© 2011 Pearson
Education, Inc.
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
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
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
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
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
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
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)
© 2011 Pearson
Education, Inc.
Sea Floor Spreading Evidence
Sea floor stripes record Earth’s magnetic polarity
© 2011 Pearson
Education, Inc.
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
Evidence to support sea floor spreading
Heat flow is highest at crest of mid-ocean ridge
Most large earthquakes occur along plate margins
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
Plates move apart
Mid-ocean ridge
Rift valley
New ocean floor created
Shallow earthquakes
Divergent boundary features
Fig. 2.15
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
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
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
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
Applications of plate tectonics model to intraplate features
Seamounts and tablemounts
Subsidence of flanks of mid-ocean ridge
Wave erosion may flatten seamount
Paleogeography
Study of ancient continents
Continental accretion
Continental material added to edges of continents through plate motion
Continental separation or rifting
Continents move apart
Future predictions
Future positions of continents and oceans
Assume same direction and rate of plate motions as now