– PW Paths for Triassic through Recent seem to converge progressively

– Differences between the paths can be reconciled if the continents are placed next to one another

– ~205Ma is the point of departure, with the opening of the Atlantic



A Scientific Revolution BeginsDuring the 1950s and 1960s technological strides permitted extensive SONAR mapping of the ocean floorDuring the Cold War, any ship of convenience towed a magnetometer (sub chasing!) Magnetic stripes were discovered.The first complete bathymetric maps of the world seafloor assembled by Bruce Heezen & Marie Tharp of the US Office of Naval Research MOR’s & Deep Sea Trenches were discoveredSeafloor spreading hypothesis was proposed by Harry Hess in the early 1960s


Geomagnetic reversals • Both in strata on land and in boreholes beneath the

sea the magnetic succession is found to be full of reversals with anti-parallel directions

• The advent of radiometric dating K/Ar & U/Pb• Earth's magnetic field periodically reverses

polarity – the north magnetic pole becomes the south magnetic pole, and vice versa

• Dates when the polarity of Earth’s magnetism changed were determined from lava flows

A Scientific Revolution Begins anew with the idea of Sea Floor Spreading

Copyright (c) 2005 Pearson Education Canada Inc. 19-16The ocean floor as a magnetic tape recorder.

Magnetic Stripes Record Sea Floor Spreading During a Succession of Geomagnetic Reversals


Geomagnetic reversals• Geomagnetic reversals are recorded both across

and within the ocean crust by the Deep Sea Drilling Program’s Glomar Challenger and other marine geoscience programs

• In 1963 Fred Vine and D. Matthews tied the discovery of magnetic stripes in the ocean crust near ridges to Hess’s concept of seafloor spreading

• Sediments are found to contain magnetic reversal stratigraphy too and with microfossils this provides the basis for a global magnetostratigraphy back through the Cretaceous

The Birth of Plate Tectonics: 1960’s


Geophysics Provided the New Data that the Older Geology Lacked

• Paleomagnetism, Magnetic Reversals, Magnetostratigraphy (evidence of past magnetism recorded in the rocks) was the most convincing evidence set forth to support the concepts of continental drift and seafloor spreading

• Bathymetry & Heat Flow: Ridges young, shallow and warm, trenches old, deep and cold with thicker sediments

• Seismics show Lithosphere thickens away from MOR’s

• Major Earthquakes were located along plate boundaries

• Volcanic Arcs behind trenches

Plate Tectonics = Continental Drift + Seafloor Spreading


Plate Tectonics: The New Paradigm

Spreading & New Crust Formed at MOR’s & CR’s, high heat flow, active volcanismSubduction zones (Wadati-Benioff zones) of deepening earthquakes behind trenchesRecycled water, gases & light elements from ArcsMuch more encompassing theory than continental drift The composite of a variety of ideas that explain the observed motion of Earth’s lithosphere through the mechanisms of subduction and seafloor spreading


Earth’s Major Plates • Associated with Earth's strong, rigid outer layer

– Known as the lithosphere

– Consists of uppermost mantle and overlying crust

– Overlies a weaker region in the mantle called the asthenosphere

– Plates are no longer just Continents, e.g. the North America Plate extends from Greenland and Iceland to Vancouver Island including both older continents and some younger seafloor

– 3 Types of Boundaries: Ridges, Trenches & Transforms



Earth’s Major Plates• ~Seven major lithospheric plates: Pacific, Eurasia,

Antarctica, N.America, S. America, Africa, Indian Ocean

• Up to a dozen smaller plates: Australia, Caribbean, Mediterranean, Scotian Sea, Juan de Fuca, Turkey, Philippine Sea…

• Plates are in motion and continually changing in shape and size

• Largest plate is the Pacific Plate• Several plates include an entire continent plus a

large area of seafloor



Earth’s Major Plates• Motions measured by Magnetic stripes & Hotspot

Tracks over Ma to 10’s of Ma and VLBI & GPS arrays over months to years

• Plate boundaries with faster motions have greater volcanism and seismicity (Java-Sumatra)

• Plates move relative to each other at a very slow but continuous rate

– Average about 5 centimetres per year

– Cooler, denser slabs of oceanic lithosphere descend into the mantle



Plate Boundaries• Almost all major interactions among individual

plates occur along their boundaries– Subduction Zone Megathrust Earthquakes

– Volcanic & Plutonic Arcs

– Rare exceptions are:

– Within plate seismicity

– Within plate hotspot volcanism

• Types of plate boundaries – Divergent plate boundaries (constructive margins)

– Convergent plate boundaries (destructive margins)

– Transform fault boundaries (conservative margins)



Plate Boundaries• Each plate is bounded by a combination of the

three types of boundaries • New plate boundaries can be created in response to

changes in the forces acting on these rigid slabs• Changes to plate boundaries require several Ma• Earthquakes on one boundary are unrelated to

those on others• Volcanoes keep their own sweet time



Divergent Plate BoundariesMost are located along the crests of oceanic ridges and can be thought of as constructive plate margins with frequent volcanism & Quakes <6

