Download Plate Tectonics

GEOL: CHAPTER 2
Plate Tectonics:
A Unifying Theory
Learning Outcomes
LO1: Review early ideas about continental
drift
LO2: Explain the evidence for continental
drift
LO3: Describe Earth’s magnetic field
LO4: Explain paleomagnetism and polar
wandering
LO5: Explain magnetic reversals and
seafloor spreading
Learning Outcomes, cont.
LO6: Explain plate tectonic theory: a unifying
theory
LO7: Identify the three types of plate
boundaries
LO8: Describe hot spots and mantle plumes
LO9: Explain plate movement and motion
Learning Outcomes, cont
LO10: Explain the driving mechanism of
plate tectonics
LO11: Recognize the role of plate tectonics
in the distribution natural resources
LO12: Recognize the role of plate tectonics
in the distribution of life
Continental Drift: Early Ideas
• Alfred Suess, late 19th C, noted similarities
on different continents of:
– Plant fossils
– Glaciation
– Rock sequences
• Gondwanaland as supercontinent
Alfred Wegener
• Continental drift hypothesis, 1915
• Pangaea break-up
• Presented geologic, paleomagnetic,
paleontologic, and climatologic evidence
• Not well received
• Alexander du Toit, 1937: proposed
supercontinent originally at South Pole
Continental Drift Evidence
• Continental fit
– South America and Africa
• Similarity of rock sequences and mountain
ranges
– Appalachian Mountains and mountains in
Europe/Greenland
Continental Drift Evidence, cont.
• Glacial evidence
– Deposits and striations
• Fossil evidence
– Same fossils on all continents
• Paleomagnetic evidence
• But no explanatory mechanism
Earth’s Magnetic Field
• Magnetism
– Spinning electrons in atoms, especially iron
– Moving electricity
• Magnetic field
– Lines of magnetic force
– North and south poles: dipolar
Earth’s Magnetic Field, cont.
• Earth as giant dipole magnet
– Magnetic field strength varies
– Magnetic poles do not align with geographic
poles
– Magnetic pole positions vary over time
Paleomagnetism
and Polar Wandering
• Paleomagnetism: remnant magnetism in
rocks – gives direction and intensity of
Earth’s field when rocks solidified
• Curie point: temperature where ironbearing minerals are magnetized
• Polar wandering: continents moved while
poles stayed relatively stationary
• Continental drift explained the data
Magnetic Reversals
• Magnetic reversals
– Magnetic field weakens and disappears
– Field reappears with pole positions reversed
• Discovered from studying paleomagnetism
South magnetic
pole (normal
position)
North magnetic
pole (normal
position)
North magnetic
pole (reversed)
South magnetic
pole (reversed)
South magnetic
pole (normal)
North magnetic
pole (normal)
North magnetic
pole (reversed)
South magnetic
pole (reversed)
Stepped Art
Fig. 2-9, p. 31
Seafloor Spreading
• Explains movement of continents
• Harry Hess, 1962
• Continents and oceanic crust on same
plate
• Thermal convection cells move plates
Seafloor Spreading, cont.
• Magnetic anomalies
– magnetic reversals explained
• Plates form at mid-ocean ridges at
divergent boundaries
• Plates are destroyed at deep-sea trenches
at convergent boundaries
Major Tectonic Plates
1.
2.
3.
4.
5.
6.
7.
Eurasian
Indian-Australian
Antarctic
North American
South American
Pacific
African
Plate Tectonics
•
•
•
•
Seven major plates
Many smaller plates
Contain both continental and oceanic crust
Vary in thickness
– Continent areas thicker than ocean areas
Plate Tectonics, cont.
• Heat transfer in asthenosphere causes
plates to move
• Separate at oceanic ridges and at some
land masses – divergent boundaries
• Collision and often subduction at
convergent boundaries
Plate Boundaries: 3 Types
1. Divergent plate boundaries
2. Convergent plate boundaries
3. Transform plate boundaries
Divergent Boundaries
• Mantle magma rises to surface
• New oceanic lithosphere forms: basaltic
• Magnetic orientation recorded as magma
cools
• Oceanic ridges most common: seafloor
spreading
Divergent Boundaries, cont.
• Continental rifting
– East African Rift Valley
– Red Sea
– Can create new ocean basins
Volcanic activity
Magma
Continental crust
Rift valley
Coastal mountain
range
Narrow fault-bounded sea
Continental “seaboard”
(coastal mountains gone)
Wide ocean
Stepped Art
Fig. 2-12, p. 35
Convergent Boundaries
1.
2.
3.
•
Oceanic-oceanic boundaries
Oceanic-continental boundaries
Continental-continental boundaries
Benioff zone: dipping plane of
earthquake foci
Oceanic-Oceanic Boundaries
•
•
•
•
•
Trench forms
Subduction complex
Volcanic island arcs
Back-arc basin
Aleutian Islands, Japan, Philippines
Oceanic-Continental Boundaries
• Oceanic plate subducts under continental
plate
• Continental volcanic arcs from rising
magma
• Plutons cool underground
• Andes Mountains in South America
• Cascade Range in western North America
Continental-Continental
Boundaries
• Begins as oceanic-continental
convergence
• Continents eventually collide
– No subduction
• Continents are welded together
• Himalayas
Transform Boundaries
•
•
•
•
•
•
Plates slide laterally past each other
Lithosphere neither created nor destroyed
Seafloor fractures
Transform faults
Earthquakes
San Andreas fault
Hot Spots and Mantle Plumes
• Intraplate: oceanic and continental
• Mantle plume: column of magma from
deep within the mantle
• Hot spot: mantle plume magma breaks
through the surface
• Form volcanoes
• Plates move over them
• Island chains: Hawaiian Islands
Plate Movement
•
•
•
•
•
Date magnetic anomalies in seafloor crust
Helps determine past plate positions
Works well only for oceanic crust
Can also use satellite laser data
Good data back to Pangaea
Plate Tectonics Driving Mechanism
•
•
Heat from Earth’s interior
Thermal convection cells, two hypotheses
1. Asthenosphere only
2. Entire mantle
•
•
Slab-pull: subducting colder slab pulls rest
of plate down into asthenosphere
Ridge-push: gravity pushes plate from
higher ocean ridges to trenches
Plate Tectonics
and Natural Resources
• Plate tectonics helps explain distribution of
petroleum and minerals
• Cooling magma from subduction zone can
concentrate metallic ores
• Copper deposits
Plate Tectonics
and Distribution of Life
• Life distribution depends on climate
• Climate depends on:
– Geographical position
– Geographic barriers
– Prevailing winds
– Ocean currents
• Plate movements and hot spot volcanoes
influence distributions of organisms