The Plate Tectonics

RevolutionA paradigm A paradigm shift in the shift in the

Earth SciencesEarth Sciences

Topics: Wegener’s Theory of continental drift

Seafloor spreading: the Vine-Matthews-Morley hypothesis

The plate-tectonics revolution

What is a plate? How many are there?

J. Tuzo Wilson and his cycles

Triple junctions

Alfred Wegener (1880-1930)

Promoted continental drift with publication of The Origin of Continents and Oceans in 1915

Alternative to land bridges as theory to explain similar fossils on different continents

Alfred Wegener

Supporting evidence: Appalachian mountains match Scottish Highlands; similar rocks in South Africa and Brazil

Coined term Pangea (Greek, meaning “all the Earth”)

Appalachians (eastern N.A.) Scottish Highlands

Harry Hess

In 1962, U.S. Navy marine geologist Harry Hess proposed that ocean crust is much younger than continental crust, and forms by “seafloor spreading”

Raff, A. D. and R. G. Mason. 1961. Magnetic survey off the west coast of North America, 40º N. latitude to 52º N. latitude. Geological Society of America Bulletin 72:1267-1270.

Magnetic surveys revealed a distinctive striped pattern in the oceans

In the meantime, scientists in U.S.A. and Australia discovered that the Earth’s magnetic field has reversed many times

Vine, F. J. and D. H. Matthews. 1963. Magnetic anomalies over oceanic ridges. Nature 199:947-949.

The Vine-Matthews Hypothesis

Magnetic Stripes = Seafloor Spreading + Magnetic Field Reversals

Basalt extruded at mid-ocean ridges is magnetized in the direction of the Earth’s magnetic field

Larry Morley

… but did they discover this first?

several months before Vines and Matthews, Larry Morley (Geological Survey of Canada) had submitted a paper to Nature with the same conclusion

His paper was rejected as “too speculative”

Revisionist history:

Vines-Matthews hypothesis is now known as the

Vines-Matthews-Morley hypothesis

The new global tectonics of the 1970’sEarth’s ultimate recycling program

New oceanic lithosphere is created at mid-ocean ridges

Oceanic lithosphere is destroyed at deep-sea trenches (subduction zones)

This process largely explains the global distribution of earthquakes and volcanoes

What is the lithosphere?

Outer part of the Earth, 100-200 km thick

Includes the crust and part of the upper mantle

Behaves rigidly

Forms plates

Large plates (6+1):

North American, South American, Eurasian, African, Australian, Antarctic

Pacific

Small plates (2 + 6):

Arabian, Indian

Scotia, Caribbean, Nazca, Cocos, Juan de Fuca, Philippine Sea

Tectonic plates

Contain continental lithosphere

Purely oceanic

J. Tuzo Wilson

• Proposed hotspot hypothesis in 1963, in support of Hess’ seafloor spreading model

• Had difficulty getting this work published!

J. Tuzo Wilson

• Proposed transform plate boundary hypothesis in 1965 to explain linear fracture zones in oceans

• Realized that motion on an oceanic transform fault is opposite to apparent offset of ridges

The Wilson Cycle1. Continental breakup (rifting)

2. Opening of ocean basin

3. Subduction zones form

4. Ocean basin closes

5. Continents collide and form high mountains

6. Crust thins by erosion

Types of plate boundaries