Earth Sciences 240A
Lecture 11
PART D: Plate Tectonics I
The Early Days
Jigsaw fit of continents
1912: Alfred Wegener
Jigsaw fit
Continuity of
mountain chains
Continuity of
glacial deposits
No mechanism!
Tuzo
Wilson (U of T): Proponent of continental drift
Concept of
plumes/hot-spots (
Concept of
spreading centers/subduction
Understood stress
system of San Andreas
Sam Carey (U of Hobart): Proponent
of expanding Earth
Earth’s Magnetic
Field
Dynamo-like action
Solid metal sphere
within liquid metal
Solid core has
different rotation speed
Similarity to inner bar magnet
Lines of flux
Inclination/latitude
Dynamo model
Intensity
variation with time
‘Excursions’ or
‘events’
Paleomagnetism
Magnetometers
Magnetite crystals in basalt: ‘Frozen’
latitude evidence
WWII sea-floor surveys
Hunt for
submarines/lost ships
Discovery of
‘normal’/’reverse’ pattern
Sea-Floor Spreading
1963: Vine and Matthews: Undersea
ridge volcano survey
Magnetic pattern
Flux reversals
over time
Thus spreading of
ocean floor
One of most significant discoveries
ever
Sun: Every 11 years
Earth: Irregular pattern; Nothing big/lasting past 780,000 y; Rapid switch?
Venus: Nil
Mars: Weak paleomag
Age Dates
Essential dates to correlate with
changing latitude evidence
Calculate rate of continental
motion
Mechanism
No evidence!
Next: Plate Tectonics
II