Download to Ch. 9 Notes

Geology Ch. 9 Notes
Objectives:
1. Describe the hypothesis of continental drift
2. Evaluate the evidence in support of continental drift
3. Identify the main objections to Wegner’s hypothesis of continental drift
4. Explain the theory of plate tectonics
5. Describe lithospheric plates
6. Identify the three types of plate boundaries
7. Explain how seafloor spreading and continental rifting cause formation of new
lithosphere
8. Describe the process of lithosphere destruction that takes place at subduction
zones
9. Differentiate among subduction at oceanic-continental, oceanic-oceanic, and
continental-continental convergent boundaries
10. Describe the action of plates at a transform fault boundary
11. Explain how Paleomagnetism and magnetic reversals provide evidence that
supports the theory of plate tectonics
12. Evaluate how earthquakes, ocean drilling, and hot spots provide evidence that
supports the theory of plate tectonics
9.1 Continental Drift
An Idea Before Its Time
 Wegener’s continental drift hypothesis stated that the continents had once
been joined to form a single ____________________________
• Wegener proposed that the supercontinent, __________, began to
break apart 200 million years ago and form the present landmasses.
 Evidence
• The Continental Puzzle
• ______________________________
- Fossil evidence for continental drift includes several fossil
organisms found on different
landmasses.
• Rock Types and __________________________
- Rock evidence for continental exists in the
form of several ________________that end at one coastline, only
to reappear on a landmass across the ocean.
• Ancient ___________________________________
Rejecting the _______________________________
 A New Theory Emerges
• Wegener could not provide an explanation of exactly what made the
continents move. News technology lead to findings which then lead to
a new theory called__________________________.
9.2 Plate Tectonics
Earth’s Major Roles
 According to the plate tectonics theory, the________________, along with
the overlying crust, behaves as a strong, rigid layer. This layer is known as the
lithosphere.
• A_________________ is one of numerous rigid sections of the
lithosphere that move as a unit over the material of the asthenosphere.
Types of Plate Boundaries
 Divergent boundaries (also called____________________) are the place
where two plates move apart.
 Convergent boundaries form where two plates move _____________.
 Transform fault boundaries are margins where two plates grind past each
other without the production or destruction of the ___________________
9.3 Actions at Plate Boundaries
Divergent Boundaries
 _____________________and Seafloor Spreading
• Oceanic ridges are continuous elevated zones on the floor of all major
ocean basins. The____________ at the crest of ridges represent divergent
plate boundaries.
• __________________are deep faulted structures found along the axes of
divergent plate boundaries. They can develop on the seafloor or on land.
• ____________________produces new oceanic lithosphere.
 Continental Rifts
• When spreading centers develop within a continent, the landmass may
split into two or more smaller segments, forming_________________.
Convergent Boundaries
 A _____________________occurs when one oceanic plate is forced down into
the mantle beneath a second plate.
 Oceanic-Continental
• Denser oceanic slab sinks into the ______________________
• Pockets of magma develop and rise.
• Continental ______________________form in part by volcanic activity
caused by the subduction of oceanic lithosphere beneath a continent
• Examples include the Andes, Cascades, and
the _________________________________________
 Oceanic-Oceanic
• Two oceanic slabs converge and one descends
beneath the other.
• This kind of boundary often forms ______________on the ocean floor.
• Volcanic ______________________form as volcanoes emerge
from the sea.
• Examples include the Aleutian, __________________, and
Tonga islands.
 Continental-Continental
• When subducting plates contain continental
material, two continents collide.
• This kind of boundary can produce new mountain ranges, such as the
__________________________
Transform Fault Boundaries
 At a transform fault boundary, plates _________________each other without
destroying the lithosphere.
 Transform faults
• Most join two segments of a_________________________.
• At the time of formation, they roughly ____________the direction of
plate movement
• They aid the movement of_______________ crustal material.
9.4 Testing Plate Tectonics
Evidence for Plate Tectonics
 __________________is the natural remnant magnetism in rock bodies; this
permanent magnetization acquired by rock can be used to determine the location of
the magnetic poles at the time the rock became magnetized.
• __________________—when rocks show the same magnetism as the present
magnetism field
• ___________________—when rocks show the opposite magnetism as the
present magnetism field
 The discovery of strips of alternating polarity, which lie as mirror images across
the ocean ridges, is among the strongest evidence of________________________.
