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
Earths Magnetic Field
The earths magnetic field
Movement of iron alloy in the outer core = magnetic field
The earths dipole - an arrow that points from the north pole towards
the south
How does a compass work?
Magnetic declination
Magnetic inclination
Earths 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 earths 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