Transcript Plate Tectonics

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)
Plate Tectonics
•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 Boundaries- Where most geological action is.
Plate boundaries -where plates contact each other
•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...
Divergent Plate Boundary
•Plates move apart
•New lithosphere is created
•Sea-floor "spreading centers" (Hess).
Continental Rift
•Above: Earliest stages
of divergence
•Below: Later, as
oceanic crust begins to
form
Convergent Boundaries
(Collision zones)
• 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)
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