Transcript Earth_Can01_ch19_Tark

Chapter 19 in 1st edition
& 1 in 2nd edition
Plate Tectonics
PowerPoint Presentation
Stan Hatfield . Southwestern Illinois College
Ken Pinzke . Southwestern Illinois College
Charles Henderson . University of Calgary
Tark Hamilton . Camosun College
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19-2
Continental Drift: An Idea Before Its
Time
Alfred Wegener
• First proposed his continental drift hypothesis in
1915
• Published The Origin of Continents and Oceans
Continental Drift Hypothesis
• Supercontinent called Pangaea began breaking
apart about 200 million years ago
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19-3
Continental Drift: An Idea Before Its
Time
Pangaea approximately 200 million years ago.
Paleozoic
Tethys Sea, Pan Thallassic Ocean
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19-4
Continental Drift: An Idea Before Its
Time
Continental Drift Hypothesis
• Continents "drifted" to present positions
Evidence used in support of the continental drift
hypothesis
• Fit of the continents
• Fossil evidence
• Rock type and structural similarities
• Paleoclimatic evidence
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19-5
Continental Drift: An Idea Before Its Time
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Wegener’s
matching
mountain
ranges on different continents.
19-6
Continental Drift: An Idea Before Its Time
Permian
Glaciation,
Tillites,
Pavements:
Copyright
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19-7drift.
The Great Debate: No Viable Mechanism
Objections to the Continental Drift Hypothesis
• Inability to provide a mechanism capable of
moving continents across the globe
• Wegener suggested that continents plowed through
the ocean crust, much like ice breakers cut through
ice
• Proposed Tidal forcing, too weak, Rocks too strong
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19-8
Lessons from Continental Drift’s Failure
Continental Drift and the Scientific Method
• Wegener’s hypothesis was correct in principle, but
contained incorrect details
• For any scientific viewpoint to gain wide
acceptance, supporting evidence from all realms of
science must be found
• A few scientists considered Wegener’s ideas
plausible & continued the search a generation later
• Perseverance is important in Science, ideas must be
supported and widely publicized
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19-9
Continental Drift and Paleomagnetism
Initial impetus for the renewed interest in
continental drift came from rock paleomagnetism
• Keith Runcorn’s Lab at Newcastle & Ted Irving
Magnetized minerals in rocks
• Record the direction of Earth’s magnetic poles
• Provide a means of determining their latitude of
origin: tan ( Inclination ) = 2 tan ( Latitude )
• Oriented strata of known age, yet from different
continents track relative motions
• Polar Wander Paths
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19-10
Continental Drift and Paleomagnetism
Polar Wandering
• Rocks from successive ages give successive N-S
poles for their continents
• The apparent movement of the magnetic poles
illustrated in magnetized rocks indicates that the
continents have moved both absolutely and relative
to each other
• Shows that Europe was much closer to the equator
when coal-producing swamps existed
• Brazil, Africa & India were much closer to the S
pole in Permian time
• Antarctica is the least travelled continent!
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19-11
Continental Drift and Paleomagnetism
Apparent
polar-wandering
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for Eurasia and North America.
19-12
Continental Drift and Paleomagnetism
Polar Wandering
• Polar wandering curves for North America and
Europe have similar paths, but are separated by
about 24 of longitude
– PW Paths for Cambrian through Permian strata are
parallel for Europe and North America
– PW Paths for Triassic through Recent seem to converge
progressively
– Differences between the paths can be reconciled if the
continents are placed next to one another
– ~205Ma is the point of departure, with the opening of
the Atlantic
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19-13
A Scientific Revolution Begins
During the 1950s and 1960s technological strides
permitted extensive SONAR mapping of the
ocean floor
During the Cold War, any ship of convenience
towed a magnetometer (sub chasing!) Magnetic
stripes were discovered.
The first complete bathymetric maps of the world
seafloor assembled by Bruce Heezen & Marie
Tharp of the US Office of Naval Research
MOR’s & Deep Sea Trenches were discovered
Seafloor spreading hypothesis was proposed by
Harry Hess in the early 1960s
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19-14
A Scientific Revolution Begins anew with the
idea of Sea Floor Spreading
Geomagnetic reversals
• Both in strata on land and in boreholes beneath the
sea the magnetic succession is found to be full of
reversals with anti-parallel directions
• The advent of radiometric dating K/Ar & U/Pb
• Earth's magnetic field periodically reverses
polarity – the north magnetic pole becomes the
south magnetic pole, and vice versa
• Dates when the polarity of Earth’s magnetism
changed were determined from lava flows
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19-15
Magnetic Stripes Record Sea Floor Spreading
During a Succession of Geomagnetic Reversals
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The ocean
floor
asInc.
a magnetic
tape recorder.
