Transcript Plate Tectonics - Johnston County Schools

Plate Tectonics
The Continental Drift Hypothesis
Proposed by Alfred Wegener in 1915.
Supercontinent Pangaea (meaning “all land”)
started to break up about 200 million years
ago.
Continents "drifted" to their present positions.
Continents "plowed" through the ocean crust.
Earth ~200 million years ago
Wegener’s Evidence for Continental
Drift
 Geographic fit of South America and Africa
(Wegener argued that the coastlines on
opposite sides of the South Atlantic Ocean
have similarities and look like they fit
together).
 Rebuttal from opponents: Erosion
continually changes shorelines over time.
Didn’t necessarily look the same millions
of years ago.
Continental
Drift:
Evidence
Tight fit of
the
continents,
especially
using
continental
shelves.
More Evidence from Wegener
 Fossils match across
oceans. Including
Mesosauraus, an aquatic
reptile whose fossil remains
are limited to eastern South
America and Africa.
 Other scientists had argued
that land bridges could
account for this (during an
ice age, the sea level
lowered to allow animals to
cross) – no evidence
supports that one existed
between South America and
Africa.
Continental Drift:
Evidence
Fossil critters and plants
Yet More Evidence – Rock Types
and Structures
 Several mountain belts
end at one coastline and
reappear on a landmass
across the ocean.
 The Appalachian belt
ends off the coast of
Newfoundland and we
find similar rocks and
structures in the British
Isles and Scandinavia.
Continental
Drift:
Evidence
Correlation of
mountains
with nearly
identical rocks
and structures
The Evidence of Ancient Climates
 Wegener was a meteorologist and he found
glacial deposits showing that between 220-300
million years ago, ice sheets covered large
areas of the Southern Hemisphere.
 He thought that if the landmasses did exist as a
supercontinent, with South Africa centered over
the South Pole, that this would explain the
conditions necessary to form large areas of
glacial ice over much of the Southern
Hemisphere (it would also place the northern
landmasses nearer the tropics, which would
account for their vast coals deposits).
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.
No Mechanism
 Wegener proposed that the tidal influence of the
Moon was strong enough to give the continents
a westward motion – physicists showed that
physically incompatible with what we know of
the tides and the Earth’s rotation.
 Wegener also proposed that the continents
broke through oceanic crust, but no evidence
existed to indicate that the ocean floor was that
weak.
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 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
Earth’s outer shell is broken into thin, curved
plates that move laterally atop a weaker
underlying layer.
The plates are called tectonic plates or
lithospheric plates (remember that the
lithosphere is the crust and the uppermost part
of the upper mantle).
Tectonic Plates on Modern Earth
Major Plates, Secondary Plates,
& Tertiary Plates
 There are 7 major plates (African Plate,
Antarctic Plate, Eurasian Plate, Indo-Australian
Plate, North American Plate, Pacific Plate, and
South American Plate).
 Secondary plates tend to be much smaller.
There are 8 (the Arabian and Indian are the biggest).
The others are: Caribbean Plate, Cocos Plate, Juan de
Fuca Plate, Nazca Plate, Philippine Sea Plate, Scotia
Plate.
Tertiary Plates
 Tertiary plates are grouped with the major plate
that they would otherwise be shown as part of
on a major plate map.
 Mostly these are tiny microplates, although in
the case of the Nubian-Somalian and AustralianCapricorn-Indian plates these are major plates
that are rifting apart.
 Different maps and references will have different
numbers of tertiary plates – it is difficult to show
what comprises separate plates and there tends
to be less consensus with the smaller plates.
Movement & Events at Plate
Boundaries
Most earthquakes and volcanic eruptions happen
at plate boundaries.
Three types of relative motions between plates:
divergent convergent transform
Divergent boundaries: Chiefly at oceanic
ridges
(aka spreading centers)
Seafloor Spreading
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
Modern examples: Andes,
(denser) material.
Cascades
Major features: trench,
biggest EQs, explosive
volcanoes
Another subduction zone—this one with
oceanic material on both sides.
Modern example: Japan
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.
Evidence Supporting Plate
Tectonics
 Earth’s magnetic field periodically reverses
polarity (the north magnetic pole becomes the
south magnetic pole, and vice versa).
 A rock solidifying during one of the periods of
reverse polarity will be magnetized with the
polarity opposite of rocks being formed today.
 Magnetometers have been towed across the
ocean floor and have revealed alternating strips
of high-intensity (normal polarity) magnetism
and low-intensity (reverse polarity) magnetism.
 This strongly supports seafloor spreading.
How magnetic reversals form at a spreading center
Alternating Magnetism
More Support – Earthquake
Patterns
 There is a close link between deep-focus
earthquakes and ocean trenches.
 No earthquakes have been recorded
below 700km, which fits with the theory
because the slab is predicted to be heated
enough to soften (without the rigid rocks,
there would be no conditions for
earthquakes to occur).
Earthquake depth indicates subduction
zones
More Support – Ocean Drilling
 Data from sea floor drilling (especially that
conducted by The Deep Sea Drilling
Project (1968-1983) revealed that the
youngest oceanic crust is at the ridge crest
and the oldest oceanic crust is at the
continental margins.
Hotspots, such as the one under Hawaii,
have also validated plate tectonic theory.
Hot Spots
 Melting of hot rock in the mantle creates a
volcanic area (hot spot) near the surface.
 As the plate moves over the hot spot,
successive volcanic mountains form.
 This is how the islands of Hawaii formed.
 Kauai is the oldest of the islands and its
volcanoes are extinct.
Why do the plates move?
Several related ideas are widely accepted:
Slab pull: Denser, colder plate sinks at
subduction zone, pulls rest of plate behind it.
Ridge-Push: Oceanic lithosphere slides down
the sides of the oceanic ridge due to gravity.
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