Plate Tectonics
Birth of a Theory
In
1911 astronomer, meteorologist, climatologist Alfred
Wegener (1880-1930) found earlier studies noting the same
fossils on opposite sides of the Atlantic.
Earlier theorists attributed this to now sunken bridges
Wegener noted the fit of South America and Africa and became
fixed on one continent
His search for other evidence resulted in The Origin of the Continents
and Oceans in 1915, with expanded versions published in 1920, 1922,
and 1929
He postulated a single continent Pangaea (Greek for “all the earth”)
Wegener was not the first to suggest the idea but the first to include
extensive evidence. American F.B. Taylor included little evidence with
his 1910 work and was ignored.
Wegener’s Arguments
Fit
of continents
Appalachian
and Caledonian mountains
Fit of Continents
Matching Mountain Ranges
Matching Mountain Ranges
Fossil
evidence
Mesosaurus,
Paleoclimatic
Widespread
Glossopteris, Cynognathus
evidence
glaciation
Glacial Evidence
Wegener’s
Argued
ideas were largely unaccepted
that continents were driven through solid oceanic crust like
icebreakers
Centrifugal or tidal forces
No acceptable mechanism
Paleomagnetism
rocks (basalt) acquire a record of earth’s magnetic field
upon cooling below the Curie Point (580° C)
Results in two types of information
Iron-rich
Location
of deposition
Time of deposition
Temporal Paleomagnetism
Earth’s magnetic field reverses polarity at irregular intervals
Polarities last 105 to 106 years
Rocks acquire current field at cooling
Polarities correlative and can be used for dating
Oceanic Paleomagnetism
Work on mid-oceanic ridges shows strong and weak magnetic anomalies
later determined to be normal and reversed magnetic events
Events form mirror images across the ridges
Dating determines that the rocks age away from ridge
Tying the Ends Together
Wegener’s
Continental Drift may be combined with Sea-Floor
Spreading
Combination provides a mechanism for movement of continents
suggested by Wegener
Combination has been developed into the Theory of Plate
Tectonics
Plate tectonics: A modern version of an old idea
Earth’s
major plates
Associated
with Earth's strong, rigid outer layer
Known
as the lithosphere
of uppermost mantle and overlying crust
Overlies a weaker region in the mantle called the asthenosphere
Consists
Earth’s
major plates
Seven
major lithospheric plates
Plates are in motion and continually changing in shape and size
Largest
plate is the Pacific plate
Several plates include an entire continent plus a large area of
seafloor
Earth’s
major plates
Plates
move relative to each other at a very slow but continuous
rate
Average
Cooler,
Plate
about 5 centimeters (2 inches) per year
denser slabs of oceanic lithosphere descend into the mantle
boundaries
All
major interactions among individual plates occur along their
boundaries
Types of plate boundaries
Divergent
plate boundaries (constructive margins)
plate boundaries (destructive margins)
Transform fault boundaries (conservative margins)
Convergent
Each
plate is bounded by a combination of these three types of
boundaries
New plate boundaries can be created in response to changes in
the forces acting on these rigid slabs
Divergent plate boundaries
Most
are located along the crests of oceanic ridges and can
be thought of as constructive plate margins
Oceanic ridges and seafloor spreading
Along
well-developed divergent plate boundaries, the seafloor is
elevated forming oceanic ridges
Seafloor spreading occurs along the oceanic ridge system
Divergent boundaries are located mainly along oceanic
ridges
Spreading
rates and ridge topography
Topographic
At
differences are controlled by spreading rates
slow spreading rates (1-5 centimeters per year), a prominent rift
valley develops along the ridge crest that is wide (30 to 50 km) and deep
(1500-3000 meters)
At intermediate spreading rates (5-9 centimeters per year), rift valleys
that develop are shallow with subdued topography
At spreading rates greater than 9 centimeters per year no median rift
valley develops and these areas are usually narrow and extensively
faulted
Continental
rifts
Splits landmasses into two or more smaller segments
Examples
include the East African rifts valleys and the Rhine
Valley in northern Europe
Produced by extensional forces acting on the lithospheric plates
Not all rift valleys develop into full-fledged spreading centers
Convergent plate boundaries
Older
portions of oceanic plates are returned to the mantle
in these destructive plate margins
Surface
expression of the descending plate is an ocean trench
Called subduction zones
Average angle at which oceanic lithosphere descends into the
mantle is about 45
Types
of convergent boundaries
Oceanic-continental
convergence
Denser
oceanic slab sinks into the asthenosphere
the plate descends, partial melting of mantle rock generates magmas
having a basaltic or, occasionally andesitic composition
Mountains produced in part by volcanic activity associated with
subduction of oceanic lithosphere are called continental volcanic arcs
(Andes and Cascades)
As
Types
of convergent boundaries
Oceanic-oceanic
convergence
When
two oceanic slabs converge, one descends beneath the other
forms volcanoes on the ocean floor
If the volcanoes emerge as islands, a volcanic island arc is formed
(Japan, Aleutian islands, Tonga islands)
Often
Types
of convergent boundaries
Continental-continental
convergence
Continued
subduction can bring two continents together
Less dense, buoyant continental lithosphere does not subduct
Result is a collision between two continental blocks
Process produces mountains (Himalayas, Alps, Appalachians)
The collision of India and Asia produced the Himalayas
Transform fault boundaries
The third type of plate boundary
Plates slide past one another and no new lithosphere is created or
destroyed
Transform faults
Most
join two segments of a mid-ocean ridge as parts of prominent linear
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
Testing the plate tectonics model
Paleomagnetism
Polar
wandering
The
apparent movement of the magnetic poles illustrated in magnetized
rocks indicates that the continents have moved
Polar wandering curves for North America and Europe have similar
paths but are separated by about 24 of longitude
Different paths can be reconciled if the continents are placed next to
one another
Apparent polar-wandering paths
Plate
tectonics and earthquakes
Plate
tectonics model accounts for the global distribution of
earthquakes
Absence
of deep-focus earthquakes along the oceanic ridge is consistent
with plate tectonics theory
Deep-focus earthquakes are closely associated with subduction zones
The pattern of earthquakes along a trench provides a method for
tracking the plate's descent
Deep-focus earthquakes occur along convergent
boundaries
Earthquake foci at the the Japan trench
Evidence
from ocean drilling
Some
of the most convincing evidence confirming seafloor
spreading has come from drilling directly into ocean-floor
sediment
Age
of deepest sediments
Thickness of ocean-floor sediments verifies seafloor spreading
Hot
spots
Caused
by rising plumes of heat from the at least the mantle
(some say the core)
Volcanoes
can form over them (Hawaiian Island chain)
Most mantle plumes are long-lived structures and at least some
originate at great depth, perhaps at the mantle-core boundary
The Hawaiian Islands: a stationary hot spot
Measuring plate motions
A
number of methods have been employed to establish the
direction and rate of plate motion
Volcanic
chains
Paleomagnetism
Very Long Baseline Interferometry (VLBI)
Global Positioning System (GPS)
Calculations
show that
Hawaii
is moving in a northwesterly direction and approaching
Japan at 8.3 centimeters per year
A site located in Maryland is retreating from one in England at
a rate of about 1.7 centimeters per year
The driving mechanism
No
one driving mechanism accounts for all major facets of
plate tectonics
Researchers agree that convective flow in the rocky 2,900
km-thick mantle is the basic driving force of plate tectonics
Several mechanisms generate forces that contribute to plate
motion
Slab-pull
Ridge-push
Models
of plate-mantle convection
Any
model describing mantle convection must explain why
basalts that erupt along the oceanic ridge
Models
Layering
at 660 kilometers
Whole-mantle convection
Deep-layer model
Mountain belts
– the processes that col-lectively produce a
mountain belt
Orogenesis
Includes
folding, thrust faulting, metamorphism, and igneous
activity
Mountain
building has occurred during the recent geologic
past
Alpine-Himalayan
chain
American Cordillera
Mountainous terrains of the western Pacific
Earth’s major mountain belts
Older
Paleozoic- and Precambrian-age mountains
Appalachians
Urals
in Russia
Several
hypotheses have been proposed for the formations of
Earth’s mountain belts
Mountain building at convergent boundaries
Plate
tectonics provides a model for orogenesis
Mountain
