Theory that continents move in relation to one another.
Proposed in the early 1900s
Supported by an array of geologic data
The complete development of the theory Earth's dynamics was stifled by a lack of understanding of the ocean crust.
Structure and Rock Type (Figure 17.3)
Fit of the continents
Continuity of rock types and structures
Paleozoic Glaciation (Figure 17.5)
Paleoclimatology (Figure 17.6)
Development of the Theory of Plate Tectonics
The development of new instrumentation in the 60's permitted
1) Map the topography of the ocean floor,
2) Determine the paleomagnetic characteristics of the ocean floor
I. Geology of the Ocean Floor
The Earth has an internally generated magnetic field, which can be thought of as a bar magnet (inclined at ~11o)
The study of ancient magnetic fields as preserved in rocks
1. Changes in position of the poles
2. Reversals of the magnetic poles
Fe-rich igneous rocks become weakly magnetized by the Earth's magnetic field as they cool.
Apparent Polar Wandering
'Paleomag' studies have shown in rocks of different ages that the north magnetic pole has steadily and systematically changed its position
III. Patterns of Magnetic Reversals on the Seafloor
At least 9 reversal Epochs in 4.5 million years
Vine and Matthews (1963) used paleomagnetic signature to test the theory put forth by Hess.
1. Basalt injected as dikes below ocean ridge, or as flows
2. Basalt crystallizes and cools, retains magnetic signature
3. As the seafloor spreads, the zone of crust is split, and migrates away from the ridge, but remains parallel to it
Evidence form Sediment on the Ocean Floor
Glomar Challenger (1968)
Without seafloor spreading, the entire ocean would be covered with a thick blanket of oceanic sediment
With seafloor spreading, the sediment pile thins towards the ridge.
Movement of rigid plates on a sphere is complex. Each
plate moves as:
1) an independent unit (???)
2) in different directions
3) at different velocities
Thought Experiment: Consider a plate that covers
an entire hemisphere. Each point on the plate moves along a line of latitude
with respect to the pole
It is a requirement that plates rotate around an
axis of spreading (rift).
Plates always move parallel to transform faults, and along circles of latitude perpendicular to the spreading axis
Rates of Plate Motion (Fig. 17.20)
Traditionally, how do we determine the velocity of plate
motion? Magnetic reversals on the ocean floor.
Plates move at different rates (1 - 18 cm/year)
Driving Mechanisms for Plate Tectonics
Classic theory is that the plates play a passive role and that movement is due entirely to mantle convection
More recent ideas consider the plates to be active participants in the convection process.
The most important forces that influence plate motion are: