Unformatted text preview: Monday Sept. 15, 2008 Plate Tectonics
•Theory of Plate Tectonics was developed in 1960’s
•outer layer of Earth’s crust (lithosphere) consists of separate plates that move
•Explains locations of •earthquakes
•mountain belts 1820’s version of the world
1820’s version of the world Oceans and continents don’t move
Oceans are old
Mountains from contraction of earth like grape to raisin too much wine?
http://www.lib.utexas.edu/maps/historical/world_cyclopedia_1820.jpg Antonio SniderPellegrini Antonio SniderPellegrini 1858 fit of continents Alfred Wegener (18801930) first proposed the Hypothesis of Continental Drift: The Origin of the Continents
and Oceans (1915)
continents have moved and are still moving across Earth. Alfred-Wegener-Institute for Polar & Marine Research What evidence supported Wegener’s
What evidence supported Wegener’s
continental drift hypothesis?
1. Obvious fit of continents
2. Paleoclimate data (glaciers, climate belts)
3. Similar fossils separated by oceans
4. Rock type and structural similarities What evidence did
250 Ma 1. Obvious fit of
W. W. Norton. Modified from Wegener A,
The Origin of the Continents and Oceans.
The What other evidence did Wegener use? Glacial striations
Glacial striations •Evidence for distribution of glaciers Glaciation (260280 Ma) striationsdirection
till depositsperimeter If continents
were connected, one large ice cap
explains record Support for continental drift
Support for continental drift
Climate belts seem to match across continents Equator Support for Continental Drift
3. Distribution of fossils …similar fossils …oceans apart Modern Distributions Each continent has unique assemblage of terrestrial species “Isolated” by large ocean basins Mesosaurus
freshwater reptile lived ~286 Ma Support for Continental Drift
Distribution of fossils Support for Continental Drift
Distribution of fossils Otherwise, fossil Otherwise, fossil distribution must be explained by migrations across land bridges Castaway? Feasible…but need
Sufficient WWII Support for Continental Drift 4. Alignment of ancient 4. Alignment of ancient mountain belts & rock types Continental drift problems…….
Continental drift problems……. But how did the continents move? Evidence for continental drift was compelling
.... but how could weak continents ‘sail’ through the strong ocean crust? continental
crust mantle Wegener’s hypothesis was largely unaccepted and untested for 40 years Alfred Wegner- Father of Continental Drift Theory of Continental Drift
mechanism: low density continents “floating” on higher density
ocean floor rocks and “sailing” through them Arthur Holmes (18901965)
in 1928 proposed convection cells in the mantle as a mechanism for moving continents However, he warned his ideas were "purely speculative" and could "have no scientific value until they acquire support from independent evidence." Harry Hess (1960) built on Holmes’ ideas, & proposed sea floor spreading
New crust forms at ridges
Old crust disposed at trenches But how to prove it? Remember about
Earth’s internal structure Earth’s magnetic field • Earth’s rotation>circulation of liquid iron alloy in outer core > creates an electric current > generates a magnetic field
• Earth behaves like a giant electromagnet (dynamo)
• Earth’s magnetism creates magnetic lines of force in space (magnetic field) Magnetic pole ≠ geographic pole!
• 11° difference today (=declination)
• moves ~0.2°/yr
• in 10,000 yrs. average magnetic pole
position equals geographic pole W. W. Norton Magnetic
• Angle between magnetic field and horizontal
• = 0° at magnetic equator
• = 90° at magnetic north pole
• Is NOT measured by a standard compass
Is Paleomagnetism - how do rocks
acquire a magnetic signature?
• common magnetic mineral
• provides weak magnetization to
• >550°C, thermal chaos prevents
organized magnetic effect in lava
organized Below 550°C
• thermal motion slows down
• magnetite grains align with Earth’s
field, providing weak magnetization
• direction of Earth’s magnetic
field is locked into cold basalt
field Analogy to Concrete!
Analogy Deposition Cooling Solid Sedimentary rocks can
also record a magnetic signature
also Remember that the Earth’s
magnetic field reverses in time
Time between reversals is fairly random with shortest at 20,000 to
30,000 years and the longest was 50 million years, Time the actual reversal averages ~7000 yrs but depends on latitude Collection of oriented rock samples in the field Collection of oriented rock samples in the field Why Igneous or Volcanic Rocks?
