Download 1915 – Alfred Wegener`s Observations - GLG310-

Plate tectonics
• Evidence and evolution of thinking about it
– Wegener and continental drift
– Magnetic reversals and sea floor spreading
– Distributions of volcanoes, earthquakes,
measured plate motions, mountain belts
• Basic premise of plate tectonics
• Plate boundaries
• History of plate motions
– Global
– Western North America
• The seafloor
GLG310 Structural Geology
GLG310 Structural Geology
Plate tectonics
The importance of Plate Tectonics
-Essential paradigm for
understanding significance of
geologic structures.
-Plate interactions create rockforming environments
-Plate motions generate stresses that
drive deformation
http://emvc.geol.ucsb.edu/index.htm
• Evidence and evolution of thinking about it
– Wegener and continental drift
– Magnetic reversals and sea floor spreading
– Distributions of volcanoes, earthquakes,
measured plate motions, mountain belts
• Basic premise of plate tectonics
• Plate boundaries
• History of plate motions
– Global
– Western North America
• The seafloor
GLG310 Structural Geology
1915 – Alfred Wegener’s
Observations


1.
2.
Pieces fit together like a puzzle
Past glaciations
a) Evidence of glaciers found at low latitudes
b) Striations (scratches carved in underlying rock by an
advancing glacier)
c) Glacial till – mud, sand and pebbles left behind by
receding glacier
d) South America, Southern Africa, South India, Australia,
Antarctica – which are now widely separated & many
lie at low latitudes
e) But, put the puzzle back together and see what you get
GLG310 Structural Geology
GLG310 Structural Geology
Evidence
Paleoclimate evidence
(ancient)
Ice sheets covered big areas of southern hemisphere
~ 220-300 million years ago
GLG310 Structural Geology
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1915 – Alfred Wegener’s
Observations
Evidence
Paleoclimate evidence
 3.
Ice sheets covered big areas of southern hemisphere
~ 220-300 million years ago
Glacial
striations
Equatorial Climate distribution
– a.
– b.
 i.
 ii.
 iii.
4.
North America, Southern Europe, NW Africa
Deposits of low latitudes
Coal
Reefs
Deserts
Fossil Distribution
– a. Several species were found to live in these regions
that are now separated by vast ocean
GLG310 Structural Geology
GLG310 Structural Geology
Evidence
Evidence
GLG310 Structural Geology
1915 – Alfred Wegener’s
Observations
 5.
Matching geologic units
– a. Same rocks occurred (same age, same
formation) on East coast of South America and
West coast of Africa
– b. Appalachians of the eastern US and
Canada match those of Greenland, UK and
Scandanavia
GLG310 Structural Geology
Continental Drift
1915
Alfred Wegener published
hypothesis of
continental drift
He hypothesized:
existence of single
“super-continent”
Pangaea (“pan - GEE - uh”)
~ 200 million years ago Pangaea broke into
smaller pieces, & “drifted” to present positions
GLG310 Structural Geology
GLG310 Structural Geology
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Continental Drift
1924
Wegener’s book translated to English
& met with hostile criticism
Main objection: no way to explain continental drift.
No one believed his explanation (Continental
Drift) because he did not come up with a
mechanism for driving the motion, nor did he
explain a few minor problems (ie Do the
continents just plough through the sea floor?).
Plate tectonics
• Evidence and evolution of thinking about it
– Wegener and continental drift
– Magnetic reversals and sea floor spreading
– Distributions of volcanoes, earthquakes,
measured plate motions, mountain belts
• Basic premise of plate tectonics
• Plate boundaries
• History of plate motions
– Global
– Western North America
• The seafloor
GLG310 Structural Geology
Revisiting the Earth’s magnetic
field
GLG310 Structural Geology
Earth’s magnetic field
 Why does the Earth have a magnetic field?
p.42-43bc
 Why does the polarity shift every 10,000
years or so?
 How might magnetic minerals on the sea
floor be affected by this magnetic field?
GLG310 Structural Geology
Earth’s magnetic Field and paleomagnetism
Wegener’s idea died until 1950’s.
Renewed interest from rock magnetism
GLG310 Structural Geology
Earth’s magnetic Field and paleomagnetism
Rock magnetism
•Certain minerals are magnetic (e.g., magnetite)
•Magnetic grains align w/ Earth’s magnetic field
Paleomagnetism: ancient magnetic field of Earth recorded
and frozen into rocks
How?
