6. Key Ideas Plate
Tectonics
Plate tectonic theory
suggests that Earth’s surface
is divided into segments
called lithospheric plates,
which move, collide, and
slide past one another.
7. Key Ideas Plate
Tectonics
One force that drives plate
motion is heat-driven
convection currents in the
mantle. The heat is generated
by the decay of radioactive
elements within Earth.
8. Key Ideas Plate
Tectonics
Plate tectonics explains the
large-scale features of Earth’s
surface, and the young age of
the seafloors.
9. Key Ideas Plate
Tectonics
Many scientific disciplines
have contributed to the
confirmation of the theory of
plate tectonics.
10. A Layered Earth - Density
Density is a key concept for understanding
the structure of Earth.
Density measures the mass per unit volume
of a substance.
Density = Mass
Volume
Density is expressed as grams per cubic
centimeter.
Water has a density of 1 g/cm3
11. A cross section
of Earth showing
the internal
layers. These
layers can be
described by
their chemical
and physical
characteristics.
Layered Earth
12. What evidence supports the idea that Earth has layers?
The behavior of seismic waves generated by earthquakes
give scientists some of the best evidence about the structure
of Earth.
(above-left) S waves cannot penetrate Earth’s liquid core.
(above-right) P waves are bent as they pass through the liquid
outer core.
Evidence Of Earth’s
Layers
13. Chemical Properties of Earth’s Layers
Layer Chemical Properties
Continental
Crust
Composed primarily of granite
density = 2.7 g/cm3
Oceanic
Crust
Composed primarily of basalt
density = 2.9 - 3.3 g/cm3
Mantle Composed of silicon, oxygen,
iron and magnesium
density = 4.5 g/cm3
Core Composed mainly of iron
Density = 13 g/cm3
Note that Earth is density stratified, that is, each
deeper layer is denser than the layer above.
15. Physical Properties of Earth’s Layers
Layer Physical Properties
Lithosphere The cool, rigid outer layer
Asthenosphere Hot, partially melted layer which
flows slowly
Mantle Denser and more slowly flowing
than the asthenosphere
Outer Core Dense, viscous liquid layer,
extremely hot
Inner Core Solid, very dense & extremely hot
A cool, rigid, less dense layer (the lithosphere) floats
on a hot, slowly-flowing, more dense layer (the
asthenosphere).
16. Why don’t tall mountains sink into the
asthenosphere? How are features such as
mountains supported?
The concept of buoyancy is illustrated by a ship on the ocean. The ship sinks until
it displaces a volume of water equal to the weight of the ship and its contents.
Isostatic Equilibrium
(right) Earth’s
lithosphere is supported
on the asthenosphere in
a similar manner, with a
few important
differences. Instead of
buoyancy, the term
isostatic equilibrium
describes the way the
lithosphere is supported
on the asthenosphere.
18. Nature of Crustal Material
• Ocean floor
– develops in 200 million year cycle
– no floor older than 200 million years
• oldest known is 175 million years
19. Nature of Crustal
Material
• Ocean floor
– develops in 200 million year cycle
– no floor older than 200 million years
• oldest known is 175 million years
• Continental Material
– less dense than ocean floor and therefore floats
atop it
– very old cores
• core of North America ~2 billion years old
– fragmented by plate tectonics
– grown by aggregation due to plate tectonics
20. •Group of rocks all moving in
the same direction
•Can have both oceanic and
continental crust or just one
kind.
Plates
21. Earth’s Crustal Plates
• About 12 major rigid plates and
many microplates
• plates slide over partially molten,
weak aesthenosphere
• continents are embedded in plates
and are carried along passively
22. The major lithospheric plates and their direction
of relative movement are shown here.
The Major Lithospheric
Plates
23. Types of plate
boundaries
• divergent: mid-ocean ridges
• convergent: collision zones
volcanic arcs
• transform: San Andreas fault,
North Anatolian Fault, Turkey.
