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 (Hawaii)

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

Normal – Reverse Flux

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