Continental Rifts:• East African Rift, Rio Grande, Baikal Rift, Rhine Graben

• Grabens, volcanic plateaux, normal faulting, hot springs

Oceanic ridges and seafloor spreading • Along well-developed divergent plate boundaries, the

seafloor is elevated forming oceanic ridges

• Mid Atlantic Rift, Lomonosov Ridge, East Pacific Rise, Juan de Fuca Ridge, Galapagos Ridge, SW Indian Ridge…


Oceanic Ridges and Seafloor Spreading• Seafloor spreading occurs along the oceanic ridge

system• Active volcanism causes black smokers, vent

communities & seafloor massive sulfide deposits (SMS)

Spreading Rates and Ridge Topography • Ridge systems exhibit topographic differences • Topographic differences are controlled by

spreading rates– Slow spreading = rugged topography

– Med-fast spreading = axial valley

Divergent Plate Boundaries

Copyright (c) 2005 Pearson Education Canada Inc. 19-27Divergent boundaries are located mainly along oceanic ridges.



Spreading Rates and Ridge Topography• Topographic differences are controlled by

spreading rates– At slow spreading rates (1-5 centimetres per year), a

prominent rift valley develops along the ridge crest that is wide (30-50 km) and deep (1500-3000 metres) Mid-Atlantic

– At intermediate spreading rates (5-9 centimetres per year), rift valleys that develop are shallow with subdued topography East Pacific



Spreading Rates and Ridge Topography• Topographic differences are controlled by

spreading rates– At spreading rates greater than 9 centimetres per year

no median rift valley develops and these areas are usually narrow and extensively faulted

– At Thingvallir in West Iceland the rift is above sea level!

Continental Rifts• Splits landmasses into two or more smaller

segments • These start with 3 arms but usually 1 or 2 of them

fail



Continental Rifts• Examples include the East African rift valleys and

the Rhine Valley in northern Europe• Produced by extensional forces acting on the

lithospheric plates • Not all rift valleys develop into full-fledged

spreading centres • There are failed Precambrian rifts beneath

southern Alberta, the East arm of Great Slave Lake and Mid Continent between Minnesota and Missouri!


Copyright (c) 2005 Pearson Education Canada Inc. 19-31The East African rift – a divergent boundary on land.


RRR Triple Junction


Convergent Plate BoundariesOlder portions of oceanic plates are returned to the mantle in these destructive plate margins

• Surface expression of the descending plate is an ocean trench

• Called subduction zones • Average angle at which oceanic lithosphere

descends into the mantle is about 45• Older colder lithosphere descends steeper up to 90°• Younger warmer lithosphere descends flatter <15°• The steeper the angle, the deeper the trench• Younger descending plates make the greatest

earthquakes ~MR > 9.0


Although all have the same basic characteristics, they are highly variable features Types of Convergent Boundaries

• Oceanic-Oceanic Convergence– Denser-older of 2 oceanic slabs sinks into the asthenosphere– Earthquakes under Island arc (Antilles, Philippines, Japan)– Primitive andesitic volcanoes– Seabed explosive volcanoes and “black smokers” + SMS (PNG)

• Oceanic-Continental Convergence– Denser oceanic slab sinks into the asthenosphere – Earthquakes are under the edge of the continent– Andean type arcs with granitic plutons (Coast Mountains, Sierras)– Big porphyry Cu-Mo deposits

• Continent-Continent Convergence– Subducted slab falls away– Massive continental mountain belt built with crustal melting (Himalayas,

Urals, Appalachians-Caledonides)– Massive earthquakes under “backstop” continent (Tibet, China)– Rare metal pegmatites

Convergent Plate Boundaries


Oceanic-Oceanic Convergence When two oceanic slabs converge, the older, colder, denser one descends beneath the otherOften forms volcanoes on the ocean floor (Marianas arc)If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands, Antilles, Java-Sumatra)Super-collosal explosive volcanoes as water hits magma (Krakatau)Active black smokers & Seafloor Massive Sulfides SMS (Papua New Guinea)



Oceanic-Continental ConvergenceAs the plate descends, water from the slab causes flux partial melting of mantle rockThis generates magmas having a basaltic or occasionally, andesitic compositionThese magmas can evolve in the crust to batholiths and explosive rhyolitesPorphyry Cu-Mo deposits in batholiths (BC)Shallow hydrothermal systems produce epithermal Au-Ag-Cu deposits (Chile, Peru, Mexico, BC)Mountains produced in part by volcanic activity associated with subduction of oceanic lithosphere are called continental volcanic arcs (Andes and Cascades)



Continental-Continental ConvergenceContinued subduction can bring two continents together

Less dense, buoyant continental lithosphere does not subduct

Result is a collision between two continental blocks

Process produces massive inracontinental mountains (Himalayas, Alps, Urals, Appalachians)

Thickening causes lower crust to melt making granites

Rare metal pegmatite deposits


Copyright (c) 2005 Pearson Education Canada Inc. 19-37The collision of India and Asia produced the Himalayas.