19-16
The Birth of Plate Tectonics: 1960’s
Geomagnetic reversals
• Geomagnetic reversals are recorded both across
and within the ocean crust by the Deep Sea Drilling
Program’s Glomar Challenger and other marine
geoscience programs
• In 1963 Fred Vine and D. Matthews tied the
discovery of magnetic stripes in the ocean crust
near ridges to Hess’s concept of seafloor spreading
• Sediments are found to contain magnetic reversal
stratigraphy too and with microfossils this provides
the basis for a global magnetostratigraphy back
through the Cretaceous
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19-17
Plate Tectonics = Continental Drift +
Seafloor Spreading
Geophysics Provided the New Data that the Older
Geology Lacked
• Paleomagnetism, Magnetic Reversals, Magnetostratigraphy
(evidence of past magnetism recorded in the rocks) was the
most convincing evidence set forth to support the concepts
of continental drift and seafloor spreading
• Bathymetry & Heat Flow: Ridges young, shallow and
warm, trenches old, deep and cold with thicker sediments
• Seismics show Lithosphere thickens away from MOR’s
• Major Earthquakes were located along plate boundaries
• Volcanic Arcs behind trenches
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19-18
Plate Tectonics: The New Paradigm
Spreading & New Crust Formed at MOR’s &
CR’s, high heat flow, active volcanism
Subduction zones (Wadati-Benioff zones) of
deepening earthquakes behind trenches
Recycled water, gases & light elements from Arcs
Much more encompassing theory than
continental drift
The composite of a variety of ideas that explain
the observed motion of Earth’s lithosphere
through the mechanisms of subduction and
seafloor spreading
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19-19
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
– Plates are no longer just Continents, e.g. the North
America Plate extends from Greenland and Iceland to
Vancouver Island including both older continents and
some younger seafloor
– 3 Types of Boundaries: Ridges, Trenches & Transforms
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19-20
Plate Tectonics: The New Paradigm
Earth’s Major Plates
• ~Seven major lithospheric plates: Pacific, Eurasia,
Antarctica, N.America, S. America, Africa, Indian
Ocean
• Up to a dozen smaller plates: Australia, Caribbean,
Mediterranean, Scotian Sea, Juan de Fuca,
Turkey, Philippine Sea…
• 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
of seafloor
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19-21
Plate Tectonics: The New Paradigm
Earth’s Major Plates
• Motions measured by Magnetic stripes & Hotspot
Tracks over Ma to 10’s of Ma and VLBI & GPS
arrays over months to years
• Plate boundaries with faster motions have greater
volcanism and seismicity (Java-Sumatra)
• Plates move relative to each other at a very slow
but continuous rate
– Average about 5 centimetres per year
– Cooler, denser slabs of oceanic lithosphere descend into
the mantle
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19-22
Plate Tectonics: The New Paradigm
Plate Boundaries
• Almost all major interactions among individual
plates occur along their boundaries
– Subduction Zone Megathrust Earthquakes
– Volcanic & Plutonic Arcs
– Rare exceptions are:
– Within plate seismicity
– Within plate hotspot volcanism
• Types of plate boundaries
– Divergent plate boundaries (constructive margins)
– Convergent plate boundaries (destructive margins)
– Transform fault boundaries (conservative margins)
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19-23
Plate Tectonics: The New Paradigm
Plate Boundaries
• Each plate is bounded by a combination of the
three types of boundaries
• New plate boundaries can be created in response to
changes in the forces acting on these rigid slabs
• Changes to plate boundaries require several Ma
• Earthquakes on one boundary are unrelated to
those on others
• Volcanoes keep their own sweet time
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Divergent Plate Boundaries
Most are located along the crests of oceanic
ridges and can be thought of as constructive plate
margins with frequent volcanism & Quakes <6
Continental Rifts:
• East African Rift, Rio Grande, Baikal Rift, Rhine Graben
• Grabens, volcanic plateaux, normal faulting, hot springs
Oceanic ridges and seafloor spreading
• Along well-developed divergent plate boundaries, the
seafloor is elevated forming oceanic ridges
• Mid Atlantic Rift, Lomonosov Ridge, East Pacific Rise,
Juan de Fuca Ridge, Galapagos Ridge, SW Indian Ridge…
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19-25
Divergent Plate Boundaries
Oceanic Ridges and Seafloor Spreading
• Seafloor spreading occurs along the oceanic ridge
system
• Active volcanism causes black smokers, vent
communities & seafloor massive sulfide deposits
(SMS)
Spreading Rates and Ridge Topography
• Ridge systems exhibit topographic differences
• Topographic differences are controlled by
spreading rates
– Slow spreading = rugged topography
– Med-fast spreading = axial valley
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Divergent Plate Boundaries
Divergent
boundaries are located mainly along oceanic ridges.
