lecture outlines physical geology, 15/esubduction.rocks/storage for lecture notes/300 spr... ·...

Lecture Outlines

Physical Geology, 15/e

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Plummer, Carlson & Hammersley

Plate Tectonics: The UnifyingTheory

Physical Geology 15/e, Chapter 19


Plate Tectonics – Earth’s surface is composed of a few large,thick plates that move slowly and change in size

• Plate boundaries – plates move away, toward, or past eachother through intense geologic activity

• Developed from the continental drift and seafloor spreadinghypotheses


Plate Tectonics

Puzzle-piece fit of coastlines of Africa and

South America has long been known

Alfred Wegener – noted South America,Africa, India, Antarctica, and Australia havealmost identical late Paleozoic rocks andfossils in early 1900s

• Glossopteris (plant), Lystrosaurus andCynognathus (animals) fossils found on all fivecontinents

• Mesosaurus (reptile) fossils found in Brazil andSouth Africa only


The Early Case for Continental Drift

Pangaea – supercontinent proposed byWegener

• Laurasia - northern supercontinent containingNorth America and Asia (excluding India)

• Gondwanaland - southern supercontinentcontaining South America, Africa, India,Antarctica, and Australia

Late Paleozoic

• Glaciation patterns were evident on thesouthern continents (Gondwanaland)

• Coal beds deposited in the northern continentsfrom swampy, probably warm environments(Laurasia)



Paleoclimate belts – suggestedpolar wandering as potentialevidence for continental drift.Polar Wandering – the apparentmovement of the poles

The evidence for this can be explained inthree ways:

1).The continents remainedmotionless and the poles actuallymoved (polar wandering)

2). The poles did not move and thecontinents moved (continentaldrift)

3). Both occurred



Skepticism About Continental Drift• Land Bridges – could explain the distribution of land-dwelling reptiles on

scattered continents• Winds or Oceans currents – could explain distribution of fossil plants on

separate continents• Polar Wandering – could be explained by moving poles rather than moving

continents• Mechanism – Wegener’s proposed mechanism was not accepted by most

geologists in the northern hemisphere• His idea that the continents plowed through the oceans propelled by centrifugal

forces from the Earth’s rotation and gravitational forces that cause the tides wereproven to be insufficient



Evidence from Paleomagnetism

• Paleomagnetism – the study of ancientmagnetic fields

• The mineral magnetite becomesmagnetized in cooling lava once itstemperature drops below the Curie Point

• Uses mineral magnetic properties todetermine direction and distance to themagnetic pole when rocks formed

• Steeper dip angles indicate rocks formedcloser to the magnetic poles

• Rocks with increasing age point to polelocations increasingly far from presentmagnetic pole positions


The Revival of Continental Drift

Evidence From Paleomagnetism

Apparent polar wander curves for differentcontinents suggest real movement relative toone another

Permian rocks in every continent show adifferent pole position which seems highlyunlikely

By reconstructing their locations to formPangaea, the polar wandering paths arenearly identical indicating that the continentswere once joined together.



Geologic Evidence for Continental Drift

Fitting of Continents

• Redefined the edge of each continent as the middle of thecontinental slope greatly improved the fit

• Isotopic ages, glacial striations, rock types, structure andsequence match

History of Continental Positions

• Rock matches show when the continents were together

• Paleomagnetic data indicate the direction and rate ofmovement

• Pangaea split apart 200 million years ago but the continentshave been in motion for much longer (2-4 billion years)



Seafloor spreading – the concept

that the sea floor is moving like aconveyor belt away from the crest ofthe mid-oceanic ridge until itdisappears by plunging beneath acontinent or island arc.

Proposed in 1962 by Harry Hess

Hess’s Driving Force

Deep Mantle Convection – circulationpattern driven by rising of hot material(hot mantle rock) and/or the sinking ofcold material (oceanic crust)


Seafloor Spreading

Explanations

The Mid-Oceanic Ridge

• Hot mantle rocks rises beneath

• Decompression melting occurs

• Circulation pattern diverges moving rock away from the ridge

• Rift valley forms due to tensional forces at the ridge crest

Oceanic Trenches

• Rock has cooled and become denser

• Crust Sinks beneath a continent or island arc back into the mantle

Age of the Sea Floor

• Overall young age for sea floor rocks (everywhere <200 million years) isexplained by this model


Seafloor Spreading

Tectonic plates – composed of the relatively rigid lithosphere• Lithospheric thickness and age of seafloor increase with distance from mid-

oceanic ridge• Float upon ductile asthenosphere• Interact at their boundaries

• Divergent boundaries – plates move apart• Convergent boundaries – plates move together• Transform boundaries – plates slide past one another


Plates and Plate Motion

Marine magnetic anomalies –alternating positive and negativemagnetic anomalies that form a stripe-like pattern parallel to the mid-oceanicridges

The Vine-Matthews Hypothesis•New basaltic magma continually extrudes atthe ridge crest and cools to record the earth’smagnetism including magnetic field reversals

•Matches pattern of reversals seen incontinental rocks which allows us to measurethe rate of movement and to predict the ageof the sea floor


How Do We Know That Plates Move?