Miocene ~10Ma Modern


Transform Fault Boundaries

The third type of plate boundary

Plates slide past one another and no new lithosphere is created or destroyed

Transform Faults all take up differential motion• Most join two segments of a mid-ocean ridge as

parts of prominent linear breaks in the oceanic crust known as fracture zones

• Others join subduction zones, offset or opposed• Can also join a ridge to a subduction zone (Queen

Charlotte Fault connects Juan de Fuca Ridge to Aleutians)


Transform Faults• A few (the San Andreas Fault in California,

the Alpine Fault of New Zealand & the Anatolian Fault in Turkey) cut through continental crust

– This type makes large earthquakes ~MR>7

• Most separate ocean crust of different rates & ages (Blanco, Sovanco, Mendocino, Clipperton)

– Only the part between ridges, trenches is seismically active

Transform Fault Boundaries


Testing the Plate Tectonics Model

Plate Tectonics and Earthquakes

• Plate tectonics model accounts for the global distribution of earthquakes

– Absence of deep-focus earthquakes along the oceanic ridge is consistent with plate tectonics theory (too warm)

– Deep-focus earthquakes are closely associated with subduction zones (cold brittle zone extends into mantle)

– The pattern of earthquakes along a trench provides a method for tracking the plate's descent

– Young buoyant subducting plates have wide shallow Benioff zones & the most damaging earthquakes

Copyright (c) 2005 Pearson Education Canada Inc. 19-41Deep-focus earthquakes occur along convergent boundaries.


Copyright (c) 2005 Pearson Education Canada Inc. 19-42Earthquake foci in the vicinity of the Japan trench.


2 Subduction Zones


Evidence from Ocean Drilling• Some of the most convincing evidence confirming

seafloor spreading has come from drilling directly into ocean-floor sediment

– Age of deepest sediments in trenches (Marianas, Peru-Chile)

– Thickness of ocean-floor sediments verifies seafloor spreading (thickens & ages away from ridges)

– Oldest sediment & oldest ocean crust is distant from MOR’s

– Age of oldest/deepest sediment = age of underlying magnetic anomaly



Hot Spots • Caused by rising plumes of mantle material

– Many from bumps on Core-Mantle Boundary

• Volcanoes can form over them (Hawaiian Island chain, Azores, Yellowstone, Fiji, Iceland)

• Most mantle plumes are long-lived structures and at least some originate at great depth, perhaps at the mantle-core boundary

• Isotopes of great “growth ages” from recycled ancient crust carried into lower mantle

• Fixed reference frame for plate motions (Pacific, Yellowstone)


Copyright (c) 2005 Pearson Education Canada Inc. 19-45The Hawaiian Islands have formed over a stationary hot spot.



Measuring Plate Motion

Currently possible with space-age technology to directly measure relative motion between plates

Two methods used are VLBI-Very Long Baseline Interferometry & GPS-Global Positioning System

Calculations show that Hawaii is moving NW and approaching Japan at 8.3 cm/year

Yellowstone shows SW motion of North America since Miocene

North America and Europe are getting 5 cm further apart per year (50 km per Ma)


The Driving MechanismNo one driving mechanism accounts for all major motions & forces in plate tectonics. Researchers agree that convective flow in the rocky 2900 kilometre-thick mantle is the basic driving force of plate tectonics

Early ideas were that convection was passive and slower than plate motionMost now believe the Mantle moves faster than the Plates

MOR’s are thermal bulges (high spots) in Upper Mantle while Trenches are (low spots)

Modern researchers believe that Lithosphere Plates slide downhill over the weak Asthenosphere

Early ideas for mechanisms generate forces that contribute to plate motion

Slab-pull (but rocks tensile strengths are weak)Ridge-push (but rocks are weak in compression & no folds or thrusts on MOR system)


Models of Plate-Mantle Convection • Any model describing mantle convection must

explain why basalts erupt along the oceanic ridge• All of the ideas are constrained by seismic

information about rock properties in the Earth’s interior: fast, slow, strong, weak, cold, hot…

• Models – Layering at 660 kilometres

– Whole-mantle convection

– Deep-layer model

The Driving Mechanism


Importance of Plate TectonicsTheory provides a unified explanation of Earth’s major surface morphology & internal processes Within the framework of plate tectonics, geologists have found explanations for the geologic distribution of earthquakes, volcanoes, and mountain beltsPlate tectonics also provides explanations for past distributions of plants and animalsRegions of Seismicity, High Heat Flow are explainedDifferent ore deposits, rock types & rare minerals are explained


End of Plate Tectonics as a

Background for much of the rest of

the course!Chapter 19

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Copyright (c) 2005 Pearson Education Canada, Inc.19-1 PowerPoint Presentation Stan Hatfield....

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