Chapter Plate Tectonics - jkaser.com 9 Plate... · Chapter Plate Tectonics. ... 9.2 Plate Tectonics...

Chapter

Plate Tectonics

An Idea Before Its Time

9.1 Continental Drift

Wegener’s continental drifthypothesis stated that thecontinents had once been joinedcontinents had once been joinedto form a single supercontinent.

• Wegener proposed that thesupercontinent, Pangaea, began tobreak apart 200 million years agoand form the present landmasses.

Breakupof

PangaeaPangaea

An Idea Before Its Time

9.1 Continental Drift

Evidence• The Continental Puzzle -matching shorelinesmatching shorelines

• Matching Fossils- Fossil evidence for continental

drift includes several fossilorganisms found on differentlandmasses.

9.1 Continental Drift

Evidence

• Rock Types and Structures

- Rock evidence for continentalexists in the form of severalexists in the form of severalmountain belts that end at onecoastline, only to reappear on alandmass across the ocean.

9.1 Continental Drift

Evidence

• Ancient Climates

- glacial deposits were found in- glacial deposits were found inlarge areas of the SouthernHemisphere- large tropical swamps in theNorthern Hemisphere

GlacierEvidence

Matching MountainRanges

Rejecting the Hypothesis

9.1 Continental Drift

A New Theory Emerges• Wegener could not provide anexplanation of exactly what made

• Wegener could not provide anexplanation of exactly what madethe continents move. Newtechnology lead to findings whichthen lead to a new theory calledplate tectonics.

Earth’s Major Roles

9.2 Plate Tectonics

According to the plate tectonicstheory, the uppermost mantle, alongwith the overlying crust, behaves as awith the overlying crust, behaves as astrong, rigid layer. This layer is knownas the lithosphere.• A plate is one of numerous rigidsections of the lithosphere that moveas a unit over the material of theasthenosphere.

9.2 Plate Tectonics

Divergent boundaries (also calledspreading centers) are the place wheretwo plates move apart. - creates newseafloor

Convergent boundaries form where

Types of Plate Boundaries

Convergent boundaries form wheretwo plates move together. - merges twoplates

Transform fault boundaries aremargins where two plates grind pasteach other without the production ordestruction of the lithosphere. - SanAndreas Fault in CA

Three Types ofPlate Boundaries

Tectonic Plates

Tectonic Plates

Divergent Boundaries

9.3 Actions at Plate Boundaries

Oceanic Ridges and SeafloorSpreading

• Oceanic ridges are continuouselevated zones on the floor of allmajor ocean basins. The rifts at thecrest of ridges represent divergentplate boundaries.

9.3 Actions at Plate Boundaries

Oceanic Ridges and SeafloorSpreading

• Rift valleys are deep faultedstructures found along the axes ofstructures found along the axes ofdivergent plate boundaries. Theycan develop on the seafloor or onland.

• Seafloor spreading produces newoceanic lithosphere.

SpreadingCenter

Divergent Boundaries

9.3 Actions at Plate Boundaries

Continental Rifts• When spreading centers developwithin a continent, the landmass

• When spreading centers developwithin a continent, the landmassmay split into two or more smallersegments, forming a rift.

East African Rift Valley

Convergent Boundaries

9.3 Actions at Plate Boundaries

A subduction zone occurs whenone oceanic plate is forced downone oceanic plate is forced downinto the mantle beneath a secondplate.

9.3 Actions at Plate Boundaries

• Denser oceanic slab sinks into theasthenosphere.

Oceanic-Continental

• Pockets of magma develop and rise.

Convergent Boundaries

• Pockets of magma develop and rise.

• Continental volcanic arcs form inpart by volcanic activity caused bythe subduction of oceaniclithosphere beneath a continent.

• Examples include the Andes,Cascades, and the Sierra Nevadas.

Oceanic-ContinentalConvergent Boundary

9.3 Actions at Plate Boundaries

• Two oceanic slabs converge andone descends beneath the other.

Oceanic-Oceanic

• This kind of boundary often forms

Convergent Boundaries

• This kind of boundary often formsvolcanoes on the ocean floor.

