Transcript Plate Tectonics

Plate Tectonics
Courtesy of Dr. Richard Sedlock
Department of Geology
San José State University
Earth ~200 million years ago
The
Geologic
Time Scale
Based 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
Continental
Drift:
Evidence
Tight fit of
the continents,
especially
using
continental
shelves.
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 a
tight polar
distribution.
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
drift’s widespread acceptance.
Conflict remained unresolved because seafloors
were almost completely unexplored.
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 ridges—submerged mountain ranges
fracture zones—cracks perpendicular to ridges
trenches—narrow, deep gashes
abyssal plains—vast flat areas
seamounts—drowned undersea islands
Dredged rocks of the seafloor included only
basalt, gabbro, and serpentinite—no continental
materials.
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
The Rise of Plate Tectonics
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 study validated the reversal hypothesis...
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.
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.
Bands of seismicity—
chiefly at trenches and oceanic ridges
The Theory of Plate Tectonics
“group authorship” in 1965-1970
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:
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 a
continent could lead
to formation of
oceanic lithosphere.
e.g., East Africa Rift
e.g., Red Sea
e.g., Atlantic Ocean
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,
Cascades
Major features: trench,
biggest EQs, explosive
volcanoes
Another subduction zone—this one with
oceanic 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
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.
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.
How does convection
work? No one
knows—but they
aren’t afraid to
propose models!
Whole-mantle convection
Two mantle convection cells
Complex convection