Classroom presentations to accompany Understanding Earth, 3rd edition prepared by Peter Copeland and William Dupré University of Houston Chapter 20 Plate Tectonics: The Unifying Theory

Peter W. Sloss, NOAA-NESDIS-NGDC

Plate Tectonics Fundamental concept of geoscience Integrates from many branches First suggested based on geology and paleontology Fully embraced after evidence from geophysics

Fig Mosaic of Earth’s Plates Peter W. Sloss, NOAA-NESDIS-NGDC

Plates Group of rocks all moving in the same direction Can have both oceanic and continental crust or just one kind.

Types of plate boundaries divergent:mid-ocean ridges convergent:collision zones volcanic arcs strike-slip:San Andreas fault Alpine fault, N.Z.

Divergent plate boundaries Usually start within continents— grow to become ocean basin

Features of Mid Ocean Ridges Central rift valley (width is inversely proportional to the rate of spreading) Shallow-focus earthquakes Almost exclusively basalt

Continental Rifts 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

Rifting and Seafloor Spreading Along the Mid-Atlantic Ridge Peter W. Sloss, NOAA-NESDIS-NGDC

Inception of Rifting Within a Continent Fig. 20.4b

Inception of Rifting Along the East African Rift System Peter W. Sloss, NOAA-NESDIS-NGDC

Fig. 20.5a Nile Delta Gulf of Suez 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 “Fit” of the Continents

Anomalous Distribution of Fossils Fig. 20.2

Convergent boundaries 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/cm 3 oceanic crust ≈ 3.2 g/cm 3 asthenosphere ≈ 3.3 g/cm 3

Convergent boundaries Three types: ocean–oceanPhilippines ocean–continentAndes continent–continentHimalaya

Ocean–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–Continent Continental arcs: Active volcanoes (andesite to rhyolite) Often accompanied by compression of upper crust

Fig. 20.6a Ocean-Continent Subduction Zone

Continent–Continent 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 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 Plateau Models Underthrusting Distributed shortening Strike-slip faulting

Spreading Centers Offset by Transform Boundary Fig. 20.7

Wilson 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-O convergent O-O divergent O-C convergent O-C convergent O-O divergent C-C divergent O-O divergent

Ocean–Continent Convergent Boundaries Fig. 20.8c

Continent–Continent Convergent Boundary Fig. 20.d

Rates of plate motion Mostly obtained from magnetic anomalies on seafloor Fast spreading Fast spreading: 10 cm/year Slow spreading Slow spreading: 3 cm/year

Fig Magnetic Anomalies

Fig Formation of Magnetic Anomalies

Fig Age of Seafloor Crust R. Dietmar Muller, 1997

Relative Velocity and Direction of Plate Movement Fig Data from C. Demets, R.G> Gordon, D.F. Argus, and S. Sten, Model Nuvel-1, 1990

Fig Opening of the Atlantic by Plate Motion After Phillips & Forsyth, 1972

Rock assemblages and plate 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 Idealized Ophiolite Suite Peridotite Gabbro Pillow basalt Deep-sea sediments

Model for Forming Oceanic Crust at Mid-ocean Ridges Fig

Fig Precambrian Ophiolite Suite Pillow basalt M. St. Onge/Geological Survey of Canada

Volcanic and Nonmarine sediments are deposited in rift valleys Fig a

Cooling and subsidence of rifted margin allows sediments to be deposited Fig b

Carbonate platform develops Fig c

Continental margin continues to grow supplied from erosion of the continent Fig d

Fig Parts of an Ocean–Ocean Convergent Plate Boundary

Fig Parts of an Ocean–Continent Convergent Plate Boundary

Continued Subduction Fig a

Fig b Continent– Continent Collision

Approaching Arc or Microcontinent Fig a

Collision Fig b

Accreted Microplate Terrane Fig c

Fig Microplate terranes Added to Western North America Over the Past 200 Million Years After Hutchinson,

Fig After Hutchinson,

Tectonic reconstructions A variety of evidence traces the motion of continents over time: Paleomagnetism Deformational structures Environments of deposition Fossils Distribution of volcanoes

Fig Assembly of Pangaea I.W.D. Dalziel, 1995

Fig a Breakup of Pangaea 200 million years ago After Dietz & Holden, 1970

Fig b Breakup of Pangaea 140 million years ago After Dietz & Holden, 1970

Fig c Breakup of Pangaea 65 million years ago After Dietz & Holden, 1970

Fig d 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 a

Fig b

Fig c

Fig d

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 Basins