• Volcanic island arcs form asvolcanoes emerge from the sea.

• Examples include the Aleutian,Mariana, and Tonga islands.

Oceanic-OceanicConvergent Boundary

Convergent Boundaries

9.3 Actions at Plate Boundaries

• When subducting plates contain

Continental-Continental

• When subducting plates containcontinental material, two continentscollide.

• This kind of boundary can producenew mountain ranges, such as theHimalayas.

Continental-Continental Convergent Boundary

Collision of India and Asia

9.3 Actions at Plate Boundaries

At a transform fault boundary, platesgrind past each other withoutdestroying the lithosphere.

Transform faults

Transform Fault Boundaries

Transform faults• Most join two segments of a mid-ocean ridge.

• At the time of formation, theyroughly parallel the direction of platemovement.

• They aid the movement of oceaniccrustal material.

Transform Fault Boundary

Evidence for Plate Tectonics

9.4 Testing Plate Tectonics

Paleomagnetism is the naturalremnant magnetism in rockbodies; this permanentremnant magnetism in rockbodies; this permanentmagnetization acquired by rockcan be used to determine thelocation of the magnetic poles atthe time the rock becamemagnetized.

Evidence for Plate Tectonics

9.4 Testing Plate Tectonics

•Normal polarity—when rocks showthe same magnetism as the presentmagnetism fieldmagnetism field

•Reverse polarity—when rocks showthe opposite magnetism as thepresent magnetism field

Paleomagnetism Preservedin Lava Flows

Evidence for Plate Tectonics

9.4 Testing Plate Tectonics

The discovery of strips ofalternating polarity, which lie asalternating polarity, which lie asmirror images across the oceanridges, is among the strongestevidence of seafloor spreading.

Polarity of the Ocean Crust

Evidence for Plate Tectonics

9.4 Testing Plate Tectonics

Earthquake Patterns

•Scientists found a close linkbetween deep-focus earthquakesbetween deep-focus earthquakesand ocean trenches.

•The absence of deep-focusearthquakes along the oceanicridge system was shown to beconsistent with the new theory.

9.4 Testing Plate Tectonics

Evidence for Plate Tectonics

9.4 Testing Plate Tectonics

Ocean Drilling•The data on the ages of seafloorsediment confirmed what thesediment confirmed what theseafloor spreading hypothesispredicted.

•The youngest oceanic crust is atthe ridge crest, and the oldestoceanic crust is at the continentalmargins.

9.4 Testing Plate Tectonics

Hot Spots•A hot spot is a concentration ofheat in the mantle capable ofproducing magma, which rises to

Evidence for Plate Tectonics

producing magma, which rises toEarth’s surface; The Pacific platemoves over a hot spot, producingthe Hawaiian Islands.

•Hot spot evidence supports that theplates move over the Earth’ssurface.

Hot Spot

Causes of Plate Motion

9.5 Mechanisms of Plate Motion

Scientists generally agree thatconvection occurring in the mantleis the basic driving force for plateis the basic driving force for platemovement.

•Convective flow is the motion ofmatter resulting from changes intemperature.

9.5 Mechanisms of Plate Motion

Slab-Pull

•Slab-pull is a mechanism thatcontributes to plate motion in whichcool, dense oceanic crust sinks into

Causes of Plate Motion

cool, dense oceanic crust sinks intothe mantle and “pulls” the trailinglithosphere along. It is thought tobe the primary downward arm ofconvective flow in the mantle.

9.5 Mechanisms of Plate Motion

Ridge-Push

•Ridge-push causes oceaniclithosphere to slide down the sidesof the oceanic ridge under the pull

Causes of Plate Motion

of the oceanic ridge under the pullof gravity. It may contribute to platemotion.

9.5 Mechanisms of Plate Motion

Mantle Convection•Mantle plumes are masses ofhotter-than-normal mantle materialthat ascend toward the surface,where they may lead to igneous

Causes of Plate Motion

•The unequal distribution of heatwithin Earth causes the thermalconvection in the mantle thatultimately drives plate motion.

that ascend toward the surface,where they may lead to igneousactivity.

Mantle Convection Models