plate tectonics courtesy of dr. richard sedlock department of geology san josé state university
TRANSCRIPT
Plate TectonicsCourtesy of Dr. Richard Sedlock
Department of Geology San José State University
Earth ~200 million years ago
The Geologic
Time ScaleBased on
*Fossils*Correlation
Later
*Calibrated with
radiometric dating
The Continental Drift Hypothesis
Proposed by Alfred Wegener in 1915.
Supercontinent Pangaea started to break up about 200 million years ago.
Continents "drifted" to their present positions.
Continents "plowed" through the ocean crust.
Continental Drift: Evidence
Geographic fit of South America and Africa
Fossils match across oceans
Rock types and structures match across oceans
Ancient glacial features
Tight fit ofthe
continents, especially
usingcontinental
shelves.
Continental
Drift:Evidence
Continental Drift: Evidence
Fossil critters and plants
Continental Drift:
Evidence
Correlation of
mountains with nearly
identical rocks and structures
Continental
Drift:Evidence
Glacial features
of the same age
restore to atight polar
distribution.
Continental Drift: ReactionsReceived 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 drift’s widespread acceptance.
Conflict remained unresolved because seafloors were almost completely unexplored.
The Rise of Plate TectonicsWW II and the Cold War: Military SpendingU.S. Navy mapped seafloor with echo
sounding (sonar) to find and hide submarines. Generalized maps showed:
oceanic ridges—submerged mountain ranges
fracture zones—cracks perpendicular to ridges
trenches—narrow, deep gashes
abyssal plains—vast flat areas
seamounts—drowned undersea islandsDredged rocks of the seafloor included only basalt, gabbro, and serpentinite—no continental materials.
The Rise ofPlate
Tectonics
Black: normal polarityWhite: reversed polarity
Both: very magnetic
Marine geologists found that seafloor magnetism has a striped pattern
completely unlike patterns on land.Mason & Raff,
1961
Hypothesis: Stripes indicate periodic reversal of the direction of Earth’s 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 Rise of Plate Tectonics
The study validated the reversal hypothesis...
The Rise ofPlate
TectonicsAnd then (1962-1963) geologists realized that the
patterns are SYMMETRICAL across oceanic
ridges.The K-Ar datesshow the youngestrocks at the ridge.
The Rise of Plate TectonicsMeanwhile, 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.
Bands of seismicity—chiefly at trenches and oceanic ridges
The Theory of Plate Tectonics
Earth’s 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:
“group authorship” in 1965-1970
divergent convergent transform
Tectonic Plates on Modern Earth
Divergent boundaries: Chiefly at oceanic ridges (aka spreading
centers)
How magnetic reversals form at a spreading center
Divergent boundaries
also can rip apart (“rift”)
continents
How rifting of acontinent could
lead to formation of
oceanic lithosphere.
e.g., Red Sea
e.g., Atlantic Ocean
e.g., East Africa Rift
Presumably,Pangea was ripped apart
by such continental
rifting & drifting.
Subduction zones form at convergent boundaries if at least one side has oceanic (denser) material.Modern examples: Andes,
CascadesMajor features: trench, biggest EQs, explosive
volcanoes
Another subduction zone—this one withoceanic material on both sides.
Modern example: Japan
Earthquake depth indicates subduction zones
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.
Most transform boundariesare in the oceans.
Some, like the one in California, cut continents.
The PAC-NA plate boundary is MUCH more complex than this diagram
shows.
Hotspots, such as the one under Hawaii,have validated plate tectonic theory.
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 Earth’s loss of internal heat.
Whole-mantle convection
Two mantle convection cells
Complex convection
How does convection work?
No one knows—but they aren’t afraid to
propose models!