Fig. 18-2, p. 419
Fig. 18-2a, p. 419
Fig. 18-2b, p. 419
Fig. 18-2c, p. 419
PLATE TECTONICS
The unifying theory of geology
Plate Tectonics explains many geologic
phenomena, such as volcanism,
earthquakes, mountain building,
changes in climate, flora and fauna
distributions, and the distributions of
natural resources.
Basics of Plate Tectonics
Earth’s surface is broken into numerous tectonic
plates (8 larger plates and several smaller pieces)
Tectonic plates are moving (variable rates from 1 cm
to 18 cm per year)
Tectonic plates interact at their boundaries
Moving tectonic plates cause the simultaneous
renewal and destruction of the Earth’s surface.
Heat-transfer mechanism known as convection in the
mantle is thought to be the mechanism for the
movement of tectonic plates.
Plate Tectonics
Continental Drift + Sea Floor Spreading
= Plate Tectonics
The continents are carried with the
moving sea floor as the lithospheric
plates travel.
The Lithospheric Plates
Tectonic Plates
Fig. 2-14, p. 38
The Earth’s Layers
The layers of the Earth result from density
differences between the layers caused by
variations in composition, temperature, and
pressure.
Continental Crust: SiAl (rock)
Oceanic Crust: SiMa (ferromagnesian rock—
basalt--mafic)
Mantle: FeMg (Peridotite—ultramafic) (rock)
Core: Fe and a small amount of Ni (metal)
Fig. 1-10c, p. 14
Lithosphere and Asthenosphere
Lithosphere is the solid, brittle outer layer of
the Earth composed of:
Oceanic and continental crust
Top part of the mantle
Asthenosphere is the plastic layer of the mantle
directly below the lithosphere over which the
lithospheric plates move.
The lithosphere is broken into many pieces called
plates.
Fig. 1-11, p. 15
Plate Tectonics and Surface
Features
Trench
Ridge
Fault
Volcanoes
Mountain Ranges
Islands or Island Arcs
Mantle Plumes and Hot Spots
The Hawaiian Islands
The Mechanism for Plate Motion is Convection in the Mantle
Fig. 1-12, p. 15
Fig. 1-17, p. 20
Three types of plate boundaries
1.
Divergent plate boundary
2. Convergent Plate Boundary
3. Transform Plate boundary
Fig. 1-14, p. 18
Table 2-2, p. 39
Table 1-3, p. 16
Fig. 2-15, p. 40
Plate Boundary Structures
Oceanic Ridges (MOR and Iceland)
Rift zones (East African Rift Zone)
Trenches (Marianas Trench)
Island Arcs (Japan, Indonesia)
Mountain Ranges (Alps, Andes, Himalayans)
Faults (San Andreas Fault)
Volcanoes (Mount St. Helens, Mont Pelee,
Vesuvius)
Mid-Oceanic Ridge: Divergent
Plate Boundary
Fig. 2-12, p. 36
Mid-Oceanic Ridge: Divergent
Plate Boundary: formation of an
ocean
Fig. 2-15, p. 40
Mid-Oceanic Ridge: worldwide
distribution
http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/
Iceland
Iceland
Iceland and the Mid-Oceanic
Ridge (MOR)
Iceland and the MOR
http://www.volcanodiscovery.com/volcano-tours/iceland_volcanoes.html
East African Rift Zone
Spreading center activity leading
to separation of continental crust
Fig. 2-16, p. 41
Convergent Plate Boundaries
Subduction
zone
Volcanic Island Arc
Convergent Zone: oceanic crust melts
and volcanoes develop
Fig. 2-17, p. 42
Convergent Plate Boundary: Himalayan Mountains or
Alps form as a result of the collision of two continental
masses
Fig. 2-19, p. 43
The Alps
The Marianas Trench
http://www.abc.net.au/science/news/stories/s897958.htm
http://www.worsleyschool.net/science/files/marianas/trench.html
San Andreas Fault, California
Fig. 2-20, p. 44
Vesuvius and Pompeii
Fig. 2-23, p. 46
Fig. 2-24, p. 47
Catch all
Explains how rocks now present in one
climatic region of the Earth actually
formed in another.
Explains that the continents and sea
floor are in motion.
Table 2-1, p. 28
Plate tectonics is
evidenced by
continental drift and
by seafloor spreading
CONTINENTAL DRIFT
Based upon geologic, paleontologic and
climatologic evidence.
The continents were one united in to a
single supercontinent called Pangaea.