Marine Magnetic Anomalies

Measuring the Rate of Motion

• Compare to known magnetic reversals from lava flows on land

• Rate of plate motion – equals distance from ridge divided by age of rocks

Predicting Seafloor Age

• Seafloor age increases with distance from mid-oceanic ridge

• Symmetric age pattern reflects plate motion away from ridge



Another Test: Fracture Zones and Transform Faults

Mid-oceanic ridges – offset along fracture zones

• transform fault – fracture zone segment between offset ridge crests

• relative motion along fault is result of seafloor spreading from adjacent ridges

Measuring Plate Motion Directly

Plate motion can be measured using satellites, radar, lasers and globalpositioning systems



Divergent plate boundaries –

plates move away from each other• can occur in the middle of the

ocean or within a continent

• marked by rifting, basalticvolcanism, and eventual ridgeuplift

• eventually creates a new oceanbasin


Divergent Plate Boundaries

Transform plate boundaries–plates slide horizontally past oneanother

Transform faults may connect:

• two offset segments of mid-oceanicridge

• a mid-oceanic ridge and a trench

• two trenches

• transform offsets of mid-oceanic ridges allow series ofstraight-line segments toapproximate curvedboundaries required byspheroidal Earth


Transform Plate Boundaries

Convergent plate boundaries – plates move towardone another• Ocean-ocean plate convergence – marked by ocean trench,

Benioff zone, and volcanic island arc

• Ocean-continent plate convergence – marked by ocean trench,Benioff zone, volcanic arc, and mountain belt

• Continent-continent plate convergence – marked by mountainbelts and thrust faults


Convergent Plate Boundaries

Plate boundaries – can move over time

• Mid-oceanic ridge crests can migrate toward or away from subductionzones or abruptly jump to new positions

• Convergent boundaries can migrate if subduction angle steepens oroverlying plate has a trenchward motion of its own

• Transform boundaries can shift as slivers of plate shear off


Do Plate Boundaries Move?

Plates can change size over time.

• North American plate is increasing in size• New sea floor is being added on the trailing edge as the Atlantic sea

floor spreads

• Most of the plate is not being subducted

• The Nazca Plate is getting smaller• Leading edge is being subducted under South America

• Trailing edge is adding sea floor but at a slower rate

The Attractiveness of Plate Tectonics• Explains distribution and composition of volcanoes,

earthquakes and mountain belts.

• Explains the major features of the sea floor


Can Plates Change Size?

Any proposed mechanism must explain why:• mid-oceanic ridges are hot and elevated, while trenches are cold

and deep• ridge crests have tensional cracks• the leading edges of some plates are subducting sea floor, while

others are continents (which cannot subduct)

Mantle convection – may be the cause or an effect ofcirculation set up by ridge-push and/or slab-pull


What Causes Plate Motions?

Ridge Push – as new plate moves away from the divergentboundary it cools and thickens and subsidesSlab Pull – cold lithosphere sinking at a steep angle throughthe hot mantle should pull the surface part of the plate awayfrom the ridge crest.Trench Suction – if subducting plates fall into the mantle atangles steeper than their dip then trenches and the overlyingplates are pulled horizontally seaward toward thesubducting plate.



Mantle Plumes and Hot Spots

• Mantle Plume - narrow columns of hot mantle rock that risethrough the mantle

• Stationary with respect to moving plates

• Large mantle plumes may spread out and tear apart the overlying plateforming a Hot Spot at the Earth’s surface (examples include Hawaii,Yellowstone and Iceland)

• flood basalt eruptions

• rifting apart of continental land masses

• New divergent boundaries may form



Mantle Plumes and Hot Spots

Mantle plume hot spots in the interior of a plate produce volcanicchains

• Orientation of the volcanic chain shows direction of plate motion overtime

• Age of volcanic rocks can be used to determine rate of plate movement

• Hawaiian Islands are a good example



Objections to Plate Tectonics

•Some seafloor objects did not seem compatible with a movingsea floor.

•The geology of many continental regions did not seem to fit thetheory

•Refinements of the theory made these feature more compatiblewith theory

The evidence for Plate Tectonics is very convincingand has changed the whole concept of Earth dynamicsin the last fifty years.


A Final Note

End of Chapter 19


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