CHAPTER 2 Plate Tectonics and the Ocean Floor

Fig. 2-32

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Chapter Overview

Much evidence supports plate tectonics theory.

Different plate boundaries have different features.

Tectonic plates continue to move today.

Plate tectonics or the new global geology

Thin, rigid blocks move horizontally

Interactions of plates build major features of Earth’s crust

Fig. 2.10

Plate tectonics explains:

Global distribution of… Volcanoes

Earthquakes

Faults

Mountain belts

Features of seafloor

Evolution of continents and oceans

http://www.geo.lsa.umich.edu/~crlb/COURSES/270/plate_tectonics.jpeg

http://www.geo.lsa.umich.edu/~crlb/COURSES/270/plate_tectonics.jpeg

Continental drift

Alfred Wegener proposed “continental drift hypothesis” in 1912

One large continent – Pangaea

Surrounded by single large ocean

Panthalassa

About 200 million years ago

http://pubs.usgs.gov/publications/graphics/wegener.gif

Fig. 2.2

Evidence for continental drift

Puzzle-like fit of continents

Sir Edward Bullard fit continents at 2000m water depth (1965)

Better fit than Wegener’s using coastlines

Fig. 2.3

Evidence for continental drift Wegener matched

sequences of rocks and mountain chains

Similar age, rock types, structures

http://geology.csupomona.edu/drjessey/class/Gsc101

http://www.lee.edu/~cguldenzopf/Images/Historical/Historicalmw32.jpg

Wegener noted glacial ages and other climate evidence

Ancient glaciation in modern tropical regions

Direction of glacial flow

Evidence for continental drift

Distribution of organisms

Same land plants and animals distributed in different continents (e.g., South America and Africa)

Evidence for continental drift

Similar fossils on separate southern continents

http://jove.geol.niu.edu/faculty/fischer/105_info/105_E_notes

Fig. 2.6

Similar fossils on separate southern continents

http://pubs.usgs.gov/publications/graphics

Objections to Wegener’s continental drift hypothesis

Continents cannot “plow” through ocean crust

Wegener proposed a celestial mechanism for moving plates

Celestial mechanism – gravitational forces associated with tides too small

Incorrect so hypothesis lost favor in 1930’s

More data were needed collected using new technologies

Sonar for mapping ocean bottom

Measurements of Earth’s magnetism

Echo-sounding ocean ridges match

continental margins

Radiometric dating of bottom cores

ocean crust < 200 my; cont. > 3.9 by

http://oceanexplorer.noaa.gov/history/quotes/tech/media

Evidence for plate tectonics

http://www.fau.edu/divdept/physics/jordanrg/LLS

Evidence for plate tectonics

Paleomagnetism

Study of Earth’s ancient magnetic field… interprets where rocks first formed

N or S magnetic alignment of magnetite particles when rock hardens

Magnetic inclination (magnetic dip)

Latitude

Evidence for plate tectonics

Magnetic inclination (magnetic dip)

Related to latitude

Dip is solidified as rock hardens

If rocks move on plates, the latitude of origin is reflected in the rock’s magnetic dip

Apparent polar wandering

When rocks from different continents were aligned as Pangea, polar wandering curves matched

shows a single stationary pole, but moving plates

Magnetic polarity reversals

Reversals aged by radiometric dating

About 1000 reversals recorded over last 76 my (once per 200K years)

Evidence to support sea floor spreading

Parallel magnetic anomalies record changes in Earth’s

magnetic polarity as sea floor created

Age of ocean floor increases away from crest of mid-ocean ridge

Fig. 2.11

Sea floor spreading

Harry Hess (1962) Depth recordings show sea floor features Proposed Mantle convection cells as driving

mechanism Mid-ocean ridge site of new ocean crust – spreading

centers Oceanic trench site of crust destruction (subduction

zones)

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Sea Floor Spreading Evidence

Sea floor stripes record Earth’s magnetic polarity

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Age of Ocean Floor

Late 1960s deep-sea drilling, plus radiometric dating of ocean rocks

Symmetric pattern of age distribution about mid-ocean ridges

Oldest ocean floor only 180 million years old

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Age of Ocean Floor

Evidence to support sea floor spreading

Heat flow is highest at crest of mid-ocean ridge

Most large earthquakes occur along plate margins

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Global Distribution of Earthquakes

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Global Plate Boundaries

Plate tectonics theory

Lithospheric plates “float” on the asthenosphere

Two major tectonic forces

Slab pull – gravity acting on upper, connected slab

Slab suction – created as lower slab descends

Large scale geologic features occur at plate boundaries

http://www.spacedaily.com/news/tectonics-02k.html

Three types of plate boundaries

Convergent

Divergent

Transform

Plates move apart

Mid-ocean ridge

Rift valley

New ocean floor created

Shallow earthquakes

Divergent boundary features

Fig. 2.15

Fig. 2.17

Divergent boundary features

Three types of plate boundaries

Types of spreading centers

Oceanic rise

Fast-spreading

Gentle slopes

Oceanic ridge

Slow-spreading

Steep slopes

Ultra-slow

Deep rift valley

Widely scattered volcanoes

Convergent boundary features

Plates move toward each other

Oceanic crust destroyed

Ocean trench

Volcanic arc

Deep earthquakes

Three types of convergent boundaries Oceanic-continental

convergence

Oceanic-oceanic convergence

Continental-continental

convergence

Oceanic-continental convergence

Ocean plate subducted

Continental arc

Oceanic trench

Deep earthquakes

Explosive andesitic volcanic eruptions

Types of convergent boundaries

Fig. 2.21a,b

Oceanic-oceanic convergence

Older, denser oceanic plate subducted below another younger oceanic plate

Island arc or archepeligo

Deep oceanic trench

Deep earthquakes

Types of convergent boundaries

Types of convergent boundaries

Continental-continental convergence

No subduction

Uplifted mountain ranges

Deep earthquakes

Himilayans – Continental-Continental Convergence

Transform boundary features

Offsets oriented perpendicular to mid-ocean ridge

Segments of plates slide past each other

Offsets permit mid-ocean ridge to move apart at different rates

Shallow but strong earthquakes

Types of transform faults

Oceanic—wholly in ocean floor

Continental—extends from mid-ocean ridge across continent

Three types of plate boundaries

Applications of plate tectonics model to intraplate features

Mantle plumes and hotspots Volcanic islands within a plate Island chains Systematic variation of age

Record ancient plate motions

Hawaiian Islands & Emperor Seamount Chain

Fig. 2.25

Applications of plate tectonics model to intraplate features

Seamounts and tablemounts

Subsidence of flanks of mid-ocean ridge

Wave erosion may flatten seamount

Hot Spots & Coral Reefs

Coral Reef Formation

Coral reefs associated with subsiding seafloor Fringing

Barrier

Atoll

Coral reef development

Fig. 2.27

Measuring plate motion by satellites

Fig. 2.30

Paleogeography

Study of ancient continents

Continental accretion

Continental material added to edges of continents through plate motion

Continental separation or rifting

Continents move apart

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Paleogeographic Reconstructions

Future predictions

Future positions of continents and oceans

Assume same direction and rate of plate motions as now

World map 50 million years from now

Fig. 2.32

Wilson cycle

John Tuzo Wilson

Life cycle of ocean basins

Formation

Growth

Destruction

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Wilson cycle

End of CHAPTER 2 Plate Tectonics and the Ocean Floor