Early Ideas For Continental Drift
Edward Suess (1885)
Recognized the presence of
Glossopteris flora on five southern
“continents” (Antarctica, Asia, Africa,
India and South America)
Called this collection Gondwanaland.
Explains this similarity among fossils by
the appearance and disappearance
of land bridges
Alfred Wegener (A German meteorologist developed the hypothesis of
continental drift in 1915)
Fig. 2-3, p. 29
Alfred Wegener (1915)
Credited with the development of the
hypothesis of Continental Drift (1915)
Proposed that all land masses were originally
united into a single supercontinent called
Pangaea.
Pangaea subsequently separated in a series of
breakups.
Based his conclusions upon geologic,
paleontologic and climatologic evidence.
He did not put forth a convincing mechanism for
the drift of the continents
Alexander du Toit (1937)
Placed the five southern
“continents”—Gondwanaland--at
the south pole to explain the
presence of glacial deposits
Placed the northern continents—
Laurasia—near the equator to
explain the presence of coal
deposits.
Evidence for Continental Drift
Continental Fit
Similarity of Rock Sequences and
Mountain Ranges
Rocks of the Appalachians end abruptly and
appear again in Greenland, Ireland, Great
Britain and Norway (figure 2.5)
Glacial Evidence
Glacial till
Glacial striations in exposed rock (figure 2.6)
Pangea (the
supercontinent)
Fig. 2-4, p. 30
Appalachian and Caledonian Mountains are same age, but have been
separated by moving plates.
Fig. 2-5, p. 31
Glacial Striations formed when great thickness of ice moved over the rock. Direction of
striations associated with this rock formation agrees with the configuration of these
continents.
Fig. 2-6, p. 31
Evidence for Continental Drift
Fossil Evidence
Glossopteris flora of Late Paleozoic
Mesosaurus of Late Permian
Lystrosaurus and Cynognathus of Early
Triassic (figure 2.7)
Glossopeteris leaves from upper Permian Period.
Found on all five Gondwanaland landmasses.
Fig. 2-2, p. 29
Paleontological and Paleobiological
Evidence for Pangea
Fig. 2-7, p. 32
Evidence for Continental Drift
--Paleomagnetism and Polar Wandering
Remnant magnetism in ancient rocks
Iron-bearing minerals atoms in magma or lava aligns
with the Earth’s magnetic pole
Record both direction and strength of the magnetic
field
The iron-bearing minerals are “frozen” into place at the
Curie point, when the magma or lava cools.
The movement of the crust away from the ridge or
spreading zone shows a pattern of magnetic pole
reversals (figure 2.10)
Arrangement of continents in Pangaea
based upon paleomagnetic mapping is
supportive of Continental Drift
Earth’s
Magnetic
Field:
Today
Fig. 2-8a, p. 34
Magnetic field lines show the position of the magnetic
north and south poles.
Fig. 2-8b, p. 34
Fig. 2-10, p. 35
The iron in the lava is aligned with the current
magnetic configuration and is frozen in place when
the lava cools.
Fig. 2-11, p. 36
Pole paths actually indicate
the motion of the plates
leading up to Pangea when
both continents were
adjacent to each other
Fig. 2-9, p. 35
Relative ages of sea floor
rocks are oldest away
from the ridge. This
supports the idea that the
sea floor is spreading
away from the ridge.
Fig. 2-13, p. 37
Fig. 2-6a, p. 31
Fig. 2-6b, p. 31
Fig. 2-12, p. 36
Fig. 2-14, p. 38
Plate Tectonic Theory
Provides a framework for interpreting the
composition, structure and internal processes of the
Earth (see Table 1.3).
Explains that Earth is broken into lithospheric plates
that move over the asthenosphere.
Shows relationship between plate boundaries and
zones of major geologic activity.
Establishes a basis for explaining cyclical and
systematic destruction and renewal of the crust.
Based upon Alfred Wegener’s Continental Drift
hypothesis, which was later revised and improved
upon by Harry Hess’s theory of sea-floor spreading.
Fig. 2-15a, p. 40
Fig. 2-15b, p. 40
Fig. 2-15c, p. 40
Fig. 2-15d, p. 40
Fig. 2-17a, p. 42
Plate Tectonic Theory
Provides a framework for interpreting the
composition, structure and internal processes of the
Earth (see Table 1.3).
Explains that Earth is broken into lithospheric plates
that move over the asthenosphere.
Shows relationship between plate boundaries and
zones of major geologic activity.
Establishes a basis for explaining cyclical and
systematic destruction and renewal of the crust.