•When cooled below Curie point (580oC for iron), grain alignment
frozen in
Need to know about 2 things:
- Earth’s magnetic field
- Magnetism in rocks
GLG310 Structural Geology
GLG310 Structural Geology
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Earth’s magnetic Field and paleomagnetism
Rock magnetism
•Certain minerals are magnetic (e.g.,
magnetite)
•Magnetic grains align w/ Earth’s
magnetic field
•When cooled below Curie point
(580oC for iron), grain alignment
frozen in
Sea floor spreading
Observations of the Sea Floor
•Magnetic reversal record in sea floor basalts
• symmetric
•age of sea floor increases with distance from ridge
Mechanism to explain observations: Harry Hess & Bob Dietz
in the early 1960’s proposed: Sea Floor Spreading
 ocean ridges are above mantle upwellings, which cause
seafloor to spread, like a conveyor belt
 magma replaces seafloor as it moves away, becoming new
oceanic crust
 deep ocean trenches are locations where oceanic crust dives
back into planet
GLG310 Structural Geology
GLG310 Structural Geology
Sea Floor Spreading
Magnetic Reversals
 The polarity of Earth's magnetic field
reverses with time. The main figure
demonstrates how sea-floor anomalies, also
known as magnetic stripes, develop during
sea-floor spreading. The inset image
records the reversal of Earth's dipole.
 This animation shows progressive stages in the
opening of the Atlantic Ocean.
 The youngest rocks (in red) clearly outline the
mid-ocean ridge system, complete with transform
faults.
 The oldest ocean crust (in blue), is confined to
offshore regions adjacent to the United States,
Canada and western Africa.
 This distribution demonstrates that the North
Atlantic began to open before the South Atlantic.
GLG310 Structural Geology
GLG310 Structural Geology
Plate tectonics
• Evidence and evolution of thinking about it
Volcano Distribution
– Wegener and continental drift
– Magnetic reversals and sea floor spreading
– Distributions of volcanoes, earthquakes,
measured plate motions, mountain belts
• Basic premise of plate tectonics
• Plate boundaries
• History of plate motions
– Global
– Western North America
• The seafloor
GLG310 Structural Geology
GLG310 Structural Geology
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Earthquake Distribution
Measured motions of benchmarks
GLG310 Structural Geology
GLG310 Structural Geology
Earth’s Plates (& evidence of their existence)
Mountain Belts
• The Lithosphere is fractured into 12 major plates + (8 to 20
microplates)
• What evidence led to the identification of these plates?
• Earthquake Belts – that’s where all the action is happening –
movement leads to Earthquakes
• Volcanic Belts
• Mountain Belts
• Measured benchmark motions
• Geologic evidence
• Most deformation occurs at the plate boundaries – relatively little
at the interior of plates
• Likewise, Plate interiors are relatively seismically free regions –
stronger, so they don’t accommodate the movement.
GLG310 Structural Geology
GLG310 Structural Geology
Earth’s Major Plates
Basic Premise of Plate
Tectonics
• Earth’s lithosphere is divided into plates
• Some plates are oceanic lithosphere and some are
continental lithosphere
• They float on and move relative to the underlying
asthenosphere
• They move relative to one another
• Anywhere from 1 to 15 cm per year
• Most of the plate remains undeformed except at the plate
boundaries
GLG310 Structural Geology
GLG310 Structural Geology
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Fig.4.26
Revisit Earth’s Layers
PLATE
TECTONICS
Driving mechanisms???
W. W. Norton
• Lithosphere – crust +
upper mantle (cooler so
viscosity goes up) –
can’t flow
–
–
Avg continental –
150 km (up to 300 km)
Avg oceanianic 100 km (old) down to
10 km at mid-ocean
ridges
http://mcnamara.asu.edu/content/educational/main/CitcomCU_plumes/akm_perspective1.mpg
• Asthenosphere – can flow (higher temp + different composition),
therefore it convects
–
–
–
–
Oceanic lithosphere – high density
Continental lithosphere – low density
New Oceanic crust –lower density (because it’s hot)
Older Oceanic crust – higher density (because it’s cold)
• Earthquakes occur to depths of 670 km – no
Structuralbecause
Geology
earthquakes below this GLG310
(probably
material is
too hot to fracture
Continental Margins -- 2 types
Earth’s Major Plates
• Active continental Margins (Plate
boundary)
• Passive continental Margins (no plate
boundary)
• Some plates are either just ocean or just
continent or a mix of both
GLG310 Structural Geology
GLG310 Structural Geology
The Grand Unifying Theory
Plate boundary movies: 4.1(Transforms), 4.2 (Divergent), 4.3 (Convergent)
Insert revised figure 3.16 here
GLG310 Structural Geology
GLG310 Structural Geology
http://geology.about.com/library/bl/maps/n_map_GSHAP1500.htm
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Plate tectonics
• Evidence and evolution of thinking about it
– Wegener and continental drift
– Magnetic reversals and sea floor spreading
– Distributions of volcanoes, earthquakes,
measured plate motions, mountain belts
• Basic premise of plate tectonics
• Plate boundaries
• History of plate motions
– Global
– Western North America
Global Paleogeography
• The seafloor
GLG310 Structural Geology
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
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GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
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GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
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GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
GLG310 Structural Geology
http://cpgeosystems.com
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GLG310 Structural Geology
http://cpgeosystems.com
Northern Pacific and North American Plates
GLG310 Structural Geology
Many cool animations:
http://emvc.geol.ucsb.edu/
GLG310 Structural Geology
http://cpgeosystems.com
The seafloor holds much of the key to understanding plate tectonics
http://topex.ucsd.edu/marine_topo/mar_topo.html
GLG310 Structural Geology
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