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Divergent Plate Boundaries
Spreading Rates and Ridge Topography
• Topographic differences are controlled by
spreading rates
– At slow spreading rates (1-5 centimetres per year), a
prominent rift valley develops along the ridge crest that
is wide (30-50 km) and deep (1500-3000 metres) MidAtlantic
– At intermediate spreading rates (5-9 centimetres per
year), rift valleys that develop are shallow with subdued
topography East Pacific
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19-28
Divergent Plate Boundaries
Spreading Rates and Ridge Topography
• Topographic differences are controlled by
spreading rates
– At spreading rates greater than 9 centimetres per year
no median rift valley develops and these areas are
usually narrow and extensively faulted
– At Thingvallir in West Iceland the rift is above sea level!
Continental Rifts
• Splits landmasses into two or more smaller
segments
• These start with 3 arms but usually 1 or 2 of them
fail
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19-29
Divergent Plate Boundaries
Continental Rifts
• Examples include the East African rift 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 centres
• There are failed Precambrian rifts beneath
southern Alberta, the East arm of Great Slave
Lake and Mid Continent between Minnesota and
Missouri!
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Divergent Plate Boundaries
RRR Triple Junction
The East African rift – a divergent boundary on land.
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19-31
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
• Older colder lithosphere descends steeper up to 90°
• Younger warmer lithosphere descends flatter <15°
• The steeper the angle, the deeper the trench
• Younger descending plates make the greatest
earthquakes
~MCanada
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Convergent Plate Boundaries
Although all have the same basic characteristics, they are highly
variable features
Types of Convergent Boundaries
• Oceanic-Oceanic Convergence
–
–
–
–
Denser-older of 2 oceanic slabs sinks into the asthenosphere
Earthquakes under Island arc (Antilles, Philippines, Japan)
Primitive andesitic volcanoes
Seabed explosive volcanoes and “black smokers” + SMS (PNG)
• Oceanic-Continental Convergence
–
–
–
–
Denser oceanic slab sinks into the asthenosphere
Earthquakes are under the edge of the continent
Andean type arcs with granitic plutons (Coast Mountains, Sierras)
Big porphyry Cu-Mo deposits
• Continent-Continent Convergence
– Subducted slab falls away
– Massive continental mountain belt built with crustal melting (Himalayas,
Urals, Appalachians-Caledonides)
– Massive earthquakes under “backstop” continent (Tibet, China)
metal Education
pegmatites
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Convergent Plate Boundaries
Oceanic-Oceanic Convergence
When two oceanic slabs converge, the older,
colder, denser one descends beneath the other
Often forms volcanoes on the ocean floor
(Marianas arc)
If the volcanoes emerge as islands, a volcanic
island arc is formed (Japan, Aleutian islands,
Tonga islands, Antilles, Java-Sumatra)
Super-collosal explosive volcanoes as water hits
magma (Krakatau)
Active black smokers & Seafloor Massive
Sulfides
SMS
(Papua
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19-34
Convergent Plate Boundaries
Oceanic-Continental Convergence
As the plate descends, water from the slab causes flux
partial melting of mantle rock
This generates magmas having a basaltic or
occasionally, andesitic composition
These magmas can evolve in the crust to batholiths
and explosive rhyolites
Porphyry Cu-Mo deposits in batholiths (BC)
Shallow hydrothermal systems produce epithermal
Au-Ag-Cu deposits (Chile, Peru, Mexico, BC)
Mountains produced in part by volcanic activity
associated with subduction of oceanic lithosphere are
called continental volcanic arcs (Andes and Cascades)
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19-35
Convergent Plate 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 massive inracontinental mountains
(Himalayas, Alps, Urals, Appalachians)
Thickening causes lower crust to melt making
granites
Rare metal pegmatite deposits
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19-36
Convergent Plate Boundaries
Miocene ~10Ma
Modern
The collision of India and Asia produced the Himalayas.
