Plate TectonicsHorizontal Movement of

Earth’s Lithosphere

Additional Reading: USGS Plate Tectonics (Pdf) – class web site

Learning Objectives 

Understand the processes that are continuously changing Earth’s surface as lithospheric plates move relative to one another. Identify the role of oceanic ridges, transform faults and deep-sea trenches in defining the edges of lithospheric plates. Explain the distribution of magnetic anomaly stripes, seismicity, and volcanism in terms of the concept of global plate tectonics. Calculate spreading rates of ocean basins.

Plate Tectonics

1. The Theory of Plate Tectonics

2. Plate Boundariesa) Spreading Centersb) Subduction Zonesc) Transform Faults

3. Plate Movement

“Continental Drift” - theory* proposed by Alfred Wagner, a German meteorologist (1915)

* Not accepted by scientific community- no mechanism to explain plate movement

The Theory of Plate Tectonics

Explained by:• geologic fit• fossils

Plate Tectonicshttp://pubs.usgs.gov/gip/dynamic/dynamic.html

Plate Tectonics - evidence for theory of continental

drift by Hess, Heezen and Tharp (1960’s) found lithospheres plate boundaries that can be 3 types: 1) ridges (spreading centers) 2) trenches (subduction zones) 3) transform faults (plates sliding past one another)

The Theory of Plate Tectonics (cont’d.)

major plates:

1. Pacific – 105 x106 km2

2. Eurasian - 70 x106 km23. Antarctic - 60 x106 km24. Australian - 45 x106 km25. S. American - 45 x106 km26. African - 80 x106 km27. N. American - 60 x106 km2

Lithospheric Plates

minor plates:

1. Cocos - 5 x106 km2

2. Phillipine - 6 x106 km2

3. Caribbean - 5 x106 km2

4. Nazca - 15 x106 km2

5. Arabian - 8 x106 km2

6. Indian - 10 x106 km2

7. Scotia - 5 x106 km2

8. Juan de Fuca - 2 x106 km2

The Theory of Plate Tectonics (cont’d.)

the Pacific Ring of Fire IS 40,000 km long chain of volcanoes caused by "convergent tectonic plates" coming together;

1) Convection cells form• Density differences – cool vs. hot

2) Convection cells cause frictional drag on lithosphere3) Lithosphere stretches due to convective movement4) Lithospheric crust weakens

Plate Boundaries

a) Spreading centers - ‘rift zones’ (cont’d.)

5) Faulting – break in overlying lithosphere6) Magma flows upward7) New lithospheric crust formed

Plate Boundaries (cont’d.)

a) Spreading centers - ‘rift zones’ (cont’d.)

Creating new ocean crust

• Plates split apart -‘divergent plate’ boundary• New crust formed - ‘constructive’ plate

boundary

a) Spreading centers - ‘rift zones’ (cont’d.)

Evolution of a mid-ocean ridge system1. Upwarping2. Rift valley3. Linear sea4. Mid-ocean ridge system

Plate Boundaries (cont’d.)

• Lithospheric Plates collide - ‘convergent’ plate boundary

• Crust destroyed - ‘destructive’ plate boundary• Forms trenches and mountains

Plate Boundaries (cont’d.)

b) Subduction zones

3 types of subduction zones:

1. Ocean crust into continental crust – form trenches and mountain ranges

Ex. a): Juan de Fuca plate into the N. American plate - forms Cascade Mtn. RangeEx. b): Nazca plate into the S. American plate - forms Peru-Chile Trench and

the Andes Mtn. Range

Plate Boundaries (cont’d.)

b) Subduction zones (cont’d.)

b) Subduction zones (cont’d.)

2. Ocean crust into ocean crust – forms trenches and island arcs

Ex. A): Philippine plate into the Pacific plate – formed the Marianna Trenchand the Marianna Island Arc system

Ex. B): N. American plate into the Caribbean plate and then the N. American plate into the S. American plate – formed the Isthmus of Panama

Plate Boundaries (cont’d.)