We can date them!!! • measured magnetic orientation in rock layers
• • sharp magnetic reversals
• Conclusion: polarity of Earth’s magnetic field suddenly reverses periodically
Normal Reversed Finding paleopoles measure direction of ‘locked in’ magnetism
declination points to paleonorth (you don’t know how far, just which way)
inclination provides paleomagnetic latitude (distance from you to paleopole) Interpreting paleomagnetic poles
•inclination, declination, location of magnetic pole through time •hypothetical continent ‘polar wander’ ends at today’s magnetic pole But has the magnetic pole really moved this much ??! Apparent polar wander paths the last 300 million years
North America & Europe Four continents Plotted on present day continent positions Only explanation: continents drifted, pole stationary..?
Only explanation: continents drifted, pole stationary..? • rotate Europe’s polar wander curve west
• curves overlap from 280180 Ma>continents adjacent
= time span Wegener proposed for Pangea
before & after this time, continents were separate DEEP BLUE
How the exploration of our ocean basins revolutionized our
understanding of the Earth So far we have…..
1920s & 1930s ocean studies expanded
1920s & 1930s ocean studies expanded
1940s greatly increased in the war years; new equipment yielded new view of oceans
1950s & 1960s rise of Plate Tectonics Matthew Fontaine Matthew Fontaine Maury the first textbook of modern oceanography 1920s & 30s sonar developed for military & industry;
1920s & 30s sonar developed for military & industry;
belatedly used for iceberg detection…..
1912 RadioAcousticRanging (RAR) navigation invented by the Canadian Reginald Fessenden was a means to transmit and receive sound waves in the sea was the seed that led to the invention of depthfinding sonars and other types of sonars for looking ahead of a vessel and out to the sides of a vessel. Echo sounder
circa 192527 Old days threw rope over the side Measure velocity & travel time to calculate distance to seafloor Beginning in the 1940’s: bathymetric profiles & magnetic surveys of the ocean floor distance = velocity x time All data put together to make a new ocean map Heezen & Tharp map
Heezen & Tharp map earthquake locations, 1953
earthquake locations, 1953 Tharp noted that all epicenters located over deep valleys in MidOcean Ridge Needed more data of ocean Dredge or grab samples
Dredge or grab samples Composition of the seabed
Composition of the seabed basalt
sediment Launching an expendable bathythermograph,
circa 1985 Heat flow
Heat flow Highest heat flow at midocean ridges,
Highest heat flow at midocean ridges,
lowest at old oceanic crust.
wwwrses.anu.edu.au/~uli/Teaching/ Heat/FouriersLaw.html Harry Hess (19061969)
Assembled wide ranging information: • existing knowledge
• his own observations
• Wegener’s hypothesis 1962 ‘Essay in Geopoetry’
• Sediment thickens above older crust
Earthquakes + central rift result of splitting of the crust at MOR
High heat flow due to magma rising to cool + form crust
Ocean crust consumed in trenches
... continents carried by not moving through spreading seafloor Hess’s concept of seafloor spreading
Hess’s concept of seafloor spreading Spreading: New oceanic crust produced at ridges.
Subduction: Old oceanic crust sinks under continents into mantle in deepsea trenches
Continents: Along for the ride
A mobile Earth – so far just HYPOTHESIS Magnetometer evidence for SFS (sea floor spreading)
• measures strength of total magnetic field (Earth+magnetic rocks)
> expected field = positive anomaly
< expected field = negative anomaly Magnetometer
circa 1964 Early magnetic stripe map of the
California coast from 1971 that
helped to decipher the nature of the
ocean Magnetic field recorded in oceanic crust is Magnetic field recorded in oceanic crust is symmetric about MidOcean Ridge (MOR) A symmetric pattern of polarity stripes develops.
A symmetric pattern of polarity stripes develops. positive anomaly crust formed when Earth had normal polarity
negative anomaly crust formed when Earth had reverse polarity The Ultimate Proof DSDP Leg 3
The Ultimate Proof DSDP Leg 3
Test: does the basalt get older away from the ridge?
• drilled through sediments and into basalt
• determined age of oldest sediment at each site using fossils
• age vs. distance from ridge proved seafloor spreading ...
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- Fall '10
- Plate Tectonics, Earth's magnetic field, Alfred Wegener