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19-37
Transform Fault Boundaries
The third type of plate boundary
Plates slide past one another and no new
lithosphere is created or destroyed
Transform Faults all take up differential motion
• Most join two segments of a mid-ocean ridge as
parts of prominent linear breaks in the oceanic
crust known as fracture zones
• Others join subduction zones, offset or opposed
• Can also join a ridge to a subduction zone (Queen
Charlotte Fault connects Juan de Fuca Ridge to
Aleutians)
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19-38
Transform Fault Boundaries
Transform Faults
• A few (the San Andreas Fault in California,
the Alpine Fault of New Zealand & the
Anatolian Fault in Turkey) cut through
continental crust
– This type makes large earthquakes ~MR>7
• Most separate ocean crust of different rates
& ages (Blanco, Sovanco, Mendocino,
Clipperton)
– Only the part between ridges, trenches is
seismically active
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19-39
Testing the Plate Tectonics Model
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 (too warm)
– Deep-focus earthquakes are closely associated with
subduction zones (cold brittle zone extends into mantle)
– The pattern of earthquakes along a trench provides a
method for tracking the plate's descent
– Young buoyant subducting plates have wide shallow
Benioff zones & the most damaging earthquakes
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19-40
Testing the Plate Tectonics Model
Deep-focus earthquakes occur along convergent boundaries19-41
.
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Testing the Plate Tectonics Model
2 Subduction Zones
Earthquake foci in the vicinity of the Japan trench19-42
.
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Testing the Plate Tectonics Model
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 in trenches (Marianas, PeruChile)
– Thickness of ocean-floor sediments verifies seafloor
spreading (thickens & ages away from ridges)
– Oldest sediment & oldest ocean crust is distant from
MOR’s
– Age of oldest/deepest sediment = age of underlying
magnetic anomaly
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19-43
Testing the Plate Tectonics Model
Hot Spots
• Caused by rising plumes of mantle material
– Many from bumps on Core-Mantle Boundary
• Volcanoes can form over them (Hawaiian Island
chain, Azores, Yellowstone, Fiji, Iceland)
• Most mantle plumes are long-lived structures and
at least some originate at great depth, perhaps at
the mantle-core boundary
• Isotopes of great “growth ages” from recycled
ancient crust carried into lower mantle
• Fixed reference frame for plate motions (Pacific,
Yellowstone)
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19-44
Testing the Plate Tectonics Model
The Hawaiian Islands have formed over a stationary hot spot.
19-45
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Measuring Plate Motion
Currently possible with space-age technology to
directly measure relative motion between plates
Two methods used are VLBI-Very Long Baseline
Interferometry & GPS-Global Positioning
System
Calculations show that Hawaii is moving NW and
approaching Japan at 8.3 cm/year
Yellowstone shows SW motion of North America
since Miocene
North America and Europe are getting 5 cm
further
apart per year (50 km per Ma)
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19-46
The Driving Mechanism
No one driving mechanism accounts for all major
motions & forces in plate tectonics.
Researchers agree that convective flow in the rocky 2900
kilometre-thick mantle is the basic driving force of plate tectonics
Early ideas were that convection was passive and slower than plate motion
Most now believe the Mantle moves faster than the Plates
MOR’s are thermal bulges (high spots) in Upper Mantle while
Trenches are (low spots)
Modern researchers believe that Lithosphere Plates slide downhill over the
weak Asthenosphere
Early ideas for mechanisms generate forces that contribute to
plate motion
Slab-pull (but rocks tensile strengths are weak)
Ridge-push (but rocks are weak in compression & no folds or
thrusts on MOR system)
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19-47
The Driving Mechanism
Models of Plate-Mantle Convection
• Any model describing mantle convection must
explain why basalts erupt along the oceanic ridge
• All of the ideas are constrained by seismic
information about rock properties in the Earth’s
interior: fast, slow, strong, weak, cold, hot…
• Models
– Layering at 660 kilometres
– Whole-mantle convection
– Deep-layer model
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19-48
Importance of Plate Tectonics
Theory provides a unified explanation of Earth’s
major surface morphology & internal processes
Within the framework of plate tectonics,
geologists have found explanations for the
geologic distribution of earthquakes, volcanoes,
and mountain belts
Plate tectonics also provides explanations for past
distributions of plants and animals
Regions of Seismicity, High Heat Flow are
explained
Different ore deposits, rock types & rare
minerals are explained
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19-49
End of Plate
Tectonics as a
Background for
much of the rest of
the course!
Chapter 19
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19-50