Plate Tectonics: The Unifying Theory
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Transcript of Plate Tectonics: The Unifying Theory
Plate Tectonics:The Unifying Theory
Peter W. Sloss, NOAA-NESDIS-NGDC
Plate TectonicsPlate Tectonics
• Fundamental concept of geoscience
• Integrates from many branches
• First suggested based on geology and paleontology
• Fully embraced after evidence from geophysics
What tectonics theory explains
• Distribution of earthquakes and volcanoes
• Relationship of age and height of mountain belts
• Age distribution of oceanic crust
• Magnetic information in rocks
What is Plate Tectonics Theory?
• Earth’s upper mantle (lithosphere) is broken into rigid plates which move with respect to each other
• Plates rest on and move in the asthenosphere
• There are 3 kinds of plate margins that are marked by earthquakes and volcanoes
– Divergent
– Convergent
– Transform
• The movement is driven by uneven distribution of heat within Earth and the mechanism that drives plate movement is not well known
Mosaic of Earth’s Plates
Peter W. Sloss, NOAA-NESDIS-NGDC
Theory evolved from 2 concepts:
• Continental DriftContinental Drift - by Alfred Wegener– Fit of continents - Pangaea– Ancient climatic evidence - glacial
deposits– Fossil evidence - Glossopteris &
Mesosaurus– Matched mineral zones & mountain
chains• (Problem - no acceptable method of (Problem - no acceptable method of
motion)motion)
“Fit” of the Continents
Anomalous Distribution of Fossils(Mesosaurus)
ANDPALEOMAGNETIC EVIDENCE
• Polar WanderingPolar Wandering - either
– the North Pole or the continents had moved
– both Europe and North America had apparently moved as a single continent for several hundred million years
Apparent polar
wandering curves
Seafloor SpreadingSeafloor Spreading
• “Geopoetry” of Harry Hess & Robert Dietz
• New seafloor forms by upwelling at the center of MOR and moves laterally
• Older crust is destroyed in the subduction zones at the trenches
• Seafloor is younger than 200 MY
• Solved Continental Drift problem
Modern Proof of Seafloor Spreading
• Deep Sea Drilling Project - sampled sea floor sediment & crust– Age & thickness increased with
distance from MOR– Ages were symmetrical about MOR
• Magnetic Anomalies - found weak & strong signal– Alternating normal & reversed
polarization– Stripes || to & symmetrical about
MOR
Magnetic Anomalies
Ocean crust records
magnetic reversals
Fig. 20.11
Age of Seafloor Crust
R. Dietmar Muller, 1997
More ProofsMore Proofs
• Hot Spots - deep, long-lived, stationary mantle magma sources
• Expressed at the surface by:– linear chain of volcanoes– aged with distance from hot spot
• Over 100 identified • Used as fixed points against which
plate motion is measured
Hawaiian-Emperor chain
Long-lived global hot spots
PUTTING IT ALL PUTTING IT ALL TOGETHERTOGETHER
I. I. PlatesPlates
• Group of rocks all moving in the same direction
• Can have both oceanic and continental crust or just one kind
• Plate interior are relatively quiet
• Most activity is located at plate margins
II. Types of plate boundariesII. Types of plate boundaries
• divergent: mid-ocean ridges
• convergent: collision zones volcanic arcs
• strike-slip: San Andreas Fault Alpine Fault, N.Z.
A. Divergent plate A. Divergent plate boundariesboundaries
Usually begin within continents as hot spot burns thru crust and eventually grows to become ocean basin
Two kinds of divergent margins
Mid Ocean Ridges (MOR)
Continental Rift Valleys
Features of Mid Ocean Features of Mid Ocean RidgesRidges
• Central rift valley (width is inversely proportional to the rate of spreading)
• Region of high heat flow
Shallow-focus earthquakes
• Almost exclusively basalt volcanism
Earthquakes Associated with Earthquakes Associated with Divergent MarginsDivergent Margins
Rifting and Seafloor
Spreading Along the
Mid-Atlantic Ridge
Peter W. Sloss, NOAA-NESDIS-NGDC
Rifting and Seafloor Spreading
Features of Continental Features of Continental RiftsRifts
• East Africa, Rio Grande rift
• Beginning of ocean formation (may not get that far)
• Rifting begins at a triple junction (two spreading centers get together to form ocean basin, one left behind).
• Rock types: basalt and sandstone
Hot Spot induced rifting
• Hot spot burns thru crust -> 3 branched rift
• Divergence begins• 2 branches are active• 1 branch is not active • Linear sea develops
Inception of Rifting
Along theEast African Rift System
Peter W. Sloss, NOAA-NESDIS-NGDC
Inception of Rifting Within a Continent
Nile Delta
Gulf ofSuez
Gulf of‘Aqaba
Red Sea
Earth Satellite Corp.