 Earthquake Patterns
• found a close link between deep-focus earthquakes and_______________.
• The absence of deep-focus earthquakes along the oceanic ridge system was
shown to be consistent with the new theory.
 Ocean Drilling
• The data on the_____________ of seafloor sediment confirmed what the
seafloor spreading hypothesis predicted.
The youngest oceanic crust is at the ridge crest, and the oldest oceanic crust is at
the____________________________________.
 Hot Spots
• A _____________is a concentration of heat in the mantle capable of
producing magma, which rises to Earth’s surface; The Pacific plate moves
over a hot spot, producing the______________________.
• Hot spot evidence supports that the plates move over the Earth’s surface.
9.5 Mechanisms of Plate Motion
Causes of Plate Motion
 Scientists generally agree that convection occurring in the mantle is the basic
driving force for plate movement
• ___________________is the motion of matter resulting from changes in
temperature
 Slab-Pull and Ridge-Push
• _________________is a mechanism that contributes to plate motion in which
cool, dense oceanic crust sinks into the mantle and “pulls” the trailing
lithosphere along. It is thought to be the primary downward arm of convective
flow in the mantle.
• ________________causes oceanic lithosphere to slide down the sides of the
oceanic ridge under the pull of gravity. It may contribute to plate motion.
 Mantle Convection
• ________________are masses of hotter-than-normal mantle material that
ascend toward the surface, where they may lead to igneous activity.
• The _____________________distribution of heat within Earth causes the
thermal convection in the mantle that ultimately drives plate motion.
Definitions:
1. Continental drift: a hypothesis that originally proposed that continents had once
been joined to form a single supercontinent broken into pieces that drifted into
their present positions
2. Pangaea: the proposed supercontinent that 200 hundred million years ago broke
apart and formed the present day continents
3. plate tectonics: the theory that proposes that earth’s outer shell consists of
individual plates that interact in various ways and thereby produce earthquakes,
volcanoes, mountains and the crust itself
4. plate: one of numerous rigid sections of the lithosphere that moves as a unit over
the material of the asthenosphere
5. divergent boundary: a region where the rigid plates are moving apart, typified by
oceanic ridges
6. convergent boundary: a boundary in which two plates move together
7. transform fault boundary: a boundary in which two plates slide past each other
without creating or destroying lithosphere
8. oceanic ridge: a continuous elevated zone on the floor of all major ocean basins
and varying in width from 1,000 to 4,000km; the rifts at the crests of the ridges
represent divergent plate boundaries
9. rift valley: deep faulted structure found along the axes of divergent plate
boundaries, rift valleys can develop on the seafloor or on the land
10. seafloor spreading: the process by which plate tectonics produces new oceanic
lithosphere at ocean ridges
11. subduction zone: a destructive plate margin where oceanic crust is being pushed
down into the mantle beneath a second plate.
12. trench: a surface feature in the seafloor produced by the descending plate during
subduction
13. continental volcanic arc: mountains formed in part by volcanic activity caused by
subduction of oceanic lithosphere beneath a continent
14. volcanic island arc: a chain of volcanic islands generally located a few hundred
kilometers from a trench where subduction of one oceanic slab beneath another is
occurring
15. Paleomagnetism: the study of changes in the earth’s magnetic field as shown by
evidence of magnetism in rocks that have formed over time
16. normal polarity: a magnetic field that is the same as that which exists during the
present day
17. reverse polarity: a magnetic field opposite to that which exist today
18. hot spot: a concentration of heat in the mantle capable of producing magma which
rises to the earth’s surface. The Pacific plate moves over a hot spot producing the
Hawaiian islands
19. convective flow: the motion of matter resulting from changes in temperature; the
convective flow within the mantle is due to earth’s unequal heating and causes the
tectonic plates to move
20. slab-pull: the mechanism that contributes to plate motion in which cool dense
oceanic crust sinks into the mantle and pulls the trailing lithosphere along
21. ridge-push: mechanism that may contribute to plate motion, it involves the
oceanic lithosphere sliding down the oceanic ridge under the pull of gravity
22. mantle plume: a mass of hotter than normal mantle material that ascends to the
surface where it may lead to igneous activity