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Chapter 2: Plate Tectonics
Fig. 2.26
OBJECTIVES
•
Outline Wegener’s idea of drifting continents and describe
the evidence he cited to support his hypothesis as well as
the primary objection to his claims.
•
Explain how paleomagnetism provides support for
continental drift by providing evidence for the concepts of
apparent polar wander and magnetic polarity reversal.
•
Summarize the process of seafloor spreading and the
evidence that indicates that new ocean floor is
continuously being created at mid-ocean ridges.
OBJECTIVES
•
Summarize the evidence that old ocean floor is
continuously being destroyed beneath deep ocean
trenches by a process called subduction.
•
Explain how the theory of plate tectonics provides an
explanation for continental drift and describe the three
types of plate boundaries.
•
Describe how volcanic hotspots are attributed to plumes of
hot material rising from deep within Earth’s interior.
Plate Tectonics: An Overview
• Earth’s rigid outer layer is made up of slabs of rock known
as tectonic plates.
• Plate movement is driven by heat flow from Earth’s interior.
• Plate movement causes
• continental drift
• earthquakes
• formation of mountains ranges
• some volcanoes
Continental Drift
• Hypothesis of continental drift proposed by Alfred Wegener
in 1912
• Wegener’s evidence for continental drift
• Jigsaw fit of the continents
• Continuity of ancient mountain belts
• Continuity of major faults or major
crustal fractures
• Patterns of ancient climates (as
interpreted in rocks)
• Distribution of certain fossil species
Fig. 2.1
Evidence for
Continental Drift
Fossils
Glacial deposits
Figs. 2.5, 2.6
Continental Drift
• Wegener claimed that
300 million years ago,
the continents formed a
single landmass called
Pangea (“all lands”).
• Continents have drifted
slowly to their current
positions.
Fig. 2.2
Continental Drift to Plate Tectonics
• Continental drift hypothesis was not generally accepted.
• No reasonable mechanism proposed
• Additional evidence for movement of the continents was
gathered between the 1920s and 1960s.
• Paleomagnetism
• Age of the seafloor
• Earthquake locations and depths
• By the 1960s, the hypothesis of continental drift had evolved
into the theory of plate tectonics.
Evidence: Paleomagnetism
• Earth’s magnetic field
resembles that of a simple
bar magnet.
• Iron-rich minerals become
magnetized as they
crystallize.
• Orientation of the magnetic
field and apparent location
of the magnetic poles over
time can be inferred from
the rock record.
Fig. 2.7
Apparent Polar Wander
•
Differences in the apparent locations of the north magnetic
pole over time are evidence for continental drift.
Fig. 2.10
Paleomagnetic Reversals
• Earth’s magnetic field periodically reverses (magnetic
polarity reversal).
Fig. 2.11
In normal magnetic polarity, the north magnetic
pole is near the north geographic pole. In reversed
magnetic polarity, the north magnetic pole is near
the south geographic pole.
Magnetic Reversals on the Seafloor
• Magnetometers reveal magnetic anomalies on the
seafloor: alternating strips of high-intensity and lowintensity magnetism.
• High Intensity: rocks formed under normal polarity
• Low Intensity: rocks
formed under
reverse polarity
• Magnetic anomalies
are symmetrical about
the mid-ocean ridges.
• Interpretation:
seafloor spreading
Fig.
2.12
Seafloor Spreading
• Partial melting beneath the mid-ocean ridge causes volcanism.
• Earth’s magnetic polarity is recorded as lava crystallizes.
• Rock moves to either side and new magma is
emplaced along the ridge.
• The seafloor moves symmetrically away
from mid-ocean ridge, moving the ocean
floor and continents.
• Basalts increase in age with distance from
the ridge (confirmed by drilling and analysis
of seafloor rocks).
Fig. 2.13
Subduction
• Roughly the same volume of ocean crust created at mid-ocean
ridges is destroyed in deep ocean trenches through the process
of subduction.
• Old, dense ocean crust dives beneath less dense ocean or
continental crust
• Subduction zones characterized
by
• Ocean trenches
• Shallow to deep earthquakes
• Volcanic arcs
• Evidence
• Inclined zone of earthquakes
• Chemistry of arc lavas
Fig. 2.17
Subduction Zones
Subduction of oceanic lithosphere beneath (a) oceanic lithosphere or (b) continental
lithosphere both result in seismic activity and volcanism.
Fig 2.20
Moving Plates
• Earth’s surface is broken into huge, moving, tectonic plates.
• Tectonic plates are made of rocky and brittle lithosphere.
• Crust plus uppermost mantle
• Plates move over asthenosphere.
• Weak zone in Earth’s mantle
• Plates may be thousands of
kilometers across but are only
50-150 km (30-100 miles) thick.
• Plates are in continuous motion.
Fig. 2.22
Major Plates and Relative Motion
Plate Boundaries
• Earthquake zones outline plate boundaries.
• Three types of plate boundaries:
• Divergent (or Constructive): plates move apart
• Continents split
• Oceans widen
• Convergent (or Destructive): plates move toward each other
• Subduction of oceanic lithosphere
• Continental collision
• Transform: plates slide past each other along transform faults
Plate Boundaries
Fig. 2.23
Hotspots: A Plate Tectonics Enigma
• Geologic activity is focused around plate boundaries.
• Hotspots (isolated areas of long-lived volcanism) also occur far
from plate boundaries.
• Appear to be stationary beneath moving plates
• Widely attributed to mantle plumes
Fig. 2.27
Fig. 2.28
Hawaiian Islands: Movement of the Pacific plate
over the fixed Hawaiian hotspot has led to the
formation of the Hawaiian island chain.
SUMMARY
• Theory of plate tectonics is the unifying theory of Earth science.
• Earth’s surface is broken into plates that move slowly relative
to each other.
• Divergence of plates at ocean spreading centers and
continental rifts
• Convergence of plates at subduction zones and
continental collision zones
• Plates sliding past each other at transform boundaries
• Explains patterns of earthquakes, volcanism, mountain belts,
and outlines of continents.
• Theory evolved from the hypothesis of continental drift proposed
in the early 1900s.
• Continental drift was initially rejected, in part because no
viable mechanism was proposed.
• Evidence accumulated over the next 50 years.
SUMMARY
• The theory of plate tectonics is supported by
• Jigsaw fit of the continents (in features such as mountain
belts, major fault lines, and distinctive rock formations)
• Matching climate and fossil patterns on continents separated
by major oceans
• Apparent polar wander, reflecting the movement of the
continents
• Magnetic stripes on the seafloor
• Deep ocean trenches
• Patterns of earthquakes
• Shallow along transforms and divergent boundaries
• Inclined zones along convergent boundaries (subduction
zones)
• Hotspot tracks