Based upon Alfred Wegener’s Continental Drift
hypothesis, which was later revised and improved
upon by Harry Hess’s theory of sea-floor spreading.
Fig. 2-17b, p. 42
Fig. 2-18, p. 42
Fig. 2-18a, p. 42
Fig. 2-18b, p. 42
Fig. 2-19a, p. 43
Fig. 2-19b, p. 43
Fig. 2-20a, p. 44
Fig. 2-20b, p. 44
Fig. 2-20c, p. 44
Fig. 2-21, p. 44
Fig. 2-22, p. 45
Fig. 2-22a, p. 45
Fig. 2-22b, p. 45
Fig. 2-24a, p. 47
Fig. 2-24b, p. 47
Fig. 2-25, p. 47
CHAPTER OBJECTIVES
1 Plate tectonics is the unifying theory of geology and has revolutionized
geology.
2 The hypothesis of continental drift is based on considerable geologic,
paleontologic, and climatologic evidence.
3 The hypothesis of seafloor spreading accounts for continental movement, and
thermal convection cells provide a mechanism for plate movement.
4 The three types of plate boundaries are divergent, convergent, and transform,
and along these boundaries new plates are formed, consumed, or slide past
one another.
5 The interaction of plates along their boundaries accounts for most of Earth’s
earthquake and volcanic activity.
6 The rate of movement and motion of plates can be calculated in several ways.
7 Some type of convective heat system is involved in plate movement.
8 Plate movement affects the distribution of natural resources.
CHAPTER OUTLINE
• Introduction
• Early Ideas about Continental Drift
• Evidence for Continental Drift
• CULTURAL CONNECTIONS: The Struggle toward Scientific Progress
• Magnetic Reversals Related to Seafloor Spreading
• Plate Tectonic Theory
• Three Types of Plate Boundaries
• Plate Movement and Motion
• The Driving Mechanism of Plate Tectonics
• Tectonics and Natural Resources
• GEO-FOCUS 2.1: Oil, Plate Tectonics, and Politics
• GEO-RECAP
Fig. 2-CO, p. 26
Fig. 2-1, p. 28
CHAPTER SUMMARY
• The concept of continental movement is not new. The earliest maps showing
the similarity between the east coast of South America and the west coast of
Africa provided the first evidence that continents might once have been united
and subsequently separated.
• Alfred Wegener is generally credited with developing the hypothesis of
continental drift. He provided abundant geologic and paleontologic evidence to
show that the continents were once united into one supercontinent he named
Pangaea. Unfortunately, Wegener could not explain how the continents moved,
and most geologists ignored his ideas.
• The hypothesis of continental drift was revived during the 1950s when
paleomagnetic studies indicated the presence of multiple magnetic north poles
instead of just one as there is today. This paradox was resolved by constructing
a hypothetical map and moving the continents into different positions, making
the paleomagnetic data consistent with a single magnetic north pole.
• Magnetic surveys of the oceanic crust revealed magnetic anomalies in the
rocks, indicating that Earth’s magnetic field has reversed itself in the past.
Because the anomalies are parallel and form symmetric belts adjacent to the
oceanic ridges, new oceanic crust must have formed as the seafloor was
spreading.
CHAPTER SUMMARY
• Seafloor spreading has been confirmed by radiometric dating of rocks on
oceanic islands. Such dating reveals that the oceanic crust becomes older
with distance from spreading ridges.
• Plate tectonic theory became widely accepted by the 1970s because of the
overwhelming evidence supporting it and because it provides geologists with
a powerful theory for explaining such phenomena as volcanism, earthquake
activity, mountain building, global climatic changes, the distribution of the
world’s biota, and the distribution of some mineral resources.
• The supercontinent cycle indicates that all or most of Earth’s landmasses
form, break up, and re-form in a cycle spanning about 500 million years.
• The three types of plate boundaries are divergent boundaries, where plates
move away from each other; convergent boundaries, where two plates collide;
and transform boundaries, where two plates slide past each other.
• The average rate of movement and relative motion of plates can be calculated
in several ways. The results of these different methods all agree and indicate
that the plates move at different average velocities.
CHAPTER SUMMARY
• The absolute motion of plates can be determined by the movement of plates
over mantle plumes. A mantle plume is an apparently stationary column of
magma that rises to the surface where it becomes a hot spot and forms a
volcano.
• Although a comprehensive theory of plate movement has yet to be
developed, geologists think that some type of convective heat system is the
major driving force.
• A close relationship exists between the formation of some mineral deposits
and petroleum and plate boundaries. Furthermore, the formation and
distribution of some natural resources are related to plate movements.