The Gulf of California Formed by
Rifting of Baja California
from Mainland Mexico
Worldsat International/Photo Researchers
B. Convergent B. Convergent boundariesboundaries• New crust created at MOR—old crust
destroyed (recycled) at subduction zones
• 2 kinds: subduction & collisionsubduction & collision
• Relative important densities:continental crust ≈ 2.8 g/cm3
oceanic crust ≈ 3.2 g/cm3
asthenosphere ≈ 3.3 g/cm3
Convergent Convergent boundariesboundaries
Three types:
ocean–ocean Philippines
ocean–continent Andes
continent–continent Himalaya
Ocean–OceanOcean–Ocean
Island arcsIsland arcs: Chain of volcanic islands
• Highly seismic tectonic belt ofshallow to deep earthquakes
• High heat flow arc of active andesitic volcanoes
• Bordered by a submarine trench
Ocean–Ocean Subduction Zone
Ocean–ContinentOcean–Continent
Continental arcsContinental arcs:
• Magmatic belt of active volcanoes (andesite to rhyolite)
• Often accompanied by compression of upper crust which builds mountains
Bordered by a submarine trench
Ocean-ContinentSubduction Zone
Continent–Continent–ContinentContinent• Continent–continent boundaries,
convergence is accommodated by• Folding (shortening and thickening)
• Strike-slip faulting
• Underthrusting (intracontinental subduction)
Continent–Continent Convergent Boundary
Continent-Continent Collision
Himalayas and Tibetan PlateauHimalayas and Tibetan Plateau
• Product of the collision between India and Asia.
• Collision began about 45 M yr. ago, continues today.
• Before collision, southern Asia looked something like the Andes do today.
Stages in the collision of India with
Asia
C. Transform Boundary Offsetting Spreading Centers
III. Rates of plate III. Rates of plate motionmotion
Mostly obtained from magnetic
anomalies on seafloor
Slow spreadingSlow spreading: 3 cm/year
Fast spreadingFast spreading: 10 cm/year
Very fast spreadingVery fast spreading: 17cm/year
Relative Velocity and Direction of Plate Movement
Data from C. Demets, R.G> Gordon, D.F. Argus, and S. Sten, Model Nuvel-1, 1990
IV. Rock assemblages and IV. Rock assemblages and plate tectonicsplate tectonics
• Each plate tectonic environment produces a distinctive group of rocks.
• By studying the rock record of an area, we can understand the tectonic history of the region.
Layer of ophiolite suite.
Precambrian Ophiolite Suite
Pillow basalt
M. St. Onge/Geological Survey of Canada
V. Exotic or Microplate Terranes
• Large blocks that contrast sharply with surrounding area
• Wrong faulting, folding, fossils, rock types, metamorphism, magnatism
• Thought to be fragments of continents, seafloor, seamounts, island arcs that rafted in & docked in new place
Approaching Arc or Microcontinent
Collision
Accreted Microplate Terrane
Microplate terranes Added to Western
North America Over the Past
200 Million Years
After Hutchinson, 1992-1993
VI. Driving mechanism of plate tectonics
• Thought to be convection of the mantle.
• Friction at base of the lithosphere transfers energy from the asthenosphere to the lithosphere.
• Convection may have overturned asthenosphere 4–6 times.
Other factors
• Trench pull
Plate sliding
• Ridge push
Three possible driving factors
Three possible mechanisms for the movement of lithosphere
over the asthenosphere
Fig. 17.17
VII. Tectonic reconstructionsVII. Tectonic reconstructions
A variety of evidence traces the motion of continents over time:
• Paleomagnetism
• Deformational structures
• Environments of deposition
• Fossils
• Distribution of volcanoes
Assembly of Pangaea
I.W.D. Dalziel, 1995
Breakup of Pangaea
200 million years ago
After Dietz & Holden, 1970
Breakup of Pangaea
140 million years ago
After Dietz & Holden, 1970
Breakup of Pangaea
65 million years ago
After Dietz & Holden, 1970
Breakup of Pangaea
Today
After Dietz & Holden, 1970
Examining Deep-sea Drill Cores
Texas A&M University
Questions about plate tectonics
• What do we really know about convection cells in the mantle?
• Why are some continents completely surrounded by spreading centers?
• Why are tectonics in continental crust and oceanic crust so different?
Cross Section of Western Canada
Fig. 20.25a
Fig. 20.10
Formation of
Magnetic Anomalies
Himalayas and Tibetan PlateauHimalayas and Tibetan Plateau
Models
• Underthrusting
• Distributed shortening
•Strike-slip faulting
Wilson cycleWilson cycle
Plate tectonics repeats itself: rifting, sea-
floor spreading, subduction, collision,
rifting, …
Plate tectonics (or something like it)
seems to have been active since the
beginning of Earth’s history.
Fig. 20.22
After Hutchinson, 1992-1993
Examples of Plate Boundaries
Fig. 20.8a,b
O-Oconvergent
O-Odivergent
O-Cconvergent
O-Cconvergent
O-Odivergent
C-Cdivergent
O-Odivergent
Volcanic and Nonmarine sediments are deposited in rift valleys
Fig. 20.17a
Cooling and subsidence of rifted margin allows sediments to be
deposited
Fig. 20.17b
Carbonate platform develops
Fig. 20.17c
Continental margin continues to grow supplied from erosion of the
continent
Fig. 20.17d
Ocean–Continent Convergent Boundaries
Fig. 20.13
Opening of the
Atlantic by Plate Motion
After Phillips & Forsyth, 1972
Idealized Ophiolite Suite - Oc. Crust
Peridotite
Gabbro
Pillow basalt
Deep-sea sediments
Model for Forming Oceanic Crust at Mid-ocean Ridges
The growth of oceanic basin 1
The growth of oceanic basin 2
The growth of oceanic basin 3
After map by Sclater & Meinke
Age of the Ocean BasinsAge of the Ocean Basins
Fig. 20.18
Parts of an Ocean–Ocean Convergent Plate Boundary
Fig. 20.19
Parts of an Ocean–ContinentConvergent Plate Boundary
Continued Subduction
Fig. 20.20a
Fig. 20.20b
Continent– Continent Collision