The theory of global dynamics in which the lithosphere is broken into rigid plates, that move in response to convection in the upper mantle

The theory of global dynamics in which the lithosphere is broken into rigid plates, that move in response to convection in the upper mantle.

Continental Drift
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.

Pangea

Alfred Wegner (1915) Evidence Used by Wegner
Paleontological Evidence (Figure 17.2)

Glossopteris (fern)
Reptiles (Paleozoic/Mesozoic)

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)

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Development of the Theory of Plate Tectonics

The development of new instrumentation in the 60's permitted scientists to:
1) Map the topography of the ocean floor,
2) Determine the paleomagnetic characteristics of the ocean floor

I. Geology of the Ocean Floor

Rift Valley

Down-dropped block along divergent plate boundaries (tensional stresses) Seafloor Spreading

H.H. Hess (1960) - Postulated (based on echo sounding data) that ocean floors are spreading apart, propelled by convection currents in the mantle, and are moving away symmetrically from the ocean ridges. II. Paleomagnetism

The Earth has an internally generated magnetic field, which can be thought of as a bar magnet (inclined at ~11^o)
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

Normal/Reversed Polarity
At least 9 reversal Epochs in 4.5 million years

Magnetic reversal - A complete 180^o reversal of the polarity of Earth's magnetic field Polarity epochs - Long intervals of changing polarity (1Ma)
Polarity events - Short duration reversals

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

Magnetic Patterns on the Ocean Floor

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.

____________________________________________ Lithospheric Plate Motion

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

Plate Motion on a Sphere (Fig. 17.19)

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)

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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:

1. Slab pull- dense oceanic slab descends under its own weight into the asthenosphere in a subduction zone
2. Ridge Push- gravity forces the lithosphere to move off the spreading ridge
3. Basal Drag- resistance to flow along the lithosphere-asthenosphere boundary (could aid/hinder movement)
4. Mantle Resistance- resistance to the movement of the subducting plate through the asthenosphere and mesosphere
5. Friction- resistance along transform faults and between converging slabs of the lithosphere in a subduction zone