building occurs at convergent plate boundaries
Of particular interest are active subduction zones
Volcanic
arcs are typified by the Aleutian Islands and the Andean arc
of western South America
Aleutian-type
mountain building
Where
two ocean plates converge and one is subducted beneath
the other
Volcanic island arcs result from the steady subduction of
oceanic lithosphere
Most
are found in the Pacific
island arcs include the Mariana, New Hebrides, Tonga, and
Aleutian arcs
Continued development can result in the formation of mountainous
topography consisting of igneous and metamorphic rocks
Active
Formation of a volcanic island arc
Andean-type
Mountain
mountain building
building along continental margins
Involves
the convergence of an oceanic plate and a plate whose leading
edge contains continental crust
Exemplified by the Andes Mountains
Andean-type
Stages
mountain building
of development - passive margin
First
stage
Continental margin is part of the same plate as the adjoining oceanic
crust
Deposition of sediment on the continental shelf is producing a thick
wedge of shallow-water sediments
Turbidity currents are depositing sediment on the continental rise and
slope
Andean-type
Stages
mountain building
of development – active continental margins
Subduction
zone forms
process begins
Convergence of the continental block and the subducting oceanic plate
leads to deformation and metamorphism of the continental margin
Continental volcanic arc develops
Deformation
Andean-type
Stages
mountain building
of development – active continental margins
Accretionary
wedge may form
accumulation of sedimentary rocks and metamorphic rocks
with occasional scraps of ocean crust
Can become large enough to stand above sea level
Chaotic
Andean-type
mountain building
Composed
of roughly two parallel zones
Volcanic
arc
Develops on the continental block
Consists of large intrusive bodies intermixed with high-temperature
metamorphic rocks
Accretionary wedge
Seaward segment
Consists of folded, faulted, and metamorphosed sediments and
volcanic debris
Orogenesis along an Andean-type subduction zone
Orogenesis along an Andean-type subduction zone
Orogenesis along an Andean-type subduction zone
Andean-type
mountain building
Sierra
Nevada and Coast Ranges
One
of the best examples of an active Andean-type orogenic belt
Subduction of the Pacific Basin under the western edge of the North
American plate
Sierra Nevada batholith is a remnant of a portion of the continental
volcanic arc
Basin and Range Created by Tension
Possible Tensional Explanation
Continental collisions
Two
lithospheric plates, both carrying continental crust
The Himalayan Mountains are a youthful mountain range formed from the
collision of India with the Eurasian plate about 45 MYA
Spreading center that propelled India northward is still active
Similar but older collision occurred when the European continent collided
with the Asian continent to produce the Ural mountains
Plate relationships prior to the collision of India with
Eurasia
Position of India and Eurasia over time
Formation of the Himalayas
Continental
accretion and mountain building
A
third mechanism of orogenesis
Small crustal fragments collide and merge with continental
margins
Responsible for many of the mountainous regions rimming the
Pacific
Accreted crustal blocks are called terranes
Terranes consist of any crustal fragments whose geologic history
is distinct from that of the adjoining terranes
As oceanic plates move, they carry embedded oceanic plateaus,
volcanic island arcs and microcontinents to an Andean-type
subduction zone
Exotic Terranes
How Orogenesis Ends
Vertical movements of the crust
In
addition to the horizontal movements of lithospheric
plates, vertical movement also occurs along plate margins as
well as the interiors of continents far from plate boundaries
Isostatic adjustment
Less
dense crust floats on top of the denser and deformable
rocks of the mantle
Concept of floating crust in gravitational balance is called
isostasy
If weight is added or removed from the crust, isostatic
adjustment will take place as the crust subsides or rebounds
(Hudson Bay rebound, continental shelf subsidence)
The principle of isostasy
Key Terms Chapter 4
Continental drift
Sea-floor spreading
Plate tectonics
Divergent margin
Convergent margin
Conservative (transform) margin
Subduction zone
Convection
Tectonic cycle
Volcanic arc
Island arc
Isostasy
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