Transcript ppt: Plate Tectonics Intro- Theory and History

Continental DriftThe Theory
The Earth’s crust is broken into about
12 rigid plates, which slide over a
semi-molten plastic layer of the
mantle.
Tectonic Plates on Modern Earth
There are three types of
plate boundaries:
•1- Convergent
•2- Divergent
•3- Transform
1. Convergent (aka subduction zone)
boundaries – where one plate overrides
another plate. Each plate is bounded by
some combination of these three plate
boundary types.
2. Divergent (aka spreading) boundaries –
where plates are moving apart
3. Transform fault boundaries – where
plates are moving past each other
The material making up the
plates can be divided in two
types:
i. Oceanic crust
ii. Continental crust
The driving force for plate
tectonics is slow moving
convection currents in the
underlying
plastic mantle material.
• lava lamp!
How does convection
work? No one
knows—but they
aren’t afraid to
propose models!
Whole-mantle convection
Two mantle convection cells
Complex convection
The History of the
Theory of Continental
Drift
1600 - development of a comprehensive
world map (missing details of some
continents such as
Australia and Antarctica).
1620 - Francis Bacon noted the similarity
of the shape of the continents on either side
of the
Southern Atlantic – Africa and South
America.
1666 - Francois Placet suggested that prior
to the Great Flood the land was undivided by
oceans and that the Atlantic formed when
Atlantis sank.
1858 - Antonio Snider developed a theory
that when the Earth cooled from a molten
mass
continents formed only on one side. This
created instability causing the Americas to be
pulled away from the rest of the continents.
1879 – Sir George Darwin said that the Pacific
Ocean was the scar left behind when the Moon
pulled away from the Earth. After this
happened, the continents moved to create a
balanced planet.
1890s- Edward Suess suggested that at one
stage all the continents were joined as one
massive
continent, which he called Gondwana. His
evidence for this was the location of
mountain ranges and common fossils.
1924 - Alfred Wegener developed the theory of
Continental Drift to explain the similarities of
rocks, fossils and other geological structures on either
side of the Atlantic. At this time it
was accepted that the continents sat like icebergs on
the mantle and as the continents
eroded they rose out of the mantle. Wegener suggested
as well as moving up and down
in the mantle, continents could move sideways in the
mantle.
-Wegener was a meteorologist and his theory was not
well accepted. (He died on an
expedition in Greenland collecting ice samples).
1928 - Arthur Holmes suggested that convection
currents in the mantle as the driving force of
continental drift. He had no evidence to support his
theory.
1950s- Extensive mapping of the ocean floor,
especially the Mid-Atlantic Ridge.
1960s- Harry Hess suggested that sea floor
separates at the mid-oceans ridges and new sea floor
is created by up welling of the mantle.
1970s- Theory of plate tectonics well accepted.
Earth ~200 million years ago
Day 2
Continental Drift:
•Geographic fitEvidence
of South America and Africa
•Fossils match across oceans
•Rock types and structures match across
oceans
•Ancient glacial features
•Earthquake Patterns
•Volcano Patterns
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.
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
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
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
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.
Major features: trench,
biggest EQs, explosive
volcanoes
Modern examples: Andes,
Cascades
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.