3. Continental crust into continental crust – form mountain ranges

Ex. A): Indian plate into the Eurasian plate – formed the HimalayasEx. B): Eurasian plate into the African plate - closing up of the

Mediterranean sea

Plate Boundaries (cont’d.)

b) Subduction zones (cont’d.)

• Plates slide past one another• Lithospheric crust neither created nor destroyed - ‘conservative’ plate

boundary

Ex. A) Pacific plate sliding past N. American plate – forms the San Andreas Fault

c) Transform faults

Plate Boundaries (cont’d.)

Type of boundary between plates:

Constructive margins Midocean ridges

Destructive margins Subduction zones

Conservative margins Transform faults

• New crust is created at spreading centers at a rate of approximately 1-10cm per year

• Old crust is destroyed at the same rate at subduction zones

How do we know these rates? (Rate=distance/time)

Plate Movement

Creating new ocean crust

Driving Mechanisms for Plate Motions

Destructive margins Subduction zones

Constructive margins Midocean ridges

SUMMARY

• Magnetic anomalies in ocean crust...look at spreading centers paleomagnetism every so often Earth’s magnetic field flips (every 300K-500K years)

Plate Movement (cont’d.)

magnetic signal recorded in crust at spreading center as it’s formed, forms bands of crust with either a weak or strong magnetic signal

determine rate of plate movement by distance of band from spreading center divided by age of rock in band (r=d/t)

More evidence of plate moving..

Hot spots Islands of Hawaii islands or sea mountains formed over hotspots (fixed area where magma comes up)

lithosphere moves over hotspot and end up have volcanic mountain over hotspot as well as a series of mountains in ‘front’ of hotspot

determine rate of plate movement by distance of mountain from hotspot divided by age of rock in mountain (r=d/t)

Plate Movement (cont’d.)

Age of Ocean Crusthttp://www.ngdc.noaa.gov/mgg/geology/geology.html

Creating new ocean crust

Oceanic crust moves away from MOR (Mid Oceanic Ridge) and cools and subsides

Driving Mechanisms for Plate Motions

Destructive margins Subduction zones

Constructive margins Midocean ridges

the Pacific Ring of Fire IS 40,000 km long chain of volcanoes caused by "convergent tectonic plates" coming together;

Type of boundary between plates:

Constructive margins Mid ocean ridges

Destructive margins Subduction zones

Conservative margins Transform faults

Conservative marginsTransform faults

The San Andreas fault in southern California

Conservative marginsTransform faults

Hot Spots?

• Mantle plumes originate deep within the asthenosphere as molten rock which rises and melts through the lithospheric plate forming a large volcanic mass at a “hot spot”.

Mantle Plume

Coral Reefs

Air view

Spreading rates

Geological Periods

Precambrian 4.6 B - 570 Ma solidificationCambrian 514 Ma Gondwana, hard shell anim.Ordovician 458 Ma separation, coldestSilurian 425 Ma Laurentia collides with BalticaDevonian 390 Ma pre-Pangea, equatorial forestsEarly Carboniferous 356 MaLate Carboniferous 306 Ma western Pangea is completePermian 255 Ma deserts, reptiles, major ext.Triassic 237 Ma Life begins to rediversify,PangeaJurassic 195 Ma Dinosaurs, Pangea starts to breakLate Jurassic 152 Ma Pangea rifts apart, AtlanticCretaceous 94 Ma New oceans, IndiaK/T extinction 66 Ma end of dinosaursEocene 50.2 Ma India collides with AsiaMiocene 14 Ma Modern look

Future World +50 Ma N. Atlantic widens, Med. vanishFuture +100 Ma new subductionFuture +250 Ma new Pangea

Geological Periods

Precambrianbreak-up of the supercontinent, Rodinia, which formed 1100 million years ago.   The Late Precambrian was  an "Ice House" World, much like the present-day.

