Classroom presentations to accompany Understanding Earth , 3rd edition

Classroom presentations Classroom presentations to accompany to accompany

Understanding EarthUnderstanding Earth, 3rd edition, 3rd edition

prepared by

Peter Copeland and William Dupré

University of Houston

Chapter 20Chapter 20Plate 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

Fig. 20.3

Mosaic of Earth’s Plates

Peter W. Sloss, NOAA-NESDIS-NGDC

PlatesPlates

• Group of rocks all moving in the same direction

• Can have both oceanic and continental crust or just one kind.

Types of plate boundariesTypes of plate boundaries

• divergent: mid-ocean ridges

• convergent: collision zones volcanic arcs

• strike-slip: San Andreas fault Alpine fault, N.Z.

Divergent plate boundariesDivergent plate boundaries

Usually start within continents—

grow to become ocean basin

Features of Mid Ocean RidgesFeatures of Mid Ocean Ridges

• Central rift valley (width is inversely proportional to the rate of spreading)

• Shallow-focus earthquakes

• Almost exclusively basalt

Continental Continental RiftsRifts

• East Africa, Rio Grande rift

• Beginning of ocean formation (may not get that far)

• Rifting often begins at a triple junction (two spreading centers get together to form ocean basin, one left behind).

• Rock types: basalt and sandstone

Rifting and Seafloor Spreading

Fig. 20.4a

Fig. 20.4a

Rifting and Seafloor

Spreading Along the

Mid-Atlantic Ridge

Peter W. Sloss, NOAA-NESDIS-NGDC

Inception of Rifting Within a Continent

Fig. 20.4b

Fig. 20.4b

Inception of Rifting

Along theEast African Rift System

Peter W. Sloss, NOAA-NESDIS-NGDC

Fig. 20.5a

Nile Delta

Gulf ofSuez

Gulf of‘Aqaba

Red Sea

Earth Satellite Corp.

Fig. 20.5b

The Gulf of California Formed by

Rifting of Baja California

from Mainland Mexico

Worldsat International/Photo Researchers

Fig. 20.1

“Fit” of the Continents

Anomalous Distribution of Fossils

Fig. 20.2

Convergent Convergent boundariesboundaries• New crust created at MOR—old crust

destroyed (recycled) at subduction zones (i.e., the Earth is not expanding)

• 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 arcs:

• Tectonic belts of high seismic ?????

• High heat flow arc of active volcanoes (andesitic)

• Bordered by a submarine trench

Fig. 20.6b

Ocean–Ocean Subduction Zone

Ocean–ContinentOcean–Continent

Continental arcs:

• Active volcanoes (andesite to rhyolite)

• Often accompanied by compression of upper crust

Fig. 20.6a

Ocean-ContinentSubduction Zone

Continent–Continent–ContinentContinent• In ocean–continent boundaries

convergence, collision convergence is taken up by subduction (± thrusting).

• Continent–continent boundaries, convergence is accommodated by• Folding (shortening and thickening)

• Strike-slip faulting

• Underthrusting (intracontinental subduction)

Fig. 20.6c

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.

Himalayas and Tibetan PlateauHimalayas and Tibetan Plateau

Models

• Underthrusting

• Distributed shortening

•Strike-slip faulting

Spreading Centers Offset by Transform Boundary

Fig. 20.7

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.

Examples of Plate Boundaries

Fig. 20.8a,b

O-Oconvergent

O-Odivergent

O-Cconvergent

O-Cconvergent

O-Odivergent

C-Cdivergent

O-Odivergent

Ocean–Continent Convergent Boundaries

Fig. 20.8c

Continent–Continent Convergent Boundary

Fig. 20.d

Rates of plate motionRates of plate motion

Mostly obtained from magnetic

anomalies on seafloor

Fast spreadingFast spreading: 10 cm/year

Slow spreadingSlow spreading: 3 cm/year

Fig. 20.9

Magnetic Anomalies

Fig. 20.10

Formation of

Magnetic Anomalies

Fig. 20.11

Age of Seafloor Crust

R. Dietmar Muller, 1997

Relative Velocity and Direction of Plate Movement

Fig. 20.12Data from C. Demets, R.G> Gordon, D.F. Argus, and S. Sten, Model Nuvel-1, 1990

Fig. 20.13

Opening of the

Atlantic by Plate Motion

After Phillips & Forsyth, 1972

Rock assemblages and 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.

Fig. 20.14

Idealized Ophiolite Suite

Peridotite

Gabbro

Pillow basalt

Deep-sea sediments

Model for Forming Oceanic Crust at Mid-ocean Ridges

Fig. 20.15

Fig. 20.16

Precambrian Ophiolite Suite

Pillow basalt

M. St. Onge/Geological Survey of Canada

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

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

Approaching Arc or Microcontinent

Fig. 20.21a

Collision

Fig. 20.21b

Accreted Microplate Terrane

Fig. 20.21c

Fig. 20.22

Microplate terranes Added to Western

North America Over the Past

200 Million Years

After Hutchinson, 1992-1993

Fig. 20.22

After Hutchinson, 1992-1993

Tectonic reconstructionsTectonic reconstructions

A variety of evidence traces the motion of continents over time:

• Paleomagnetism

• Deformational structures

• Environments of deposition

• Fossils

• Distribution of volcanoes

Fig. 20.23

Assembly of Pangaea

I.W.D. Dalziel, 1995

Fig. 20.24a

Breakup of Pangaea

200 million years ago

After Dietz & Holden, 1970

Fig. 20.24b

Breakup of Pangaea

140 million years ago

After Dietz & Holden, 1970

Fig. 20.24c

Breakup of Pangaea

65 million years ago

After Dietz & Holden, 1970

Fig. 20.24d

Breakup of Pangaea

Today

After Dietz & Holden, 1970

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

• Ridge push

Fig. 20.25a

Fig. 20.25b

Fig. 20.25c

Fig. 20.25d

Cross Section of Western Canada

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

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?

Examining Deep-sea Drill Cores

Texas A&M University

After map by Sclater & Meinke

Age of the Ocean BasinsAge of the Ocean Basins