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