30. The Gulf of
California
Formed by
Rifting of
Baja
California
from
Mainland
Mexico
Worldsat International/Photo Researchers
31. Convergent boundaries
• New crust created at MOR—old crust
destroyed (recycled) at subduction
zones (i.e., the Earth is not
expanding)
• Relative important densities:
continental crust ≈ 2.7 g/cm3
oceanic crust ≈ 2.9 - 3.2 g/cm3
asthenosphere ≈ 4.5 g/cm3
41. Himalayas and Tibetan Plateau
• Product of the collision
between India and Asia.
• Collision began about 45 M
yr. ago, continues today.
• Before collision, southern Asia
looked something like the
Andes do today.
44. Locations where crustal plates move past one
another, for example, the San Andreas fault.
Transform Boundary
(above) Translation at transform boundaries causes shear.
45. How old is Earth?
The age of Earth has been subject to debate.
Scientists now use an age of 4.6 billion years.
The principle of uniformitarianism was
introduced in 1788. This principle sates that the
forces which shaped Earth are identical to forces
working today.
Catastrophism is the thought that Earth is very
young, and events described in the Bible are
responsible for the appearance of Earth’s features.
-- Usher 9:00am Oct. 23, 4004 BC
46. T. C. Chamberlin and R. D. Salisbury,
1904
Computed the amount of shrinkage (of Earth) for a
given amount of cooling and concluded that “It is
obvious that if the Earth shrinks, its crust must
become too large for the reduced spheroid, and
must be compressed or distorted to fit the new form.
48. Where does the heat within Earth’s
layers come from?
Radioactive decay
This heat travels in convection currents in the
mantle.
Internal Heat
49. New Evidence Supports Wegener’s Theory
of Continental Drift
Wegener’s theory of continental drift was out
of favor with the scientific community until new
technology provided evidence to support his
ideas.
• Seismographs revealed a pattern of
volcanoes and earthquakes
•Radiometric dating of rocks revealed a
surprisingly young oceanic crust.
• Echo sounders revealed the shape of the
Mid-Atlantic Ridge
50. Seafloor spreading was an
idea proposed in 1960 to
explain the features of the
ocean floor. It explained the
development of the seafloor
at the Mid-Atlantic Ridge.
Convection currents in the
mantle were proposed as
the force that caused the
ocean to grow and the
continents to move.
(right) The Mid-Atlantic
Ridge conforms to the
shape of the adjacent
continents. The inset shows
the central rift.
Seafloor Spreading - A Key Idea
51. (above-left and above-right) The patterns of paleomagnetism support plate
tectonic theory. The molten rocks at the spreading center take on the polarity
of the planet while they are cooling. When Earth’s polarity reverses, the
polarity of newly formed rock changes.
Paleomagnetism: strips of alternating magnetic polarity at spreading regions.
Confirmation of the Theory of
Plate Tectonics
52. Apparent Polar wandering:
plate movement causes the
apparent position of the
magnetic poles to have
shifted.
Confirmation of the Theory of
Plate Tectonics
53.
54. Hot Spots: Surface expression of plumes of magma.
Confirmation of the
Theory of Plate Tectonics
A volcanic island chain can form when a plate passes
over a hot spot and a stationary mantle plume.
55. Atolls and Guyots: Coral formations and submerged volcanic mountains.
(above) Guyots were once volcanic peaks above sea level. They were
eroded by wave action as they sank beneath the surface of the water.
Confirmation of the
Theory of Plate Tectonics
56.
57.
58.
59. Age and distribution of ocean sediments:
The sediment in the ocean is thinner and
younger than the age of the ocean indicates
it should be.
The Oceanic ridges: Oceanic ridges are
clear indicators of past events.
Terranes: Oceanic plateaus that form by
uplifting and mountain building as they strike
a continent.
Confirmation of the
Theory of Plate
Tectonics
60. Through the great expanse of geologic time, this slow
movement remakes the surface of Earth, expands and splits
continents, and forms and destroys ocean basins.
Summary
