Earth ~200 million years ago

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Earth 200 million years ago
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The Geologic Time Scale
Based on
Fossils Correlation
Later
Calibrated with radiometric dating
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The Continental Drift Hypothesis
Proposed by Alfred Wegener in 1915. Supercontinen
t Pangaea started to break up about 200 million
years ago. Continents "drifted" to their present
positions. Continents "plowed" through the ocean
crust.
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Continental Drift Evidence
Geographic fit of South America and
Africa Fossils match across oceans Rock types
and structures match across oceans Ancient
glacial features
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Continental Drift Evidence
Tight fit of the continents, especially
using continental shelves.
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Continental Drift Evidence
Fossil critters and plants
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Continental Drift Evidence
Correlation of mountains with nearly identical
rocks and structures
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Continental Drift Evidence
Glacial features of the same age restore to
a tight polar distribution.
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Continental Drift Reactions
Received well in Europe and southern
hemisphere. Rejected in U.S., where scientists
staunchly preferred induction (incremental
progress built on observation) over what they
perceived as speculative deduction. Lack of a
suitable mechanism crippled continental drifts
widespread acceptance. Conflict remained
unresolved because seafloors were almost
completely unexplored.
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The Rise of Plate Tectonics
WW II and the Cold War Military Spending
U.S. Navy mapped seafloor with echo sounding
(sonar) to find and hide submarines. Generalized
maps showed oceanic ridgessubmerged mountain
ranges fracture zonescracks perpendicular to
ridges trenchesnarrow, deep gashes abyssal
plainsvast flat areas seamountsdrowned
undersea islands
Dredged rocks of the seafloor included only
basalt, gabbro, and serpentiniteno continental
materials.
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The Rise of Plate Tectonics
Marine geologists found that seafloor magnetism
has a striped pattern completely unlike patterns
on land.
Mason Raff, 1961
Black normal polarity White reversed
polarity Both very magnetic
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The Rise of Plate Tectonics
Hypothesis Stripes indicate periodic reversal
of the direction of Earths magnetic field.
To test this hypothesis, scientists determined
the eruptive ages AND the polarity of young
basalts using the newly developed technique of
K-Ar radiometric dating.
The study validated the reversal hypothesis...
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The Rise of Plate Tectonics
And then (1962-1963) geologists realized that the
patterns are SYMMETRICAL across oceanic ridges.
The K-Ar dates show the youngest rocks at the
ridge.
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The Rise of Plate Tectonics
Meanwhile, U.S. military developed new, advanced
seismometers to monitor Soviet nuclear tests. By
the late 1950s, seismometers had been deployed in
over 40 allied countries and was recording 24
hrs/day, 365 days/year. Besides the occasional
nuclear test, it recorded every moderate to large
earthquake on the planet. With these
high-precision data, seismologists found that
activity happens in narrow bands.
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Bands of seismicitychiefly at trenches and
oceanic ridges
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The Theory of Plate Tectonics
group authorship in 1965-1970
Earths outer shell is broken into thin, curved
plates that move laterally atop a weaker
underlying layer. Most earthquakes and volcanic
eruptions happen at plate boundaries. Three
types of relative motions between plates
divergent convergent transform
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Tectonic Plates on Modern Earth
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Divergent boundaries Chiefly at oceanic
ridges (aka spreading centers)
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How magnetic reversals form at a spreading center
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Divergent boundaries also can rip apart (rift)
continents
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How rifting of a continent could lead to
formation of oceanic lithosphere.
e.g., East Africa Rift
e.g., Red Sea
e.g., Atlantic Ocean
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Presumably, Pangea was ripped apart by such
continental rifting drifting.
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Subduction zones form at convergent boundaries if
at least one side has oceanic (denser) material.
Modern examples Andes, Cascades
Major features trench, biggest EQs, explosive
volcanoes
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Another subduction zonethis one with oceanic
material on both sides.
Modern example Japan
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Earthquake depth indicates subduction zones
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Collison zones form where both sides of a
convergent boundary consist of continental
(buoyant) material.
Modern example Himalayas
This probably used to be a subduction zone, but
all the oceanic material was subducted.
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Most transform boundaries are in the
oceans. Some, like the one in California, cut
continents. The PAC-NA plate boundary is MUCH
more complex than this diagram shows.
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Hotspots, such as the one under Hawaii, have
validated plate tectonic theory.
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Why do the plates move?
Two related ideas are widely accepted Slab
pull Denser, colder plate sinks at subduction
zone, pulls rest of plate behind it. Mantle
convection Hotter mantle material rises beneath
divergent boundaries, cooler material sinks at
subduction zones. So moving plates, EQs,
volcanic eruptions are due to Earths loss of
internal heat.
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How does convection work? No one knowsbut they
arent afraid to propose models!
Whole-mantle convection
Two mantle convection cells
Complex convection
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Field Trip Briefing
The California subduction zone (9 seismic
line) Subduction to transform (Atwater
animation) Faults of the Bay Area (SF-SJ
maps) Rock types well encounter
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Landslide north of Mussel Rock
Occurred between 2 am and 8 am, 2/21/05