EESA06 Final Exam Ch2 Summary + 40 MCQ/T or F

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19 Apr 2012
Chapter 2: Plate Tectonics
Chapter Summary:
Plate tectonics is a theory that suggests Earth's surface is divided into several large plates that
change position and size. Intense geologic activity occurs at plate boundaries.
Plate tectonics combines the concepts of sea-floor spreading and continental drift.
Alfred Wegener proposed continental drift in the early 1900s. His evidence included coastline fit,
similar fossils and rocks in now-separated continents, and paleoclimatic evidence for apparent
polar wandering. Wegener proposed that all continents were once joined together in the
supercontinent Pangea.
Wegener's ideas were not widely accepted until the 1950s, when work in paleomagnetism
revived interest in polar wandering.
Evidence for continental drift includes careful fits of continental edges and detailed rock matches
between now-separated continents. The positions of continents during the past 200 million years
have been mapped.
Hess's hypothesis of sea-floor spreading suggests that the sea floor moves away from the ridge
crest and toward trenches as a result of mantle convection.
According to the concept of sea-floor spreading, the high heat flow and volcanism of the ridge
crest are caused by hot mantle rock rising beneath the ridge. Divergent convection currents in the
mantle cause the rift valley and earthquakes on the ridge crest, which is a spreading axis (or
centre). New sea floor near the rift valley has not yet accumulated pelagic sediment.
Sea-floor spreading explains trenches as sites of sea-floor subduction, which causes low heat
flow and negative-gravity anomalies. Benioff zones and andesitic volcanism are caused by
interaction between the subducting sea floor and the rocks above.
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Sea-floor spreading also explains the young age of the rock of the sea floor as caused by the loss
of old sea floor through subduction into the mantle.
Plates are composed of blocks of lithosphere riding on the plastic upper mantle or asthenosphere.
Plates move away from spreading axes, which add new sea floor to the trailing edges of the
An apparent confirmation of plate motion came in the 1960s with the correlation of marine
magnetic anomalies to magnetic reversals by Morley, Vine, and Matthews. The origin of
magnetic anomalies at sea apparently is due to the recording of normal and reverse
magnetization by dikes that intrude the crest of the mid-oceanic ridge, then split and move
sideways to give anomaly patterns a mirror symmetry.
The Morley-Vine-Matthews hypothesis gives the rate of plate motion (generally 1 to 6 cm/year)
and can predict the age of the sea floor before it is sampled.
Deep-sea drilling has apparently verified plate motions and the age predictions made from
magnetic anomalies.
Earthquake distribution and first-motion studies on transform faults on fracture zones also verify
plate motions.
Divergent plate boundaries are marked by rift valleys, shallow-focus earthquakes, high heat flow,
and basaltic volcanism.
Transform boundaries between plates sliding past one another are marked by strike-slip
(transform) faults and shallow-focus earthquakes.
Convergent plate boundaries can cause subduction or continental collision. Subducting plate
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boundaries are marked by trenches, low heat flow, Benioff zones, andesitic volcanism, and
young mountain belts or island arcs.
Continental-collision boundaries have shallow-focus earthquakes and form young mountain belts
in continental interiors.
The distribution and origin of most volcanoes, earthquakes, young mountain belts, and major
sea-floor features can be explained by plate tectonics.
Plate motion was once thought to be caused by mantle convection, but is now attributed to the
cold, dense, leading edge of a subducting plate pulling the rest of the plate along with it (slab-
pull). Plates near mid-oceanic ridges also slide down the sloping lithosphere-asthenosphere
boundary at the ridge (ridge-push). Trench-suction may help continents diverge.
Mantle plumes are narrow columns of hot, rising mantle rock. They cause flood basalts and may
split continents, causing plate divergence.
An aseismic ridge may form as an oceanic plate moves over a mantle plume acting as an eruptive
centre (hot spot).
1. The southern supercontinent was called: (Page 23)
A. Laurentia
B. Glossopteris
C. Gondwana
D. Pangea
2. The sliding of seafloor beneath a continent or island arc is known as: (Page 43-46)
A. Obfuscation
B. Obduction
C. Subduction
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