Class# 6

•Plate tectonics- General Theory•Plate boundaries- 3 types•Evidence to support the theory

Plate Tectonics

• The earth’s crust and the uppermost layer of the mantle move together (this package is the Lithosphere)

• The lithosphere is broken into brittle plates that slide past each other, collide, and slip under/over each other.

• Driving force for this: Convection; the mantle “turns over” because it is cooled at the top and heated within

Plate Tectonics

Take time outside of class to review all of this so you come to understand how the many parts of the story fit together.

The Lithospheric Plates of the Earth (Fig. 2.13)

•Lithosphere consists of rigid plates•Plates move over the soft (solid) asthenosphere.•Plates are either

–(1) oceanic only or

–(2) continental and oceanic.

•Thus, continents move about ("drift") as passengers on

plates.

Plate Tectonics

Plate Boundaries- Where most geological action is.

•In some places they move apart from each other “Divergent”

•In some places they move toward each other “Convergent" -- Collision zones"

•In some places plates slide past each other “Transform”

Each boundary has distinctive features...

Plate boundaries -- where plates contact each other

•Plates move apart•New lithosphere is created •Sea-floor "spreading centers" (Hess).

Divergent Plate Boundary

•Above: Earliest stages of divergence

•Below: Later, as oceanic crust begins to form

Continental Rift

• What happens depends on whether the leading edge is continental or oceanic crust

1) If Oceanic plate -- "subducted" into mantle.

2) Continental plate -- deformed and thickened

-(too light and thick to be subducted)

Convergent Boundaries (Collision zones)

Fig. 2.20

Fracture Zones

Fracture Zones

"Transform" -- plates slide past one another.

Back to Harry Hess: Why wasn’t he dismissed as Wegener had been??????

New Evidence Forced the Scientific Community to Question the Old Paradigm