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Plate Tectonics
„ Fundamental
Concept and Unifying
Theory in Earth Science
„ Idea is > 100 yrs old
„ Acceptance only within the past 30
yrs
1
Drilling into the seafloor provided more
evidence supporting sea-floor spreading
„
„
Micropaleontology
of sediments
Dating of the
underlying lavas
Drilling ship Glomar Challenger
Age of Seafloor Crust
Realizing that
the ocean
basins are
very young
was key to
acceptance
of Plate
Tectonics
Theory
2
Ocean
crust
records
magnetic
reversals
3
Seafloor Spreading Hypothesis
„
„
„
„
New seafloor forms by upwelling at the
center of MOR and moves laterally
Earth is not expanding so that must mean
that older crust is destroyed in the
subduction zones at the trenches
Seafloor is younger than 200 MY
Solved Continental Drift problem
Plate Tectonics: The
New Paradigm
„
Earth’s major plates
„ Associated with
Earth's strong, rigid outer
layer:
„ Known as the lithosphere
„ Consists of uppermost mantle and overlying
crust
„ Overlies a weaker region in the mantle called the
asthenosphere
4
Inside the Earth
Fig. 1.12
Plate Tectonics: The
New Paradigm
„
Earth’s major plates
„ Seven major lithospheric plates
„ Plates are in motion and are continually
changing in shape and size.
„ The largest plate is the Pacific plate.
„ Several plates include an entire continent
plus a large area of seafloor.
5
Earth’s Tectonic Plates
Mosaic of Earth’s Plates
6
Rates of plate motion
„ Mostly
obtained from magnetic
anomalies on seafloor
„ Fast
„
spreading: 10 cm/year
Up to 20 cm/year
„ Slow
spreading: 2 cm/year
Plate Tectonics: The
New Paradigm
„
Earth’s major plates
„ Plates move relative to each other at a very
slow but continuous rate.
„ About 5 centimeters (2 inches) per year
„ Cooler,
Cooler, denser slabs of oceanic lithosphere
descend into the mantle.
7
Plate Tectonics: The
New Paradigm
„
Plate boundaries
„ Interactions among individual plates
occur along their boundaries.
„ Types of plate boundaries:
„ Divergent plate boundaries (constructive
margins)
„ Convergent plate boundaries (destructive
margins)
„ Transform fault boundaries (conservative
margins)
Magnetic
Anomalies
in the
Atlantic
8
We can map
spreading
centers with
swath
bathymetry…
Divergent Plate Boundary
9
We can look at new “sea floor” on land
Thingvellir,
Thingvellir, Iceland: America to the
left, Eurasia to the right, a rift
down the middle.
Iceland is being
pulled apart as it
sits astride the
Mid-Atlantic
Ridge.
Gudmundur E. Sigvaldason,
Nordic Volcanological Institute
10
Nothing beats going down there in person, or at
least observing from a remotely-operated vehicle
Mir
(sub)
Alvin
(sub)
Jason II (ROV)
Ropos (ROV)
UH/HURL Submersibles
Pisces IV
Pisces V
11
At spreading centers, you can look at the
ocean crust in cross section
Pillow lava
Feeder dikes
At fracture zones, you can look at the
ocean crust in cross section
Surface
flows
12
Sometimes, sea floor gets thrust up on
land to form what is called an ophiolite
Oman, on the Persian
Gulf
Pillow
lava
Sheeted
dikes
How we think the oceanic crust forms
„
„
Hot rock rises and partially melts
The melt is erupted to form a layer of
basalt lava flows and pillows
13
How we think the oceanic crust forms
„
„
The feeders to the flows are vertical
sheets called dikes
Below the dikes, massive gabbro (like basalt,
but with larger crystals) solidifies from the melt.
How do we figure out the deep structure?
Remote Sensing from the surface:
„
„
„
Echo sounding using high-frequency sound
(several KHz = thousand cycles per second
But high-frequencies bounce off the hard rocks
We need low frequency sound (5-100 Hz) to
penetrate through the rock beneath the seafloor –
this is called “seismic” prospecting
14
How do we figure out the deep structure?
Then:
explosives
Now: Air
Guns
Maurice
Ewing
Seismic streamer
Multichannel seismics - measure structure in
two and even three dimensions
15
Sea floor structure
What do we find right at the spreading axis?
Black smoker:
Hydrogen sulfide
Giant tube worms
and clams live on
the Black smokers
16
What do we find right at the spreading axis?
A single spot on the East Pacific Rise
(pictures about a year apart)
How does it work?
ƒ Cold sea water
circulates down
through cracks
ƒ Water heats up as it
passes through hot
rock
ƒ Water interacts with
rock -- dissolves
minerals and becomes
laden with dissolved
sulfides
17
How does it work?
ƒ Sulfides precipitate on
exposure to cold water
(black smoker)
ƒ Bacteria oxidize the
sulfides
(chemosynthesis)
ƒ Worms have a
symbiotic relationship
with the bacteria in
their guts
How does it work?
ƒ Crabs live on dead
worms, bacterial mats,
and “snow”
ƒ A complete complex
food web is
established
18
What are those offsets in the spreading
center, and what are those parallel ridges?
The problem was solved by
the Canadian Geophysicist
J. Tuzo Wilson
19
Transform Fault Boundaries
„
„
Plates slide past one another and no new
lithosphere is created or destroyed.
Transform faults
„ Most join two segments of a mid-ocean ridge
along breaks in the oceanic crust known as
fracture zones.
„ A few (the San Andreas Fault and the Alpine
Fault of New Zealand) cut through
continental crust.
Transform Faults offset Spreading Centers
20
Transform Fault Boundaries
Notice how Wilson’s theory explain
depth offsets across fracture zones
21
The San Andreas
Fault:
A transform fault
separating the Pacific
and North American
Plates
Earthquakes in
mid-ocean areas
22
23
Divergent Plate Boundaries
„
Continental rifting
„ Splits landmasses into two or more
smaller segments along a continental rift
„ Examples include:
„ East African Rift Valleys
„ Rhine Valley in Northern Europe
„ Produced
by extensional forces
Continental Rifting
24
Anatomy of a Plate
Convergent Plate Boundaries
„
Older portions of oceanic plates are returned
to the mantle at these destructive plate
margins.
„ Surface expression of the descending plate is
an ocean trench.
„ Also called subduction zones
„ Average angle of subduction = 45 degrees.
25
Convergent Plate Boundaries
„
Types of convergent boundaries:
„ Oceanic–continental convergence
„ The denser oceanic slab sinks into the
asthenosphere.
asthenosphere.
„ Along the descending plate, partial melting of
mantle rock generates magma.
„ The resulting volcanic mountain chain is called a
continental volcanic arc.
arc. (The Andes and the
Cascades are examples.)
Oceanic–Continental
Convergence
26
Convergent Plate Boundaries
„
Types of convergent boundaries:
„ Oceanic–oceanic convergence
„ When two oceanic slabs converge, one descends
beneath the other.
„ Often forms volcanoes on the ocean floor
„ If the volcanoes emerge as islands, a volcanic
island arc is formed. (Japan, the Aleutian islands,
and the Tonga islands are examples.)
Oceanic–Oceanic Convergence
27
Convergent Plate Boundaries
„
Types of convergent boundaries:
„ Continental–continental convergence
„ Continued
subduction can bring two continents
together.
„ Less dense, buoyant continental lithosphere does
not subduct.
subduct.
„ The resulting collision produces mountains. (The
Himalayas, the Alps, and the Appalachians are
examples.)
Continental–Continental
Convergence
28
Continental
Collision
forms
Mountains
Types of Plate Margins
29
Tuzo Wilson’s other great contribution was
the concept of a stationary hot spot
Testing the Plate
Tectonics Model
„
Hot spots and mantle plumes
„ Caused by rising plumes of mantle material
„ Volcanoes can form
over them (Hawaiian
Island chain).
„ Mantle plumes
„ LongLong-lived structures
„ Some originate at great depth.
30
Hawaiian-Emperor
chain
31
Long-lived Global Hot Spots
What Drives Plate Motions?
„
„
Researchers agree that convective flow in
the mantle is the basic driving force of
plate tectonics.
Forces that drive plate motion:
„ Slab-pull
„ Ridge push
32
Three possible
mechanisms
for the
movement of
lithosphere
over the
asthenosphere
Forces Driving Plate Motions
33
What Drives Plate Motions?
„
Models of plate–mantle convection
„ Any model must be consistent with
observed physical and chemical properties
of the mantle.
„ Models:
„ Layering at 660 kilometers
„ WholeWhole-mantle convection
Convection in a Pot
34
Convection in the Mantle
Increased Heat with Depth
35
Convection in the Mantle
Upper Mantle Convection as a Possible
Mechanism for Plate Tectonics
36
Importance of
Plate Tectonics
„
The theory provides explanations for:
„ Earth’s major surface processes
„ Distribution of earthquakes, volcanoes,
and mountains
„ Distribution of ancient organisms, rock
types, geologic structures and mineral
deposits
37