Source: www.scotese.com

CambrianAnimals with hard-shells appeared in great numbers for the first time during the Cambrian.  The continents were flooded by shallow seas.  The supercontinent of Gondwana had just formed and was located near the South Pole.

OrdovicianDuring the Ordovician ancient oceans separated the barren continents of Laurentia, Baltica, Siberia and Gondwana.  The end of the Ordovician was one of the coldest times in Earth history.  Ice covered much of the southern region of Gondwana.

SilurianLaurentia collides with Baltica closing the northen branch of the Iapetus Ocean and forming the "Old Red Sandstone" continent.  Coral reefs expand and land plants begin to colonize the barren continents.

DevonianBy the Devonian the early Paleozoic oceans were closing, forming a "pre-Pangea".  Freshwater fish were able to migrate from the southern hemisphere continents to North America and Europe.  Forests grew for the first time in the equatorial regions of Artic Canada.

Early CarboniferousDuring the Early Carboniferous the Paleozoic oceans between Euramerica and Gondwana began to close, forming the Appalachian and Variscan mountains.   An ice cap grew at the South Pole as four-legged vertebrates evolved in the coal swamps near the Equator.

Late CarboniferousBy the Late Carboniferous the continents that make up modern North America and Europe had collided with the southern continents of Gondwana to form  the western half of Pangea.  Ice covered much of the southern hemisphere and vast coal swamps formed along the equator.

PermianVast  deserts covered western Pangea during the Permian as reptiles spread across the face of the supercontinent.

TriassicThe supercontinent of Pangea, mostly assembled by the Triassic, allowed land animals to migrate from the South Pole to the North Pole; and warm-water faunas spread across Tethys. The first mammals and dinosaurs appeared;

JurassicBy the Early Jurassic, south-central Asia had assembled.  A wide Tethys ocean separated the northern continents from Gondwana. 

Subduction zone Rocky Mountains

Formation of the Rocky Mountainshttp://wrgis.wr.usgs.gov/docs/parks/province/rockymtn.html

Late JurassicIn the Late Jurassic the Central Atlantic Ocean was a narrow ocean separating Africa from eastern North America. 

CretaceousDuring the Cretaceous the South Atlantic Ocean opened.  India separated from Madagascar and raced northward on a collision course with Eurasia. Notice that North America was connected to Europe, and that Australia was still joined to Antarctica.

Dinosaur extinctionThe bull's eye marks the location of impact site of a 10 mile wide comet caused global climate changes that killed the dinosaurs and many other forms of life.  By the Late Cretaceous the oceans had widened, and India approached the southern margin of Asia.

Eocene50 - 55 million  years ago India began to collide with Asia forming the Tibetan plateau and Himalayas (destroying the last of Tethys ocean).  Australia, which was attached to Antarctica, began to move rapidly northward.

Collision of continental crust – formation of Himalayas

• Whereas oceanic ridges indicate tension, continental mountains indicate compression forces are squeezing the land together.

Sedimentary Rocks Squeezed by Compression

Miocene20 million years ago, Antarctica was covered by ice and the northern continents were cooling rapidly.  The world has taken on a "modern" look, but notice that Florida and parts of Asia were flooded by the sea. Arabia moved away from Africa forming Gulf of Aden and Red Sea;

Last Ice AgeWhen the Earth is in its "Ice House" climate mode, there is ice at the poles.  The last expansion of the polar ice sheets took place about 18,000 years ago. 

Modern World

If we continue present-day plate motions the Atlantic will widen, Africa will collide with Europe closing the Mediterranean, Australia will collide with S.E. Asia, and California will slide northward up the coast to Alaska.

Future +100

Earth is ~ 4.6 bill years old – suggested cyclic of 500 mill year pattern of assembling and disassembling the land masses;

Future +250

The Wilson Cycle

uses plate tectonic processes to show development and creation of ocean floor and ocean

basins;

The Wilson Cycle