Plate Tectonics
Historical Development of A Scientific Revolution
& How the Earth Works

Historical development of Plate Tectonics
• For 30 years - stationary continent model (geosynclinal theory)

• Continued study of the earth eventually led to discovery of plate tectonics

• Sea Floor Topography
• Earth’s Magnetic Field

The earth’s magnetic field
• Movement of iron alloy in the outer core = magnetic field

• The earth’s dipole - an arrow that points from the north pole towards the south
­ How does a compass work?

• Magnetic declination

• Magnetic inclination

Earth’s past magnetic field preserved in rocks
• Rocks that contain iron-rich minerals can preserve the magnetic field of earth that exists at the time that they form
­ dipole

• Basaltic lava flows

• Ocean crust

• Direction & inclination

Apparent polar-wonder
• Paleomagnetism measured in samples from around the world - began to see interesting pattern

• The dipole (direction & inclination) preserved in lava rocks from different ages and places appeared to show that the position of the north pole had changed over time

• Continents changed position through time - not the poles
The ocean floor and the nature of oceanic crust
• New technology allows for determining the nature of the ocean floor - sonar

• Bathymetric profiles and maps

• Features - abyssal plains, mid-ocean ridges

Puzzling observations - oceanic crust
• Thin sediments

• Different rocks than on continents - basalt & gabbro

• Heat flow highest near mid-ocean ridges

• Distribution of earthquakes

The solution = sea floor spreading
• 1960 - Harry Hess proposes sea-floor spreading

Magnetic Anomalies and Reversals - the final straw
• Magnetic anomalies discovered on the ocean floor
­ Imaginary stripes of + and - parallel to the mid-ocean ridges

• Paleomagnetism demonstrates that the polarity of the earth’s magnetic field changes over time = magnetic reversals

Magnetic anomalies and reversals = related
• Oceanic crust recording magnetic reversals

• As new oceanic crust added at the mid-ocean ridges -material acquires polarity that exists at the time

• Illustrates sea-floor spreading